The rumours about MSL, mentioned a while back, seem to be getting stronger and stronger. Since the most recent MEPAG meeting a number of people have started more or less assuming that MSL is going to slip back at least one launch opportunity, to 2011; there's also talk of it being doubled up into an MSL A and an MSL B, like MER. It's not been announced yet and may not be a done deal, but the delay, at least, seems very much to be the way that the wind's blowing. It's possible that a formal decision is going to wait until NASA's new administrator, Mike Griffin, weighs in.
An ancillary rumour suggests that if this happens there will be serious consideration given to moving the next Scout opportunity for a small mission up from the 2011 launch opportunity to the 2009 opportunity. Four years is a very short time in which to go from a sheet of paper to a spacecraft, and a truncated selection process might be put in place, in which only the scouts that were shortlisted last time need apply. Those three were: Scim, a mission to pull dust from the upper atmosphere; Ares, an aircraft that would do magnetometry, inter alia; and Marvel, an orbiter with which to measure atmospheric trace gases, including methane, based on the bus for Mars Odyssey. If it were done that way, then the case for Marvel would be extremely strong: the science basis is all the stronger now we have reports of methane, and the technical risk has to be low compared to the other two, since it's a fairly straightforward orbiter, content to look at the atmosphere from above rather than rip through its edges with ramscoops flaring, or extend its wings and try and fly through the stuff. If you wanted to be really practical about the matter, you could save time by just selecting Marvel straight out of the gate -- but I'm not sure that that's actually legal for a scout mission.
Marvel would get really accurate data on methane levels, it would measure any seasonal variations, and it would pick up a bunch of other trace gases if they're there. It would also have a good chance of measuring the carbon-12/carbon-13 ratio in the methane, a measurement which, if it is different from the isotope ratio in the carbon dioxide, would offer strong evidence for a biological source. (The idea is that the methane-making organisms would feed on a well-mixed global reservoir of carbon dioxide, but would discriminate between isotopes as they did so, just as methane makers and photosynthesisers do on the earth; most non-biological processes don’t make such discriminations.) But in an ideal world -- say, for the sake of argument, a world in which the aim was not to fly an already accredited Scout design, but to fly the best methane mission you could -- you'd want to at least consider an alternative mission that looks for the same data a different way.
Marvel would be in an orbit that kept it close to the terminator; its spectrometer, instead of looking straight down, as most spacecraft instruments do, would look at the horizon -- the planet's limb -- in order to see the sun through the atmosphere. This means it gets a strong signal, because it's got a very bright light source shining through a lot of gas. An alternative, which Mike Mumma at Goddard has thought about, is to put a telescope in a halo orbit around the Martian L1 point, which is on a line between Mars and the sun. At L1 a telescope could spend all its time looking at the whole sunlit face of Mars -- and paying attention to any region that was of particular interest.
The challenge of an L1 mission is that the signal is a lot weaker. Your light source is the surface of Mars, which is a lot less bright than the sun. And the light passes through much less of the atmosphere. Light coming to Marvel on a tangent to the planet's surface goes through something like 40 times as much of the atmosphere as light reflected from the surface to the L1 point, and thus the spectral features due to methane, or anything else, will be considerably stronger.
The advantage of L1 is that you could look at specific places -- at resolutions down to a few tens of kilometres -- and look at them at any time of day that you chose. Marvel has to look only at sunsets or dawns, and it can't choose to focus down on a particular region (though it will see all the regions of the planet on a fairly regular basis). If methane levels are strongly variable in space and time, that would seem to be an advantage for the L1 mission. The lower signal at L1 should be something that can be compensated for in principle simply by taking data for longer; in principle, an L1 mission could stare at a particular spot all day.
I really don't know which mission might be better. If I knew for sure that Marvel's signal to noise ratio made it significantly more capable in terms of isotope analysis, I think I'd go with that. There again, the idea of being able to look down wherever and whenever you want is certainly appealing, especially if it were to turn out that there are point sources and complex surface chemistry. But developing a mission from scratch to fly in 2009 would be very, very hard, and I think that the greater amount of design work already done on Marvel would probably trump all other factors. So if there is to be a methane mission at all, it may well be a case of love the one you're with - not too hard, since Marvel is pretty damn loveable.