Why is chirality of organic molecules the key for determining whether there was life on Mars?

Chirality of what? If you are referring to proteins, be aware of the possibility of life being based on ribozymes instead of protein enzymes. This is the basis of "RNA world" which is generally believed to be a precursor of "life as we know it," perhaps the penultimate precursor.

I am not aware of an adequate substitute for structural proteins, however, if one is looking for organisms of the level of sophistication of bacteria.

These matters are discussed from time to time in talk.origins, and occasionally in sci.bio.paleontology, to which I returned in December 2010 after a decade of absence -- unfortunately too late to catch you, Cal, who quit a little over a year earlier. The latter newsgroup is still hanging on, with lots of discussion of paleontology and systematics, and occasional other topics as well. There are six regulars there now -- three of them honest (including myself).

Peter Nyikos

Living things have particular molecular structures where the exact molecular structure is formed from enzymes. It is that precise nature of enzyme pathways in living organisms that would make molecules not random so not chiraly (is that a word) neutral. If they are randomly made they should have half one way, half the other. A precise enzyme pathway like exists in our life (probably a relevant example) makes the molecules very precisely. Now if they were looking at samples and saw a shift of light or whatever it is that chiral molecules causes as a valid test of life?? Not a hundred percent but I don t know how much. Calling it key is overstating in my not really well considered opinion on the matter.

I don't know exactly, and so can't answer exactly relative to this specific question....

But, life is funny biochemically, and is often preferential for specific isomers and isotopes when synthesising organic molecules.

So I should imagine, the thinking is, if there was a 50-50 distribution of a specific biological marker, then that would infer that it has formed through non-living means, as geology, weather etc is not usually as preferential. There can of course be exceptions.

For example, life on earth tends to prefer Carbon-12 over Carbon-14 for protein synthesis. But this could be a question of the abundance of C12 compared to C14.