NASA has found three discrete regions where Mars is releasing methane at this time.
Methane can only survive a short time in the atmosphere until it is destroyed by sunlight, and so its continued presence means it is being replenished.
"The production (of methane) is likely due to only one of two possibilities. The first is geochemistry, the second is biology. That raises much interest on which one is the dominant production mechanism."
If the methane is produced by geological activity, it could either originate from active Martian volcanoes or from a process called serpentinisation.
The latter process occurs at low temperatures and occurs when rocks rich in the minerals olivine and pyroxene react chemically with water, releasing methane.
We've demonstrated there are regions of active (methane) release," said Dr Mumma.
"What we'd really like to do is map the entire planet identifying all such regions of release and validating that some are reproducible from year to year.
"In addition, we'd like to map the gases being released to establish which of those are more likely biological in origin versus others that may be geochemical in origin.
"On this basis we could target several interesting sites for lander missions."
Nasa's Mars Science Laboratory (MSL) rover, due to launch to the Red Planet in 2011, will carry instruments that have the potential to distinguish between carbon in gases produced by biological activity and those with a geochemical origin.
Unequivocal determination may have to wait for the next rover mission after MSL as explained at Astrobiology Magazine.
Astrobiology Magazine Explains the Science
Although the amount of martian methane is small (10 parts per billion compared to 1,800 parts per billion on Earth), it appears to be concentrated in regions around the equator. Because these methane "clouds" only last a year before dispersing, the methane sources must be fairly localized and constant.
Microbes called methanogens produce this greenhouse gas as part of their metabolism. Onstott estimates that this localized generation is comparable to that of Earth's Arctic permafrost, which is one of our planet's main sources of this greenhouse gas.
The building blocks of methane (carbon and hydrogen) exist in different forms, called isotopes, that differ in mass. Geochemistry isn't picky and will use whatever isotopes it finds to make methane. Life, however, prefers to consume lighter isotopes.
"Enzymatic processes work faster on compounds of lighter weight," Onstott said.
In the case of methanogens, they will select molecules with hydrogen (rather than its heavier isotope deuterium) and carbon-12 (rather than the heavier carbon-13).
The result is that biogenic methane should be lighter than abiogenic methane.
One confounding factor is that organisms that eat methane may also inhabit Mars. These so-called methanotrophs have a preference for light-weight methane, thereby removing the evidence of methanogen activity.
The Mars Science Laboratory (MSL) — now scheduled to launch in 2011 — will carry such an optical spectrometer (the Tunable Laser Spectrometer, or TLS). This device may be able to measure the carbon isotope ratio in martian methane, but Onstott does not think it will be able to say unequivocally whether life or geology is the source.
For this reason, he and his colleagues are designing a special kind of optical spectrometer, called a cavity ring-down spectrometer (CRDS), that will be 1,000 times more sensitive than TLS. The CRDS works by illuminating an atmospheric sample with a laser whose frequency can be tuned to resonate with methane molecules of a particular isotopic configuration.
Although the CRDS is a mature technology, Onstott and his group need to develop a portable device that can reach a high sensitivity. They have already built a test version that weighs 70 pounds, about a fifth of what a typical mass spectrometer weighs.
The goal now is to make the instrument smaller and more compatible for space missions — such as the next rover mission after MSL.
"We plan to make modifications that will ensure it functions on Mars, where there's lower pressure and lots of dust," Onstott said.