(This is the eighth in the series of posts on methane that started just over a week ago back here, and as well as being rather long it's probably going to be the last for a little bit. It's an attempt to look at what might happen next, and follows on pretty directly from the recap of what's happened so far in the previous post.)
So there's methane in the atmosphere of Mars that seems likely to be coming from the subsurface. What more would we like to know?
Ultimately, the thing that matters most is whether it comes from living things or not. And this is a question that is probably answerable, but not quite yet. On the earth, methane made by living things is lighter than methane made by purely physical chemistry: the biochemical processes that make the methane prefer to use the lighter of the stable isotopes of carbon, carbon-12, rather than the heavier one, carbon-13. If the methane producers of Mars are related to those of the earth (which is quite plausible if life hitched a ride on a meteorite passing from one of the planets to the other) then they will share this preference.
Among biologists, astro- and otherwise, a clear distinction is made between that hitchhiking possibility and the possibility that life on Mars could be completely unrelated to life on earth, marking a "second genesis". A second genesis would make Marslife much more interesting in terms of biology -- a whole new way of being alive, rather than some unusual examples of the way of being alive that all organisms on earth already share. It would also have more implications for religion and environmental ethics. Despite these differences, though, it still seems a very good bet that, whatever their genesis, life forms interested in making methane will use a molecular mechanism that shows some degree of preference for one isotope or the other.
Within less than a decade, carbon-isotope analysis of the martian methane should be possible; I'll go into the options a little later. First, though, let's look at questions that might be tackled a little sooner. The obvious ones are the patterns of methane production in time and space. Vittorio Formisano, the lead investigator on the Mars Express Planetary Fourier Spectrometer (PFS) will keep measuring methane whenever he gets the chance, and so he should be able to pick up any large seasonal variations. Seasonal variations might reflect a biological source -- but they might also reflect differences in the rate at which methane is taken out of the atmosphere that have nothing to do with biology.
Formisano is also trying to follow up the original detection by seeing how the methane levels vary between longitude bands. Good luck to him, but it will be a hard measurement to make; at 10 parts per billion the average methane level is pretty close to the lower limits of the PFS's power to detect it, and small fractional differences will be very difficult to measure.
So far, the nearest to a location we have is from Michael Mumma's poster presentation at the DPS meeting last fall. In their observations Mumma and his colleagues saw rather more signal from the equatorial regions than might have been expected. (I misreported this in an earlier post as a longitude effect, and sort of implied that Steve Connor had too: sorry.) It's conceivable that this could be related in some way to the unexpected detection of ice, or more properly hydrogen, in some low latitude regions by the Mars Odyssey Gamma Ray Spectrometer. If there was liquid water in the deep subsurface in these regions, you might expect a little ice in the shallow subsurface, since water vapour in the pore spaces above that liquid water would percolate up through the rocks until it got to the very cold soil near the surface. There most of it would freeze, though some might conceivably escape to the atmosphere.
Intriguingly, there are some hints in the data from Mars Global Surveyor's TES instrument that there are sources of water vapour away from the poles and the obvious high latitude ice deposits, but this is very preliminary, and it is not yet possible to show that it's anything other than some sort of seasonal effect in which the water vapour shuffles backwards and forwards between the atmosphere and the top layers of soil. But if there are places where water vapour is seeping up from the deeps, the methane might be coming up with it.
If it's not possible to locate the methane directly, it might be possible to see something associated with the source -- a faint infrared signature associated with a geothermal hotspot, say. The spectrometers looking at the surface -- Omega on Mars Express and Crism, which will be on the NASA Mars Reconnaissance Orbiter (MRO) due to fly in 2005 -- might pick up some sort of surface mineralogical signal if there are areas methane seeps up through regularly. It's also possible that the other atmospheric instrument on Mars Express, Spicam, might see something. If an area of particular interest was discovered, it's possible that something telling might be seen there by one of the cameras -- MOC on MGS, Themis on Mars Odyssey, HRSC on Mars Express or HiRise on MRO. I rather doubt the cameras will see anything easily linked into the methane story -- but even so it's worth pausing simply to appreciate the fact that there are now three magnificent cameras in orbit round Mars, with another slated to join them soon, and a handful of spectrometers too, not to mention a radar. It's a golden age.
The other instrument that could reveal something relevant is that radar, Mars Express's Marsis. Marsis hasn't yet started operations -- the antenna gets deployed later this month -- but if it detects subsurface water once it's up and running, the areas where the water is found will jump to the top of the list of places to study further for all sorts of reasons, methane production included. Geological methane sources would either need liquid water or be likely to produce it; biological ones would need it too, though they could get by on amounts much smaller than those Marsis can detect.
Meanwhile, back on earth, the excellent instrument that Krasnopolsky and his colleagues used at the Canada-France-Hawaii Telescope has unfortunately now been decommissioned. Michael Mumma and his colleagues will be applying to make more observations with various telescopes, including the Keck telescope, where they have made some preliminary observations but haven't yet mounted a major Mars-focused campaign. Observations from earth might speak to seasonal changes, and all the spectrometers capable of looking for methane will also be looking for other traces gases that might point to geology or biology. (Krasnopolsky has been looking for some sulphur compounds, but has not seen any yet.)
That's about it, I think, for the missions now up and running. But what of those in the planning stage -- or of those that should now be getting moved into the planning stage to follow up the methane detection and track down the possibility of life.
As mentioned above, in 2005 MRO will take HiRise and Crism off to Mars (along with a different sort of radar, Sharad, which looks into the top few metres of the soil). None of these is obviously going to contribute much to the methane hunt, but if Sharad were to detect near surface liquid water that would certainly liven things up. Anyway, who's to say they won't find something that ties into it from an unexpected angle? That's half the fun of this business.
The next lander is NASA's 2007 Phoenix mission, the first of a planned series of relatively cheap "Scout" missions (Bruce Moomaw's definitive guide to the Scout proposals starts here). Phoenix carries equipment with which to look for organic molecules frozen into the ice in the high northern latitudes, but it's probably not going to be able to pick up little traces of atmospheric methane, and certainly won't be able to do isotopic analysis on it.
The very ambitious Mars Science Laboratory planned for 2009 is being conceived, as I understand it, as a much more capable successor to Spirit and Opportunity, able to analyse samples it takes from the environment in an onboard lab (and with a microscope sharp-eyed enough to spot microfossils). It has a mass spectrometer on board as part of the lab payload, but whether that could make measurements of atmospheric methane at low concentrations I don't know. If it can't, maybe it can be redesigned so that it can.
But it's important to remember that the purpose of MSL is not just to carry as many neat instruments as possible to the surface of Mars. Planetary landers can't be all things to all people; they need a purpose. For MSL the purpose is to do a detailed survey of a particular habitable martian environment, either past or present. (In practise, since MSL is a surface mission, present would mean a mission to the gullies). Last year's final report by the MSL Project Science Integration Group was particularly enthusiastic about past habitability. That's why the discovery of water's role in the distant past of Meridiani Planum led NASA's science chief, Ed Weiler, to say it was now top of the list of targets for the 2009 mission.
So what of missions more tightly focused on methane? There seem to me to be three possibilities.
A Beagle reflight
Colin Pillinger wants to fly a Beagle 2 follow-on/replacement as soon as possible. As I've said elsewhere, I'm not sure this is a very good idea or a very sellable proposition. But the methane news is obviously going to be very helpful to the Pill's campaign. Beagle 2's Gas Analysis Package (GAP), which contained a mass spectrometer, was designed to pick up methane and the atmosphere and do isotopic analysis on it. That said, Beagle 2 was designed to land safely on Mars. The designers' intentions aren't necessarily what matter. Before reflying the GAP, let alone the whole Beagle 2 spacecraft design, I'd imagine ESA would want to have it thoroughly picked over by outside experts looking for all the ways it could go wrong. (The ESA report on what may have happened to Beagle 2 is now slated for release sometime in mid April.)
If you trusted the GAP to do the science required, how would you refly it? One possibility would be to combine it with the late lamented French Netlander mission which would have put a small network of weather stations and seismometers on to the planet's surface (defunct shell of a website here). If one of the possibilities raised by the methane is magma in the depths, that makes seismic studies even more interesting than they were before. A set of Netlanders with GAP packages on a couple of them -- maybe stripped down GAPs designed just to do atmospheric work -- might be quite an appealing mission, though maybe a rather pricey one.
The Organic Origins Observatory
Michael Mumma has been planning to propose a space telescope called the Organic Origins Observatory (of which there's a slightly out of date description here) to NASA's "Discovery" programme for a while, and there'll be an opportunity this summer. (Discovery is the line of relatively cheap planetary missions that the Mars "Scout" line is based on.) Triple-O, as it's probably most easily called, would be a telescope designed to work in the infrared with a spectrometer on board as good as those Mumma uses at telescopes on eart. Freed of the problems caused by looking through the earth's atmosphere this spectrometer would be capable of all sorts of wonderful things. It would be able to categorise comets according to the organic molecules they carry, revealing how the components of life were distributed in the earliest days of the solar system. It would also be able to look at other stellar systems currently in the throes of formation and make similar measurements. Most amazingly, it would be able to analyse the atmospheres of planets around other stars. Kepler, one of the missions currently being developed for Discovery, is designed to spot planets orbiting other stars by picking up the faint dimming a planet causes on the rare occasions when it moves between its parent star and the patiently watching telescope. Triple-O could follow up Kepler's discoveries by watching stars while the planets passed in front of them and analysing the backlit planetary atmospheres.
Along with all this other cool stuff, Triple-O should also be able to do a bang-up job of analysing the martian atmosphere, achieving a sensitivity hundreds of times greater than its earthly counterparts. It would be able to trace relatively small changes in the amount of methane and also pick up other gases associated with volcanism, or even possibly life. And it would be just about sensitive enough to distinguish light and heavy methane, thus standing a good chance of seeing whether the methane is biogenic or not.
If I were running the world, Triple-O would be a shoo-in for the next Discovery slot, which would mean it would be up and observing by the end of 2009. In fact, if I were running the world I'd be tempted to throw a little more into the pot and give Triple-O the bigger mirror its creators wanted at first, rather than the smaller compromise mirror that will make it fit into a Discovery-programme budget. If it came down to it, I'd happily swap a few more years of the Hubble for funding for something like Triple-O -- the Hubble is a great instrument, but there's been a lot of investment in the sort of astronomy and cosmology it is mostly used for. An instrument with as much astrobiological potential as Triple-O strikes me as one whose time has come.
As-yet-unnamed Canadian project
Indeed, it's not only not named -- as far as I know it's not a project. But it's a cool possibility that the people involved are undoubtedly aware of.
One of the other proposals for the 2007 Scout mission slot that was eventually awarded to the Phoenix lander was a mission called Marvel, proposed by Mark Allen of JPL. Marvel had at its heart a very capable spectrometer that would orbit Mars watching the sun rise and set. Instead of analysing light which had passed down through the atmosphere, bounced off the martian surface and come back up, as PFS does, the Marvel instrument would have looked straight at the sun, which offers significant advantages. Allen thinks it would have been about a thousand times more sensitive than PFS -- an instrument that could pick up just a few cows-worth of methane. That sensitivity would allow it to monitor very small changes in methane levels, locate methane sources, pick up a bunch of other gases and, crucially, tease out the isotopic make-up of the methane.
I believe Ed Weiler said that choosing between the shortlisted Scout proposals was one of the hardest decisions he'd been called on to make. But in the end he chose Phoenix, and currently Marvel is going nowhere. The next opportunity to fly a Scout mission, as I understand the current plans, is 2011. But there is an alternative. Marvel grew out of a project for doing spectroscopic analysis of the earth's atmosphere called Atmos, which produced instruments for flights on the space shuttle and on high altitude balloons. And another descendant of Atmos is already flying -- a fourier transform spectrometer on a Canadian satellite called ACE devoted to studying the earth's atmosphere.
Build a suitable copy of the ACE instrument, put it on a spacecraft capable of orbiting Mars and you have something very like Mark Allen's Marvel mission. If you brought ESA on board -- building an interplanetary spacecraft on its own might be a stretch for Canada's space budget -- and used the Mars Express spacecraft bus, you could have a mission all the parts of which had already flown before (the same would apply if you used the Mars Odyssey spacecraft bus). It would be cheap, too, especially if it was launched on the (admittedly still on the drawing board) Falcon V from SpaceX. Such a mission is probably the most cost effective way to get from methane detection to the all-important isotope analysis (though it doesn't do all the other neat stuff that Triple-O could). It would be a lovely thing to announce at this October's IAF meeting in Vancouver.
Marc Garneau, the former astronaut who now runs Canada's space agency, is keen for the country to set its sights on Mars. This is a perfect opportunity for it to take the lead in a small, reasonably cheap international mission that would have a shot at uncovering strong evidence for extraterrestrial life.
And when it comes down to it, that's the real prize on offer. If there is life below the surface of Mars, then this methane is almost certainly its calling card. And if the methane is not from life, that's important too. As I argued in this post, if there's an active, wet subsurface with the sort of chemistry that makes methane production a possibility, but no methane producing bugs, then it's hard to imagine where else they would be. And a thin smattering of methane producing bugs in the subsurface has for a while been the most plausible of the possibilities for life on (OK in) Mars today. So if the methane were to be found not to be biogenic, it would be something more than just an absence of evidence for life; it would be something close to evidence for the absence of life.
MSL might show that Mars once teemed with life. As I hypothesised in this post -- the post that first brought mention of methane (and, in the comments, of Michael Mumma -- thanks again Bruce) to MainlyMartian -- the methane that life produced might even have explained why Mars was then warm enough for surface water. And at the same time as MSL tracks down what microfossils it can, a methane mission of some sort could provide the strongest evidence yet that their descendants, if any, are gone for good. Mars might not just give us our second insight into how a planet lives -- it might give us our first detailed understanding of how one dies.
Or it might show us how, remarkably, life can cling on even when the biosphere it was once part of collapses.