Tuesday, November 18, 2014

Why We Need to Study Comets

News wires are full of stories today about the detection of organic molecules by the comet lander Philae. The more exciting news, arguably, is that Rosetta scientists are "very confident" Philae will wake up again as the comet gets closer to the sun. It should be noted that in October, Rosetta had already detected formaldehyde (an organic compound), sulfur dioxide, and hydrogen sulfide (which is nasty-smelling stuff, and quite chemically reactive), plus carbon monoxide and carbon dioixide, in the comet's vicinity.

How excited should we be about finding organic molecules on comet 67P/Churyumov-Gerasimenko? It's hard to know, actually, until the identities and abundances of the organic molecules are determined in greater detail, but we should bear in mind that the finding of organic molecules in space rocks is nothing new. That's not to say it's not exciting, though. It always is, IMHO.

Hydrocarbons are actually surprisingly abundant in space. The majority of stars in the Milky Way (and possibly elsewhere) are red and brown stars, including many brown dwarfs that are so cool (think room temperature) you wonder why they qualify as stars. Many of the brown dwarfs, in turn, are super-rich in methane (the simplest of hydrocarbons).

Carbon chemistry is amazingly complex in meteors. (For a great overview, I recommend the 2002 paper by Mark A. Sephton.) Only about 5% of meteorites are iron meteorites (and that percentage is probably greatly inflated by discovery error). Most of the rest are carbonaceous chondrites.

The Murchison meteorite is among the best-studied meteorites and shows monocarboxylic acids at 332 parts per million; amino acids at a concentration of about 60 ppm; sugar-related compounds at 60 ppm; urea at 25 ppm; alcohols, aldehydes, and ketones at 11 to 16 ppm each; purines at 1.2 ppm; pyrimidines (uracil and thymine) at 0.06 ppm; and a zoo of other minority constituents. The fact that many of the amino acids discovered in meteorites do not occur in proteins has been taken as evidence of an abiotic source chemistry. Also, the fact that the meteors' chiral amino acids tend to occur in racemic mixtures (with no enantiomeric excess of L- over D-forms) has been taken as evidence of abiotic chemistry, although this view should be tempered by the finding that on earth, D-enantiomers in natural sediments tend to increase in concentration with the age of the sediment. (In other words, as sediments age, natural racemization tends to even out the ratios of D- and L- amino acids.) Also, it should be noted that enantiomeric excess has, in fact, been observed for some meteorite amino acids. The presence of enantiomeric excesses of various kinds in various amino acids from various meteorites, and the possible reasons for those excesses, are still a matter of contentious debate. (See Sephton's paper for details.)

What's perhaps just as interesting (or more interesting) than the finding of amino acids in meteorites is the fact that much of the organic matter in carbonaceous chondrites is tied up in high-molecular-weight insolubles. Thus, the controversy over amino acids and their stereochemistry is somewhat like trying to understand the architecture of a house by analyzing the mortar holding together the bricks.

Aromatic compounds detected in Cold Bokkeveld and Murchison meteorites
using thermal degradation plus gas chromatography and mass spectrometry.
We know relatively little about the higher-molecular-weight components of carbonaceous meteorites. Sephton notes, however: "The majority of the carbon in meteoritic macromolecular materials is present within aromatic ring systems." Indeed, many species of aromatics have been recovered from meteorites using thermal degradation (pyroloysis).

Where do all these compounds come from? We know that interstellar clouds contain methane, formaldehyde, water, molecular nitrogen, and ammonia. These molecules are known to condense around dust grains (and comets), but there's also a lot of UV light, ionizing radiation, and heat in environments like the early solar system; and these could have led to a lot of chemistry. Studying comet 67P/Churyumov-Gerasimenko should tell us more about all this, which in turn could help us understand how life arose on earth. We know that every day, something like a million kilograms of extraterrestrial material rains down on earth. The fall-rate was probably much higher, early in earth's history. It's not inconceivable that 10% or more of the biomass on earth got its carbon from "somewhere else." (See the final paragraph of Sephton's paper.) Arrival of complex hydrocarbons from meteors, asteroids, and comets may well have jumpstarted life on earth. This isn't the only reason to study comets, but for me, it's one of the most compelling.

41 comments:

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