Has anyone put forth a good theory as to why the various large objects in the Solar System–which presumably accreted from the same stew of ingredients–are so varied in their present compositions?
I think I’ve a handle on why the inner planets are so different from the gas giants: because of the great differences in size and distance from the Sun (tellingly perhaps, if true, the gas giants seem to be pretty similar). But the variation between Earth and Venus is pretty great, and then there’s Mars and our own Moon. And then looking at the largerer Jovian and Saturnine moons, it’s like a menagerie. How did they get to be so different; or rather, why aren’t they more alike?
It seems to mostly come down to size and distance from the Sun.
Mercury: It’s both close to the Sun and small, so it cannot hold onto any decent atmosphere which the intense solar wind would blow away anyway.
Venus: It’s likely that Venus and Earth started out mostly the same, but Venus is slightly warmer, so its oceans evaporated and started the runaway greenhouse effect. In fact, it’s thought that in a billion years or so, as the Sun gradually heats up, Earth will evolve into a Venus-like state.
Mars: Like Venus, Mars probably started out similar to Earth. But Mars is much smaller than Earth and Venus, so its atmosphere gradually escaped out into space. With its lack of atmosphere, its oceans boiled away, and we’re left with the cold, dead Mars of today.
There are three classes of objects in the solar system–rockballs, gasballs, and iceballs.
Objects close to the sun are rocky because the volatiles have evaporated. Gasballs are objects massive enough to accrete a lot of volatiles from the early solar system. They are also so massive that their internal heat keeps the volatiles as gases. Iceballs are almost all volatiles, but small enough and far enough from the sun that the volatiles are in solid form. These form the outer dwarf planets like Pluto and also comets.
The moons of the various gas giants are small rockballs or iceballs or mixtures, depending on their origin and how much volatiles they have lost or gained over their history.
It’s probably not a coincidence that in our solar system we have rockballs close to the sun, then gasballs, then only iceballs. Any rockballs or iceballs with orbits near the gas giants will eventually probably end up captured as a moon, ejected into another orbit, or accreted. But there are still a few iceballs like Chiron within the orbits of the gas giants.
However, in other solar systems we know there are superjovian planets that are very close to the star, closer than Mercury. The current theory is that these objects started out farther away, but have migrated closer. Our solar system may not be a typical specimen. Or maybe it is, since it’s a lot easier to detect exoplanets the more massive they are and the closer they are to the star, and our sample of exoplanets is biased.
Okay, forget about the planets, whose differences are apparently fairly easily explained. Now, why are Jupiter’s and Saturn’s big moons so varied? Ganymede, Io, Titan, Europa–right there you’ve got some serious diversity (and of course I could name others). How come? Maybe Io is a simple case, but still.
Ganymede, Europa, and Titan are all pretty much made of the same stuff: a mix of water ice and silicate rocks. Io would probably have some water on it too if the effects of Jupiter’s tidal kneading weren’t quite so large. I would guess that this kneading also imparts enough energy to Europa to keep its water liquid rather than solid (and allows it to differentiate from the silicates.)
Now, that still leaves the matter of Titan’s atmosphere; my understanding is that the fact that Titan is colder than the Jovian system allowed it to hold on to lighter molecules that would otherwise have escaped. But one could argue that the atmosphere is a minor detail: since the majority of Titan’s mass is the moon itself, and it’s not largely different from the composition of Ganymede, Callisto, and so forth.