Finding such "prebiotic compounds" on Mars -- and thus finding out how close ancient Mars did come to developing life -- is every bit as crucial a scientific goal in Mars exploration as finding actual life on the planet is. Without finding this type of evidence on Mars, we may well never be able to figure out how life first appeared on our own planet.

However, the fact that Earth and Mars may very well have exchanged living microbes -- especially during their early years -- raises another problem: if either "extant" (present-day) or fossil life IS found on Mars, how will we know whether it actually evolved separately there or is just a descendant of transplanted Earth microbes?

Only if there are radical biochemical differences will we be able to have any confidence of this at all -- and even then there will always be some doubt, since such inter-planet meteorite transfers happened far more often during the earliest years of both planets, when life on Earth had not yet evolved firmly into the forms it has now taken even among our most primitive remaining microbes.

Alternatively, life might have originated only on Noachian Mars, and then been transferred to Earth -- all Earth life may be the evolutionarily modified descendants not of ancient terrestrial microbes but of ancient Martian microbes, which may since have become extinct on their home planet. This is a serious possibility -- and, as I say, even if we find proof of Martian life, we may well never be able to know with any confidence what really happened.

The problem of pinning down life's precise planet of origin is made more complex by the fact that both early Earth and early Mars during the heavy bombardment period may well have been a lot more capable of supporting and preserving already-living germs transferred to them on meteorites than they were capable of evolving life on their own, given the short time life had to evolve out of nonliving molecules between impacts (and the other problems it may have had evolving in the more limited water supply of Noachian Mars).

For this reason, there's a growing suspicion that the discovery of life on Jupiter's moon Europa might actually be much more scientifically important than finding it on Mars. If life is found on Mars, that fact by itself may do nothing to disprove the serious possibility that life is an extremely rare chance chemical phenomenon in the universe -- it might very well have evolved by such a long-shot chance on either Earth or Mars in our solar system, and then just been transferred to the other world on meteorites.

But the transfer of living microbes from either world to distant Europa is tremendously more difficult -- of every billion rocks ejected into solar orbit by a giant impact on either Earth or Mars, only about 20 or so are thought to wander out into the outer Solar System and hit Europa within 10,000 years. And so, if we find life on Europa -- while we may never be able to prove beyond absolute doubt that it didn't just originate once on Earth or Mars and then get transferred to Europa as well (or even have originated on Europa and then gotten transferred to the inner worlds!) -- its existence on that distant world will nevertheless be far stronger evidence that life did evolve out of nonliving matter separately in at least two places in just a single solar system, which in turn would mean that it must be very common in the Universe.

Returning to Mars, however, there is the other major discovery by Mars Odyssey -- its confirmation that Mars, in its near-polar regions, has an even denser and more spectacular near-surface layer of ground ice than had been anticipated. In my next installment, I'll discuss why this discovery has now turned out to be more puzzling than had initially been thought, how that new puzzle ties in with modern Mars' hundred-thousand year obliquity cycles and with the other puzzling fact that Mars' soil is much more water-weathered than its rocks -- and how all this means that, if ancient Mars DID evolve microbial life, some small traces of that life may possibly still be holding out not only deep underground, but just a fraction of a meter below large parts of the Martian surface.

Related Links
LPSC 2001: A Martian Odyssey - By Bruce Moomaw
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