But the Florida air isn't the only possible source of contamination. There's also the problem of the contamination by N-Methyl-N-tert-butyldimethylsilyltrifluoroacetamide (abbreviated MTBSTFA) that has leaked from one of the SAM wet cells and caused all kinds of problems for the SAM team. Whenever SAM cooks a sample in its oven, it sends a portion of the generated gas to the tunable laser spectrometer for isotopic analysis. The evolved gas usually contains quite a lot of methane -- 10 parts per million rather than a fraction of a part per billion -- and the SAM team has concluded that most of this methane comes from the MTBSTFA.

So how can they be certain that the methane that they detect at the few part-per-billion level is actually Martian methane? In the supplementary materials for the paper (which you can download for free here, all 62 pages of it), they go through a number of hypothetical sources of terrestrial contamination, and argue against them. For example, maybe the methane is there because it's leftover from a SAM evolved gas analysis, and the Herriot cell hasn't been completely pumped out. But they show that the levels of methane that they measure -- low or high -- are not correlated with when they did SAM evolved gas runs. Or maybe MTBSTFA contamination builds up on surfaces inside the Herriott cell over time, so it doesn't go away when the cell is pumped out, and then it produces methane when it reacts with the introduced Mars atmosphere for some reason. But they argue that they would detect this contamination as a reduction in the power of the laser, which they don't detect; in fact, if anything, the laser has increased in power (by cleaning off the mirrors inside the Herriott cell) over time. More generally, they just don't see any correlation between when SAM did evolved gas runs and when they saw the low or high methane levels.

They looked for all kinds of other correlations. The high methane levels happened at a time when Curiosity was driving a lot -- actually, during the period of peak wheel degradation. Methane couldn't have come from the wheels themselves -- they're solid aluminum -- but did the wheels somehow make methane by crushing rocks? They looked for, and found, no correlation between how long the rover was sitting around at one spot before a methane reading; the different kinds of terrrain it passed over; or APXS measurements of the chemical content of different rocks. They looked for correlations with season, dust opacity, ground and air temperature, pressure, humidity, water vapor abundance, direction that their inlet was pointing, and background radiation levels. They found a couple of slight possible relationships -- an anti-correlation with water abundance, temperature, and atmospheric opacity, for example -- but all those correlations are wrecked by the low level measured in the first enrichment experiment.

What now?

SAM has, at least, made a good measurement of the background level of methane in the atmosphere at Gale crater, and that is a step forward. And they did detect a transient higher-methane signal -- but it's not at all clear where that came from, or how it vanished so quickly. It's not an entirely satisfactory result; it's still too mysterious to clear up any of the questions about Mars methane that Nick summarized. But the SAM team will continue to do atmospheric analyses as time and power permit, and hopefully they will see another "puff" and find some way to correlate it with events on Mars -- or within the rover. And from above, the Methane Sensor for Mars on Mars Orbiter Mission may be able to make some contribution, someday. When NASA's chief scientist Ellen Stofan visited ISRO in November, one of her goals was to advance NASA-ISRO cooperation that could enable the SAM and Mars Orbiter Mission teams to work together on this problem.

We're missing something. There's something we don't understand here. But that's nothing new, in planetary science. The only thing to do is to keep collecting data and hope it brings us closer to an answer!