Posted on behalf of Rick Lovett
Recent findings of methane on Mars using Earth-based telescopes may be an artifact of the Earth’s atmosphere, a team of scientists argued at a meeting of the American Astronomical Society’s Division for Planetary Sciences, this week in Pasadena, California.
In order to see Martian methane through the Earth’s atmosphere, which itself contains the same gas, it is necessary to make observations when the two planets are moving toward or away from each other in their orbits. That way the Doppler effect allows spectroscopes to distinguish Martian methane lines from earthly ones.
But the Martian methane was seen only when the two planets were converging, and not when they were drawing apart.
One way to explain the difference is that there was a seasonal variation of methane on Mars. But Kevin Zahnle, a planetary scientist from NASA Ames Research Center, Moffat Field, California, notes that during the convergence the Martian line would have been blue-shifted right into a line from a rare isotope of methane in the Earth’s atmosphere, comprised of carbon-13, rather than the more plentiful carbon-12.
This overlap is well known, but there’s 20 times more earthly methane-13 than reported Martian methane-12, making it very difficult to tease out the difference, Zanle says. “I think it’s too difficult. I have no confidence that [they] are actually detecting Martian methane.”
Another problem is that methane should persist on Mars for hundreds of years. To disappear as quickly as the now-you-see-it-now-you-don’t reports would require, Zanle says, some process must be destroying it 1000 times faster than expected.
Other scientists have also reported seeing spectroscopic signatures of methane from the Mars Express orbiter, but Zanle doesn’t find their results compelling either, arguing that the methane signal, if it exists, is hard to distinguish from background noise. And he notes that Mars Express reports suggest that methane is appearing, and then disappearing so rapidly that known processes for destroying it — even if they could operate that quickly — would deplete the entire supply of Martian oxygen within 7,000 years. That, he says, requiring an unknown source of oxygen to balance out the unknown source of methane — one too many unknowns for his taste.
The Martian methane caused a stir when it first was detected, since methane could be a byproduct of deep micrbial life under the red planet’s harsh surface. Other non-biological explanations exist, but they would also suggest the existence of geologically activie hot-spots where water and minerals can react, which might, in turn, provide a save haven for life.
Michael Mumma, of the NASA Goddard Center for Astrobiology was the leader of the team that made the original finding. He agrees that there is a mystery to the rapid disappearance of the methane. “We agree that it’s not being destroyed by photochemistry,” he says.
But he thinks that the Phoenix lander might have found a more rapid methane-destroyer in the soil, in the form of perchlorates. Since the soil blows around as dust, he notes, these chemicals could go a long way toward destroying atmospheric methane.
He also notes that his team did many complex tests (many unpublished) on their data to make sure they weren’t artifacts of the Earth’s atmosphere. “I welcome his attention to this,” Mumma says. “I think it’s very important that theorists have a good deep look at this.”
Image: Space Telescope Science Science Institute/NASA