Update: this was my first post about methane: for a fuller version of the story you might want to try this post written a week later
Dick Kerr of Science magazine, who's been writing planetary science a good bit longer than most of us in this game, has a remarkable story up on the Science Now site -- something potentially far more striking than the crossbedding announcement. The team on the Mars Express Planetary Fourier Spectrometer (PFS) has announced the discovery of what look like methane absorption lines in the Martian atmosphere at 3.3 microns. Kerr quotes the PFS principal investigator, Vittorio Formisano saying it's "A very little amount," -- 10.5 parts per billion -- "but the result is clear." If this is indeed methane, then it's evidence that something is going on: either volcanic activity or life.
Methane is not a stable molecule in the Martian atmosphere. Left in the sun it will fairly quickly react with hydroxyl ions in the atmosphere; estimates suggest that it has an atmospheric lifetime of a few hundred years. And while some chemicals can be made in one part of the atmosphere and used up in other parts, methane isn't one of them, at least not to any appreciable degree. So if there is an even moderately substantial level of methane in the Martian atmosphere, then something on or below the surface is putting it there. Kerr mentions two possibilities; volcanic activity or bacteria. Methane in the atmosphere suggests that one or other of these processes is going on now. (A third possibility comes from above, not below: the recent impact of a comet, since comets contain methane. But there doesn't seem to be any sign of the very fesh looking crater that would be associated with such an event in any of the datasets, and it seems unlikely that a comet small enough to be destroyed by Mars's thin atmosphere, and thus not leave a crater, would be big enough to leave a global methane signature for any length of time.)
On the earth, methane from rocks is for the most part methane that derives ultimately from life -- natural gas, and the like, formed from the bacterial decay of organic matter. There is apparently also, though, a very low level of methane from non-biological stuff, which I believe is thought to be methane left over from the planet's formation. (Some people think there are vast amounts of this primoridal methane, and there would be on Mars, too. But most don't, and I'm not going to get into that here.) The average outgassing flow of methane from the martian depths expected by most people would be a lot less than the flow from the deep earth, and it would not be able to put substantial amounts of methane into the atmosphere. But during active volcanism, or other geothermal activity, the flow would be higher. So that's a possible source.
If it's volcanoes or similar sources, though, we have to wonder where they actually are. Nothing that looks like an active volcano has been seen. And even if the activity were more subtle and less splashy -- the injection of lava below the surface somewhere, say -- it's hard to see how it would avoid giving off heat that TES on Mars Global Surveyor or Themis on Mars Odyssey would have picked up. A small and not very warm spot of geothermal energy might escape TES, which divides the surface up into fairly big parcels. But with a resolution of 100 metres in the infrared Themis should be able to pick up such things, and so far it hasn't, even though it's taken infrared data on large parts of the planet.
The other obvious possibility is life. On earth, almost all the methane in the atmosphere is produced by bacteria, specifically methanogenic archae. These are anaerobic organisms, which would suit them to Mars. Their metabolisms depend on making methane from carbon dioxide and hydrogen (the hydrogen itself sometimes a product of other bacterial life). These have long been seen as the most likely forms of life to be found on Mars.
It would be a jump to go from a single announcement of a single piece of evidence from a single instrument to assuming life on Mars. But there is not just one piece of evidence. As Bruce Moomaw pointed out in the comments to this post (which made some remarks about the role of methane and life early on in martian history), Michael Mumma of NASA's Goddard Space Flight Center presented a poster on methane on Mars at the American Astronomical Society's Division of Planetary Sciences (DPS) meeting last fall. The poster (abstract here) presented results from observations of the Martian atmosphere using two different telescopes, NASA's Infrared Telescope Facility in Hawaii and the international Gemini South observatory in Chile. These showed some evidence for methane absorbing infrared light in the 3.3 micron wavelength, which is what the PFS observations show, too. Mumma and his colleagues have since refined their analysis of the data from Gemini, and taken more data from IRTF. They have also taken data using the NIRSPEC spectrometer on on of the vast Keck telescopes, but not yet reported results from those observations, as far as I know.
In the DPS poster, Mumma and his colleagues note that in one set of observations the strongest methane signal seems to come from the equatorial regions of Mars where the GRS has shown surprising amounts of hydrogen, and thus possibly water, in the surface. That fits the possibility of life quite nicely.
In his piece on Science Now, Dick Kerr quotes Mumma as saying that the PFS result "bears additional confirmation" -- which I assume the work he currently has in hand might provide. (That's if the PFS data don't come to be seen as confirming the initial observations he and his colleagues have already reported). In an email to me earlier this month, Mumma pointed out some of the difficulties of observing at these wavelengths when, as is the case for PFS, you have a relatively poor ability to make fine distinctions between wavelengths; ozone, water vapour and carbon dioxide can all complicate your results. The big spectrometers on earthly telescopes make such fine distinctions rather better. But he also said the PFS team was expert in dealing with these issues, and that he was "guardedly optimistic" they'd be able to measure methane were it there.
As Dick also mentions, there is another set of observations. Vladimir Krasnopolsky and his colleagues have been using the Fourier Transform Spectrometer at the Canada-France-Hawaii Telescope, and they too have detected evidence of methane on Mars. They'll be presenting it at the European Geophysical Union's meeting in Nice next month. Their results are right in line with the PFS measurement -- 11 parts per billion. In their abstract (pdf here) they argue that the best explanation for this is very sparse bacterial life on, or rather in, Mars. With three independent sets of observations, and with no evidence of current geothermal activity, that looks like the best explanation to me as well.
Nor need the life necessarily be quite that sparse. While some bacteria produce methane, others eat it up. If I were a martian ecosystem living in the shallow or deep subsurface, I would want to lose as little methane as I could, and so I would have layers of methane eaters above the methane producers, keeping the net flow to the atmosphere as low as possible. (Yes, that's an absurdly anthropomorphic way of putting it. So sue.) Earthly bacteria are very good at organising themselves this way.
Another possibility is that the methane output might be sporadic; the amount given off might change, for example, with the cycle of the Martian ice ages, or some other ongoing climate change. We might be seeing fallow-period methane levels. It's also possible that we are seeing methane that has been in storage for some time. This could conceivably be the methane from the martian equivalent of natural gas, laid down as biomass and stored in a deep geological formation for hundreds of millions of years ago. That seems unlikely, though -- you need a really productive biosystem to lay down vast fossil fuel reserves (probably one that photosythesises, too) and there's no evidence that anything like that could have been going on on Mars in the recent past. It would basically mean that there would have to have been an earthlike biosphere on Mars in the past 500 million years, and that just didn't happen.
A more plausible reservoir that might have stored up methane in the past would be the ice. Methane can be stored in water ice in the form of something called a clathrate. This is what happens to quite a lot of the methane produced by earthly bacteria -- there are methane-storing clathrates scattered all over the sea floors of the earth. These clathrates may be a potential source of trouble in a greenhouse world; they may also be a potential energy source. Martian ice might be a methane reservoir, storing it up under some circumstances, releasing it under others. So if methane production on Mars was spasmodic, methane stored in clathrates at the time when it's being made might be released from them more slowly later on.
That's probably enough speculaton for this first post. Except to note that even if the methane is coming from a gethermal source and not from bacteria, that itself would be big news. And it would have implications, too. If there's geothermal energy being pumped into a Martian crust which we now know to contain a lot of water ice, then that means there is almost certainly some liquid water down below. So even if the methane is not evidence of bacteria, it sort of has to be evidence of a wet and therefore habitable niche on Mars right now.