The last post has kept me thinking about how different the solar system is from what we thought it was, even just fifty years ago. Was a time when Venus and Mars were the odds-on favorite for life.
My personal odds-on favorites for places most likely to support life in the solar system? Not what you'd think. First off, I wouldn't bother looking in the inner system. Mercury is blasted by a sun so close its apparent disc fills half the sky; even extremophiles would have to wear sunglasses. Venus? Oy. It's hell, pure and simple: victim of millions of years of rampant greenhouse effects that have turned the surface into seething lava beds and the atmosphere into a dense and boiling ocean of sulphuric acid. Mars? The best chance of all the inner worlds, but still not the way to bet. If only it were as big as Earth; but, as they said on Firefly, "if wishes were horses, we'd all be eating steak." Mars was simply too far from the sun, and too small to hang onto its core warmth, and thus its magnetosphere dissipated. With no magnetosphere to protect the planet (think of the magnetosphere as a planetary-sized deflector shield) from the sterilizing UV rays, there wasn't much chance for life to develop.
So we have to look further out. A lot further.
The two biggest planets in our solar system are Jupiter and Saturn. Together, they have over a hundred moons (hell, they probably have thousands if we count rocks from the size of baseballs on up. If you count all the rock and ice fragments making up the rings, we're into millions. This way lies madness. We're only interested in two: Europa and Enceladus.
They're very similar (despite Europa belonging to Jupiter and Enceladus to Saturn); So similar that we can generalize both of them into one. The major difference is size; Europa's about twice the size of Enceladus. Other than that you can barely tell 'em apart. They're both huge balls of ice; snowball moons. Utterly inhospitable on their surfaces, they nevertheless represent the best chances for extra-terrestrial life.
How? they're both covered with ice. Ice several miles deep, in fact.
Ah, but UNDER the ice ...
... is water. WARM water.
Wait, what? How is this possible? We're not talking colder than your Frigidiare cold, or even Antarctic cold; we're talking minus 300 odd degrees below zero cold. that's not just cold enough to freeze the balls off a brass monkey, that's cold enough to shatter it into powder with one blow of a ball peen hammer.
But nevertheless, if you could somehow drill down through several miles of rock hard ice, you would find water. Water warm enough, and filled with enough amino acids that, given a few billion years of sloshing around, the chances of life -- complex life -- developing are almost inevitable. How do we get warm liquid water in such an inhospitable place?
One word: torsion.
Europa and Enceladus have one other thing in common: they're the inmost orbiting moons of both their primaries. their orbits are close enough for Jupiter's and Saturn's gravitational pull, working against the gravity of the other moons, to cause massive tidal forces. And what does that mean?
It means the same thing that happens to any closed volatile system when outside energy is introduced: it produces heat. A lot of heat. Enough, over the eons, to keep an entire ocean liquified.
So, with kinetic heat generated by the gravitational equivalent of a huge taffy-pull and a nice insulating cap of ice sitting on top of them, the oceans of Europa and Enceladus could be sitting there, nice and cozy, for millions of years. More than enough time for complex life to develop.
Of course, "complex" is a -- well, complex concept. It could define anything from simple multi-cellular lifeforms you need a microscope and a powerful lot of squinting to see, all the way up to critters that (given the high salinity of the water plus the lesser gravity), could make a blue whale look like a rainbow trout. Hell, why not? As Sagan was known for saying, "Extraordinary claims require extraordinary evidence." True enough, but if I might humbly propose a codicil: "Extraordinary evidence demands extraordinary imagination to be recognized." Most planetary geologists, exobiologists and the like will pee up one leg and down the other if they find so much as an ice moon's equivalent of a paramecium Out There. Me, I'm voting for the marine equivalent of Argentinosaurus. Something that can pick its teeth with Nessie.
If you're going to dream, why not dream big? We used to think that the clouds of Venus hid a steaming swampy world that teemed with exoraptors. We got rogered real good on that one. But maybe, even though the inner worlds weren't exactly the stuff of which dreams -- or at least masthead novelettes for a nice, shiny nickel a word -- are made, perhaps, in the black depths of those exolunar oceans, a second chance worthy of the name Leviathan may yet lurk.