QUOTE (ngunn @ May 10 2006, 01:45 PM)
Absolutely right. The question is do we limit what we mean by life (and contamination) to carbon based organisms? As others have long ago pointed out if in fact carbon based biochemistry never took hold on Mars that would make it the ideal place to test the clay theory.
IF the clay theory proves sound and IF indeed clays did start to evolve on Mars in the absence of the more high-tech carbon biochemistry then that raises numerous questions. The fossil remains of these evolved clays could be quite extensive across the planet, but would we recognise them? Are we even looking? What sorts of instruments should be used for the search? Have we already found them without realising what they are? How vulnerable might they be to contamination?
I hope very much that someone is onto all this, but I'm not sure if clay minerals specialists participate in Astrobiology conferences, or whether the equation BIO=CARBON holds. Issues on the fault line between academic disciplines can for this reason alone be all too easily treated as peripheral, even off-beat, in both communities.
Eventually, we could find, not life, but one of its primary steps. It is well known that clay play a role of catalists, favourising certain amino acids and certain nucleic bases among tens of similar bodies. As we could expect, our life is precisely made of those amino acids and nucleic bases, strongly hinting to the role of clays into early pro-life chemistry.
We could eventually find on mars one of these steps, for instance clays having concentrated the right amino-acids, but which not evolved into actual living being, or even some more advanced step. Even if life itself is not present, it would be a great discovery to find one of its primary stades.
As to non-carbon life, it is a bit speculative, but we cannot dismiss it for simply this reason. By definition we don't know its chemistry. So its existence could be detected only through the discovery of living beings, or their effect onto the environment. This should be anyway included in any search strategy.
Eventually we could find in clay some exotic things, such as a self-catalytic mixture which evolved through a selection, but never gave cells. If such a thing ever existed on earth, it would of course have disappeared for long. But finding such a thing would give us clues on early prebiotic stages, or on other evolution paths than just cells or carbon-based life.
By the way I have some doubt about finding 4 billion years old amino acids. Perhaps we should search for degraded products. Oil shales?