NASA’s Curiosity rover has come up empty-handed in its search for methane in the atmosphere of Mars, researchers report September 19 in Science.
During eight months of data collection, the rover detected average methane concentrations of 0.18 parts per billion. The researchers say that, because of the measurement’s margin of error, the finding translates to essentially no methane in the Martian atmosphere.
“It’s disappointing because [methane] is a potential sign of biological activity,” says study coauthor Christopher Webster, a planetary scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. Microbes have produced up to 95 percent of the methane in Earth’s atmosphere, where the gas’s concentration is roughly 1,800 ppb.
Although the results dampen hopes that methane-making microbes now live on Mars, microbes that don’t generate methane may still live there. Or life (methane-producing or otherwise) could have existed on the planet in the past (SN Online, 3/12/13).
Scientists had previously identified methane on Mars using Earth-based telescopes and spacecraft orbiting the Red Planet. In 2003, researchers detected a concentration as high as 45 ppb. Since then, experiments have reported much lower levels of less than 10 ppb.
To explain the diminishing methane measurements, researchers suggested the gas was periodically released and then rapidly cleared from the atmosphere. Geologic activity could have created the methane. Or microbes beneath Mars’ frozen surface could have produced the gas, with seasonal surface melting unleashing the methane into the air.
Curiosity’s findings don’t preclude those scenarios, says Michael Mumma, a planetary scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md. Mumma was part of the team that detected methane in 2003 and in some subsequent studies but was not involved with the Curiosity measurements. If methane ejections are indeed periodic and fleeting, Curiosity may still detect a release as it explores Mars, he says. NASA recently extended the rover’s original two-year mission indefinitely.
Other scientists are skeptical of the earlier methane reports. Kevin Zahnle, a planetary scientist at the NASA Ames Research Center in Mountain View, Calif., notes that Curiosity’s methane detector is more sensitive than those based on Earth or in space. Another problem with previous studies is explaining why any methane would disappear so rapidly. The gas should last hundreds of years in the Martian atmosphere.
Mumma suggests that Martian soil could contain compounds that oxidize methane when they’re lofted into the air in dust.
Next, Curiosity will start looking for the gas with an even finer precision. Webster says the rover could sniff out as little as 50 or 100 parts per trillion. If that’s all the methane Mars has, he says, meteorites or comets containing organic molecules are a likely source of the gas.