Earth's oceans would have been able to absorb much of the heat from these impacts -- but on those occasions when an impact was big enough to boil them into steam, it would have taken thousands of years for the planet to cool down enough for surface liquid water to reappear.

Mars lacked big oceans -- but because of its weaker gravity, big impacts would have been rarer, and when they did occur much of the hot vaporized rock would have been flung into space instead of clinging to the planet.

Thus, subsurface Martian microbes might have had less trouble surviving than the microbes of Earth.

Zahnle concludes: "Overall, early Mars may have been safer from impact sterilization than early Earth, and probably was habitable before the Earth-Moon system formed."

As Horneck pointed out during her talk, this theory might also explain the remarkable speed with which life appeared on Earth after the heavy bombardment ended and the Earth's crust cooled down enough to allow it to remain -- a few hundred million years at most.

Until recently, it's been assumed that this simply means that the evolution of life is easy enough that it's likely to be common in the universe.

But if Zahnle's view is correct, life may very well have evolved earlier -- and a good deal more slowly -- on Mars, and Martian microbes were then transferred to Earth and seeded this planet before any native Earth life got the chance to evolve at all.

While it wasn't stated at the Conference, I think this has big astrobiological implications.

Even if we do find present or past life on Mars, unless it's radically different biochemically it may not constitute any evidence that life is common in the universe -- it may well be that life evolved by an extremely rare chance on either Earth or Mars, and that it was then transferred to the other planet by meteorites.

By contrast, the meteorite transfer between either planet and Europa is virtually nonexistent -- so if we find life on Europa, it really will serve as evidence that life is common in the universe.

That, however, takes us to our next topic: just what is the best way to look for life on Mars (or Europa or other worlds)? As you might expect, this was one of the Conference's main topics, and there was much interesting news on it.

JPL's Ken Nealson provided a rundown on recent research into this major problem.

As he pointed out, intriguing shapes can point toward possible living microbes or microfossils -- but by themselves they are not an infallible guide; there are quite a few nonliving mineral processes that produce spheres and ovules that look strikingly like microbes.

One aspect of this problem is the claims made by Robert Folk and some other biologists -- and repeated by them at this Conference -- that they have proven the existence of extremely tiny "nanobacteria", far smaller than previously known bacteria, and in fact as small as the possible microbial "microfossils" seen in ALH84001 and some other Martian (and even non-Martian) meteorites.

Most biologists say flatly that there is no way such tiny bodies could possibly contain biochemical processes complex enough to support life -- and David Blake repeated here his claim that many similar-looking objects are created by provable nonliving processes -- but there's still enough doubt that the debate goes on.

Hojatollah Vali and some other scientists repeated their claim that they have in fact identified complex biochemicals in "living" nanobacteria. If this is confirmed, a whole new revolution is about to open up in biology -- and one with enormous astrobiological implications.

And, as their supporters have pointed out, certainly full-blown complex cells did not suddenly appear out of nonliving molecules -- there had to be some kind of intermediate stage in which living things were smaller and less complex, and nanobacteria may possibly be the survivors of that early age, both here and on other worlds.