The more we look at the surface of the Red Planet, the more interesting things we're finding there. The latest results haven't even required a trip to Mars. Instead, a series of spectroscopy readings from telescopes in Hawaii have tracked changes in the composition of Mars' atmosphere over time. To the researchers' surprise, the Martian summer triggered large releases of methane into the atmosphere from three distinct regions on the planet. Right now, the researchers say they don't have sufficient data to determine if the source of the methane is biological or geological.
The data themselves are being reported in a paper that is being released by Science today, and NASA hosted a press conference to describe them. The basic technique involved using a spectrometer hooked up to one of the large Hawaiian telescopes (over the span of several years of observations, they used several individual telescopes). They took samples from multiple pixels along the north-south axis to get one dimension of spatial resolution; the rotation of the planet itself provided a second. Absorption peaks from the gasses in the earth's atmosphere were subtracted, leaving the signal from Mars' atmosphere behind.
During the winter, the area of the spectrum that includes a signal of methane was basically a blank. But, as Martian summer arrived in the northern hemisphere, three distinct areas of the planet started showing signs of methane production. Eventually, three distinct atmospheric plumes appeared, representing a total of 19,000 metric tons of methane; the research suggests a production rate of over a half-kilogram of a second at its peak, a rate comparable to that of hydrocarbon seeps on earth.
The W. M. Keck Observatory
As might be inferred from the low levels present in the winter, Mars doesn't provide a hospitable environment for methane. The researchers estimate that, should this be a one-time only event, the methane that's released would have a half life of four years; if this is an annual event, it must last less than a year. Given the oxidants found by the Mars Phoenix Lander and the dust storms that sweep that material into the sky on a regular basis, the authors suggest that it probably shouldn't be viewed as a surprise that the methane doesn't last.
So, where is it coming from? At the press conference, the NASA scientists noted that no plumes came from the area that's normally covered by an ice cap during the Martian winter, so it's not simply a matter of the source being blocked for part of the year. The areas where it does come from are within regions of the planet that have been found to have had liquid water in the past. Beyond that, however, the three plume sources seem to have little in common. The authors note that Terra Sabae has sub-surface hydrogen, Nili Fossae has hydrated minerals, and Syrtis Major has a volatile-rich substrate. Of course, it's possible that there are small regions within these areas that do share some geochemistry.
Mars is now tectonically dead, and the lack of sulfur dioxide suggests that the gas is not coming from any volcanic activity. Still, there is no way to rule out that some hot mantle material hasn't come into contact with the regions' residual water, which could create a steam reaction with minerals that could liberate water. On earth, however, these sorts of faults tend to create notable surface features, and cool fairly rapidly.
The alternative that springs to everyone's mind is a biological source. The finding that bacteria can live at the bottom of South African mines provides a model cited by the authors. Radioactive activity could break apart water and release hydrogen, which the bacteria react with carbon dioxide to release energy, with methane as a byproduct.
The problem for either of these cases is that the production of methane would be occurring several kilometers down in the crust, so it's nothing that would be accessible to machinery NASA plans to send to Mars any time soon. Biological activity tends to involve lighter isotopes of elements, so it's possible that mass spectroscopy of the methane might provide a clue, but getting a concentrated sample would be very difficult. It came out during the press call that one of the regions was a potential site for the Mars Science Lab that was eliminated late in the planning stages; since that mission has now been delayed until 2011, there's a chance that decision will be revisited.
Although there's no good way to distinguish among the possible sources, the finding is a clear indication that Mars remains an active planet, and liquid water is almost certainly associated with that activity. As Martian years take two on earth, scientists will be anxiously waiting to see whether the plumes return next Martian summer, an event that would rule out some of the more unlikely sources. In the mean time, the discovery will almost certainly inspire a lot of scientists to start thinking about a variety of chemical processes that could produce methane, and trying to determine ways to test whether any of those are operating on the red planet.