The results were presented this week at the American Astronomical Society's Division of Planetary Sciences conference in Louisville, Kentucky. This evidence for methane on Mars has also been corroborated with data gathered by the European Space Agency's Mars Express spacecraft.
Methane gas is a potential biomarker because various photochemical and other processes destroy it on Mars. Without being continually replenished, it would disappear from the atmosphere in about 340 years or less. On Earth, microorganisms are by far the dominant source of methane gas.
Using NASA's 3-meter Infrared Telescope Facility atop Mauna Kea, Hawaii, Mumma and his colleagues detected methane at a level of 250 parts per billion in equatorial latitudes north of the Hellas impact basin. Further observations conducted with the 8-meter Gemini South telescope in Chile also revealed elevated methane concentrations over Valles Marineris. The team found methane concentrations of 20 to 60 parts per billion at higher latitudes. These methane concentrations are much higher than those previously reported. In both locations, Mumma's group identified two spectral lines due to methane, which
minimizes the possibility of a false detection.
"I'm shocked by this result; we didn't expect this," says Mumma. "At these two points on Mars the data imply that there were significant methane releases. We could be seeing methane released by methanogens, although we cannot exclude abiotic origins."
"This is a very enticing result," adds independent commentator David H. Grinspoon (Southwest Research Institute). "I want to know more about the distribution of methane in the atmosphere so we can better understand where it is being produced and destroyed."
Krasnopolsky's group added several spectral lines of methane to improve the detection limit while observing Mars with the 3.6-meter Canada-France-Hawaii Telescope in Hawaii. His team found a global methane level of about 10 parts per billion.
Scientists cannot state with certainty the source of the methane. Other possible sources besides microorganisms include volcanic or geothermal activity, or a recent comet impact. But Krasnopolsky points out that the THEMIS instrument on NASA's Mars Odyssey orbiter has found no evidence for volcanic or
geothermal hot spots, despite the fact that the instrument was specifically designed to find them. Krasnopolsky also says that there is only a 2 percent chance that a comet impact could have occurred recently enough to account for the observed methane.
Krasnopolsky's calculations suggest that Martian organisms would be producing only 270 tons of methane per year, which is 100 million times less than the amount produced by terrestrial organisms. This yields a Martian biomass of at least 20 tons, but possibly higher. "Mars is generally sterile except for small
oases," says Krasnopolsky.
Grinspoon questions whether a mostly dead planet could sustain isolated pockets of life for long time scales. In a paper published in the 1970s, atmospheric chemist James Lovelock and biologist Lynn Margulis proposed that once life gains a foothold on a planet, it will literally take over the planet by altering its geological and atmospheric cycles. Such a process happened on Earth, where life dominates the surface and even subsurface environments. But even the most optimistic assessments of extant Martian life would agree that this did not
happen on Mars.
"These results are already stimulating a great deal of theoretical work," says Mumma. "The areas being explored range all the way from geophysics to atmospheric processes."