It remains far from certain that Europa today has enough geothermal heat to produce volcanice hot-water vents. Its tidal heating is remendously less than Io's.

All estimates are that it emits, at absolute most, only about three times as much geothermal heat (from all sources) per square kilometer of its surface as the Moon does -- only two-thirds as much as Earth does -- and most of that tidal heating comes from tidal flexing of its ice and liquid-water layers themselves, rather than the underlying rock.

However (as is pointed out in the 1997 report "Exploring the Ice and Ocean Environments", which can be found at JPL's spendid Websites on Cryobot/Hydrobot development, accessible through its Europa Orbiter site), even if Europa is geologically almost dead now -- indeed, even if it is still frozen and has only an underlying layer of soft ice rather than a liquid ocean -- it will still be extremely important exobiologically.

The reason is simply that Europa (like Mars) was a far friendlier place during its ancient days. There is no doubt that, while it was still cooling off during the first few hundred million years after its formation, it was riddled with volcanism and hot-water vents.

It is very possible that life could have evolved during that period, and then evolved to survive in the cooling and deteriorating environment, so that some remnants of it may exist today (and maybe even be able to take advantage of mineral energy sources not requiring active volcanism).

In fact, given the speed with which life appeared on Earth after the planet cooled down enough to become habitable, I think it downright probable that early Europa did have life! It is virtually certain that only small amounts of life could exist there today -- but I think there is a real chance that such remnants of life do exist there.

And even if life doesn't exist there today, Europa's ice would be a far better place to look for fossil life than Mars' surface -- large numbers of microbes may be frozen and almost perfectly preserved in it. I even wonder whether some of them might be revivable, although the high Jovian radiation level presents a problem unless you probe down to moderately great depths.

Dr. Chris McKay has said that he thinks the only thing that would prevent ancient microbes frozen in Mars' permafrost from being revivable is the genetic damage they would undergo from billions of years' exposure to the traces of radioactive uranium, thorium and potassium in Mars' rocks -- and Europa's ice must have much less rock mixed in with it.

Or -- if life started to evolve from organic chemicals on ancient Europa but never made it all the way -- the resultant "prebiotic" chemicals preserved in the ice would be almost as important scientifically (as NASA has officially said they would be on Mars).

The reason is that we have only the vaguest idea of how life might have evolved on Earth -- in fact, nobody has been able to propound a really satisfactory theory -- and all the fossil chemicals that might serve as evidence have long ago either been eaten by other living things or destroyed by Earth's crustal recycling and weather.

In the cold inert environments of Mars and Europa, they are much more likely to still exist -- and so we may have to go to those worlds to find out how life first appeared here.

Bruce Moomaw is retired and lives in Cameron Park east of Sacramento. He has been an avid reader of platerary exploration research and provides occassional public talks on the solar system.

Bruce Moomaw is retired and lives in Cameron Park east of Sacramento. He has been an avid reader of planetary exploration research and provides occasional public talks on the solar system.