Worlds of Controversy, discussed several recently controversial topics in planetary science.
One of the topics was the issue of methane on Mars -- or lack thereof. We've all read the articles about how the measurements of Mars methane points to possible signs of life, woohoo! But none of the articles cover the problems with those measurements, as described in a recent paper by Kevin Zahnle, Richard S. Freedmana and David C. Catling: Is there methane on Mars?
Lack of life on Mars isn't sexy, I guess; The Economist was the only mainstream publication covering Kevin's paper, in an excellent article, Methane on Mars: Now you don't...
Here's the short summary from my column last month:
I'm sure you've seen articles on Martian methane. Methane doesn't last long in the atmosphere -- only a few hundred years -- so if it's there, it's being replenished somehow. On Earth, one of the most common ways to produce methane is through biological processes. Life on Mars! Whoopee! So everyone wants to see methane on Mars, and it makes for great headlines.
The problem, according to Kevin, is that the Mars measurements show changes on a scale much shorter than hundreds of years: they fluctuate on a seasonal basis. That's tough to explain. Known atmospheric oxidation processes wouldn't get rid of methane fast enough, so you'd need to invent some even more exotic process -- perhaps methane-eating bacteria in the Martian soil? -- to account for the drops.
Worse, the measurements showing methane aren't very reliable. The evidence is spectroscopic: methane absorbs light at several fixed wavelengths, so you can measure methane by looking for its absorption lines.
But any Earth-based measurement of Martian methane has to cope with the fact that Earth's atmosphere has far more methane than Mars. How do you separate possible Mars methane absorption lines from Terran ones? There's one clever way: you can measure Mars at quadrature, when it's coming toward us or going away from us, and any methane spectral lines would be red- or blue-shifted compared to the Terran ones. But then the lines overlap with other absorption lines from Earth's atmosphere. It's very difficult to get a reliable measurement. Of course, a measurement from space would avoid those problems, so the spectrograph on the ESA Mars orbiter has been pressed into service. But there are questions about its accuracy.
The published evidence so far for Martian methane just isn't convincing, especially with those unlikely seasonal fluctuations. That doesn't mean there's no methane there; it means we need better data. The next Mars Rover, dubbed "Curiosity", will include a laser spectrometer which can give us much more accurate methane measurements. Curiosity is set to launch this fall and arrive at Mars in August of next year.
It gets worse: the kapton tape issue
But it gets worse. That Curiosity rover whose sensitive equipment is going to answer the question for us? Well, check out an article in Wired last week: Space Duct Tape Could Confuse Mars Rover.
It seems the materials used to build Curiosity, notably the kapton tape used in large quantities to hold the rover together ... emit methane! Andrew C. Schuerger, Christian Clausen and Daniel Britt, in Methane Evolution from UV-irradiated Spacecraft Materials under Simulated Martian Conditions: Implications for the Mars Science Laboratory (MSL) Mission (abstract), take a selection of materials used in the rover, plus bacteria that might be expected to contaminate it, and subject them to simulated Mars conditions. They conclude
... the large amount of kapton tape used on the MSL rover (lower bound estimated at 3 m2) is likely to create a significant source of terrestrial methane contamination during the early part of the mission.
A skeptical eye
So let's sum up:
* We desperately want to see methane on Mars, because it might point to
biological processes and that would be cool.
* But we don't currently have any reliable way to measure Martian methane.
* So we build a special mission one of whose primary purposes is to get accurate measurements of Martian methane.
* But we build the probe with materials that will make the measurements unreliable.
It's apparently too late to fix the problem; so instead, just shrug and say, well, it might not be so bad if we measure at night, or if we wait a while (how long?) until most of the methane outgasses. The methane emission from the kapton tape is fairly small -- though it's hard to know exactly how small, since it's impossible to test it in a real Martian environment.
So in a couple of years, when you start seeing news releases trumpeting Curiosity's methane measurements and talking about life on Mars, read them with a skeptical eye.
Maybe Curiosity will see methane levels on Mars so large that they swamp any contamination issues. Maybe not. But we won't be able to tell from the reports we read in the popular press.
[ 12:38 Mar 13, 2011 More science/astro | permalink to this entry | comments ]