The orbits, spins and motions in the Solar System also provide clues to its formation. As we saw when discussing the night sky, the Solar System has a lot of regularity to it. Most of the planets (and Sun) orbit and spin in the same direction (counterclockwise as viewed from above the Ecliptic). Planets have nearly circular velocities, and the planets stay close to the ecliptic -- the path of the Earth round the Sun. In other words, the Solar System is a flattened spinning system. There are exceptions to this, notably Pluto among the planets, which has a tilted eccentric orbit, and Venus, Uranus and Pluto, which have retrograde spin.
Disk formation appears to be common around newly forming stars. This is significant, because it suggests that planet formation (out of the material of the rotating protoplanetary disk) may also be a common phenomenon. Recall our study of conservation of angular momentum and conservation of energy. As the early solar nebula formed, it's heat increased with gravitational collapse, and it formed a flattened, spinning disk. This structure and these motions are reflected in the current constituents of the Solar System. The temperature differences will result in the formation of two distinct types of planets. Our model can be tested by studying the properties of disks around other forming stars.
Inner Solar System bodies (including the belt asteroids) are rocky bodies, and quite small (none is bigger than the Earth), while the outer Solar System bodies are icy, where they are solid at all. The giant planets have huge atmospheres of hydrogen and helium around predominantly icy cores. They are surrounded by many moons, and by rings of ice and dust.
The clue to why the inner and outer Solar System bodies have such different sizes and composition despite forming from a common nebula lies in realizing that all this planet formation was going on at the the same time as the proto-Sun was "turning on"; the inner Solar System was much hotter than the outer Solar System. The key idea is that the solids that eventually made up the planets condensed as small grains out of the nebular gas. The condensation sequence of materials with temperature meant that the kinds of grains that could condense out as solids would depend on the temperature of the nebula at that location. Near the terrestrial planets, where nebular temperatures reached about 1500 degrees K, only metallic grains, and silicates (the material of rocky and iron cores) could form solids, so the inner planets and asteroids are made of silicate rocks and metals. Only in the outer Solar System could the lighter solids (water ices, carbon dioxide, methane, and ammonia ices) condense as well. This explains the difference in composition between the planets in the inner and outer Solar System. There is a size difference because the ices are made up of C, N, H, O -- elements which are much more abundant in the solar nebula than Fe, Si, Mg, metals that formed the grains in the inner Solar System. This made it easy for the outer Solar System planets to grow into giant worlds. The outer planets are large enough that they probably formed their own "mini-disks" around themselves, that eventually evolved into their own miniature "solar systems", with moons and rings. The ices formed low-density worlds worlds, with compositions much like the Sun (mainly hydrogen), while the metallic grains and silicates formed high-density rocky terrestrial planets.
We think all the bodies in the Solar System were formed initially from dust grains sticking to one another to form larger and larger bodies. These larger bodies are called planetesimals. Initially, dust grains just stick to one another, but as really big bodies form, they can also gravitationally attract neighboring particles and bodies to grow even bigger. Eventually, the biggest bodies win: these become the planets. Some of this material might not go on to form planets. (For example, the asteroid belt is thought to be a planet which did not succeed in forming, due to the gravitational disruption of Jupiter).
Once the Sun `turned on', radiation pressure and a dense wind from the Sun probably cleared out most of the material. Some of this left over `debris' from the planet making process survives as asteroids and comets. Their compositions (rocky or icy) reflects the sites of their original formation in the inner or outer regions of the Solar System. The early clearing out of debris created a period of early cratering. Some of the later collisions result in `captured' moons, which may have unusual orbits that do not reflect the original patterns of motion in the forming Solar System.
Dating the Solar System is accomplished by investigating naturally occurring radioactivity in solid rocks. We are then really considering the time to the solidification of the rock in question. A parent nucleide spontaneously undergoes a radioactive process, that alters the nucleus and gives off energy. The time for half the sample of nuclei to decay is the half-life of the species in question. By considering the ratio of parent to daughter nuclei, the time to the solidification can be calculated, provided the initial composition can be guessed at. This guessing is done by knowing something about the naturally occurring isotopic ratios in the sample (nuclei with the same number of protons -- same element -- but different numbers of neutrons -- different isotopes.) For example, Uranium-238 decays into Lead-206 with a half life of 4.5 billion years (Gyr) and the Lead-206 to Lead-204 ratio gives the initial state, since Lead-204 is not a daughter species. The oldest Earth rocks dated in this way are 3.8 Gyr old. The age of the Solar system is generally reckoned to be that of the oldest meteorites, about 4.6 Gyr.
The chronology of the Moon is quite well determined by radioactive dating of rocks brought back by the Apollo Program astronauts. The heavily cratered Highlands contrast with the younger, less cratered Maria floors which are upwellings of basalts that are much younger than the impacts that formed the basins themselves. The craters on the Moon and Mercury are impact craters from meteorite impacts and, the older the surface, the more impacts it shows. However, the lunar record shows that there must have been a period of very heavy bombardment about 4 billion years ago to account for the very high crater density in the Highlands. The youngest features on the Moon are the craters like Tycho and Copernicus, about 200 million years old; ejecta from these impacts overlies older terrain --- the principle of stratigraphy: new stuff is on top of old stuff. We believe that all these objects formed at roughly the same time.
The idea of "comparative planetology" is that we would like to understand why the planets of the Solar System got to be the way they are, although they have a common origin, presumably, in the Solar Nebula. Specifically, in the inner solar system, the Earth is fairly active geologically, the Moon and Mercury are dead worlds, and Mars and Venus are in between. The fundamental question is how can these differences have arisen on worlds subject of the same physical laws and processes as on Earth, and can this be traced to the properties of the planets, their size, mass, composition and location, rather than just being treated as accidents.
We think that all the terrestrial planets have differentiated interiors. Early on in their history, while they were still hot from formation, the denser material in these planets probably sank to the center, forming a metallic core, with a less dense rocky mantle above it, and even less dense rocky crust on top. The outer part of this structure is what is called the "lithosphere", and it can react to stresses and energy flows from the interior if it is thin enough. The thickness of the lithosphere is directly related to the temperature of the interior, in the sense that it is thinner in the case of a hotter interior. This reaction in the lithosphere is what shows up as surface geology -- volcanos, lava flows, earthquakes and so on. So the key idea is that a hotter planet will be expected to have a more geologically active surface.
The crucial property of a planet that determines its inner heat, and therefore its surface activity, turns out to be the size of the planet. This comes about because the planet's interior temperature is a balance between heating and cooling. Rocky planets are predominantly heated by radioactivity in rocks distributed throughout their interiors, once the heat of formation and differentiation has radiated away. This means that the heating of a planet is a function of it's volume. On the other hand, a planet cools through its surface by radiation. Because of the balance between heating and cooling, the important quantity is the ratio of the surface to the volume of the planet. For a spherical planet, this ratio is inversely proportional to the size of the planet. In other words, a large planet keeps its heat more efficiently than a smaller planet. This ties in nicely with the planets of the inner solar system: the small worlds, like the Moon and Mercury, and to some extent Mars, will have cooled off more rapidly than Earth or Venus, and it is these small world that are less active, or dead
The four processes that modify a planet's surface are impact cratering, vulcanism -- volcanos and other lava flows, tectonics -- the effects of the motions and interactions of crustal plates, and erosion -- the effects of water and atmosphere. On Earth, all these processes have been important, though the effect of early impact cratering has been largely lost by later geological activity. All the rocky planet surfaces have been modified by impact cratering, but it is only on the Moon and Mercury that this can be fully seen without subsequent modification. Vulcanism is also ubiquitous in the inner solar system (and in the outer solar system as well)but among the inner planets, only the Earth and Venus probably still have active volcanos. Modification by lava flooding is more common: the "maria" on the Moon are vast impact basins subsequently filled in partially by huge floods of lava. The active "plate tectonics" of the Earth is probably unique in the solar system, though some other features on Mars and Venus might have a tectonic origin. Both Mars, with its thin atmosphere, and Venus with its very thick atmosphere, show signs of erosion.
It is very unlikely that the primitive inner planets were large enough to hold onto the gases of the Solar Nebula as atmospheres, but all the inner planets have some form of atmosphere, even the Moon and Mercury. In the case of the Moon and Mercury, this is though to be the result of bombardment: a very small amount of gas is intermittently present on the surfaces. In the case of Earth, Mars & Venus, the atmospheres are thought to have been outgassed from the planet's interiors during the course of early volcanic activity. These would have been primitive reducing atmospheres, of carbon dioxide, methane, and Mars and Venus still have atmospheres that are predominantly carbon dioxide. The Earth is unique in having an oxygen and nitrogen atmosphere, although carbon dioxide and water remain as important trace constituents.
The Greenhouse Effect occurs when a planetary atmosphere is transparent to visible light (and so the planet can heat up) but has greenhouse gases that absorb the infrared light that is trying to escape to cool the planet. Principle greenhouse gases are water, CO2, and methane and ammonia. Greenhouse warming affects all three planets. Even Venus has a transparent enough atmosphere for sunlight to reach the ground, and its thick, CO2-dominated atmosphere keeps the surface very hot. On Earth, 33K of greenhouse warming keeps the surface warm enough for liquid water and life.