Something is happening beneath the surface of Mars that causes substantial amounts of methane gas to burst out regularly, a discovery that NASA scientists said today represents the strongest indication so far that life might exist, or once existed, on the planet.
The methane is released into the atmosphere in specific areas and at regular times, they found, in a pattern that would be consistent with the gas being a byproduct of biological activity beneath the planet's parched surface.
Principal investigator Michael Mumma, of NASA's Goddard Space Flight Center, said the detection does not mean that life definitely exists on Mars, because the gas can be produced by subsurface geological or chemical processes as well.
Nevertheless, "we believe this definitely increases the prospects for finding life on Mars," said Mumma, whose findings are being published online today by Science Express. "No other discovery has done as much to increase the chances of finding life."
The scientists detected plumes of methane during two Martian summers, when the planet's large formations of subsurface ice might melt and release the gas.
Most of the methane in Earth's atmosphere is produced by bacteria in living creatures large and small. Even if it turns out that the Martian methane is the result of non-biological processes -- a far less dramatic prospect -- that would nonetheless reshape thinking about the planet, which scientists thought to be geologically dead and chemically unlikely to produce much of the gas.
Scientists have been working to confirm the presence of methane on Mars since it was preliminarily detected in 2003, first by Mumma and then by scientists working with the European Space Agency. Today's findings confirm that discovery and describe intense, recurring, but relatively brief releases that are consistent with either biological or active geological origins.
The new data was gleaned by NASA's Infrared Telescope Facility (IRTF) atop Mauna Kea, Hawaii, and another telescope in Chile. The earlier reports of methane were made by the European Space Agency's orbiting satellite, the Mars Express. The scientists used instruments called spectrometers to detect the "fingerprints" of methane molecules by the way they absorb sunlight.
Mumma said the methane doesn't last long in the Martian atmosphere, which is made up largely of carbon dioxide and breaks down the gas much more quickly than on Earth. That means, he said, that the gas detected was almost certainly released recently from underground methane reservoirs, although it could have been stored there for a long time.
The plumes were detected above a handful of Martian hotspots hundreds of miles apart, including Nili Fossae, Syrtis Major and Arabia Terra. Previous explorations have determined that liquid water once covered some of that area and have detected mineral deposits that require standing water to form. Images taken by a Mars orbiter in 2005 also suggest that water, or liquids of some kind, might still flow at times on the surface of Mars.
The methane discovery comes as researchers are finding previously unknown colonies of "extremophiles" living far below Earth's surface and in conditions long considered to be uninhabitable. One of the experts invited to comment today at NASA's news conference was Lisa Pratt of Indiana University, who was part of a team that identified a microbe two miles down in a South African gold mine that lives without drawing energy from the photosynthesis of sunlight. Its energy source is the radioactive decay of nearby rocks, a process known as radiolysis.
In its paper, Mumma's team points to the South African findings, saying that it might be possible for similar life forms "to survive for eons" below Mars's sub-surface ice, "where water is again liquid, radiolysis can supply energy, and CO2 can provide carbon. Gases accumulated in such zones might be released to the atmosphere if pores or fissures open seasonally, connecting these deep zones to the atmosphere at scarps, crater walls, or canyons."
Pratt called the methane find "a breathtaking discovery." She noted that on Earth, methane is not only a waste product of some microbes but also a potential nutrient for specialized organisms.
"So much attention has been given recently to looking for evidence of past life on Mars, and now it appears that we need to seriously consider the possibility of a present-day subsurface Martian ecosystem," Pratt said. "Mars just got a whole lot more interesting."
Mumma, a senior investigator with Goddard's Center for Astrobiology and its Solar System Exploration Division, has been looking for methane on Mars since the late 1980s. The gas has been difficult to detect because it is relatively scarce and breaks down quickly, and some earlier reports of detections have been controversial. But Mumma said the methane signature in his data this time is "very strong indeed," and the fact that it occurs only locally and at specific times is also well documented.
Because venting methane was found at several locations, Mumma has been urging NASA to consider one of them as a landing site for the Mars Science Laboratory, a $2 billion rover scheduled to launch in 2010. He said some of the terrain around the sites would be difficult for a landing, but he hopes the methane discovery will focus attention on how NASA might further explore its origins.
"Suppose we put a probe into a fissure at one of the release sites and we could get measurements from some extant life form," he said. "We could then sequence the life form and see if it had the same origins as Earth life. What could be more compelling?"
The most common non-biological way that methane gets into the Earth's atmosphere is volcanic eruptions that spew it out along with rock, magma and a great deal of heat. Sulfur dioxide accompanies volcanic eruptions on Earth, but scientists have not found much of that compound in the Martian atmosphere -- leading them to conclude that the planet is no longer volcanically active. As a result, they do not consider volcanoes to be the source of that planet's methane.
But methane can also be created by the interaction of water and subsurface minerals such as olivine in the presence of heat. This process, call serpentization, is known to occur beneath Earth's surface.
Methane is also known to be much more broadly present on Titan, one of Saturn's moons and another target of study for scientists looking for life beyond Earth.