In the latest chapter of the now-you-see-it-now-you-don’t-story of the quest for methane on Mars, two teams of scientists have reached opposite results in findings reported only a few days apart.
One team, using instruments on the European Space Agency’s ExoMars Trace Gas Orbiter, found no signs of methane, at least above 50 parts per trillion, the detection limit of their instrument, in much of the Martian atmosphere. (In some spots, the instrument found no signs of methane down to concentrations as low as 12 parts per trillion.) Their results appear in the journal Nature.
The other team, examining archived data from another European Space Agency mission, the Mars Express orbiter, found a “firm detection” of methane at about 15 parts per billion — roughly 300 times higher than TGO’s detection limit.
The research was published in the journal Nature Geoscience.
That data, collected in June 2013, traced the source of the methane to a region not far from Gale Crater, where NASA’s Curiosity rover had also detected the gas only a day before the Mars Express measurements were made. It was the first time that two independent sets of instruments had ever detected methane at the same time, let alone in the same place.
Methane on Mars is of interest because on Earth it is a sign of life. Here, it comprises about two parts per million in the atmosphere and is mostly produced from biological activity, ranging from cow farts and belches to decomposing plants. Biological processes are not the the only way it can be produced, but it has long been one of the things that people searching for signs of life on Mars want to track.
It’s also been incredibly elusive.
“Previous observations have failed to detect methane in most cases, and only occasionally recorded plumes of this gas,” says Marco Giuranna of the National Institute for Astrophysics and the Institute for Space Astrophysics and Planetology, in Italy, and lead author of the Nature Geoscience paper.{%recommended 8732%}
Part of the confusion stems from the fact that the various instruments aren’t looking at exactly the same parts of the Martian atmosphere.
Curiosity’s gear, for example, is mounted on the rover. It operates only a metre above ground level, and can only measure the composition of the air in the immediate vicinity.
The instruments on Mars Express and the Trace Gas Orbiter are in space. Those on Mars Express point at the ground and use reflected sunlight to seek out the spectroscopic fingerprint of methane in the intervening column of air.
The Trace Gas Orbiter, on the other hand, looks directly at the sun when it is just above the Martian horizon, slanting toward the spacecraft horizontally through various layers of the atmosphere.
Thus, each is sampling methane levels at different elevations, and with differing degrees of spatial precision.
Also, Giuranna says, it is likely that methane on Mars originates in the subsurface, from which geological processes release it only in occasional puffs, rather than continuous streams.
Therefore, he adds, “methane spikes on Mars may only be detected occasionally, when rovers, landers, or orbiters happen to be at the right place at the right time.”
But there’s still a problem because the gas should persist in the Martian atmosphere for several hundred years before being destroyed by solar radiation. During that time, enough should have been released one puff at a time – and been thoroughly enough mixed into the air by wind and weather — to have accumulated at levels detectable by the Trace Gas Orbiter’s ultra-sensitive instruments.
“All the methane that is there, whether it’s biological, volcanic, or whatever, is supposed to be mixed and stay there for a long time,” says Oleg Korablev, a planetary scientist and geophysicist at the Russian Academy of Sciences’ Space Research Institute, Moscow, and lead author of the study that did not find methane.
Nor should releases of methane from below ground be the only source of it in the atmosphere.
Mars is constantly being bombarded by interplanetary dust particles and meteorites that contain organic compounds which, when exposed to solar ultraviolet light shining on the planet’s surface, should partially decompose into methane, says Christopher Webster, a senior research fellow at NASA’s Jet Propulsion Laboratory in California, US, and a member of the Curiosity team.
Conventional wisdom held that there should have been enough methane from this source alone to produce a background level far above the Trace Gas Orbiter’s detection limit, Webster adds. This makes its failure to detect it all the more startling.
Not that anyone doubts the accuracy of the Trace Gas Orbiter’s findings.
“The spectra collected by the Trace Gas Orbiter are of superb quality, and spectra do not lie,” Webster says.
One possibility is that a giant dust storm that arose shortly after the Trace Gas Orbiter started taking its measurements may have interfered with the results.
“Martian dust storms represent a possible sink for atmospheric methane,” says Giuranna.
But that’s not likely, Korablev notes, because the spacecraft was already taking measurements when the dust storm arose.
“We did not detect methane before the dust storm,” he says. Furthermore, he adds, now that the dust storm has abated, “we continue looking and we still see nothing”.
More likely, he and others say, some presently unrecognised process is rapidly destroying methane close to the surface, thereby preventing background levels to build up high enough to be detected by Trace Gas Orbiter.
“Indeed, methane on Mars seems to appear and disappear quickly,” Giuranna says, “suggesting the presence of a destruction method capable of efficiently removing this gas from the atmosphere.”
Meanwhile, researchers from all three teams are already planning ways to make joint Mars Explorer/TGO/Curiosity observations.
“Methane remains a big puzzle,” Korablev says.
