22 January 2009

Mars Methane: the Paper

Posted by Ryan

After all the to-do about the confirmation of methane on Mars and its possible implications, I decided that I should take a look at the actual Science article and post a distillation of it here.

The paper that caused this uproar is called “Strong Release of Methane on Mars in Northern
Summer 2003″, by Mumma et al. Before this paper, methane had been detected on Mars, maybe. The evidence was pretty weak and at best gave an upper limit to the amount of methane in the martian atmosphere.

The new measurements by Mumma et al. are much more complete: they observed Mars for seven earth years (3 mars years), with enough resolution to not only detect the methane, but to figure out where on the planet it was concentrated!

How does one look for methane on Mars? Get a really big telescope and a really good prism. Mumma et al. used the IRTF and Keck-2 telescopes in Hawaii, armed with super high-resolution spectrographs that allowed them to gather lots of light from Mars and split it up into a very detailed spectrum, as shown in this rather intimidating figure:

a) Mars; b) Spectra from March 20, 2003; c) Spectra from March 19, 2003; The squiggle across the top is the spectrum of Earth's atmosphere, and the smaller squiggles are the spectra of Mars. Methan (CH4) and water vapor (H2O) are marked.

a) Mars; b) Spectra from March 20, 2003; c) Spectra from March 19, 2003; The squiggle across the top is the spectrum of Earth's atmosphere, and the smaller squiggles are the spectra of Mars. Methane (CH4) and water vapor (H2O) are marked.

Essentially, what the figure is showing is that the spectrum of earth’s atmosphere (the top line in b and c) has methane and water vapor in it, as demonstrated by the deep dips in signal at the appropriate wavelength. The spectra of Mars also show these signatures but you’ll notice that they are much smaller and are shifted slightly. They’re smaller because Mars has a very thin atmosphere and is somewhat farther away from the telescopes than earth’s atmosphere. They are shifted because Mars is moving. Just as the siren of a moving ambulance changes pitch due to the doppler effect, the spectrum of a moving object also changes frequency.

So, Mars officially has methane in its atmosphere! This is already a big deal, because methane breaks down relatively quickly in the martian atmosphere, so the fact that we can see it at all means that something is actively producing it. But there’s more to the story! Mumma et al. made lots of observations and found that the amount of methane varied and was highest during northern summer. The March 2003 plume was estimated to be due to the release of ~19,000 metric tons of methane!

The fact that the amounts of methane varied from north to south suggests that there were discrete sources of methane in the northern hemisphere, as shown in this figure:

Regions where the methane was localized during northern summer.

Regions where the methane was localized during northern summer.

So of course the question is, where is all this methane coming from? On earth, most methane is produced by living things, and the rest is produced geologically. It’s impossible to tell what the source is on Mars. Mumma et al. mention that there are colonies of bacteria on earth that can live as deep as 3 km beneath the surface and get their energy by combining CO2 and H2 to get CH4. But they also point out that the association of the methane plumes with the Syrtis Major volcano and  Nili Fossae may point to a geologic origin, related to the interaction of volcanic rocks with water.

Of course, most media outlets ran with the “Life on Mars”headline, but the truth is, we have no evidence that this methane is or isn’t related to living things on Mars. What we do know is that Mars is active in some way, which is pretty exciting no matter what that activity is!

ResearchBlogging.org
Michael J. Mumma, Geronimo L. Villanueva, Robert E. Novak, Tilak Hewagama, Boncho P. Bonev, Michael A. DiSanti, Avi M. Mandell, Michael D. Smith (2009). Strong Release of Methane on Mars in Northern
Summer 2003 Science