I recently updated my blog with some information about amino acid racemization (often called AAR). By looking at the proportion of L-amino acids to D-amino acids, one can use this as a basic "clock" to compare the relative ages of organic material. There are some important caveats, however. I will cover those in more detail later. But Aydin, over at Snail's Tales had an interesting comment about using the D/L ratio of amino acids to search for life on other planets.
To follow up on his two main points:
1. There may be inorganic processes that can concentrate a particular isomer of amino acids. However, most physical processes that we know of that sort various items involve a kinetic process. Sorting of river sediments from coarse to fine grains involves gravity, fluid dynamics, and the differing abilities of particles to be moved. Isotopic fractionation - where heavier isotopes of a particular element are concentrated - also involve sorting by the ability of a system to move them. Size (and mass) play an important role. For amino acids to be sorted by their stereo isomer through some inorganic process, there needs to be something that has a greater (or lesser) effect upon that isomer. Since the two isomers are essentially identical in mass and size (albeit with a different orientation of ), kinetic influences are not likely to concentrate a particular stereo isomer.
However, Aydin has an important point. There could be a process that sorts by isomer we are not aware of. Outer space is a pretty wacky place, and we would be fools to think we've accounted for all potential chemical/physical processes. So, when looking for extraterrestrial organic molecules, we should not take a racemic mixture as reason to think there is no evidence for life out there. Conversely, while finding a non-racemic mixture is interesting and could be a sign of life, we can't take it as incontrovertible evidence for biological processes. But it would be much harder to explain using a non-biological mechanism.
2. It is also entirely possible that (for the most part) only earth-bound organisms exclusively produce L-amino acids, and other "exobiological" processes utilize both. For example, some earth-bound organisms produce D-amino acids (Cone Snails, for example, produce some D-amino acids). Perhaps on some distant planet, all organisms utilize both isomers. Or, production of both isomers is the norm, and single-handed aminos are the exception. Reason again to think carefully about accepting or rejecting an exobiological hypothesis.
For an interesting technical paper, see: Córdova, Armando; Engqvist, Magnus; Ibrahem, Ismail; Casas, Jesús; Sundén, Henrik (2005). "Plausible origins of homochirality in the amino acid catalyzed neogenesis of carbohydrates". Chem. Commun.: 2047–2049."
Consider the hypothesis: "extraterrestrial life will produce amino acids of one stereo isomer." A null hypothesis of this statement could be phrased: "if I find an extraterrestrial sample with a single stereo isomer, I have found evidence of life." Thus, we could easily reject a null hypothesis if we find a racemic mixture on some distant planet; likewise, we would accept our null hypothesis if we find a concentrated mix of one stereo isomer. But what about the two alternatives mentioned above? Perhaps there is a non-biological process that produces a non-racemic mixture. We would, perhaps falsely, accept our null hypothesis. This would be a Type II error.* Conversely, we may have a situation where exobiological systems use both isomers. We would reject our null hypothesis even though (in "reality") it is correct. This would be a Type I error.*
Given the magnitude of unknowns regarding extraterrestrial racemization, we should certainly take into account other lines of evidence before settling on a pronouncement of the existence/non-existence of exobiological activity.
*I use "Null Hypothesis," "Type I," and "Type II" error more loosely than in the true statistical sense, but the analogy applies just the same.