This morning I spoke to Vittorio Formisano, the principal investigator on the Mars Express PFS, about the methane results. (If you're just coming to this topic, best take a look at my first post about Dick Kerr's subscription-required online report for Science, available in a slightly fuller form here now the magazine has been published).
Of the follow-up strategies, the one Formisano says he is pursuing most keenly (given the constraints of time and the fact that there is a lot of other work for PFS to do) is localising the source. PFS averages out the atmosphere, taking data pretty much continuously when the satellite is near Mars (Mars Express has a quite eccentric orbit, swinging from 11,500km away to just 260km away and back on each orbit). So, because the orbit is south-to-north, at every close approach PFS sees a swathe of the planet’s daytime side along a given line of longitude. Formisano is currently looking to see if some longitude bands provide a stronger methane signature than others, as a first step towards seeing where the methane is coming from. He gave the impression that it is possible there might be some preliminary findings on this to report by the time he gives his invited talk at the European Geophysical Union's Nice meeting at the end of April. He also said that he understood ESA would be putting out a press release around then.
My guess would be that a source localised in one (or more) bands of longitude would favour methane made by life or something like volcanism over methane made from meteoritic debris, which you'd expect to find scattered evenly all over the surface (see previous post). Meteoritic organics might behave differently at different latitudes -- different temperatures and sun intensity could effect their stability, I suppose -- but I can't see why they'd care about longitude.
Another possible follow-up strategy we talked about is looking for other volcanic gases that you'd expect to see if the methane was coming from a geothermal/volcanic source. Formisano thinks this may well be a good avenue to pursue, but hasn't had the time to start work on it as yet.
He says he is pretty much agnostic as to the source, but seems to have a certain sympathy for volcanism. As he points out, Mars has a long, proud history of volcanism. It would be a bit strange to think that this history, having endured for four and a half billion years since the planet's creation, would have come to a complete stop just before we arrived and started taking measurements. A very small amount of volcanism might be an a priori expectation for Mars, and might explain what's being seen. It would also (this is me, not Formisano, though I've no reason to think he wouldn't agree) argue for pockets of warmth and wetness in the crust.
As it happens, Formisano went on to point out, one of PFS's original objectives is to look for hotspots on Mars that might be associated with geothermal activity, so it can tackle both ends of the question. (Yesterday I asked Phil Christensen of Arizona State University, who's the principal investigator on NASA's two orbiting infrared instruments, TES and Themis, whether his data was able to put any limits on current hotspots. He said they hadn't seen anything clearly geothermal as yet, but that there's a lot of variability in the night-time Themis data that could either be internal heat or something explicable through surface properties -- different materials having different cooling rates, and so on. Phil says that looking for geothermal heat with Themis is a high priority for him -- but with two orbiting instruments and the mini-TES instruments on Spirit and Opportunity to deal with, being a high priority doesn't necessarily get something done right away...)
Looking for hotspots is something PFS will be able to do particularly well after mid-May, when Mars Express's orbit will be shifted so that it is closest to the planet in the nighttime, rather than the daytime. That shift is also good for the radar, Marsis (which is why it's being done), and I imagine that Marsis might pick up any subsurface water associated with a hotspot. The orbital shift means there will be no close-up observations with the HRSC camera for a while, though, and it means that PFS will stop doing methane measurements for a while.
On another track, I think it's worth pointing out (and I should have done so before) that none of the work on detecting methane I've been talking about here has yet been published in a peer-reviewed journal. The PFS results came out via a press conference, and have not yet been submitted for publication. Michael Mumma and his colleagues presented a poster at a conference last September and have since been working on a piece for peer-reviewed publication that will include some new observations; I don't know if it's been submitted yet. Vladimir Krasnopolsky and his colleagues have provided an abstract of their work in advance of presentation at the EGU meeting in Nice, and I would guess they've submitted it somewhere but I don't know for sure. Making this clear isn't a way of saying the data's no good. People produce abstracts and speak at conferences and sometimes give press conferences before peer review all the time. It’s the way things are done. But as I said, I think it’s worth noting simply because in general I think science writing should take note of such things.