December 6, 2004
The Martian Methane Surprise
Is the methane on Mars coming from deep underground? Why is the finding a clue in the search for biochemistry elsewhere? Astrobiologist Mike Mumma discusses some possibilities while explaining how to measure methane on another world.
Astrobiology Magazine -- At the recent Division of Planetary Sciences conference in Louisville, Kentucky, Michael Mumma, Director of the Center for Astrobiology at NASA's Goddard Space Flight Center, announced that relatively high levels of methane had been detected on Mars.
Methane on Earth is mainly produced by life, but also can be released from volcanoes or tectonic activity. Having methane appear on Mars is something of a mystery, because the planet is not believed to have active volcanism or tectonics. Could the methane be evidence of martian life forms buried underground?
In this interview with Astrobiology Magazine editor Leslie Mullen, Mumma explains how they detected the methane, and what it could mean for the chance for life on Mars.
Astrobiology Magazine (AM): I remember when a detection of methane on Mars was reported last April. Now you're saying there's a background methane level of 20 to 60 parts per billion (ppb). You've also found spikes of methane on Mars measuring 250 ppb.
Mike Mumma (MM): Right. Our results were obtained in March of 2003 using the NASA Infrared Telescope Facility in Hawaii, and in May 2003 using the Gemini South telescope in Chile. But the history of the search for methane on Mars is long and storied.
Most of the searches have been done at wavelengths near 3.3 or 7.7 microns, where methane has two strong vibrational bands. Comparing different results is complicated by the fact that many of the early searches were done with globally averaged fields of view, so they were less sensitive to latitudinal and longitudinal variations. In other words, they were insensitive to local sources, because they averaged over everything.
AM: So those were studies done from telescopes on Earth, where Mars was viewed as one big pixel?
MM: Exactly. Even in the case of Mariner 9's IRIS, the infrared orbiting spectrometer, Bill McGuire from our lab formed a grand average of the spectra regardless of the latitude and longitude, taken at all times of the year, and said, "Here's the spectrum." He estimated the maximum possible abundances for a dozen or so trace constituents, including methane. He found that the average mixing ratio was no more than 20 ppb.
That's also what the Mars Express is now claiming to detect, but in their case one of the things that concerns me is the presence of a cloud deck.
There was a major dust storm in December of 2003, and it raised the scattering level from the surface to a higher altitude - probably about 20 kilometers above the surface - and this reduced the signature of both water and methane spectral lines of reflected light. Those conditions continued until June of this year.
That means that when Mars Express tried to measure methane, they were looking against that background of dust and airborne ice. That would've affected their measurements and made derived abundances appear smaller than they would otherwise be. So far, they have reported data taken in January and February and again in May, and both data sets show the effects of extinction by ice aerosols.
But in March 2003, the martian atmosphere was fairly clear. We were able to measure both water vapor and methane in the same spectra at the same time. We compared the water at each position with the amount detected by the TES spectrometer on the orbiting Mars Global Surveyor.
AM: To verify that what you were measuring was accurate?
MM: Correct. Our water abundances were a factor of three smaller than those of TES. We always have to add a reference level to our spectral measurements to get the true value. I didn't add those numbers in my presentation this year [at the DPS conference]. Instead, I showed the minimum amount we were seeing.
AM: Why do you have to add reference values to your readings?
MM: The reason we do it that way is because the telescope is looking through a column of Earth air. The photons collected from Mars traverse the same terrestrial column of air, regardless of their position along the spectrometer entrance slit. By choosing one spectrum as a reference, and subtracting it from every other measured spectrum, we can cancel these terrestrial atmospheric features perfectly. In that way, we can isolate the Mars spectrum.
So we did this to find the methane abundance, and discovered that the methane shows a significant enhancement at the equatorial region.
At high latitudes in the north and south, there is much less methane. It's 20 to 60 ppb in the north, and even lower in the south. But it was more than 250 ppb at the equator.
AM: Does the mixture of methane in the atmosphere naturally change as you rise in altitude, or as the temperature changes?
MM: No, not really. On Mars, the mixing ratio of water vapor undoubtedly changes greatly with altitude, because it condenses. But methane does not condense at martian temperatures, so it must be uniformly mixed with altitude.
Now, there are caveats. There could be processes on Mars that destroy methane. If airborne dust coated with oxidants is lofted into the atmosphere, then methane could collide with that dust and be converted to other hydrocarbons, such as methanol or formaldehyde. A new study by Sushil Atreya suggests that hydrogen peroxide created by dust devils could act to scrub methane out of the atmosphere. But even if methane is being destroyed in this way, it doesn't affect our measurement.
AM: Right, because there would be even more methane on Mars than the high level that you found.
AM: But if methane is being actively destroyed, then doesn't that suggest that the methane you found is very current?
MM: It is definitely current.
AM: I heard that methane has a lifetime of about 300 years, which is very current astronomically speaking, but I meant "current" as in "being released right now."
MM: We think it is being released right now. We think that's why we're seeing this intense enhancement at equatorial latitudes. Such an intense release will, over time, naturally diffuse outward in the atmosphere and be transported elsewhere, spreading around the planet and to the poles.
AM: So your reading of 250 ppb in the equatorial region, that was confined to a small area?
MM: Yes, it was about minus 10 degrees south to 10 degrees north.
AM: And what's the topography of this region?
MM: It's a transition region from the highlands to a plain - Syrtis Major Planitia. There are many scarps, or cliff faces, where the topography changes drastically.
That's interesting because the other region where we showed evidence of enhanced methane was over the deep rift valley Vallis Marineris - another region with steep, high cliffs. One working model is that methane is diffusing under the permafrost and emerging at the cliff face.
You wouldn't see it emerge unless there was a cliff face, or if there were fissures or ancient volcanic pipes reaching down below the permafrost.
AM: On Earth, a lot of methane has an organic origin.
MM: Right. Methane on Mars could be produced by non-biological methods or by biological ones. We don't yet have the evidence to support one or the other.
One possibility is drawn from Earth, where one tectonic plate is subducted under another. The subducting plate carries with it carbon dioxide, water, organic material, and so forth. When it reaches the hot magmatic region, that material reacts with olivine and converts it to a different mineral - magnetite - releasing hydrogen in the process. That hydrogen reacts with carbon to form methane, which then percolates upwards and is released.
That process requires active tectonics, and we don't see any evidence for that on Mars at present. But we can test this idea by searching for other higher order hydrocarbons, and by measuring the D/H ratio - the ratio of deuterium to hydrogen - in methane. A tectonic process would most likely be consuming juvenile water stored from a time when Mars was young, and in that case it should have a lower ratio of deuterium to hydrogen than present-day water. So if methane on Mars has a low D/H ratio, that would suggest it is geothermally produced, or at least produced from the deep reservoir.
Another possibility is active biology. Here you have a choice as to whether the bio-release is at the surface layer or deep below the permafrost. If it's below the permafrost, and if the permafrost is an impervious cap, then you should have sideways diffusion, with the methane later being released at the cliff faces.
You could have bioforms consuming carbon dioxide and water that is relatively younger than the deep stuff, and then you'd expect to see a higher deuterium abundance. You would also expect to see depleted carbon-13, heavy carbon, in the methane released by bioforms.
AM: Are these predictions based on how methanogens behave on Earth?
MM: That's right. They'd have to be similar in nature, and of course we have no idea whether that would be true or not.
Members of the University of Rhode Island's NASA astrobiology team have shown that methane and ethane are produced in similar abundance in cold deep-sea sediments. In that environment, the gases are probably produced by life. That's potentially important, because if life forms on Mars are similar, ethane should be released along with methane.
So I'm suggesting a chemical search, using ground-based telescopes, to look for other hydrocarbons on Mars. Whether detected or not, we can fold the results into the model to constrain the possibilities.
The measurement of isotopic ratios, like carbon-13 or carbon-12, probably can't be done from the ground with the accuracy required. That measurement would require an orbiter around Mars, or an airplane flying over the vents. So if the present results are upheld, they could define the course of Mars exploration for years to come.
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