The MSL/Curiosity team just announced that “So far we have no definitive detection of methane.” They’ve limited it to less than 5 parts per billion at a 95% confidence level. Why should you care? Why shouldn’t you care? Read on…
Methane is important to astrobiologists, because it is a gas made either by geological activity or by life… so a detection of methane would imply the planet is either geologically or biologically active. The former would be HUGE news for the whole planetary sciences community… the latter HUGE news for, well, the whole Earth community.
Now, to give a little context, there has been a debate over the last decade or so as to whether or not Mars has methane. The most publicized of the papers claiming there’s methane on Mars is one by Mike Mumma and colleagues. In addition to detecting methane concentrations well over 10 parts per billion, they also claim to see a seasonal and regional variability in the methane concentrations.
These claims – in particular those of varying methane – have been rejected by photochemical modelers led by Kevin Zahnle. They claim that methane variations on the time scaled and distances reported by Mumma et al. are implausible, because there’s no way to generate nor eliminate the methane rapidly enough to explain the observations. In short, if Mumma’s observations are correct they necessitate some physical or chemical process to be active on Mars that we do not understand.. and that a simpler explanation is that there’s something wrong with the observations of methane. (Full disclosure: I’m partial to these arguments, because my “major” toolkit is the same sorts of photochemical models used to cast these doubts.)
Enter MSL/Curiosity, which has a laser onboard tuned to the wavelengths needed to detect methane. In theory, it should be able to detect methane concentrations down to at least 1 part per billion. What we learned today is that using this instrument, the Curiosity team has limited methane to be less than 5 parts per billion. This is well below the 10’s of ppb’s Mumma claim to observe.
I remembered after posting this that it Gale Crater is in the *Southern Hemisphere*. The fact that it is Winter at Gale Crater means it is SUMMER in the Northern Hemisphere. So the methane SHOULD be there if Mumma et al. are correct. To be fair, this might be on the tail end of their “seasonal” signal, or it could be a signal that was around in the early 2000’s but is now gone, or it could be that Gale Crater is far away from their methane plumes for it not to appear there. Basically, they could claim “wrong place and/or wrong time.” But whatever their explanation for this measurement, this is pretty bad news for the seasonal methane hypothesis. If it’s Northern Summer, they should be seeing at least a *little* methane down in Gale Crater.
There might be a stronger signal come the next Gale Crater winter (2 Earth years from now), but it would be surprising. I’m eagerly anticipating the publication on this, because it will put all of this in better – and more carefully thought out – context.
(That’s a pretty big OOPS on my part. I always get this wrong because the crater is just NORTH of the Highland/Lowland boundary, yet still SOUTH of the equator. Below is what I originally (and mistakenly) wrote, thinking Gale Crater was in the Northern Hemisphere.)
So… game over, right?!?! I know I’ve seen some of my colleagues use this as support for rejecting the hypothesis by Mumma and crew that Mars has methane at the 10+ppb level…. BUT not so fast, my friends. Mumma claimed methane has seasonal variations on Mars, and that it appears in the Northern Summers. Well, right now it’s Northern Winter on Mars. So had they found methane, it would have surprised even those that are claiming the high concentrations and seasonal and regional variability. In other words, this doesn’t tell us much, other than that we’ve started to compile the data set we need to address these questions. That much is news, but let’s not read too much into it. In other words, this just confirms what everyone agreed on: there’s no methane in the Martian Northern Hemisphere during Mars’s northern winter. Don’t get me wrong – on the whole I side with Zahnle, Catling, and my other fellow photochemists that doubt Mumma’s claims. But this data set – on its own – is not a rejection of what he has claimed to observe. On the contrary, it’s entirely consistent with what they’ve reported.
The good news is there’s a decent chance we’ll get resolution of this question over the next Earth year (the next couple Martian seasons), as the Martian summer comes into full (methane-rich?) bloom. But we can’t reject Mumma and others that claim to see these seasonal variations. Not yet, anyways.