We’re going far afield for chemistry news this morning: all the way to Mars. As many readers will have seen, there’s some very interesting (and long-awaited) news – deposits of organic compounds have been conclusively identified. (Here’s the paper, free full text). This really is of great importance, for several reasons, and involves a few twists and turns of scientific history that you might not have expected.
We immediately land clavicle-deep in definition-of-terms stuff, though, because (as chemists well know) “organic” in our nomenclature doesn’t necessarily mean “associated with life”. It would help if we had another commonly usedd adjective for carbon-chain-based compounds that didn’t have such an association with living things, but for very understandable historical reasons, English and most other languages lack one (breaking out “abiogenic” in casual conversation is not a pathway to clear communication, for the most part). It was, in fact, a sensation back in 1828 when the first molecule that was known to be associated with living creatures was synthesized from definitely nonliving precursors: Wöhler’s urea synthesis. And any attempt to define what might be different about (for example) “natural” vitamin supplements versus “synthetic” ones will show that the issue is hopelessly confused for a lot of the general public.
At any rate, what the Curiosity rover has found by drilling into the mudstone of an ancient lake is what technically would be called “kerogen“, that is, carbon-based compounds found in rock deposits. At least on Earth, that can be a sign of life (current and/or previous), and it can be produced totally by inorganic “mineral” chemistry. The living part is easy enough to understand, since we see fossils of ancient forests in coal beds and so on, and it’s pretty widely known that there are all sorts of bacteria living and metabolizing in various rock formations and thus leaving their metabolic traces. That link goes into detail about how various types of earthly kerogen can be traced back to ancient terrestrial plants, to marine deposits, etc. There’s a huge variety of chemical composition in these things in terms of molecular weight distributions, element ratios, behavior on heating, and so on.
But how could you get “inorganic organic” kerogen deposits? This takes us into the very controversial field of abiogenic petroleum. There have been many claims over the years for deposits of oil and natural gas that are not fundamentally derived from deposits of ancient organisms. This was a big field of research in the old Soviet Union, and in the US, Thomas Gold – a very unusual man indeed who tended to be either spectacularly right or spectacularly wrong, but was never boring – was a vocal proponent of the idea.
Evidence has mounted over the years that the huge majority (very likely all) such deposits known on Earth are in fact biogenic. If there are substantial amounts of abiogenic gas, etc., on our planet, they’re probably located/produced in deep rock formations, where methane chemistry has been shown to be feasible. But it’s also become abundantly clear that there is a lot of methane floating around on the other planets in our solar system, for starters (often literally, at least when it’s not graining up and forming dunes). That methane can be converted by purely abiotic processes (thermal and photochemical) to higher hydrocarbons, which appears to be where the startling lakes of ethane on Saturns’ moon Titan originate. Small organic compounds (acetylene, acetonitrile, etc.) are well-known in interplanetary space around in clouds around other stars. As for more complex materials, look no further than the Murchison meteorite (which landed containing amino acids and many other complex organics besides), the Tagish Lake meteorite (which resembled nothing so much as a smelly lump of odd-looking asphalt or a big charcoal briquette), and the results of the Rosetta comet mission and others.
So, to Mars. The deposits found by the Curiosity rover are not exactly commercial grade oil shale, but the mass spec signatures are unmistakeable. Substituted thiophenes, for example, were directly detected, as well as benzene, small-branched aliphatics, thiols, sulfides, and more. The samples were heated strongly for this analysis, so there are surely decomposition products in there. But we now have enough understanding of Martian soil to get around the presence of perchlorates (which confused earlier observations) and to make these data definitive. The profile is very similar to what you get if you pyrolize kerogen-containing samples (biogenic ones) from Earth, but it’s also very similar to that obtained from pyrolizing carbonaceous meteorite samples.
You can easily hypothesize a biogenic origin – after all, these are deposits from the fossilized mud of an ancient lake. But you can easily hypothesize abiogenic ones, too. Mars has had plenty of volcanic activity over its history (Olympus Mons!), and carbonate rocks could have produced methane and other hydrocarbons under these conditions. And don’t forget that Mars, like all the other planets, surely had carbonaceous gunk, of the kind found in those meteors and comets, involved in its formation. We don’t know enough about the geology (and the geologic history) of Mars to say that the whole planet isn’t layered in low-level abiotic kerogen material if you dig a little. And given conditions near the Martian surface, the deeper you dig the better the chances you have of finding even more. It’s very much an open question, as open as it gets.
There’s another interesting Martian observation that might bear on all this: for many years now, it’s been clear that there actually is methane in the Martian atmosphere. Not much – but you wonder why there’s any at all, because the planet’s gravity is not sufficient to hold it. Neither is ours, but the methane in Earth’s atmosphere is apparently mostly biogenic (from microorganisms) and being continually replenished. Is that a steady supply on Mars?
If this is coming only from surface rocks, you wonder if it would have dissipated by now – and if it’s coming from deeper reservoirs, that would be pretty interesting, too (Thomas Gold would presumably have loved to have heard about these observations, but he died right about the time they were being confirmed, I believe). The latest results (ground observations from Curiosity again) show that the methane levels have a strong, reproducible seasonal variation at Gale crater, and it’s larger than you can probably explain by variations atmospheric pressure throughout the year. What’s more, there are occasional “plumes” of higher concentration. Overall, this suggests small reservoirs of the gas are being released steadily, but in bursts. The seasonal variation may well be increased sunlight heating the Martian dust, or it could be something else. And the origin of this methane is. . .another extremely open question. We’re at a very interesting, wide-open, tense time in the history of our attempts to answer the question of extraterrestrial life. Personally, I am extremely happy to be living though it!