> > And I'm going to start with what I think is a pretty reasonable > > assumption: Europa may actually fit the profile of a very average, > > life-bearing planet in the universe. Most life in the universe > > may originate from oceans on planets around gas giants.
Robert Bradbury: > Huh? May I suggest "facts not in evidence". Europa is around > a gas giant that is relatively far from our sun that would be > a frozen iceball were it not for gravitational heating. Almost, > if not all, of the other "gas" giants we are aware of are in *very* > tight orbits around their respective stars where the surface > temperature has long since boiled off any water. I was under the impression that closely-orbiting gas giants were a relatively recent and surprising discovery. See table at end of http://www.public.asu.edu/~sciref/exoplnt.htm Quite a few gas giants have been inferred despite the relatively greater difficulties of detecting the ones that orbit at distances comparable to or greater than Jupiter's distance from the sun. There have been dozens of planets inferred, if not confirmed, but the closely-orbiting gas giants are big news recently because people thought they were impossible. The fact that people are finding gas giants (some say they may be tidally-stripped brown dwarfs) in such highly unlikely thermal conditions argues, in my mind, for a very great prevalence of gas giants, over a wider range of temperatures than was thought possible. It may be that most *Earthlike* planets are in orbit around gas giants, not directly in orbit around stars. And that the number of Earthlike planets, whether in orbit around gas giants or not, is much lower than the number of Europoids. As well, a high percentage (the majority?) of star systems are multiple-star systems, which has been used to argue against stable "life zone" orbits like Earth's, which are expected to be very narrow bands indeed. However, planets around gas giants with tidal heating of ice-covered oceans may be less exposed to the more unstable orbital dynamics of multiple-star systems. If most gas giants are larger than Jupiter, gas giants may offer fairly wide Europoid-friendly orbital ranges themselves - i.e., that range between 'too volcanic to allow an ice shield' and 'not tidally activated enough to keep water liquid and geothermal activity going.' See here, under the discussion of (implicitly Earthlike) habitable zones. http://www.markelowitz.com/exobiology.htm In particular, the galactic environment that makes Earthlike planets and the emergence of Earthlike life more likely involves a number of factors that don't seem quite so significant for a biosphere evolving in water under a thick shield of ice, perhaps on a body too small to hold a significant atmosphere, but still large enough to be tidally heated. Note metallicity as a factor - our sun is unusually metallic. Otherwise-sun-like stars with significantly lower metallicity than ours are unlikely to have terrestrial-type planets large enough and/or dense enough to hold an atmosphere. But Europa doesn't have the problem of holding down an atmosphere, because it has ice holding down a hydrosphere. (Not many substances are less dense in solid phase than in liquid phase - H2O is one of very few. Ice floats - we forget how weird that is.) >From what little data I've seen so far, I think someone could make a reasonable case that the galaxy hosts more Europoids than terrestrial planets. Maybe a lot more. And terrestrial planets are no picnic - our own system has four, and only one of them has obviously produced a significant biosphere. You might have about the same frequency of Europoids in the galaxy, but much more likelihood of life on Europoids. (Yet another reason to get under Europa's ice mantle: to get a rough handle on the probabilities.) > (Unstated presumption -- life requires water...) You work with what you know. I don't assume that life requires water. Who knows? Maybe gas giants teem with some form of life we can't conceive of right now. But ... intelligent life? Life we could communicate with? > > A distribution of gas-giant distances from stars will > > virtually guarantee that on some of the possible Europoids, > > the water will be covered by radiation-shielding ice. > > Ice doesn't do any more shielding than an equivalent amount > of mass between you and the radiation source. Europa's > radiation shield could just as well be water or gas (if > it could hold onto it in the long term). Actually, ice is better in some ways, because it has a lot of hydrogen. If you want to shield yourself from cosmic rays, and have a choice between lead and the equivalent mass of hydrogen, take the hydrogen. Not that this matters with an ice shield as thick as Europa's. And again: ice floats. Liquid water as a shield leaves life (or its originating conditaions) fully exposed to the dangers of the universe. Gas as a shield, ditto. If H2O is the ticket (or *a* ticket, anyway), it's got a neat self-reinforcing property: it can cover itself under a fairly wide range of thermal and gravitational conditions, provided there's a heat source underneath. Tidal forces working on small rocky worlds orbiting gas giants that themselves orbit at a distance from the home star that permits a permanent ice cover - this may be as good as it gets in this galaxy, 99% of the time. > [snipped much that I'm going to have to think about how > to respond to...] Well, think hard. ;-) -michael turner [EMAIL PROTECTED] > == > You are subscribed to the Europa Icepick mailing list: [EMAIL PROTECTED] > Project information and list (un)subscribe info: http://klx.com/europa/ > == You are subscribed to the Europa Icepick mailing list: [EMAIL PROTECTED] Project information and list (un)subscribe info: http://klx.com/europa/
