(This is the sixth of the posts on methane, and will make more sense in the context of some of the earlier ones. The first,
" Methane (and thus life?) on Mars" and the fifth, "Two thousand cows" probably provide most of the necessary background)
Time to look at the possible sources again, especially abiogenic sources -- those that don't involve living organisms. The obvious non-living source, which seems to be the preferred interpretation at ESA, is volcanism. The idea is that although there are no eruptions going on right now volcanism has obviously been a big part of Martian history, and it has continued up until very recently. Indeed, it is almost certainly, in some sense, still continuing -- just getting more weak and more intermittent as the planet loses ever more of its internal heat.
How recent were the most recent of these intermittent events? Bill Hartmann, scientist and artist, says its a very hard question. The most recent lava flows will be small, one assumes, and -- by definition -- young. And because the way you estimate the age of a Martian surface is by counting the craters on it (the longer it has been exposed the more craters it will have), it is peculiarly hard to date small young bits of the surface; there has not been time for that many crater-inducing meteorites to hit them. So getting a reliable age for the most recent volcanism is hard.
Alfred McEwen, of the University of Arizona, has been studying the youngest volcanic plains on Mars, those of the Cerberus region in southeastern Elysium, for years -- at one time he was an advocate of sending one of the MER rovers to them. He's convinced that there have been no eruptions there for millions of years, perhaps a hundred million, so any gas that came to the surface in that lava will be long gone. However, he points out that not all the molten rock -- magma -- that rises from the planetary depths gets to the surface. Some pools within the crust, forming "intrusions" that geologists call dikes and sills. A near-surface intrusion forming today might be a plausible source, and might show up in the infrared as a surface hotspot . As an intrusion ages it will cool down it and stop giving itself away that way, and ut will also emit less and less methane. The relative speed of these processes -- the question of whether an intrusion can cool down enough to be thermally inconspicuous while still emitting methane -- is something that hardly seems worth guessing at. Someone will have to build a numerical model of the process and see what it says. And then everyone else with any interest will have to weigh in and criticise the model
If the overall atmospheric methane level turns out to be declining year on year, an intrusion some time ago would come to seem more and more likely. There's also the possibility, though, I suppose, that the methane might be coming out not after an intrusion but before an eruption. On the earth, monitoring the gases over a volcano is one of the ways people try to predict eruptions. I think it's only used on active volcanoes -- but do people try to predict the eruptions of dormant volcanoes? If the methane were an eruption precursor that would be undeniably cool. Localise the source to the top of a volcano and I think we can guarantee some attention.
Anyway, whatever the details, methane coming from magma has some testable implications. It suggests a localised source, or sources; Professor Formisano, on the Mars Express PFS, is trying to see whether he can assign a location to the methane's source. Vladimir Krasnopolsky, one of those who's observed the methane from earth, thinks that because methane gets mixed up in the martian atmosphere pretty quickly, assigning a location to the source isn't going to be possible, and other experts Ive talked to about the subject agree with him. Magma also implies a thermal signature, though it might be hard to pick out. It's clear there's no big signature, but Phil Christensen, the man who sucks up more infrared photons from Mars than anyone else, says he can't rule out the possibility of patches that are subtly warmed from below. I wondered for a bit whether intrusions like this might show up in the Marsis radar data, but apparently they wont.
Probably the most detectible telltale of a volcanic source would be the other gases (hydrogen, carbonyl sulphide, and other things) that would be given off with the methane. Some of these might be picked up by one or other of the teams now observing; they could certainly be picked up by next-generation instruments on a future orbiter.
Volcanism is not the only possible geological source, though. There's also the possibility of making the methane in the crust through chemical reactions. Mostly this involves a lot of heat -- a rather good article on the French Mars site Nirgal.net, for example, suggests the Fischer-Tropsch reaction, which when used to make synthetic fuels on earth requires high temperatures. Because this sort of heat could only be provided by magma, most such mechanisms can sort of be subsumed under the volcanism/intrusion stuff above. But there is a crucial exception.
I've been in touch with Allan Treiman, of the Lunar and Planetary Institute in Houston, who's thought about these things a fair bit, and like John Eiler of Caltech (in the second post, here) he points to the possibility of a form of low temperature geological metamorphosis called serpentinisation. Olivine is the most abundant mineral in both the earth's mantle and that of Mars, and iron-rich forms of it will react with water at low temperatures to produce a mineral called serpentine, some magnetite, and some hydrogen. If the water in question carries dissolved carbonates the hydrogen can go on to make methane. On earth, rocks from the mantle brought to the surface (best seen in ophiolite sequences) will normally have been at least partially serpentinised in this way.
Treiman points to various reasons for liking this hypothesis. One is that data from TES have shown that there is indeed olivine on Mars. (This discovery, made a couple of years ago, was taken as an interesting indication that Mars really was a very dry place, precisely because water would destroy the olivine.) So water in the crust would have something to serpentinise. Another reason to take serpentinisation seriously is that it seems to have happened to some of the martian meteorites that have been revoered and studied on earth -- bits of olivine in them have been replaced by serpentine-like minerals and magnetite. In a field that can get wrapped up in theory, a rock in which the proposed process has actually happened is a pretty good argument. (The late great geologist Marshall Kay used to like to put it this way: "What has happened, can happen".)
Serpentinisation is thus a very intriguing possible methane source: it has happened, so it can happen. (According to Everett Gibson of NASA's Johnson Space Center, the most zealous of the team that claimed to have found evidence of life in the martina meteorite ALH 84001, olivine in the Nakhla meteorite had its composition altered within the past million years. I don't know how widely accepted this interpretation is.) But how can we tell if serpentinisation or something similar going on now is the source that accounts for the methane signal?
Like volcanic intrusions, serpentinisation would probably be localised. Indeed, it would have to follow on from something like an intrusion -- there needs to have been a fresh supply of olivine sometime. Unlike large, fresh, shallow intrusions, a serpentinisation site would probably not be accompanied by a hot spot (though the chemical reactions involved do give off some heat -- these reactions are the source of some spectacular sea-floor geological activity). And it would not necessarily give off much in the way of other gases, though it sounds as if some hydrogen might escape.
In short, telling serpentinisation apart from a subsurface bacterial biome might be pretty hard with the instruments currently available. And it's possible the dividing line isn't even really there. Though serpentinisation can produce methane wothout life, it can also feed the appetities of methane making bugs. The reactions between hydrogen and carbon compounds that mean serpentinisation can give off methane are exactly the reactions that methanogens such as these ones living under the Beaverhead Mountains in Idaho use to make a living. There is no reason why bacteria might not make use of serpentinisation as a source of energy.
In general, after all, life makes use of chemical energy wherever the environment provides it. Living organisms make their way in the world by acting as estate agents for electrons ready to take part is chemical reactions. They tell electrons which are not entirely happy in chemical compound A that there's a nicer home for them in compound B, and then skim off a commission in the form of energy when the electrons make their move. For this to work, you need a constant supply of chemicals that are out of equilibrium with each other. On earth, the oxygen supplied by plants does the job brilliantly. There are always electrons around that would rather move to oxygen atoms, and so there's always a chance for life to get some energy. In the Martian crust, though, the most obvious ways to resupply life with chemicals ready to react would be through the arrival of hot magma, or through reactions brought about by water. The processes that are alternatives to life in terms of methane production are also necessities for life in terms of providing fuel.
I'm beginning to think that this may come to feel very significant if future research -- research on the carbon isotopes in the methane -- shows that the methane was not made by life. In earlier posts I pointed out that even without life, the methane would show that there were processes going on in the crust, processes involving magma and/or water, that made the crust habitable. And I'm conditioned enough in the astrobiological mindset to think that habitability is always a big deal, because it seems like a step along the road to finding life.
But the real implication here might be life's absence. If we showed that there were habitable regions in the planet's crust but that the methane coming from them was produced without the help of life, we'd effectively be showing that the habitable regions were uninhabited. If the chemical reactions that produce methane are going on, but no methanogens are getting in on the act in order to cream off some of the energy, then you have to assume it's because the methanogens aren't there. This wouldn't necessarily mean that there was no life deep in the crust of Mars; but of the lifeforms we know, methanogens have always looked like those best suited to underground life on Mars. If there's a niche for them and they're not in it, that looks bad. When you walk into a cold house with a cosy fire in just one room and that room's empty, it starts to look like nobody's home.
If the methane doesn't show that Mars is alive, it may come to look like grounds for presuming that it is truly dead.