Flummoxed is pretty fair in this case. The expectation is that any methane released into the Martian atmosphere should persist on long timescales. Either there is a unknown process which scrubs it quickly or previous measurements were in error.
At some point I'd love a developer to use 'flummoxed' in their status update :-)
The methane question is an interesting one and for me an example of how hard it is to re-purpose science missions when you don't have a human there to implement the new experiment. I feel that it illustrates the limits of robotic exploration for now.
Gas spectrometers are really very universal instruments. There's a lot of experiments you can do with one, especially if you're happy to use an arc to vaporise bits of solids and test the gasses that come off.
Hard to do that from orbit though, getting those solid bits up there takes some work. And when you're on the ground if you're relying on auto-navigation your average speed is less than 1km/day so hard to get a wide survey.
Take the current curiosity mission as an exemplar. It is coming up on having been on the ground for 7 years (August '12). A human team, with the same instruments, and a rover could have achieved all of the science results from Curiosity to date, in less than a month. Further they could have repaired instruments that have become non-functional from spares carried with them.
Opportunity and Spirit covered a combined 36 miles or so in 10+ years of operations, also doable in about a month by a human crew.
So we have nearly two decades of robotic exploration that we could do in about 3 months with a group of people and some transport. Plus they could do additional research that the robots can't because they could combine the instruments in ways the robots cannot, or use different processes the robotics cannot implement.
I realize this sounds like a dismissal of the huge accomplishment and contribution to science that the rovers have made. It is not my intent. Those are valuable things, and some science is always better than no science. What I'm trying to point out that putting people there would have 100x the science return for less than 100x the budgetary impact.
The simplest explanation that I can think of is that some form of methane is trapped in geologic formations which periodically release (such as ice), but that the mentioned chemical/dust reactions end up sequestering (scrubbing) it in to more solid forms that are harder to detect.
Mars would be a much more interesting place if we'd send over enough infrastructure to get mostly autonomous robots going and scaling up local infrastructure for some kind of terra-forming effort.
A solid [heh] explanation. The methane could simply be absent because of seasons. It has very slightly larger axial tilt than Earth, and a far greater eccentricity. The sun could just be liberating methane from geological formations during the correct conditions (close to the sun, and in summer).
Seasons could also affect life (just like it does plants on Earth), but it's never aliens.
Mars is currently at its aphelion and the southern ice cap is not receiving sunlight, so anything liberating Methane might be inert.
> the mentioned chemical/dust reactions end up sequestering (scrubbing)
A good explanation, there is another possibility. Given that CO2 (the main component of the Martian atmosphere) is so much heavier than methane, the minuscule concentration could have floated to the top of the atmosphere and could have been mostly blown away by solar wind. Methane could still be there, just far below the noise floor.
Both of you make good points, I just want to point out that since the mission has been observing mostly during a period of intense dust storm activity. If a dust scrubbing effect is real, the orbiter may still have a chance at detection.
That's funny... just a few days ago the Europeans (Mars Express) were "definitely finally confirming methane on Mars... now the Americans are saying "nope" without directly addressing the Mars Express findings?
This back and forth about methane on mars has been going on for a while. Not divided politically either, the US has gotten both results over various missions. The speculation I've heard about this is that the methane isn't global and isn't being released constantly, leading to confusing readings.
The answer is that the sensors are two spectrometers that detect all sorts of gases, not just methane:
[0] https://en.wikipedia.org/wiki/Nadir_and_Occultation_for_Mars...
[1] https://en.wikipedia.org/wiki/Atmospheric_Chemistry_Suite