Mark,
Per the current theory of solar system formation from a dust cloud and the known ages of the Earth (oldest rock ages here are minimum estimates), the Moon, and of meteorites, all the fun started for the solar system at about 4.5 billion years before present. That includes Jupiter's moons. After that, the exact history of either early Earth or Europa is more subject to speculation. Why? No or very little rock record is preserved until about 3.8 billion years ago on Earth. Added to this problem is the tidal dynamics of the various Jovian moons over time, so that all that exterior water/ice could have had a very complicated thermal history of freeze/thaw and the exciting possibilities of global oceans, separated seas, more global oceans, and "complete"* freezes that could have stopped and restarted any large-scale (macro) biological evolution. The present Europa surface suggests an active, briney interior ocean currently exists there, which is supported by the magnetic field measured by the Galileo spacecraft, as well as the near-infrared spectrometer data (for the salts) published in Science, Icarus, JGR, and other journals.
Gary
* Actually, there is probably no such thing as a complete freeze of liquid water to solid ice with salts present. On Earth, salts cause small liquid environments in the vicinity of any concentrations. Microbial life can take advantage of these micro-environments, and can even exist in liquid brines inside of salt crystals!
[On the subject of non-Europa posts: on other mailing lists, I've seen the convention of labelling posts that aren't directly concerned with the subject at hand with an "OT:" (meaning "Off Topic")at the start of the subject line. This makes it easier for people who aren't interested to spot (and filter out) unwanted posts. Just a suggestion. There are obviously people here (like myself) who enjoy the non-Europa posts.]
Question (on-topic) I've been wondering about.
How much time has life (as we know it) had to evolve on Europa, as compared to the earth? In other words, based on our best guess as to its history, at what point did things become stable or hospitable enough for life to have (conceivably) started evolving? Would they be millions of years ahead of us in this regard, millions of years behind, or somewhere close? Do we even know enough about Europa's history to harbor a guess at this?
I realize this is assuming a lot of things; we wouldn't expect evolution to progress at the same rate, nor would life necessarily spring up at the moment that it possibly could (though my understanding is that that's pretty much what happened on Earth).
--Mark
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