Science Notes on the Moon Project

Foreword: Described below are the generally accepted scientific theories and hypothesizes regarding the creation of the universe, the solar system, various bodies in the solar system and life. Some of these theories are heavily supported by scientific evidence (namely those which deal with the solar system) and others are still highly speculative and debated (those that deal with whole universe). Many competing theories (especially in cosmology) exist, and if history has taught us anything, the addition of new data and new minds will at the very least require modifications to that which is presented below.

The Universe: [The Biggest Perspective on Things | Solar System Formation | Star Death & Matter in the Universe | How Will it End?]
The Solar System: [Sun | Mercury | Venus | Earth | Moon | Mars | Jupiter | Saturn | Uranus | Neptune | Pluto & Debris]
Tech and Notes: [Telescopes & Telescope Types  |  Measuring Distance | Life in the Universe]

Part 1: A Summary of the Universe

The Biggest Perspective on Things
Perhaps when discussing "Life the Universe and Everything", it's best to start at the beginning. Around 14 Billion years ago, the Big Bang occurred. The Big Bang didn't happen in any particular place, rather the whole universe was the size of an infinitely small point where the Big Bang occurred. Only after the Big Bang did the universe have a "size".

For the first 10^-43 seconds of the existence of the universe we simply have no clue what happened because physicists have been unable to find a combination of quantum mechanics and special relativity. From that time until 10^-37 s after the big bang the universe underwent a period of "hyper-inflation" during which its size increased by a factor of 10^50. During this period, the size of the universe is thought to have increased from about one millionth of the size of a proton to many hundreds of millions of lightyears. This incredible expansion occurred many times faster than the speed of light but does not violate special relativity because the universe itself can expand faster than light travels. The hyper-inflation period clears up many problems with the big bang model which I will not discuss here.

Up until 10^-35 s after the big bang, the four fundamental forces of the universe (gravity, electromagnetic, strong and weak) were merged into a single Grand Unified Force, but they then broke apart. By 10^-12 s, strong, gravity and electroweak were distinct forces and electroweak soon broke up into the electromagnetic and weak forces. The separation of forces allowed the universe which up until this point had consisted solely of radiation to begin forming matter.

After the first minute or so of the universe, very high energy photons were converting themselves into the fundamental building blocks of matter such as protons, neutrons and electrons as well as more exotic particles. Since protons themselves are hydrogen nuclei, at this point hydrogen nuclei were forming. Approximately two minutes after the big bang, conditions became ripe for helium and deuterium to fuse from hydrogen along with a few heavier elements. By the end of the first 15 minutes, helium formation ceased and the universe consisted of roughly 75% hydrogen and 25% helium.

Several thousand years after the big bang, dark matter, a kind of matter which doesn't interact very much with radiation, began to form clumps due to gravity. Normal matter was unable to form similar clumps because it interacts radiation, and these interactions prevented atoms, let alone clumps of atoms, from forming.

For the first 380,000 years or so, the universe was radiation dominated. This means that density (mass per unit volume) of radiation was greater than the density of matter. As the universe expanded, the densities of both matter and radiation decreased but amount of matter was conserved. However, in addition to having to occupy more volume, the radiation also experienced a phenomenon called the cosmological redshift which decreased its total energy and thus the density of radiation decreased faster than the density of matter. 

Before the 380,000 year mark, the universe was opaque to electromagnetic radiation because atoms had yet to form. Both elemental nuclei and electrons had formed, but the universe was too hot for them to combine to form atoms. At this time, photons (particles of light) would simply scatter off the free electrons. Around the 380,000 year mark, the universe had expanded and cooled enough for atoms to form. Electrons and nuclei merged to form atoms and the universe became transparent to most frequencies of light. As a result, the light could then escape. As the universe expanded, the light became more redshifted, but we can still detect this early radiation emitted from this early highly ionized universe in the form of Cosmic Microwave Background Radiation (CMB). CMB has also been used to verify many predictions regarding the early universe, dark matter and the structure of the universe.

After the formation of atoms, the hydrogen and helium atoms in the early universe were gravitationally attracted to the clumps of dark matter which had formed while the universe was still radiation dominated. These clouds of atoms are the breeding grounds for star formation and thus the locations of galaxies. Dark matter clumps, which formed only a few thousand years after the big bang, determined the large scale structure of the entire universe.

About 200 million years after the big bang, the first stars and galaxies began to form. (Galaxies are clumps of stars and other interstellar material including dark matter which are held together by gravity.) These early galaxies were relatively small by modern standards, but they formed in chunks in the same areas so they often merged to create the larger galaxies we have today. These galactic mergers continue to this day, and as we examine the universe we see that many galaxies including our own have undergone numerous mergers.

Galaxies are often members of clusters of galaxies and these clusters are often members of superclusters of clusters. The galactic superclusters are the largest units in the universe which are bound (held together) by gravity. Superclusters, despite being the largest gravitationally bound structures, are not the largest structures in the universe. Galaxies appear to be arranged in non-random filaments with large voids of empty space between the filaments. This structure is probably reflective of the early dark matter structures which appeared just a few thousand years after the big bang.

The universe is about 14 billion years old, and it continues expand. A survey galaxies indicates that those which are close to Earth (but not gravitationally bound to our supercluster) are receding slowly, and those which are further away are receding faster. This result does not indicate that Earth (or our supercluster) is the center of the universe, but rather that the expansion of the universe itself continues. If you were to measure the expansion of universe from any point anywhere in the universe, you would find that all galaxies are receding from that point with farther ones receding faster. This is roughly analogous to a raisin in a very large loaf of bread. Before the bread expands all of the raisins in the bread are close together but as the bread expands, the distance from any raisin to all other raisins has increased.

An ongoing question is whether the universe will continue to expand forever or whether gravitational forces will eventually cause it to contract back to a point. Originally, scientists thought that outcome of the universe would be caused by the gravitation attraction between all matter in the universe. If the average density of matter in the universe exceeded a certain constant (known as the critical density), the universe would contract; if the average density of matter was below the critical density, the universe would expand and if it were exactly at the critical density, the rate of expansion would approach zero as time tended toward infinity. It should be noted that in all three cases, the rate of expansion of the universe decreases due to gravitational attraction. The question is simply whether the decrease is sufficient to eventually stop the expansion.

Theoretical models along with some investigations of Cosmic Microwave Background Radiation show that the universe should have exactly the critical density of matter. However, attempts at actually measuring visual matter fall short of this amount by a factor of nearly 25 times. We can explain some of this discrepancy with the aforementioned dark matter which does not radiate energy or interact with radiation. Scientists believe that only about 15% of matter in the universe is normal matter and the other 85% is dark matter. Even when accounting for dark matter, the density of the universe falls considerably short of the critical value.

Unfortunately for theorists, careful attempts at measuring the expansion of the universe by comparing the present rate of expansion to past rates of expansion seem to demonstrate that the universe is in fact expanding faster now than it was before. This cosmic acceleration is problematic because the previous paragraph indicates that any amount of matter should slow down the expansion of the universe due to gravitational attraction. This expansion has been attributed to a phenomenon known as dark energy. Although the exact nature of dark energy is still a matter of debate, it is thought be more or less constant throughout space and to exert a "vacuum pressure" that forces the universe to expand at an increasing rate. If the total amount of matter in the universe were expressed in terms of energy (or dark energy in terms of matter) using Einstein's E=MC^2, we find that the universe is made up of 73% dark energy, 23% dark matter and 4% normal matter. Dark energy, dark matter and normal matter combined are thought to have enough mass to keep the universe at exactly critical density. However, since dark energy does not behave like matter, the universe appears to be destined to continue expanding.

Observationally, we can examine the universe throughout it ages with several techniques. First of all, the observable universe is approximately 14 billion light years in size. Since light travels one light year every year, the distance to an individual celestial object also reflects its age. The light from a star which is one light year away is one year old when it arrives at Earth, and the light from a galaxy which is 500 million light years away is 500 million years old. Thus, by looking at more distant objects we can see further back in time. A telescopic image taken by the Hubble Space Telescope called The Ultra Deep Field may show some galaxies only 400 million years after the big bang. To peer further back, telescopes which can observe at lower frequencies (in the infrared) are necessary. We can also examine the Cosmic Microwave Background Radiation and from its varying intensity at different positions in the sky, we find physical evidence for theoretical predictions regarding the role of dark matter in influencing the structure of the universe, and we can confirm other fundamental assumptions and theoretical predictions.

Solar System Formation
Most theorists believe that our solar system and indeed all star systems in the universe formed using the same basic method. Scattered throughout interstellar space there are clouds of gas at extremely low densities, far below the lowest densities of the best vacuums we can create in labs. These clouds primarily consist of hydrogen and helium gas, which were created by the big bang, but they also have traces of heavier elements which were created by stars and then ejected upon the stars' deaths.

These interstellar clouds occasionally are disturbed by the influence of nearby stars or internal pressure variations due to temperature. These disruptions cause the cloud to collapse into denser fragments whose gravity is sufficient to attract more dust from the cloud, so the fragments grow. As a fragment accumulates more matter, the pressure at the central begins to form a protostar which heats up from the gravitational collapse. As gravity adds more matter to the protostar, its temperature continues to increase until its core becomes hot enough to begin nuclear fusion, the conversion of hydrogen in to helium and energy.

Although matter at the center of the fragment forms the protostar, much of the matter from the interstellar cloud forms as disc (also known as a solar nebula) around protostar. The disc begins with very low density, very far from the star but gravitationally shrinks toward the central protostar. As the disc shrinks, angular momentum is conserved so it begins to rotate faster (this is akin to the situation with a figure skater who speeds up her spin by retracting her arms and slows her spin by extending them). The higher speeds of disc particles effectively puts them into orbit around the protostar, and thus counterbalances shrinkage. So, for the time being, the disc is more or less stable.

Within the disc, the smaller particles begin to collide forming larger particles in turn forming larger planetesimals by gravitational attraction. The planetesimals themselves begin an intense period of collision in which the larger bodies gather more and more mass which increases their gravitational force, allowing them to become even more massive faster. This process is known as accretion.

In the outer reaches of the solar system, where heat from the star is low, many more volatile materials such as water and carbon dioxide become solids and contribute to the mass of the outer bodies. These outer planets eventually become so massive that they can attract and retain the hydrogen and helium gases from the solar nebula. Eventually, the mass of gas "swept up" by these outer bodies becomes so great that it becomes the principle component of these Gas Giant planets, with a great envelope of gas surrounding a small solid core. In our solar system, Jupiter, Saturn, Uranus and Neptune formed in this manner.

The inner planets are too hot for volatile molecules to solidify so they are less massive than the outer planets. As a result, they are composed of rocks and metals which have high densities but are far rarer than the volatile elements in gas giants. These rocky (or terrestrial) planets are much smaller and less massive than the gas giants, but have far higher densities (more mass per unit volume). Overtime, internal geological processes may create gases and release an atmosphere through a process called outgassing and impacts with bodies from the outer solar system may bring water and other volatiles. In the case of Earth, certain life forms and geological processes further modified the atmosphere by removing carbon dioxide and creating oxygen.

As the protostar grows in size and stabilizes, its begins to emit a solar wind, a stream of very highly charged particles at extremely high speeds (400 km/s). The solar wind blows the remaining gas in the protoplanetary disc into interstellar space and the solar system we know is left.

The early planets themselves were made predominantly of molten material for they had not had a chance to cool. Like a latte, this liquid state allowed the heavy metals to fall to the centers of the planets while lighter rocky materials floated on top of the metals. As a result, almost all large bodies in the solar system have a differentiated structure with a metallic core at their centers surrounded by a rocky mantle and perhaps an extremely light atmosphere. However, some smaller bodies cooled too quickly to differentiate so they are composed of a more or less uniform mixture of metals and rocks.

Star Death & Matter in the Universe
Stars are the universe's nuclear fusion reactors. After they form, they undergo nuclear fusion in their cores. For much of a star's life, the nuclear fusion transforms primordial hydrogen, hydrogen which was created by the big bang, into helium. Stars do not have an unlimited supply of hydrogen in their cores and eventually they run out. More massive stars burn faster, hotter and brighter, and despite their larger sizes they die sooner, perhaps in as little as ten million years. Less massive stars burn slowly, are cooler and less luminous and can last 1 trillion years.

It is important to note that fusion reactions take place only in a star's central core. Although stars are composed mainly of hydrogen, only a very small portion of the total hydrogen in a star undergoes fusion. The hydrogen outside of the core does not reach a sufficiently high temperature to fuse, but it is instrumental in generating the pressure for fusion and conveying the heat produced by the central fusion reaction to the exterior where it can be radiated into space.

After hydrogen stops burning, the star is left with a helium core. Hydrogen fusion releases an enormous amount of radiation which creates pressure to counteract the contractive force of gravity. With the end of hydrogen fusion, the central core which is now mostly helium begins to contract, raising its temperature and pressure. The hydrogen surrounding the core is heated up so much that it too undergoes fusion. Thus, the star has a core non-burning of helium surrounded by a small shell of burning hydrogen. The extremely high temperatures mean this hydrogen shell burns very quickly. This hot and fast burning shell causes the star to become brighter and increases the star's internal pressure, causing it to expand. For this reason, a pre-helium burning star is often referred to as a red giant.

Helium does not burn (fuse) as easily as hydrogen because a helium nucleus has 2 protons so the electrostatic repulsion between nuclei is greater. Nevertheless, in all but the least massive stars, the gravitational contraction is eventually sufficient to push helium to its fusion temperature. When helium fusion starts the incredible amounts of energy released in the core cause an explosion and the star's core quickly expands under the pressure of helium fusion. The expansion is so great that the star actually cools down and leaves the red giant state. The star then stabilizes and for a while (perhaps 10 million years) fuses helium into carbon. All the while the core is surrounded by a shell of burning hydrogen.

As the central core of the star becomes dominated by the carbon, the helium fusion ends. At the end of helium fusion, the results are similar those at the end of hydrogen fusion. A shell of burning helium surrounds the non-burning carbon core and the helium shell is surrounded by a shell of burning hydrogen. The gravitational forces cause the core of the star to contract driving it hotter and hotter and the shells of helium and hydrogen burn even more fiercely than before, and it becomes a red giant again.

At this point, the behavior of a star diverges down two separate paths. Most smaller stars, those which are less than 8 times the Sun's mass, are not massive enough to fuse carbon. The pressure generated from the helium and hydrogen fusion creates intense radiation which begins to blow away the star's own non-burning envelope of material surrounding the core. This material is ejected into space at enormous speeds, and it is heated by the now dying core, creating ionized gases. These gases are known as the star's planetary nebula (even though they have nothing to do with planets). The planetary nebula is blown off in roughly equal amount in all directions, forming a sphere of shining gas around the extinct star's core.

Most of the remains of the core do not get blown into space. Instead, fusion ends with a final few nuclear reactions which create some heavy elements. The core then shrinks because the pressure of nuclear fusion no longer counterbalances gravity. It shrinks until electron degeneracy pressure, a pressure due to a quantum mechanical effect called the Pauli Exclusion Principle, prevents the atoms from condensing any further.  What is left is a very hot carbon ball known as a white dwarf. Eventually, the white dwarf cools and its corresponding light emissions dwindle. In this final state, it is known as a black dwarf. Such is the eventual fate of our Sun.

While the small stars live long lives and slowly fade away, larger stars (those with 8 times our Sun's mass or more) live short lives and die quick and violent deaths. Unlike lighter stars the heavy stars are massive enough to fuse carbon into oxygen. However, fusion does not stop there; the star is massive enough to continue fusion of even heavier elements. Oxygen can be fused to neon then, neon to magnesium then, magnesium to silicon and finally silicon to iron. As the core burns each element, it is surrounded by shells which burn all of the elements which fused previously. Also, each heavier element undergoes fusion more and more quickly. Because less energy is released from the fusion of heavier elements is less than that of lighter elements, much larger quantities must be burned to cancel out the gravity. A star that took 10 million years to fuse hydrogen to helium may fuse all of its silicon to iron in a week. Over this period, the star's radius increases and its surface temperature drops. For this reason, stars in this phase are known as red supergiants.

Once a massive star has an iron core, fusion cannot continue. All elements with an atomic mass below that of iron can be fused together to create more energy. In stars, this energy creates pressure which counterbalances the gravitational forces which try to collapse the star. However, iron cannot be fused to create to energy. Instead, fusion of iron requires more energy than it produces. So, the force of gravity is more or less unchecked and the star begins to implode. The iron in the core of the star actually causes the star to cool down by absorbing photons which break the iron into lighter component elements. This "photodistingration" decreases the star's internal pressure further and accelerates the implosion. The collapse continues until finally the neutrons themselves are so closely packed that the collapse can go no further due neutron degeneracy pressure, an effect which like electron degeneracy pressure which also is derived from the Pauli Exclusion Principle in quantum mechanics.

This dense packing of neutron causes an absolute barrier past which the core cannot collapse any further. The collapse at this point has considerably exceeded the equilibrium point between pressure and gravity and it begins to rebound incredibly quickly. The shockwave generated by this rebound causes an explosion which blows much of the mass of the star, including the shells of elements surrounding the core into space. This explosion is known as a Type II supernova or core collapse supernova. Such events are visible to the human eye over distances of hundreds of thousands of light years.

After a core collapse supernova, the core of the star itself survives. The end result of a core collapse supernova depends on the mass of remaining core. If the core is reasonably low density, below three times our Sun's mass, it shrinks until the neutrons are packed so closely that neutron degeneracy pressure prevents them from collapsing any further. This core remnant is referred to as a neutron star. Inside a neutron star, the density of matter is so great that a thimbleful worth of matter would have as much mass as a mountain on Earth. Also, the angular momentum in a neutron star was conserved when it contracted, so the neutron star spins incredibly quickly, completing rotations in mere fractions of a second. It is likely that all neutron stars also have intense magnetic fields which coupled with their high rotation rates cause them to emit high intensity and very directional radiation. On Earth, we are only able to observe these radio emissions if we happen to be in the plane of the very thin cone over which the radiation is emitted. Such sources have been observed and are referred to as pulsars. Many researchers believe that all neutron stars behave as pulsars, but we are simply not in the appropriate position to observe the radio emissions from most of them. Interestingly, the fast repetitive signals emitted from pulsars are thought to be the most accurate natural clocks in the universe.

If the mass of the core is greater than about three times our Sun's mass, even neutron degeneracy pressure is no longer sufficient to stop the core collapse. Without the nuclear fires which generated enormous amounts of pressure during the star's life nothing prevents the core from collapsing. Eventually the collapsing core's density reaches the point where the force of gravity is so great that not even light can escape and the object becomes known as black hole. In theory, the central mass should continue shrinking all the way to an infinity small point which is known as the singularity. However, singularities are impossible to observe because nothing, including light, can ever exit the black hole to indicate its internal structure.

The principle characteristic of a black hole is its event horizon, the sphere around the singularity beyond which no light can escape. The size of the event horizon is proportional to the mass of the black hole. Only two other physical properties of a black hole even measurable: its rotation rate and net charge. All other information, including the composition of the matter which condensed to form it is not retrievable.

Black holes are often surrounded by an accretion disc, a disc of matter which orbits the black hole. For black holes which resulted from star collapse, this matter is probably the stellar material which was ejected during the supernova. Some matter from the accretion disc is heated up by the incredible gravitationally forces to create a radiation in the form of X-Rays or Gamma Rays. Since observing the blackhole itself is essentially impossible (no light can escape), the best technique for detecting black holes is searching for their accretion discs. Scientists believe that there are black holes are the cores of galaxies partly due to observations of very bright radiation which was likely caused by the accretion discs in active galaxies and quasars.

One interesting consequence of supernovae is that in the brilliant explosion some elements form. The explosion is so energetic that some atomic nuclei are torn apart to produce some free protons and neutrons. These protons and neutrons are then fused with other nuclei to create heavy elements. This process is similar to less violent process by which heavy elements are created at the end of low mass stars which was discussed previously. The heavy element formation in both cases is an energy losing reaction, more energy is required than is produced, but large amounts of energy are available at the times of these reactions. Without these rare events, no elements with atomic numbers greater than iron's 26 could be produced. Elements such as nickel, copper, zinc, silver, lead, gold, titanium and uranium were formed during stars' deaths, and almost all of the elements in the universe other than helium and hydrogen were produced in stars. The calcium and oxygen in our bodies, the nitrogen and carbon in our atmosphere and the silicon and iron in our rocks were all made by stars.


How Will It End?
In the Babylon 5 Episode "The Coming of Shadows", the dying Centauri Emperor comes to Babylon 5 in hopes of reaching a peace accord with the hated Narns. He fails because members of his own court would like to use war to further their own political agendas. As he lies on his deathbed, he is visited by Kosh, the representative of the Vorlons, the oldest and most mysterious race. He asks, "how will it end", and the Vorlon replies, "In fire".

For Earth at least, things probably will end in fire. Our Sun is about 5 Billion years old and will last for roughly 5 Billion more. At that point it, its core will deplete its supply of hydrogen and it will expand into a red giant. The bloated Sun will probably be outputting enough energy to evaporate all water on the Earth and end all life. The Sun will eventually expand beyond Earth's present orbit, but as the Sun expands, it will lose some mass to space so it is possible that the Earth will avoid being swallowed by the Sun. However, even if the Earth avoid this fate, it will be a dead and lifeless planet. One needn't worry though, human beings have only been around for about 1 million years and multicellular organisms are about 1 billions years old. The odds of anything that even remotely resembles us being around 5 billion years are extremely low.

In the grand scheme of things, the beginning and end of the Earth/Sun system is not very significant. Stars will continue to form and die over the next 100 trillion years or so. At that point, most of the lighter elements in the universe will have been fused by stars into heavy elements which do not generate much energy through fusion. As a result, no more stars will form. The universe as we know it will be gone.

In the extremely long term, there are three plausible scenarios which are known as the "Big Rip", "Big Crunch" and "Heat Death of the Universe". Which of these scenarios ultimately occurs requires a better understanding of dark energy and gravity than we presently have. The "Big Crunch" would occur only if the universe is past the critical density and the force of gravity is sufficient to make the universe collapse back to a point (even after accounting for dark energy). The "Big Rip" would occur if dark energy is so great that it overwhelms gravity, then the electromagnetic force which hold molecules together and finally the strong nuclear force which holds atoms together. Finally, the "Heat Death of the Universe" occurs if gravitational attraction is insufficient to cause the "Big Crunch" and dark energy is insufficient to cause the "Big Rip." Most theories seem to center around the heat death scenario, so, for the remainder of this section, I will discuss the heat death scenario.

The heat death of the universe is perhaps a slightly incorrect term. It seems to imply that the universe will heat up a lot when it will in fact be extremely cool. The name is derived from an area of physics known as thermodynamics which basically states that over time, the universe will become less and less ordered. The disorder is predominately in the form of heat. For an example on a macroscopic scale, I could break up a rock but putting it back together will require energy which I must get from somewhere. Getting that energy may require hydrogen to be fused into helium, plants to die or some other effect which produces some waste heat. Ultimately, I cannot increase the order in the rock without creating more disorder elsewhere.

After most stars have died, the remaining matter in the universe will be gradually consumed by black holes. Objects in the universe will gradually be scattered by their mutual gravitational interactions. By 10^40 years after the big bang, most of the remaining protons in the universe will have decayed into gamma rays and almost all matter will have been absorbed by black holes. The black holes themselves do not last forever. By 10^150 years, all Black Holes will have evaporated by a quantum mechanical oddity referred to as Hawking Radiation. After this point, the universe gets quite boring. Other than the occasional particle popping up due to quantum mechanical effects, the universe is nothing but subatomic particles such as photons and neutrinos.

Of course, all of this is an extraordinarily long time away. The universe is only about 1.4*10^10 years (14 Billion) old. It will take 100,000 times longer than that before the age of stars ends and it will be 10^30 times longer than that before black holes absorb all matter and protons decay. However, it is important to note that not only the universe as we know it, but matter, the stuff we are made of and the stuff that universe as we know it is made of seems to have a finite lifetime.

Part 2: Specific Moon Site Science Notes

The Sun
The Sun is the producer of nearly all of the energy in our solar system and the largest and most massive object in our solar system. The Sun has an enormous mass of 2*10^30 kg, 330,000 times that Earth, and a radius of nearly 700,000 km, 109 times that of Earth. The Sun's large mass gives it extraordinary gravitational force which is sufficient to retain orbiting objects at distances of over 1 lightyear. Despite its incredible power, mass and gravitational force, the Sun is actually a low density body, its 1410 kg/m^3 density is comparable to that of the gas giant planets and much lower than all of the Earth-like terrestrial planets. Measurements of the Sun's rotation period are difficult. The Sun is not a solid a ball but rather a gaseous one, and different part of it rotate at different speeds. Depending on what you parts of the Sun you use to measure its rotation, you get a result between 25 and 36 days.

The Sun is broken into severals concentric regions. At its center lies its core. In the core, the Sun fuses hydrogen to create helium and enormous amounts of energy. Every second the Sun fuses 600 million tons of hydrogen into helium and in the process it releases an enormous 4*10^26 Watts of power. The energy from these fusion reactions is so hot that the core reaches a temperature of 16 degrees million K (29 million F).  Despite the enormous amount of matter the Sun converts into energy every second, it has been converting hydrogen to helium for about 5 billion years, and it still has enough hydrogen to continue to do so for another 5 billion.

The core of the Sun is the only place where hydrogen fusion takes place, but the Sun is surrounded by many non-burning zones which must carry the intense energy from the core to the solar surface. The first such zone is the radiation zone which surrounds the core. Here, the extreme solar heat and makes the interior of the sun relatively transparent to radiation so most of the energy from the reactions in the core can travel freely for the first 300,000 km away from the core in the form of very high frequency light.

As the distance from the core increases and the amount of energy per unit area drops, the Sun cools down and soon reaches the point where regular atoms with electrons orbiting their nuclei form and block the radiation from traveling any further. Here the process of convection takes over. Warm hot solar material rises to the higher cooler regions where it cools and falls back to the lower warmer regions, forming a loop. In this conductive zone of the Sun, many of these convection cells are stacked on top of eachother and serve to move all of the solar heat energy from a distance of 500,000 km from the Sun's core to 700,000 from the Sun's core. The top layer of these convection cells can actually be resolved by Earth based instruments.

The solar "surface" which we see as the extremely bright portion of the Sun is called the photosphere. In the photosphere, solar material is too thin to continue convection so the heat must again be released in the form of radiation. The laws of physics describe a phenomenon known as blackbody radiation. Blackbody radiation is naturally produced by all objects in the universe at a frequency proportional to their temperature, but the total energy released is proportional temperature to the 4th power. The photosphere's temperature is approximately 6000 K (10,000 F), a temperature at which it releases most of its energy in the form of visible light. This is the sunlight we see on Earth, which illuminates the planets and makes our Sun visible for distances of many light years.

Beyond the photosphere, the Sun still has an atmosphere in the form of the chromosphere, the transition zone and conora. The chromosphere is a thin and relatively cool zone (4500 K / 7500 F) which emits some of its own light though at a much lower intensity than the lower photosphere. Above the chromosphere are the transition zone and conora where, for unknown reasons, the Sun once again heats up dramatically to the 1 million degree K range (1.8 million F). In these regions, the ionized atoms add strange emission lines to the Sun's spectra. In this hot conora, the temperatures are so extreme that some highly ionized gas can escape into space. These ionized particles move extraordinary quickly at speeds of around 500 km/s (1 million mph) and effect the entire solar system. This stream of ionized particles is known as the solar wind.

The Sun's composition can be infered from spectroscopy, the study the frequencies of light absorbed by certain atoms and molecules. These studies show that the Sun is dominated by hydrogen (71% of its total mass) and helium (27.1% of its total mass) with relatively small portions of oxygen, carbon, nitrogen, silicon, magnesium, neon, iron and sulfur. This composition should not be too surprising. Hydrogen and helium are the most abundant elements in the universe as they formed as direct products of the big bang. Oxygen, carbon, nitrogen, silicon, magnesium, neon and iron were all formed at the cores of supergiant stars which lived and died long before our Sun was burned. Our Sun is not just composed of the products of the big bang, but also the ashes of the stars which came before it.

The Sun is a place of enormous amounts of energy and isn't entirely stable. These instablities range from slightly cooler regions on the Sun's surface known as sunspots to ejections of large amounts of radiation and ionized coronal matter into space. Sunspots cooler and darker regions on the Sun's surface which tend to show up in pairs of opposite magnetic polarities. Sunspots follow an 11 year long cycle which is thought to be evidence of 22 year long magnetic activity cycles. More intense activity takes the form of explosions near sunspot pairs. These explosions can take the form of 100,000 km long prominences, large loops of solar material which folow the Sun's magnetic field in which lots of energy is involved but little material is actually released into space. Far more serious are solar flares which involve the same amount of energy as promiences but occur over minutes instead the hours of days that prominences take. Solar flares eject a sizable amount of highly ionized solar material into space. If a flare is in the proper direction, the flare material may strike Earth and disrupt communications and power systems.

The Sun is relatively easy, albeit dangerous to observe. Staring at the Sun with your naked eye can result in permanent retina damage, and you should never use unfiltered binoculars or telescopes to view the Sun because they focus enormous volumes of light down to a single point.  Telescopes with special filters and special purpose spacecraft are routinely used to observe the Sun and monitor its surface activity. Today several special purpose spacecraft are observing the sun including NASA's Ulysess and Genesis missions and ESA's Solar and Heliospheric Observatory (SOHO). In addition to their research roles, these spacecraft act as warning system for incoming solar flares which are inconveniences on Earth but potentially devastating to manned or unmanned spacecraft beyond the protection of Earth's magnetic field.

Mercury
Mercury is the planet closest to our Sun. Therefore, it is bombarded with more intense solar radiation than any other planet. Despite this huge amount of sunlight, its average temperature (440 K) is actually much cooler than that of the second planet, Venus. Mercury also has the most eccentric (least circular) orbits of any planet, save Pluto (which debatably may not be a planet). Mercury's closest approach to the sun is a mere 46 million km (31% of the Earth's distance), but it can also be as far away as 70 million km (47% of Earth's distance to the Sun). These close orbits also give Mercury a very short year of 88 Earth days. Interestingly, the Sun causes large gravitational deformations which have slowed Mercury's rotation to a mere crawl. The combination of Mercury's highly elliptical orbit and large solar tidal forces cause it to complete a mere 3 rotations every two orbits. A day on Mercury is 2/3rds of a year on Mercury.

Compositionally, Mercury is an extremely dense planet (5430 kg/m^3) and has a very high iron content. Astronomers believe that Mercury is fully differentiated with a large iron core surrounded by a rocky mantle and crater saturated crust. Interestingly, spacecraft have observed a very low intensity magnetic field originating from Mercury. The cause of this field is presently unknown, but is likely the result of a past or present dynamo effect, similar to the one seen on Earth. Despite its density, Mercury's small size means that it has a low mass and thus low surface gravity. This gravity is too low for Mercury to retain a significant atmosphere. Mercury's lack of an atmosphere also allows for very wild temperature swings. Sunlit parts of the planet may reach 700 degrees K (800 F or 425 C), only to cool to temperatures of 90 degrees K (-300 F  or -180 C) once in the dark.

Mercury's surface is surprisingly similar to our Moon's. Like the moon, Mercury has been geologically dead for 4 billion years. Since then, the planet's surface has been changed by meteor impacts and wrinkles referred to as scarps which probably result from the cooling and associated shrinking of the planet. Recent evidence suggests that there may be some craters in Mercury's polar regions that contain water ice. Water could remain in these craters because their floors are never exposed to direct sunlight.

Mercury is a very difficult planet to observe. The planet's small physical size and great distance mean that it has a small angular size when observed from Earth. Also, the planet's orbit keeps it very close to the Sun, so it never sets more than two hours after sunset or rises more than two hours before sunrise. Since Mercury's orbit lies within the Earth's, the point where Mercury is closest to Earth is also where it is directly between the Earth and the Sun. Mercury was visited by the Mariner 10 spacecraft which provided closeup images and information regarding its magnetic field. Presently, NASA's Mercury Messenger is en route to Mercury.

Venus
Venus is the second planet from the Sun, and in many ways is very similar to Earth. Venus orbits in a nearly circular orbit about 100 million km from the sun (72-73% of the Earth/Sun distance), and its density (5240 kg/m^3), surface gravity, and mass are all similar to Earth. A year on Venus is 224 Earth days long, but the planet has a very odd rotation. Venus rotates in the retrograde (opposite) direction of almost all other solar system bodies, but it does so extremely slowly. As a matter of fact, a year on Venus is shorter than the planet's day (243 Earth days). Scientists believe that this backward rotation is the product of a glancing collision with a large body far back in the planet's history. Basic physics show that such a collision could produce the rotation rate we observe.

Given its other similarities to Earth, it is not surprising that Venus is thought to have a virtually identical composition:  an iron core, rocky mantle, and a thick crust. Venus probably has a liquid outer core, but its incredibly slow rotation mean prevents the dynamo effect from generating a sizable magnetic field. Although Venus has two large elevated regions which resemble Earth's continents, it lacks plate tectonics. That said, Venus is not geologically dead. There are ample examples of geological activity ranging from mountain ranges formed by forces in the plant's crust, to volcanoes and lava flows.

Topographically, Venus is an extremely smooth planet with a young crust. The planet's features are actually more mild than those found on Earth. For instance, the distance between the lowest point on Venus's surface and highest point is a mere 14 km, far less than the 20 km distance between the bottom of Challenger Deep and the top of Mount Everest. The planet's thick atmosphere shields it from all but the largest of impactors so impact craters are usually 10 km or larger. Radar images show volcanic structures of several types on Venus: massive pancake shaped lava domes, shield volcanoes, lava flows and crown shaped circular regions called coronae which are hundreds of kilometers in size. To date, no volcanic eruptions have been observed on Venus, but there is reasonably good circumstantial evidence that eruptions have recently occurred. Venus's young surface is likely the result of periodic massive volcanic upwellings which resurface large parts of the planet by covering it with new lava.

Perhaps the most interesting attribute of Venus is its atmosphere. At the planet's surface, atmospheric pressure is nearly 90 times greater than that on Earth's surface. This pressure is so great that early Russian probes sent to Venus actually imploded before reaching the surface. Although Venus itself has a very slow rotation rate, its atmosphere has high altitude winds of 300-400 km/hr. Because of the extreme thickness of the atmosphere and circulation patterns, surface temperature on Venus is more or less uniform, even on the night side and polar regions. Venus's atmospheric composition is mainly carbon dioxide (96.5%) with some nitrogen (3.5%) and trace amounts of other gases. This incredible amount of carbon dioxide is an extremely effective greenhouse gas and raises the planet's temperature by an incredible 400 degrees C. Without the extreme greenhouse heating, Venus's ambient air temperature would be comparable to Earth's. Oddly, above the planet's troposphere and greenhouse gases, clouds of sulfuric acid actually reflect a very large amount of incident sunlight back into space. While the lower part of the planet's atmosphere is very effective at retaining heat and keeping it warm, the upper atmosphere is actually cooling Venus down.

Venus is brighter than everything else in the sky other than the Sun and the Moon. Due to its appearance before sunrise and after sunset, Venus is known as both the morning and evening star. Unfortunately, Venus's thick clouds completely obscure its surface. So, little was known about Venus until radar studies were conducted in the later half of the 20th century. Orbiting spacecraft have been able to use radar techniques to map Venus's surface at high resolutions, and several Russian Venera landers successfully landed on its surface. A European Space Agency spacecraft called Venus Express is scheduled to begin observations in 2006.

Earth
Earth is our home planets and thus the best studied body in the solar system. Earth has a radius of about 6380 km (3960 miles) and relatively high mean density of 5520 kg/m^3. Like the rest of the solar system, our planet formed around 4.5 Billion years ago though it has changed considerably since its early formation. Earth also rotates quite quickly, giving us our 24 hour days and causing the planet's substantial magnetic field. Earth is also the only planet we know of that supports life, but this result is heavily influenced by our lack of conclusive data about other worlds.

Earth is fully differentiated--the heaviest materials lie closest to the center and the lightest are furthest away from the center. Earth consists of a solid inner core which is surrounded by a liquid outer core, then a rocky mantle and finally the rocky crust on which we live. Beyond the crust, Earth has a gaseous atmosphere. Further away, our planet's magnetic field protects us from the harmful ionizing radiation of space.

The inner core has a radius of about 1300 km ( 800 miles) and the outer core has a radius of about 3500 km (2175 miles). Both the inner and outer core consist mainly of nickel and iron although some lighter elements may also be present. Near the center of the planet, temperatures reach 5000 degrees Kelvin, but the pressure is so great that the nickel iron alloy cannot melt. The outer core is slightly cooler but it is also at lower pressure, so it is molten. The rotation of the Earth coupled with the liquid outer core causes the magnetic field.

The vast majority of our planet's total mass is the rocky mantle. Unlike the core which has an estimated density of around 12,000 kg/m^3, the mantle's density is a relatively light 3000 kg/m^3. The mantle consists mainly of rocks of composed of oxygen, silicon and magnesium with smaller amounts of iron, aluminum and calcium. The most common compounds in the mantle are SiO₂ and MgO.

The mantle is involved in the transfer of heat from the hot core to the cooler crust through a process known as convection. Convection is most common in fluids where a hot fluid expands and thus rises into a lower density and cooler region. The fluid then cools and circulates back down to the warmer region where the process repeats. Although the mantle is made of solid rock, not fluids, geologists believe that over long periods of time, the convective process occurs in solid rocks as well.

The crust which makes up Earth's surface is not a single solid region which floats on top of the mantle but rather several tectonic plates which change the arrangement of Earth's surface over time. The plates are driven by the convective forces in the mantle which cause them to move at the creepingly slow rate of about 2 cm (less than 1 inch) a year. On the geological time scale, the plates can move thousands of miles. Roughly 200 million years ago, Earth consisted of a single large continent called Pangaea which was slowly torn apart by plate motion. Pangaea probably wasn't the first supercontinent on Earth. Geologists believe that supercontinents have formed, been torn apart and reformed many times in Earth's past.

Plates are constantly colliding to cause unique geological features. When continental plates collide, they can cause massive mountain ranges such as the Himalayas. When an oceanic plate and a continental plate collide, the oceanic plate subducts (sinks) beneath the lighter continental plate an goes into the mantle. These collisions often cause deep oceanic trenches, and they can result in volcanism because the water in the subducting oceanic plate decreases the melting point of the mantle material to produce magma. Lastly, plates can also rub past each other instead of colliding. In this case, the energy is released suddenly in the form of earthquakes.

In order for plates to collide and subduct, plates must move apart elsewhere. Plates move apart at midocean ridges. Here, hot mantle material rises up to fill the gap between the separating plates. Our planet's crust consists mainly of the igneous rocks basalt and granite which were formed at these midocean ridges. The other two types of rocks, sedimentary rocks and metamorphic rocks are ultimately derived from igneous granite and basalt. Sedimentary rocks are formed from the weathering and erosion of igneous rocks, and metamorphic rocks are formed through the transformation of sedimentary or igneous rocks due to high pressures or temperatures.

The crust's depth ranges from 5 to 10 km in the oceans to 20-70 km on the continents. Because of plate tectonics, the Earth has a relatively young crust. Approximately every 500 million years, most of the Earth's crust is recreated so old rocks and fossils are difficult to find.

As we all know, Earth has a substantial atmosphere above its crust. The atmosphere consists of nitrogen (N₂, 78%), oxygen (O₂, 20%), argon (Ar, 1%), carbon dioxide (CO₂, .03%) and widely varying amounts of water vapor. The atmosphere is thickest near the surface but its density drops of rapidly; approximately half of Earth's atmosphere is below 5,000 m (15,000 ft), the height of many moderately sized mountains. The atmosphere is divided into four separate zones: the troposphere, stratosphere, mesosphere and ionosphere.

The troposphere is the zone closest to Earth's surface with a height of 7-17 km. In the troposphere, solar radiation heats the Earth's surface and the heat is carried away from the surface through the convection of fluids in the troposphere. On Earth, you would expect flows from the warmer equator toward the cooler poles, but there are two problems. First of all, the Earth is rotating so the air can not travel in the solely in the north/south direction; it also has an east/west component. This deflection due to Earth's rotation is known as the Coriolis Effect. Second, Earth is too large to a support a single convection cell in each hemisphere. Instead, the hemispheres are each broken into three convection cells: the Hadley, Ferrel and Polar Cells. The Hadley Cells which are closest to the planet's equator have a westerly motion and move surface air toward the equator. The Ferrel cells are between the Hadley and Polar cells and move surface air easterly and toward the poles. The Polar cells are over the poles and move surface air westerly and toward the equator. The motion of air due to these convection cells results in the prevailing winds.

Note: Convection cells are in fact cells so air has to make a complete loop. Although we commonly just describe the motion of surface air, the air rises above the surface, cools and follows a return path in the direction opposite the flows near the surface. These return paths for the cool air are usually 10-15 km above the surface and are noticeable in aircraft.

In the troposphere, temperature drops as altitude increases. This is why you may feel cold near the top of a mountain even if it is hot day at the base. This cooling trend ends at the top of the troposphere. The stratosphere contains the ozone layer which absorbs most solar ultraviolet radiation and converts it into heat. The largest amount of ultraviolet light is available near the top of the stratosphere, so the top is hottest. At the bottom of the stratosphere, much of the ultraviolet light has been absorbed by the ozone above it, so the bottom is coolest. The top of the stratosphere reaches a temperature of around 270 K (-3 C or 26F) which is only about 20 degrees K cooler than Earth's surface average of 290 K (17 C or 62 F). In contrast, the coolest point in the troposphere is a very chilly 221 K (-52 C or -62 F).

At about 50 km above sea level, the stratosphere ends and the mesosphere begins. The end of the stratosphere corresponds to the end of heating due to absorption of ultraviolet light in ozone, so the temperature again begins to fall with height, reaching a low temperature of a cold 200 K ( -73 C or -99 F). In this region of the atmosphere most falling meteors (shooting stars) burn up.

Beyond the 80 km (50 miles) high top of the mesosphere, the X-Ray and ultraviolet bombardment of Earth's atmosphere is so intense that  ultraviolet radiation can knock electrons out of their orbitals around atoms to form positive ions. For this reason, this region is known as the ionosphere. The ionization of the ionosphere also has the useful ability to reflect reasonably low frequency radio waves including those in the AM band. This allows us to hear radio stations which are further than the horizon, and facilitates long distance radio communications without satellites. Because of the absorption of UV and X-Ray radiation in this layer, substantial heating occurs and temperature rises with altitude.

Earth's magnetic field extends far beyond the atmosphere and deep into space. The Earth's magnetic field is like an imaginary bar magnet aligned approximately along the rotational axes of our planet. Of course, there is no bar magnet in the center of the Earth. Instead, the magnetic field is caused by Earth's rotation which drives movements of conducting liquid metal in the outer core. Geological evidence shows that Earth's magnetic field reverses itself every 250,000 years. Such behavior has also been observed in complex computer models of Earth's interior.

Note: By convention we refer to the north pole of a magnet as the pole which will point toward the north pole of the planet. In magnets, opposite poles attract and likes repel. This leads to the curious result that north pole of the Earth actually behaves like the south pole of magnet. Thus, when we imagine a bar magnet through the center of the planet, the imaginary magnet's north pole is actually at our planet's south pole and the magnet's south pole is at Earth's north pole.

The magnetic field deflects charged particles in the solar wind (mostly electrons and protons) and prevents them from hitting Earth directly. Some of the solar wind particles can be trapped within the Earth's magnetic field in two regions known as Van Allen belts. The inner Van Allen belt contains the heavier protons and is about 3000 km from Earth. The outer Van Allen belt contains the lighter electrons and is about 20,000 km from Earth. Particles from the Van Allen belts can only enter the Earth's atmosphere near the magnetic poles; everywhere else the magnetic field prevents them from getting close to Earth. At the polar regions, these particles interact with the atmosphere and excite atoms in the atmosphere to create auroae.


Moon Formation & Appearance
The most generally accepted theory of the formation of the moon follows:

Shortly after the formation of the solar system (about 4.68 Billion Years ago), a proto-Earth probably existed alone without any moons. Then, a Mars sized body struck Earth in a glancing blow. The force of the impact blew an enormous amount of matter into space, but a good deal of it fell back to Earth in the following days. The remaining matter remained in orbit and gravitationally clumped into a molten moon. Although the early moon was probably very hot, it did not have a substantial quantity of metal because the metallic core of the Mars sized object was too heavy to be heaved into orbit. Instead, it combined with the already molten Earth and eventually was added to Earth's core. Thus, although the moon is differentiated, it's iron core is very small.

In the early solar system, many pieces of interplanetary matter floated around and impacted with the moon, the Earth and the other planets. The oldest pieces of lunar crust, the bright highlands, date back to this violent stage in solar system history as is evidenced by their high density of impact craters. Between 3.9 and 3.2 Billion Years ago, lunar volcanism filled the lunar lowerlands or maria with lava, erasing old cratering and providing dark and smooth surfaces. After the maria cooled, they too were occasionally struck by impacts but at a much lower rates. So, the number of craters in the maria is relatively low.

Today, for all intents and purposes, the moon is geologically dead. There are no resurfacing events such as volcanoes, there is essentially no atmosphere and there are no plate tectonics. There are some "moonquakes" due to the movement of substances deep down inside of the moon but these are extremely low in magnitude. Also, meteor bombardment occasionally creates a new crater and bombardment by micrometeorites has slowly pulverized the rocks on the lunar surface into a layer of fine "soil" known as regolith. Despite often being referred to as "lunar soil", regolith does not have any organic constituents, so it does not resemble terrestrial soil in that sense.

The moon and Earth have peculiar relationship known as tidal locking which causes one side of the moon to permanently face the Earth. This is because, the gravitational force of the Earth on the moon causes it to deform slightly, creating a bulge on the Earth facing side. This bulge allows Earth's gravity to effect the moon's rotation and force one face to permanently face Earth. In order to keep one side facing the Earth at all times, the moon completes exactly one rotation every time it orbits the Earth.

The phases of the moon are a result of the moon's orbit around the Earth. When the moon's orbit takes it between the Earth and the sun, a "new moon" is seen because the side that faces away from the Earth is illuminated by sunlight and the side that faces the Earth is in shadow. When the moon is on the opposite side of the Earth from the Sun its disc is fully illuminated by the sun, and it appears full. At positions between these extremes, only part of the disc of the moon is visible.

It is important to note that the moon does not have "dark side" and a "light side" as some people incorrectly believe. Instead, it has an "near side" which always faces the Earth and a "far side" which always faces away from the Earth. However, both sides of the moon are illuminated by sunlight for equal portions of time.

Solar eclipses occur when the moon happens to pass directly in front of the sun relative to some observers on Earth. By coincidence, the relative sizes of the moon and Sun are virtually identical so the moon nicely blots out the sun, but only for a few precious minutes. Lunar eclipses occur when the moon passes into Earth's shadow so sunlight cannot reach it directly. Because the Earth has quite a large relative size on the moon, lunar eclipses last much longer than solar eclipses. Both lunar and solar eclipses are somewhat infrequent because the orbital plane of the moon around the Earth is not exactly the same as the orbital plane of the Earth around the Sun. Most of the time, a new moon is above or below the sun and does not blot it out and a full moon is above or below the Earth's shadow and thus is not blotted out. Only occasionally do the moon, the sun and the Earth align for an eclipse.


Mars
Mars is the 4th planet from the Sun. It is only about 11% of the Earth's mass, 53% of the Earth's radius and 33% of the Earth's gravity. Despite having a low density of only 3903 kg/m^3, Mars has a substantial amount of iron present on its surface which gives it its reddish color. However, this low density does indicate that Mars is predominantly a rocky body so its metal core must be small.

Mars has an atmosphere that is 95% carbon dioxide with some nitrogen, argon and trace amount of oxygen, carbon monoxide and water vapor. The Martian atmosphere is extremely thin; pressure at the Martian surface is only .6% of the air pressure at sea level on Earth. This pressure is too low for standing liquid water to exist on its surface. Liquid water on the Martian surface would either freeze or evaporate. Earlier in the planet's life, it probably had a thicker and warmer atmosphere which provided sufficient temperature and pressure for standing liquid water to exist on the planet's surface. Indeed, there is ample geological evidence on both the microscopic and macroscopic scales of past liquid water.

Today, much of Mars' carbon dioxide and water appear to be stored in its large polar ice caps which are composed of solid carbon dioxide (dry ice) and water ice. Carbon dioxide and water vapor are both greenhouse gases, gases which assist in warming a planet by reflecting some of the planet's infrared heat back toward the surface, thus preventing heat from escaping into space. Mars may have been victim of a "run away greenhouse effect" in which the condensation of a little bit of carbon dioxide resulted in a cooling which caused more carbon dioxide to cool and condense and so forth. Now, the planet's surface temperature, though widely varying averages a cold 210 degrees Kelvin (-63 C,  -81 F).

The Martian surface is home to some extremely large and remarkable features. The planet's entire northern hemisphere are lowlands which are nearly 5 km lower than the southern highlands. The 10 km (6.2 mile) high Tharsis Bulge and 6 km deep Hellas Basin lie on the equator but on opposite sides of the planet; both are comparable in size to North America. Mars is home to the largest and tallest mountain in the solar system, the volcano Olympus Mons which rises 25 km (15 miles) high and is 700 km (434 miles) wide. This shield volcano's extreme dimensions are at least partly due to the low gravity on Mars. Mars is also home to Valles Marineris, a "canyon" which with a length of 4000 km is as long as the United States. (NOTE: Despite early speculation, none of these features are indicative of either intelligent life or running water. They have less dramatic geological explanations.)

There is an on going debate about whether Mars was ever home to any living organisms. Experience from Earth suggests that anywhere there are nutrients, an energy source (usually the sun) and liquid water there is life, but it is  unclear if this axiom would carry over to Mars. Electron microscopy on a Martian meteorite that found its way to Earth shows tiny structures that could potentially be the fossilized remains of single-celled organisms. Scientists seem to agree that if life ever did exist on Mars it was probably in the form of extremely primitive single celled organisms, and the prospects for complex life were essentially zero. It is also possible that some Martian life still does exist; perhaps it is embedded in permafrost or in underground aquifers where there is sufficient pressure and warmth to keep water in a liquid state. Life testing experiments on the Viking landers appeared to show negative results though some debate this conclusion.

It appears that Mars could potentially be terraformed, converted into a more Earth-like planet that could support many forms of terrestrial life. Such a project has been discussed in both serious research papers and science fiction. Any such project would probably entail melting the polar ice caps to release carbon dioxide into the atmosphere in order to attempt to jump start a greenhouse effect although other indirect approaches have been suggested. No existing power technology (including nuclear fission reactors) could provide enough energy for this approach, but constructing extremely large orbiting mirrors and using the Sun as the power source appears to be a workable concept. These orbital mirrors could increase the thickness of the atmosphere and melt ice into liquid water, but they would not introduce oxygen. Instead, cyanobacteria, bacteria which were present in early Earth and converted much of Earth's carbon dioxide to oxygen, would have to be introduced to Mars. However, estimates of the amount of time it would take to transform Mars's atmosphere to a human breathable one are on the order of thousands, if not millions of years. It is possible that future power technologies such as nuclear fusion reactors could provide sufficient energy to speed the process along.

For an amateur, Mars is a very difficult plant to observe. Even when it is at opposition, it still appears as a small faint reddish sphere through moderately sized telescopes. The difficulty in observing Mars is perhaps partly the reason that early astronomers believed they saw vegetation and canals on the red planet. In the past few decades, Mars has been visited by a variety of flyby, orbiter and lander spacecraft which have radically increased our understanding of the Red Planet. Presently, Mars is being observed by three orbiters, NASA's Mars Global Surveyor and Mars Odyssey and ESA's Mars Express along with the rovers Spirit and Opportunity. Recently NASA launched another orbiter, Mars Reconnaissance Orbiter which should reach Mars in 2006 and the space agency intends to launch the Phoenix Lander in 2007.

Jupiter
Jupiter is the 5th planet from the Sun and the first of the gas giant planets, it is also the largest and most massive planet in our solar system. Jupiter's 71,500 km radius is over 11 times greater than Earth, and its volume is over 1400 times Earth's. The planet's mass of 1.9*10^27 kg is so great that the solar system can be crudely approximated as a two body system of the Sun and Jupiter. Despite its incredible radius and mass, Jupiter rotates much faster than any of the terrestrial planets--it only has a 10 hour day. Jupiter's gravitational influence probably greatly affected the early solar system, and it continues to play a role in structure of the asteroid belt which lies between the orbits of Jupiter and Mars.

Structurally, Jupiter and all of the gas giants have a thick atmosphere which is mainly molecular hydrogen (H₂) and helium gas with traces of water vapor, methane (CH₄) and ammonia (NH₃). Scientists believe that three cloud layers exist on the planet with ammonia making up the top two layers and water ice making up the bottom layer. Because Jupiter rotates so rapidly, there are strong prevailing winds in the upper atmosphere which can reach speeds of nearly 400 miles per hour. Observers from Earth can also see light colored zones and dark colored belts in the atmosphere which are the result of the convective movement of internal heat to the planet's surface. Hot material rises in the light zones cools and descends in the dark belts. Jupiter also has a Great Red Spot, a massive storm system that is at least 300 years old and driven by heat and the Coriolis effect, but it still is poorly understood. The belts, zones and Great Red Spot can all be observed from Earth through small telescopes. Closer observations also show that smaller storm systems including lightening arise in the Jovian atmosphere.

There is no definite bottom of the Jovian atmosphere. It just keeps getting thicker and thicker and turns into a layer of molecular hydrogen (H₂). The interesting clouds and other atmospheric activity is probably only in the first 200 km. About 20,000 km below the top of the atmosphere, the pressure inside of Jupiter reaches 3 to 4 million times Earth's atmospheric pressure. This pressure is so great that the hydrogen liquefies and begins to exhibit the properties of a liquid metal. This hydrogen is referred to as metallic hydrogen because it is an excellent conductor. At the very center of the planet, there is a rocky core of incredibly high density which is estimated to be about 20,000 km in diameter and weighs about 5-15 times as much Earth.

Jupiter's high rotational speeds and massive layers of conducting metallic hydrogen give it an incredible magnetic field. While Earth's magnetic field, is the strongest of the terrestrial planets, Jupiter's is 20,000 times stronger. This magnetic field affects the solar wind, the stream of charged particles emitted from the sun, so much that its effects have been observed beyond Saturn's orbit. The field also accelerates charged particles in the Jovian system, causing intense radiation which can be observed from Earth and is a threat to both spacecraft and any human beings who venture to the Jovian system.

Of Jupiter's 61 known moons, the four large Galilean moons--Io, Europa, Ganymede and Callisto--are by far the most interesting. They were discovered by Galileo and Marius and are easily visible through binoculars or a small telescope. All of these moons are also quite large. They are all comparable in size to Earth's moon and have unique properties such as volcanism, and possible subsurface oceans.

Io is the only known volcanically active moon in the solar system and the only body in the solar system other than the Earth with observed active volcanoes. The volcanism isn't caused by plate tectonics as it is on Earth but rather by the intense gravitational tugging by Jupiter the other Galilean moons. This tugging causes Io to deform and the deformations produce enormous amounts of heat which drive the volcanism. This volcanism is so intense that some scientists claim that its surface is unmappable because its features can change so quickly. Io is fully differentiated has a rocky mantle and solid core. Io is the only one of the Galilean moons which lacks water.

Europa is perhaps the best candidate for life in our solar system because magnetometer data from the Galileo spacecraft indicates that it may have an enormous liquid ocean below its icy surface. Europa's ocean is probably kept in a liquid state by a less intense form of the same gravitational heating that causes Io's volcanoes. We believe that liquid water coupled with a source of energy and nutrients are necessary for life. Europa with its liquid oceans and tidal heating provides both water and energy. It is unclear whether the nutrients exist or not. On it's surface, Europa is covered with an ice sheet with occasional rocky deposits. There is also a good deal of surface evidence of liquid oceans such as crisscrossing lines which resemble ice flows on Earth. There may also be "ice volcanism" or geysers on Europa, but they has not been observed directly. Europa, like Io is thought to have a rocky mantle and an iron core.

Ganymede is not only the largest moon in our solar system, but at around 5300 km in radius, it is larger than Mercury and Pluto. Although Ganymede probably has a similar composition to Europa, an icy crust with a subsurface ocean, rocky mantle and an iron core, its crust is probably much thicker than Europa's. Numerous impact craters indicate that Ganymede has a much older surface than Europa. Indeed, some parallels have been drawn between Ganymede and our moon because Ganymede seems to have young regions which resemble our moon's maria and older regions which resemble the lunar highlands. Ganymede also has evidence for past plate tectonics but that was billions of years ago.

Callisto is basically a less interesting version of Ganymede. Unlike Ganymede, Callisto isn't differentiated so it has no definite core or mantle. The lack of substantial gravitational heating probably means that it lacks any substantial volume of liquid water. Ice however appears to be quite prevalent. Callisto has no evidence of any past plate tectonic activity and its surface is very old.

Jupiter was one of the first objects observed through early telescopes, and it remains the focus of intense study. Even the least expensive amateur telescopes give spectacular views of Jupiter and the moons are clearly visible in binoculars. Professional telescopes can resolve many atmospheric features and have successfully tracked the Great Red Spot for hundreds of years. Volcanism on Io was originally observed by the Voyager 1 spacecraft and can now be seen from Earth using our most advanced telescopes such as Keck and Hubble. Jupiter was visited by the several flyby missions: Pioneers 10 & 11, Voyagers 1 & 2, Ulysses and Cassini. From 1995 until 2003, the Galileo spacecraft orbited Jupiter and used a large array of scientific instruments, including an atmospheric to probe to examine the giant planet and its moons. NASA had planned a nuclear reactor powered mission known as Jupiter Icy Moons Orbiter (JIMO), but it was deemed too ambitious and postponed. A much less costly but less advanced spacecraft known as JUNO has been proposed and it may be launched around 2010.

Saturn
Saturn is the second largest planet in our solar system and the farthest naked eye visible planet from Earth. It lies nearly 10 times farther from the Sun than the Earth and thus receives only about 1/100th the visible light per unit area that the Earth does. Its orbit is 29.4 Earth years long, its mass of 5.68*10^26 kg is 95 times greater than Earth's and its radius of 60,270 km is about 9.5 times that of Earth. Like Jupiter, Uranus and Neptune, Saturn is a gas giant planet, but it has the lowest density of the group. At 687 kg/m^3, Saturn is the only planet in the solar system which is light enough to float in water. Like Jupiter, Saturn rotates extremely quickly. It completes a day approximately every 10 hours. This fast rotation coupled with the planet's low gravity serves to dramatically flatten the planet like a piece of pizza dough being tossed in the air by a chef. Although this effect is visible in all of the planets, Saturn is the most flattened; its equatorial diameter is nearly 10% greater than its polar diameter.

Saturn's atmosphere is generally comparable to Jupiter's though its colder location and lower surface gravity have some interesting effects. Like Jupiter, Saturn shares three layers of clouds, but lower gravity means that the layers are much thicker and harder to see through. This makes Saturn less colorful than its more massive sibling. Saturn also has cloud bands and storms, but none of its storms have the size or endurance of the Great Red Spot.

Compositionally, Saturn's atmosphere consists of hydrogen, helium, methane and ammonia, but helium makes up only about 7.4% of Saturn's atmosphere while it makes up about 14% of Jupiter's. Scientists believe that Saturn formed with the same structure as Jupiter, but it's cooler temperatures have caused the helium to slowly precipitate and fall deep into Saturn. This helium precipitation decreases the helium content in the upper atmosphere and results in heating. Heat from helium precipitation allows Saturn to radiate nearly three times much energy as it absorbs. Eventually, all the helium will precipitate and the heating will cease.

Saturn's interior is also resembles Jupiter. It has a rock and ice core, which is surrounded by a layer of electrically conductive metallic hydrogen, then molecular hydrogen and finally the atmosphere. The metallic hydrogen layer is much smaller than Jupiter's due to Saturn's lower gravity, so Saturn also has a weaker magnetic field. The magnetic field has an intensity of about 1/20th of Jupiter's but is still about 1,000 times greater than Earth's. This magnetic field extends beyond the rings and into the reaches of the inner moons where it interacts with the solar wind.

Most observers probably know Saturn best for its rings. Although all of the gas giants have rings, Saturn's are by far the most brilliant and pronounced. The rings are actually small particles of orbiting ice which range in size from a fractions of millimeters to tens of meters. Gravitational tidal forces from Saturn prevent the ring particles from ever combining into a single solid moon, but interactions between the particles prevent them from escaping and force them into circular orbits.

Astronomers have two competing hypothesizes for the source of the ring particles. The first hypothesis suggests that a small to medium sized satellite strayed too close to Saturn was torn apart by the planets strong gravitational forces. In this case, the rings were formed within the last 50 million years and will degrade slowly over the next tens of millions of years. The second hypothesis is that the rings are constantly replenished by particles which are chipped off the planet's moons. In this case, it appears to be possible that the rings could last for an extremely long time.

Saturn has seven major rings which from the inside out are the D, C, B, A, F, G and E rings. Two large gaps exist in the ring system, the Cassini division between the B and A rings and the Encke Gap between the A and F rings. Gaps in the rings are likely caused by the gravitational tug of Saturn's inner medium sized moons such as Mimas. Additionally, spacecraft images show that the rings are composed of tiny ringlets. The ringlets are probably the result of wave motion within the rings, but certain smaller gaps are likely caused by small moonlets which orbit in the ring plane and capture ring material. Lastly, the F ring and perhaps the G ring are held together by pairs of shepherd moons whose gravitational influence prevents the ring materials from escaping on either side of these narrow rings.

Saturn has an interesting moon system which consists of a single large moon, Titan, six medium sized moons--Mimas, Enceladus, Tethys, Dione, Rhea and Iapetus--and a plethora of smaller less interesting moons. Both Titan and the medium sized satellites are sufficiently large that gravitational forces have shaped them into spheres. Most of Saturn's satellites are covered with very reflective water ice so they are extremely bright for their sizes.

The medium sized moons all have unique characteristics. Mimas has a large crater which is nearly a third of its diameter and its gravitational forces create the Cassini division in Saturn's rings. Enceladus reflects nearly 100% of incident sunlight and may have some ice volcanism (geysers) which resurface it. Tethys has large cracks and trenches which may have been caused by either cooling and shrinking or impact events. The heavily cratered Dione has some younger plains which resemble lunar maria and indicate that some geological resurfacing occurred its past. Rhea is largest of the medium sized Saturian satellites and has odd whispy markings which may result from geological processes long ago. Iapetus has one extremely reflective (high albedo) face and one very dark colored (low albedo) face. The darker face of Iapetus is formed from internal materials, not debris the moon swept up. Scientists believe that this material may be organic hydrocarbon compounds.

Titan is debatably the most interesting moon in our solar system. It is the second largest moon in the solar system after Jupiter's Ganymede, and it is larger than our own moon and the planets Mercury and Pluto (if Pluto's a planet). It is probably structurally similar to Ganymede with a rocky core and water ice mantle, but is unique in our solar system in that it possesses an atmosphere.

Titan's atmosphere is actually much thicker than Earth's and its surface pressure is 50% higher than Earth's. Interestingly, Titan's atmosphere, like Earth's, is nitrogen dominated, but Titan lacks other gases such as O₂ and H₂O which are common in our atmosphere. Titan's atmosphere is 95% nitrogen and 5% organic hydrocarbon compounds such as methane, ethane and cyanide. The planet's atmosphere also contains a thick haze layer that makes the atmosphere nearly opaque to visible light. Despite the moon's incredible distance from the sun, its hydrocarbons are generated through reactions driven by the Sun's ultraviolet radiation.

Titan's surface appears to be a reasonably young ice surface which may have some hydrocarbon lakes or even oceans. These purported oceans were thought to have been observed using long distance radar images, but were elusive when the moon was observed by the Cassini spacecraft and Huygens probe. At any rate, there are clearly geological processes underway on Titan because its surface has few craters and Huygens data shows evidence of recent liquid activity. Recent observations have also raised the possibility of ice volcanoes, found a continent like landmass which is called Xanadu and some engimatic features which cannot presently be explained.

Some scientists believe that Titan may undergo a methane cycle similar to Earth's hydrological (water) cycle. A mixture of hydrocarbons and water in certain places on Titan's surface is possible, and researchers believe that hydrocarbon compounds such acetylene could potentially be used as a source of energy by biological organisms. Any such life would probably have to exist in confined niches of warmth in the moon's surface, because its ambient temperature of 95 K (-288 F or -178 C) would slow down chemical reactions to an unmanageably slow rate. Titan's extremely low temperature has previously made many researchers to believe that life on the moon is unlikely.

Saturn itself is relatively easy to observe even with a small to moderately sized telescope. Large research telescopes can easily detect small atmospheric and ring features and observe the orbits of its moons to discern Saturn's mass very accurately.  Radio and infrared telescopes can pierce Titan's smoggy clouds and provide images of its surface. Saturn has been visited by several spacecraft which have contributed substantially to our knowledge of the planetary system. Pioneer 11 and both Voyager spacecraft flew by Saturn and observed the planet, its rings, its known moons and even discovered new satellites. Unfortunately, their cameras could not penetrate Titan's atmosphere to image its surface. In 2004, the Cassini spacecraft was inserted into orbit around Saturn and uses a large array of instruments including cameras, radar, spectrometers and magnetometers to examine Saturn, its rings and moon system in depth. Cassini also released the Huygens probe which landed on Titan and transmitted data from its onboard instruments through descent and landing on the moon's surface. Although Huygens only lasted a few hours (it wasn't designed to last any longer), the Cassini mission is still young and many discoveries will likely be made using its powerful array of instruments. Using a combination of Cassini and advanced Earth based instruments, our picture of the Saturnian system and especially Titan will probably change a great deal over the next decade.

Uranus
Uranus is the 7th planet from the Sun and the first to be discovered by telescopic observation. Uranus is a gas giant like Jupiter and Saturn, but it far lighter at a mere 8.7*10^25 kg, about 15 times Earth's mass. Its radius is approximately 25,000 km, which is also far smaller than Jupiter's or Saturn's and only about 4 times that of Earth. It's mass and radius yield a density of around 1300 kg/m^3 which is comparable to Jupiter and much lower than the terrestrial planets. It lies nearly 2.9 Billion km away from the Sun, 20 times farther than the Earth and thus has very little solar heating. So, its average surface temperature is a mere 58 degrees K (-355 F, -215 C). This incredible distance means that a year on Uranus is nearly 84 Earth years long.

Like all the other gas giants, Uranus rotates extremely quickly--it has a roughly 17 hours day (depending on how you measure days on gas giants)--but Uranus's day has an odd twist. Although Uranus orbits in roughly the same plane as all of the other planets, along the Sun's equator, its rotation (spin) is nearly perpendicular to its orbit. This phenomenon has some very interesting effects on Uranus's days. Depending on the time of year, the Sun may never set, rise and set, or never rise. Researchers think this odd rotation may be the result of a collision between Uranus and a large body, but we have no evidence to support this hypothesis.

Internally, Uranus consists of a rocky ice core, which is thought to surrounded by a conductive "slushy" region, then molecular hydrogen (H₂) and its atmosphere. Uranus isn't massive enough to force hydrogen into its metallic state. So, scientists believe its magnetic field is instead generated by the movement of electrically conductive water "slush". Voyager spacecraft measurements indicate that the Uranus's field is neither aligned with the planet's rotational axis or centered in the middle of the planet. This data seems to support the suggest that field originates in the slush.

Uranus's atmosphere is similar to Jupiter's. It contains 84% hydrogen, 14% Helium and 2% methane. Scientists believe the methane gives Uranus its bluish green tint. Uranus's low upper atmospheric temperature coupled with its large amounts of haze mean that it is very difficult to see atmospheric features. However,  computer enhanced images can show some features.

Uranus has 27 known moons, five of which are medium sized: Miranda, Ariel, Umbriel, Titania and Oberon. These moons are composed of a mixture of rock and ice. They are similar to Saturn's medium sized moons though they are darker. Miranda is the most interesting of the group, its terrain is very disordered perhaps due to it breaking up several times and reforming. Other theories suggest less violent geological processes may be the cause of its odd appearance. Ariel and Titania have evidence of geological resurfacing, and large networks of interconnected valleys which may have been caused by shrinkage associated with cooling. Umbriel and Oberon are very cratered have no evidence of past geological activity.

Uranus also has a ring system which consists of 9 extremely narrow and dark rings which are thought to be held together by the gravitational influence of shepherd moons. All but one of the rings seem to be a mere 10 km in width.

Due to its relatively small size and extreme distance from Earth, Uranus is a hard planet to observe. Even in the largest amateur telescopes, Uranus is only slightly larger than a dot. The best professional telescopes can resolve Uranus well enough to see large atmospheric features, rings and some satellites. The only space mission to Uranus was the Voyager 2 spacecraft whose data told us much of what we know about the planet. There are no public plans any future missions to Uranus.

Neptune
Neptune is the 8th and debatably the last planet in the solar system (depending on whether you count Pluto or other Kuiper Belt objects as planets). Neptune is so similar to Uranus that many astronomy texts discuss the two worlds in the same section. Neptune lies about 4.5 Billion km from the Sun, some 30 times Earth's distance. So, by the laws planetary motion, Neptune takes some 163.7 Earth years to complete a single orbit. Neptune is slightly more massive than Uranus at 1.0*10^26 kg, but it is smaller with a radius of around 25,000 km. Neptune thus has the highest density of any of the gas giant planets, 1638 kg/m^3. Neptune has a 17 hour day which is similar to Uranus's. While its rotation is not perfectly parallel to its orbital plane, it lacks the incredibly pronounced tilt of the 7th planet.

Internally, Neptune's makeup is virtually identical to Uranus. It has a rocky core which is surrounded by slushy conductor which is surrounded by molecular hydrogen which is surrounded by its visible atmosphere. Neptune has a magnetic field which is roughly 100 times stronger than Earth's and like Uranus, the field appears to originate both off center and away from the planet's axis of rotation. This leads us to conclude that it too originates in the conductive slush layer near the planet's core. Unlike Uranus, Neptune appears to still store some of the heat from its formation because it releases more heat than it absorbs from the Sun. Despite being nearly 50% farther from the Sun than Uranus, Neptune has a 59 K (-353 F, -214 C) surface temperature, which is one degree higher than Uranus.

Perhaps because of this extra heat, Neptune's upper atmospheric features are more visible than those on Uranus. Neptune has clouds and storms and the Voyager 2 spacecraft discovered a "Great Dark Spot", a storm that appeared to be similar to the Great Red Spot on Jupiter. However, recent observations from the Hubble Space Telescope show that the Great Dark Spot has vanished. Clearly the Great Dark Spot wasn't as long lived as its Jovian counterpart. Ongoing observations of Neptune indicate that storm systems develop frequently, but we do not understand their cause.

Neptune has some 13 known moons and likely has more that we don't know about. Most of the moons are extremely small and in odd orbits around the planet. The only moon of any noteworthy size is Triton, the smallest of our solar system's large moons (the others, ordered by size, are Ganymede, Titan, Callisto, Io and Europa). Triton is about 25% ice and 75% rock and has an extremely tenuous nitrogen atmosphere which has about 100,000th the pressure of Earth's. Triton appears to have an extremely young surface with lakes of water ice. The Voyager 2 spacecraft observed nitrogen geysers on Triton, and many observers believe it may have ice volcanoes.

Triton orbits Neptune in the retrograde direction (the opposite direction of Neptune's rotation). This orbit is not stable because Neptune's gravitational forces are gradually dragging Triton closer to the planet. In about 100 million years, Neptune's gravity will tear Triton apart and probably turn it into ring similar to Saturn's. Triton couldn't possibly have formed in this unstable orbit for it would have been been torn apart long ago. Astronomers believe that Triton was either recently captured by Neptune's gravity or knocked out of a more stable orbit by a cataclysmic event.

Neptune has a ring system which consists of three narrow rings and two wider rings. Observations of the rings from Earth show that the rings have clumps. The source of the clumping is likely shepherd satellites or other nearby moons. Recent evidence suggests that some of the ring clumps may be deteriorating rapidly.

Neptune is extremely difficult to observe from Earth given its small size and extraordinary distance. Only the largest telescopes with adaptive optics systems can image Neptune with any useful resolution. The only spacecraft to visit Neptune was Voyager 2 which returned the data we use to reach most of our conclusions regarding the planet. There are no publicly announced plans of another mission to visit Neptune.

Pluto & The Solar System's Leftovers
As you might have noticed from the many small moons around the gas giant planets, the solar system didn't form perfectly. Lots of material was left over and still floats around the solar system. Astronomers have come to include Pluto, an object previously regarded as the ninth planet as part of this "debris". Indeed, in 2005, planet hunters found a larger and more distant object than Pluto which would probably have to be deemed the 10th planet if we accept Pluto to be the 9th. There may be hundreds or even thousands of additional objects in the far reaches of the solar system whose size is comparable to Pluto. These frigid worlds makes our picture of the solar system very complex. Furthermore, solar system debris isn't limited to the outer solar system. In the inner solar system a large number of rocky asteroids remain locked in orbits between Jupiter and Mars, and comets occassionally stray into the warm inner solar system.

Pluto, or more accurately the system of Pluto and its large moon Charon, is an exceedingly odd place which differs greatly from other planets in our solar system. Pluto like the other 8 planets orbits the Sun (as opposed to a moon which orbits a planet), but does so in a very elliptical orbit. At its closest point to the Sun (perihelion), Pluto is 4.4 Billion km away, about 30 times the Earth-Sun distance, and 7.4 Billion km at its farthest point (aphelion), about 50 times the Earth-Sun distance. This orbit is actually so elliptical that for a brief period during every one of Pluto's orbits it is closer to the Sun than Neptune. The extreme distance to Pluto makes its year extremely long, 367 Earth years. Pluto is also extremely small, only some 2300 km in diameter and 1.3*10^22 kg in mass. Pluto and its moon Charon are tidally (gravitationally) locked to each other, so Pluto's day is equal to Charon's orbital period of 6.3 days.

No space probe has ever visited Pluto, so our information regarding Pluto comes solely from telescopic observations which given the extreme distance is quite inaccurate. Charon is roughly half of Pluto's diameter (1200 km) and about 1/7th the mass (1.3*10^21 kg). Pluto seems to be covered with methane but probably consists of rocky core and an ice mantle. This hypothesis is supported by Pluto's density of about 2000 kg/m^3.  Telescopic observations also indicate that Pluto have have an extremely tenuous atmosphere of methane gas. Pluto also appears to have some surface features in telescopic images, but these features are very poorly defined.

Pluto is in of a region of the outer solar system which is known as the Kuiper belt. The Kuiper belt lies at a distance between 5 Billion kilometers and 150 Billion kilometers from the Sun (between 30 and 1000 times the Earth-Sun distance). Here, many bodies of various sizes, but composed mainly of water ice orbit the Sun. Objects in the Kuiper belt are similar to the planets in that they orbit close to the plane of the Sun's equator. Most Kuiper Belt objects (KBOs) are probably quite small and undetectable in our present telescopes, but we can find largest objects with current instruments. These KBOs include some large objects such as Pluto, the recently discovered Quaoar and a new object called 2003 UB₃₁₃ (unofficially called Xena). All of these have a diameter much larger than 1000 km. Some or all of these object could be classified as planets depending on the taxonomy used. There is no good estimate on the number of KBOs, but some researchers believe there may be hundreds of Pluto sized objects in the Kuiper Belt.

Beyond the Kuiper Belt, lies a region known as the Oort cloud. Here, objects cease orbiting in the plane of the Sun's equator and assume arbitrary orbits. Objects in the Oort cloud are similar to those in the Kuiper Belt. They probably formed closer to the Sun, perhaps between Jupiter's present orbit and Neptune's, but the gravitational forces of the gas giants threw them out into deep space. Oort cloud objects still orbit our Sun, but do so at extreme distances ranging from .5 light years to 1.5 light years, half way to the nearest star. We believe we have discovered a single Oort cloud object, which is called Sedna. Sedna follows an extremely elliptical (eccentric) orbit, and even though it is presently in the Kuiper Belt, it will drift far out into the Oort cloud during its 10500 Earth year orbit.

Objects in both the Kuiper Belt and Oort clouds are collectively known as trans-Neptunians, and most of them will stay far away from the Sun. However, a trans-Neptunian occassionally gets yanked from its current orbit and put on an orbit which is so eccentric that it visits the inner solar system. In the Kuiper Belt, these orbit changes are likely due to the complex interactions between Kuiper Belt Objects, while objects in the Oort cloud are probably perturbed by passing stars. These altered orbits take these rare trans-Neptunians close enough to the Sun that solar heat begins to act on them. Volatile ices begin to evaportate and they soon are surrounded envelope of evaporated of gases. They have become comets.

The center of a comet is solid nucleus which is the remains of the original trans-Neptunian object. Astronomers generally regard the nucleus of a comet as something like a dirty snowball; they are mostly ice with small concentrations of some primitive carbon compounds. Comets are too small to have a substantial atmosphere so as the Sun heats them up, the solid ice that makes up the nucleus changes directly from solid to gas without going through the liquid phase (it sublimates). The evaporated gases remain in the same orbit as the comet, but expands under its internal pressure to form a coma around the nucleus which many grow to a diameter of 1 million km.

The coma particles are effected by the radiation pressure of sunlight and the steady stream of charged particles in the solar wind. The natural momentum of the comet particles would lead them along the comet's orbital path, but their interactions with sunlight and the solar wind push them away from the Sun. As a result, these particles tend to form a tail which points in the direction roughly away from the Sun. Complicating matters further, comets tend to have two distinct tails: a dust tail and an ion tail. A bright dust tail is formed from dust particles from the comet which reflect sunlight, while the ion tail consists of ionized molecules of water, carbon monoxide and nitrogen which are so excited that they emit they tend to emit fainter colorful light. The ion tail tends to be more heavily influence by the solar wind, so the two tails point in slight different directions and appear distinct to terrestrial observers.

Comets aren't particularly long lasting as astronomical objects go. They can come so close to the Sun that they evaporate on their first pass into the inner solar system. Other comets do not come so close to the Sun and thus do not die as quickly. Instead, they lose some material to evaporation on each orbit around the Sun until they finally are destroyed. In some cases, comets lose a good deal of volatile material and are surrounded by an envelope of refractory material. As they approach the sun again, the volatile ice may heat up and explosively expand causing a comet to suddenly flare up in brightness. These explosions can also destroy comets. Lastly, it is possible that some asteroids in the solar system are actually comet nuclei with all of the volatiles evaporated away.

Trans-Neptunians may be the most common kind of solar system debris, but comets are not incredibly common in the inner solar system. The temperatures in the inner solar system are sufficiently high than comet like bodies would lose their watery content quickly. Instead, the debris in the inner solar system takes the form of rocky and metallic asteroids. These too are left overs from the formation of the solar system; they are material which wasn't used in planet construction.

The planets formed through collisions of reasonably small planetesimals of several km in size. As larger proto-planets formed, their gravity allowed them to attract more planetesimals so they continued to grow. Not all of the planetesimals were involved in planet formation. Most asteroids are in a region of the solar system between Mars and Jupiter called the asteroid belt. Here, Jupiter's strong gravitational forces yank and pull on the asteroids to prevent them from ever combining to form a planet.

The asteroid belt is often inaccurately protrayed in science fiction as extremely densely populated region with asteroids so close to eachother that they are practically touching. In reality the asteroid belt has an extremely sparse population. The odds of an observer on a spaceship passing which is through the asteroid belt seeing a single asteroid are extraordinarily low. However, over on the geological time scale, asteroids do interact relatively frequently. They collide, break apart, orbit eachother in binary pairs and gravitationally effect each other's orbits. Compared to other objects in the solar systems asteroids live fast and die young.

Asteroid orbits can be more elliptical (eccentric) than most planetary orbits. Most asteroids never cross the orbit of Mars, but a group of asteroids known as the Amor asteroids do. An even smaller minority, of asteroids cross Earth's orbit. The Earth crossing asteroids are broken into two groups, those whose orbits last less than one Earth year are known as Atens, and those whose orbits last more than one Earth year are known as a Apollos. Collectively, Amors, Atens and Apollos are known as Near Earth Asteroids. Another special group of asteroids share the same orbit as Jupiter and are held in place by the mutual gravitational forces of Jupiter and the Sun. These Trojan asteroids, occupy extremely stable orbital locations known as 4th and 5th Lagrangian Points which respectively precede and trail Jupiter by 1/6th of a Jovian year.

Compositionally and structurally, asteroids seem to vary widely. Several asteroids are as large as 1000 km and others are only a few hundred meters, but most asteroid mass is probably concentrated in bodies which are several tens of km in size. Spacecraft observations show that asteroids range from dense and differentiated to porous piles of solar system rubble. Astronomers divide asteroids into three types based on spectroscopic measurements of their composition. Very dark carbonaceous C type asteroids comprise 75% of known asteroids. C types are dominated by carbon compounds and reflect a mere 3% to 9% of incident sunlight. Brighter silicon S types make up 17% of known asteroids and are brighter--they reflect 10% to 22% of incident sunlight. Metallic M type asteroids make up most of the remaining 8% of known asteroids, are composed primarily of iron, and reflect 10% to 18% of incident sunlight.  There are also some less common types of asteroids.

Note: When discussing asteroids, it is worth noting that our data on asteroids revolves around the asteroids we have observed. It is easier to find larger and brighter asteroids than small dark ones, so information such as the distribution of mass and relative abundance of asteroids types may not reflect the actual composition of the asteroid belt.

Other planets and moons in our solar system which have less geological activity than Earth show a history of collisions. Some bodies such as our moon are so riddled with craters that they have reached saturation point--new craters simply cover up old ones without increasing the total number of visible craters. Even though the rate of these collisions has decreased substantially as the solar system ages, they still occur. Most of the Atens and Apollo Earth crossing asteroids will eventually hit Earth. Interactions within the asteroid belt will create more Earth crossing asteroids and the occassional comet will be perturbed from its distant orbit and wander into the inner solar system on a unlucky collision course with Earth.

Every day, lots of solar system debris strikes Earth. These pieces of debris are known as meteoroids. Looking up in the night sky, you will often see meteor (colloquially referred to as "shooting star"), a streak of light from a small piece of comet or asteroid which strikes the upper atmosphere and burns up. The rate at which meteors strike the atmosphere increases as the Earth passes through tails of comets which contain many small rocky fragments. These events are known as meteor showers. When larger meteoroids strike the atmosphere, they can survive the incredible atmospheric heating and impact the surface.

Meteoroids which survive the journey to the surface are known as meteorites . On occassion, observers can follow a meteoroid all the way to its eventual landing spot. Even if we do not witness meteorites striking the atmosphere we can still find fallen ones in polar regions where there are no terrestrial rocks. Because of their incredible size and speed, larger meteorites strike with incredible fury and can do enormous damage. Although few impactors strike head-on (non-glancing) blows to the Earth, they produce enormous impact craters which are approximately 10 times the diameter of the impactor. Therefore, regardless of the angle at which impactors strike the Earth (or any other body), they form large circular craters. Although rare, extremely potent impact events can result in substantial devastation on a local or even global scale.

Most research into trans-Neptunians must be done by large telescopes due to the incredible distances involved and relatively small size of these object. Inner solar systen debris such as comets and asteroids are much easier to reach and numerous spacecraft have visited and imaged them. Retrieving meteorites on Earth is a common practice in polar regions where they are difficult to confuse with terrestrial rocks (meteorwrongs). Asteroids which are too small to imaged directly can still be investigated by examining their visual and infrared light curves as they rotate. This technique provides information concerning their reflectivity (albedo) and shape, but it is very imprecise. NASA has plans for a mission to Pluto known as New Horizons which is set to launch in 2006.

Part 3: Technology & Notes of Interest

Telescopes & Telescope Types
Telescopes are important to astronomy for two reasons: they can magnify small objects and increase the brightness of faint objects. Although most people think of telescopes simply as magnifying instruments, their light gather potential in many cases is actually more important because most astronomical objects that interest amateurs are not particularly small, just faint.

The main metric used for describing telescopes is usually their diameter (aperture) which is proportional to both light gathering power and maximum magnification. The other important characteristic for most telescopes is their focal length which effectively determines the magnification of the telescope tube itself. We can derive most of the other important characteristics of telescopes from focal length and aperture. These characteristics include speed (f/ratio), the darkest objects that can be see (limiting magnitude), and the smallest objects that can be see (Dawes limit). Telescopes are also often described with characteristics that describe the quality of their construction including aberrations in the image and the accuracy of the focusing mechanism.

While the above describes the characteristics of the telescope's optical tube, a human observer always uses eyepieces to look through a telescope. Swapping eyepieces allows astronomers to change both magnification and actual field of view (the area of the sky visible through the eyepiece). The actual magnification of a telescope/eyepiece combination can be found by dividing the telescope's focal length by the eyepiece's focal length. The field of view can be found by dividing the eyepiece's field of view by the magnification. It is also possible to use the telescope tube alone (without an eyepiece) as a camera lens with that focal length.

There are three common types of telescopes: refractor, classical reflector and cassegrain.

Refractors are the oldest type of telescope. They were initially used by Galileo and other early observers to view many solar system objects and prove that Earth isn't the center of the universe. Refractors consist of a large glass lens at the front which focuses light down to a focal point at the end of the tube. In a refractor, the length of the tube is the focal length of the telescope, so high focal length refractors are extremely long. Refractors are problematic both because it is hard to manufacture and hold extremely large lenses, and glass lenses tend to bend different colors of light different amounts (this problem is known as chromatic aberration). Also, since the focal length of the refractor is proportional to its physical length, long focal length reflectors are long and bulky. As a result, refractors are not used for serious professional astronomy anymore. However, they are still popular with amateurs, especially for wide field viewing.

Reflecting (Newtonian) telescopes use a mirror instead of a lens to focus light. The mirror is generally positioned at the rear of an optical tube where it gathers light and focuses it on a secondary mirror which then reflects the light to an eyepiece at the side of the telescope. Reflecting telescopes have several advantages over refractors in that they do not suffer from chromatic aberration and the mirror cell is easier to manufacture and physically support. Reflecting telescopes do have the disadvantages that long focal lengths require long and bulky tubes, and the image in the eyepiece is rotated by a certain angle and appears inverted.

Cassegrain telescopes use some combination of mirrors to provide long focal length in a compact design. The design usually has a primary mirror near the bottom of the optical tube which reflects light on a secondary mirror. The secondary mirror can further magnify the light, and it reflects the light to the eyepiece which is positioned in a  hole at the center of the primary mirror. Some versions of the telescope such as the Schmidt Cassegrain use spherical mirrors, which suffer from aberrations but are easier to build, but use a corrector lens at the front of the telescope which compensates for the spherical aberration. Others such as the Ritchey-Chrétien Cassegrain use harder to construct, but higher quality, hyperbolic or parabolic mirrors which eliminate the need corrective lenses. Almost all professional observatory telescopes in the world (including the Keck telescopes and Hubble Space Telescope) are Ritchey-Chrétien Cassegrains.

When we discuss telescopes, we think mainly of visual instruments which astronomers look through. Indeed for most of the history of astronomy this was the case. Photography was invented considerably after the telescope and very sensitive films and detectors were not invented until the 20th century. It was not even possible to capture images of objects which we can see with our naked eyes, let alone ones at other frequencies.

Imaging is relatively simple. Either an electronic detector (usually a charged coupling device or CCD) or film is put at the focus of the telescope instead of an eyepiece and a human eye. The telescope basically acts like a massive camera lens which focuses light onto the film (detector) plane where an image is encoded. The larger the telescope and the longer exposure, the more light strikes the film (detector) and fainter objects become brighter. The efficiency of the detectors also plays a key role in imaging because many detectors fail to capture a large percentage of incident light. Good CCD detectors can capture as much as 70% of incident light while films only capture between 2% and 10% of incident light.

One of the major difficulties in telescopic imaging is that the Earth rotates. In magnified telescopic images, this rotation is extremely pronounced, but even minute long exposures using regular cameras show streaky stars. In order to counter this effect, astronomers use precisely built tracking mounts to follow deep sky objects accurately. Because high magnifications also magnify errors, even relatively small defects in mounts cause major problems in imaging over long exposures. So, even the best mounts often use either electronic or human guiders which adjust the mount to keep it pointed at the same spot in the sky.

Today, astronomers observe the universe at all the wavelengths of the electromagnetic spectrum: radio, infrared, visual, ultraviolet, X-Ray and even gamma rays. Observing in the infrared and ultraviolet is relatively straightforward. Regular telescopes can usually focus light at these frequencies so detectors which are sensitive to these frequencies can simply be placed at their focuses instead of an eyepiece or visual light detector. Radio astronomy is done with large radio dishes which focus the radio waves to a detector.

X-Ray and gamma ray astronomy is much harder to do than radio, ultraviolet, infrared or visual because X-Rays tend to pass through material instead of bouncing of it. Modern X-Ray telescopes use many concentric X-Ray reflectors to try to focus the light on a detector. There is no known scheme for focusing gamma rays so gamma ray so astronomers simply collect them on a detector without focusing then and thus only get very coarse observations.

Almost all frequencies of light other than visible light, radio and some near visible frequencies of infrared are blocked by the atmosphere. As a result, telescopes that work at other frequencies need to be put at mountain tops, on aircraft or high altitude balloons or in some cases, space. Another complication is that the laws of physics tells us that larger telescopes are capable of higher maximum magnifications, but lower frequency light also decreases the maximum magnification. For example, a 1 meter visual light telescope can magnify distant objects 10 times more than .1 meter telescope and about 1000 times more than a 1 meter radio telescope. As a result, high powered observations of distant objects at lower frequencies require extremely large telescopes. A technique known as interferometry is often used to combine multiple smaller telescopes so they can have the magnifying power of a single big one.

Measuring Distance & Standard Candles
Without knowledge of distance, the science of astronomy would probably be impossible. When star gazing, we could see objects in the sky, but we would have no idea if they were part of our solar system, our galaxy or if they were extremely far away. For instance, early star gazers believed that comets were an atmospheric phenomenon. Only when astronomers realized that they were far away, were they able to determine that they are left over fragments from the formation of the solar system.

Astronomers use different techniques for measuring distance on different scales. Methods that work for the planets don't work for nearby stars and techniques that work for nearby stars don't work for distant stars. Also, techniques often build on each other. A method for measuring far objects relies accurate distances to closer objects. Due to this stacking, our methods for measuring distances to extremely distant objects are quite imprecise while we can determine the distance to nearby objects with incredible accuracy.

The simplest procedure for finding distances is known as radar ranging. Powerful radio telescopes on Earth send a radar signal to a planet, asteroid or the sun and we measure the amount of time it takes for the signal to be reflected back to Earth. Since the signal had to go in both directions, we divide the time by two and then multiply it by the speed of light. We can measure both time and the speed of light very accurately, so this method is extremely accurate. However, radio signals spread out as they travel farther and the Earth's orbit moves it away from the bounce back point of radar signals off distant objects. As a result, radar ranging is only effective inside of the solar system.

For measuring the distance to nearby stars, we rely on a method known as stellar parallax. Stellar parallax works in a manner similar to a human being's depth perception. Our eyes are separated by a distance of several inches so the images of far away objects are more or less identical while near objects will produce a different image in each eye. Our eyes are only effective for a few meters (yards) before their parallax becomes ineffective. However, if we use the Earth's orbit around the Sun as our baseline instead of the few inches between our eyes, we can get quite accurate results for nearby stars. We observe a star on either side of Earth's orbit around the Sun and determine the angle by which it's position appears to change. The distance to the object can be computed from the radius of Earth's orbit around the sun and the angular change in the star's position using high school trigonometry: distance=(Earth Sun Distance) / tan(angular change/2). The use of this technique is so common that a special unit of distance is often used in astronomy called a parsec (parallax arc second). A parsec is the distance to an object which experiences angular change of 1 arc second, about 3.3 light years or 3.12*10^16 meters.

Objects which are further away than 600 light years experience so little angular change that stellar parallax is useless. Beyond 600 light years, we generally use standard candles to measure distance. Standard candles are objects in space whose luminosity, their brightness when measured from a fixed distance away, is nearly constant. If we can find these objects and measure their apparent brightness, how bright they appear to observers on Earth, we can compare the luminosity and apparent brightness to determine distance. For example, street lights all have more or less the same luminosity so if you see one in the distance at night you can roughly gauge how far away by how bright it appears.

For stars which are too far away from stellar parallax, we commonly use a technique called spectroscopic parallax which is effective up to 30,000 light years, enough to measure distances to most stars in our galaxy. Most stars have a very strong relationship between their luminosity and their temperatures. So, provided we know a star's temperature, we can use it as a standard candle. Fortunately, we can easily measure the temperature of a star by simply determining its color, the peak frequency of light a star emits. Bluish stars are very hot and reddish stars are cooler. By using an instrument known as a spectrograph, we can determine the star's peak frequency (its color) very accurately. In order to calibrate this technique, we measure nearby stars using stellar parallax and find their luminosity and temperature.

The spectroscopic parallax technique is only effective where enough light from a star can be captured for us to know that individual star's color. This is sufficient for measuring the distances to stars in our galaxy, but to measure the distances in nearby galaxies, isolating ordinary individual stars and determining their colors accurately is impossible. Instead, we use a special kind of a star which is known as a variable star for our standard candle. Variable stars fluctuate in brightness over short periods of time (days) and the rate at which they fluctuate is proportional to their average luminosity. Variable stars are also extremely bright so they are visible over great distances. To calibrate this technique, we can find variable stars in our galaxy, observe their fluctuation rates and find their luminosities (found using spectroscopic or stellar parallax). With the luminosity and pulse period relationship for variable stars, we can find the luminosity of any variable star for which we can measure the pulse period. The variable star technique is effective up to about 80 million light years, enough to measure the distances to galaxies in our local cluster and local supercluster.

For measuring distances to galaxies, a method known as the Tully-Fisher relation uses not stars, but whole galaxies as standard candles. The Tully-Fisher relation is a very strong relationship between a spiral galaxy's rotational speed and its luminosity. The relationship was discovered and calibrated by using the variable star technique, or in some cases the supernova technique which is discussed below. The rotational speed of a galaxy can be determined by examining the galactic disc's Doppler shift, the variations in the frequencies (colors) of light due to relative motion. Parts of the disc which are rotating toward us have an increased frequency (more blue) and parts which are rotating away from us have a decreased frequency (more red). The amount by which the frequency of light is shifted tells us how fast the galaxy is spinning. Provided that we can measure the Doppler shift of various parts of a spiral galaxy's disc, we can determine its luminosity and find its distance. The Tully-Fisher relation is effective up to about 650 million light years.

A final form of standard candle is the supernova. From observations of nearby supernovae, we find that they all have roughly the same peak luminosity and then dim over time in a very predictable manner. Supernovae are also incredibly bright, so bright that they may be brighter than all of the stars in an entire galaxy. Unfortunately, supernovae are incredibly rare. Since the invention of the telescope there has not been a single supernova in our own Milky Way galaxy. This means, that we cannot choose which galaxies we measure, we simply get whatever galaxies happen to be experiencing supernovae. According to Chaisson and McMillion, the supernova technique is effective up to distances of about 3 Billion light years, but logic seems to suggest that its effective range is actually equal to that the faintest galaxies we can observe. The Hubble Ultra Deep Field observed galaxies which were thought to be about 14 billion years old. Had a supernova happened to occur in one of these galaxies, I'm sure we could have been able to determine its apparent brightness and thus the distance. It is however doubtful that we could systematically find such supernovae, we would simply have to stumble upon them by dumb luck.
 
There is a last technique for finding distances which is known as Hubble's Law. Hubble's Law does not use standard candles but rather the expansion of the universe to measure distance. As the universe expands, the frequency of light from distant objects is redshifted (decreased) through a phenomenon known as the cosmological redshift. We can determine how much the light from a galaxy has been redshifted but looking for special spectral lines which are emitted by common elements, such as hydrogen, and seeing how far away the lines are from those of nearby samples of the same element. Hubble's Law isn't very accurate because studies disagree over the exact value of the constant in the equation. Hubble's Law also does not take the dark energy acceleration of the universe into account. As a matter of fact, comparison between the supernova technique and Hubble Law's discovered the acceleration which is attributed to dark energy. Hubble's Law has no limit on its range and can be applied to almost anything.

In summary

• For planets in our solar system we use radar ranging
• For stars within 600 light years we use stellar parallax
• For stars within 30,000 light years we use spectroscopic parallax (standard candle)
  
• For galaxies within 80,000,000 light years we use variable stars (standard candle)
• For galaxies within 650,000,000 light years we use the Tully-Fisher relation (standard candle)
• For galaxies within 3,300,000,000 light years we use supernovae (standard candle)
• For everything further away we use Hubble's Law
  

The accuracy of the techniques generally decreases as we go down the distance scale. The only exception might be that supernovae may be more accurate than the Tully-Fisher relation.

Life in the Universe
To date, the only known world with life on it is Earth. Although it appears certain that Earth in the only body in our solar system with macroscopic organisms, it is entirely possible that life exists or did exist on Mars, Europa or Ganymede. This does not account for the rest of the galaxy, it simply examines our solar system which is centered around only one of 100 billion stars.

Perhaps the first question in contemplating life in the universe is "what exactly is life?" Depending on which reference you use, you get slightly different answers which could have radically different implications if they were applied. Generally, the abilities to grow, reproduce, respond to stimuli and metabolize (absorb and store energy) are sited as key characteristics of life. However, if these rules are loosely interpreted, objects which we consider inorganic such as stars may be considered life forms while mules, which are incapable of reproduction, would be considered non-living. Alternative definitions of life rely on extremely Earth centric biochemistry which may not apply to the universe as a whole. As our ability to explore other worlds expands, we may well discover entities which may or may not be alive depending on what definition we use.

Biologists believe that all life on Earth contains amino acids and nucleotide bases. Amino acids build proteins while nucleotide bases form DNA and RNA. Interestingly enough, it appears to be quite easy to form amino acids in laboratory from common non-biological molecules such as water, methane, carbon dioxide and ammonia. This is done by introducing energy sources such as electricity (lightning), or heat. While nothing produced thus far in the lab can be considered alive, these experiments do show that given a sufficient amount of time, biological molecules could result from reactions of non-biological molecules.

According to the fossil record, life has existed on Earth for a very long time. The oldest fossils show that single celled organisms existed around 3.5 billion years ago, less than a billion years after the planet formed. However, evolution was very slow at the beginning. Fossils show that multicellular organisms appeared less than 1 billion years ago. Only about 500 million years ago did vertebrates, animals with backbones begin to appear and only around 300 million years ago did insects, reptiles and sharks first appear. Dinosaurs roamed the Earth about 100 million years ago and became extinct only 65 million years ago. Only 50 million years ago early monkeys appeared, and our descendants began using stone tools around 2 million years ago. Our species, the homo sapiens sapiens, has only existed for perhaps the last 200 thousand years though the homo erectus first appeared 2-1 million years ago. These results seem to indicate that although primitive single cellular organisms are quite easy to form, complex organisms are much more difficult. Thus, if we were to find life elsewhere in the universe, chances are that it would resemble bacteria or other single celled organisms and not the larger lifeforms such as animals, trees or even moss which we are accustomed to on Earth.

Within our own solar system, the best candidates for life appear to be Mars, Europa and Ganymede. Mars may have once been an Earth-like planet with standing liquid water, and a much warmer and denser atmosphere. A Martian meteorite revealed some structures that may have been the fossilized remains of microorganisms, but we are not certain of this. It is possible that some organisms may still live in aquifers on Mars but no standing water exists near the surface. Europa and Ganymede are both thought to have liquid water oceans below their icy surfaces. These oceans are kept above the freezing point of water by tidal heating which could also provide energy for life. Some astrobiologists have also speculated that life on Earth may actually have originated elsewhere in the solar system, perhaps on Mars, comets or interstellar dust. Indeed, large amount of organic materials have been observed on comets, so it possible that the solar ultraviolet radiation could have been the source of energy which created biological molecules.

By examining the hardiest organisms on Earth, we can better judge the limits of known life to see whether it is possible for organisms like those on Earth to survive other planets and moons. Earth's biomass is dominated by mass, number and genetic diversity by very simple single-celled organisms known as prokaryotes (bacteria). Almost all places on or near Earth's surface have some form of prokaryotic life. Bacteria have been found in pure acid and highly saline environments, subsisting from the 120 degree Centigrade heat of deep sea vents in regions of the ocean where light cannot penetrate and living in ice which is -12 to -17 Centigrade at depths of up to 3.6 km (2.2 miles) below the surface. Other organisms are capable of remaining dormant under extremely harsh conditions and being revived later. For instance, an organism known as a Tardigrade can be revived after being exposed 1000 thousand times the fatal dose of radiation to humans, chilled to nearly absolute zero or exposed to the vacuum of space.

If we assume that all life has the same basic components and needs as ours, we can expand our focus beyond the solar system and examine the possibility of life existing elsewhere in the galaxy. First of all, life requires basic elements which must be produced in stars. Regions of space where the has been limited stellar activity will probably have insufficient quantities of elements such as carbon, oxygen and nitrogen for life to form. Regions which are near galactic centers may also have very intense radiation, so life may not be able to form there either. The region between the high radiation of the inner galaxy and heavy element poor outer galaxy is referred to as the galactic habitable zone. Within a particular solar system, astrobiologists considers a region known as the stellar habitable zone. This is the area in which a planet with liquid water could exist. If a planet is too close to the central star its water will boil off, and if it's too far away the water will freeze. In our solar system, Earth is near the center of the stellar habitable zone with Venus and Mars at the fringes. There may also be habitable pockets such as Europa and Ganymede which are not inside the stellar habitable zones but special circumstances allow liquid water to exist there. Both habitable zones and habitable pockets rely on the underlying assumption that all life requires water. While this is true on Earth, it may not be true elsewhere in the universe.

We do yet not have the technology to detect primitive life outside of our solar system, but we can potentially detect radio signals from technologically advanced life. The Search for Extraterrestrial Intelligence (SETI) uses several techniques to look for evidence of alien civilizations. SETI uses arrays of radio telescopes to search candidate stars for alien radio signals and optical instruments to search for light signals or waste heat from alien civilizations. The name SETI is misleading, SETI doesn't search for Extraterrestrial intelligence but rather technologically advanced extraterrestrial civilizations. The difference is quite substantial. Although the intelligence of human beings has not changed noticeably over our 200,000 year existence we have only had radio technology for the past 100 years. There are also other animals on Earth such as certain marine mammals and apes which exhibit some characteristics which we might consider intelligence but clearly could not construct radio, cities which produce waste heat or lasers. Additionally, we appear to be moving away from classical radio techniques with alarming speed. Inventions such as cable television and frequency hopping radio signals would be undetectable by alien in civilizations, and it is likely they too would quickly invent such techniques.

Even if alien civilizations do exist in our galaxy, the prospects for two way communications are poor. The galactic disc is approximately 100,000 light years in diameter and the closest star to Earth, Proxima Centauri, is about 4.2 light years away. Light takes one year to travel one light year, so it would take 4.2 light years for a radio signal from Earth to reach Proxima Centauri and another 4.2 for a reply (if any were sent) to reach Earth. Thus, the time required for us receive a response for a communication from Earth is twice the distance between our Sun and the alien civilization's star when the distances are measured in light years. Researchers expect that even given the relatively optimistic assumption that 1 million advanced civilizations exist in our galaxy, the average minimum distance between civilization would be about 100 light years. This would mean it would require 200 years for us to receive a response to our radio signals. Using considerably less optimistic assumptions the distances shoot up. For 1000 civilizations in the galaxy, the average minimum distance to a civilization becomes 3000 light years so the round trip communications time is becomes 6000 years, almost as long as all of human civilization.

Is there a mathematical way of predicting the number of advanced civilizations in the galaxy? The answer is "sort of". In the 1960's, Dr. Frank Drake came up with his famous Drake Equation which breaks down the chances of finding technologically advanced extraterrestrial civilization into simpler terms. This allows us to concentrate on determining the values for the terms instead of solely searching for exosolar intelligence. Drake's equation is:

N = R_stars * frac_{star_planet} * avg_{planets_hab} * frac_{hab_life} * frac_{life_int} * frac_{int_tech} * avg_techlifetime

Where the values of the coefficients are

• N: The number of intelligent civilizations which are capable of interstellar communication in our galaxy
  
• R_star: Rate of star formation in our galaxy (stars per year)
  
• frac_{star_planet}:The fraction of stars having planetary systems (planetary system per star, <= 1)
• avg_{planets_hab}:The average number of habitable planets in a planetary system
• frac_{hab_life}:The fraction of habitable planets on which life evolves (planets with life per habitable planet, <= 1)
• frac_{life_int}:The fraction of planets with life where organisms develop intelligence (planets with intelligent life per planet with life, <= 1)
• frac_{int_tech}:The fraction of planets with intelligent life that develop technology to communicate (planets with communications technology per planets with intelligent life, <= 1)
• avg_techlifetime:The average lifetime of a technologically advanced civilization
  

Estimates for the values of the values of the coefficients in the Drake Equation vary incredibly widely. The only term for which we think we have a reasonably accurate value is R_star which thought to be approximately 10 stars per year. We have almost no data for the other terms, so it is impossible to give accurate estimates for their values. However, over the next decade, advances in telescopy may be able to better address frac_{star_planet} and avg_{planets_hab}. Very optimistic estimates of the coefficients put N in the millions while very pessimistic estimates put N at in the one millionth range. Many variations on the Drake Equation exist.

The far right terms in the Drake Equation may well be the most troubling. Given our experience on Earth, it extremely difficult, or at least time consuming, for life to develop from simple organisms into intelligent organisms. Consider this:

• Life in its most primitive form has existed on Earth for 75% of Earth's history
• Multicellular organisms have existed on Earth for about 20% of the planet's history
• Vertebrates have existed on Earth for about 10% of the planet's history
• Insects, Reptiles and Sharks have only existed for about 7% of the Earth's history
• Dinosaurs were the dominant species for about 100 million years, 2% of Earth's history
• Monkey and Whale like species have existed for only about 1% of the planet's history
• Species in the genus homo have existed for only about .04% of the planet's history
  
• Homo sapiens sapiens have existed for only about .004% of the planet's history
• Human civilization has only existed for about .0002% of the planet's history
• Radio technology which is necessary to communicate with other worlds has only existed for .000002% of the planet's history
  

Although life formed virtually immediately, we have been capable of communicating with other worlds for an extremely short period of time. As a matter of fact, life has been around 35,000,000 times longer than radio technology. Even if human civilization will to survive and maintain radio technology for 1 million years, life would still have been around 3,500 times longer than radio. From these results, we should consider the possibility that although life may be ubiquitous in the universe, life capable of communicating over interstellar distances is extremely rare.