Mars methane mystery deepens with ExoMars data

An artist’s impression showing the ExoMars Trace Gas orbiter using repeated dips into the upper atmosphere of Mars, a technique known as aerobraking, to reach its intended science orbit. Recent observations indicate a surprising absence of methane in the red planet’s atmosphere. Image: ESA

The ESA-Roscosmos ExoMars Trace Gas Orbiter – TGO – spacecraft has found a surprising lack of methane in the martian atmosphere. In fact, the spacecraft’s two spectrometers, NOMAD and ACS, found 10 to 100 times less methane than all previously reported detections by a variety of spacecraft and ground-based telescopes.

Martian methane is an area of high interest because it can be a signature of biological activity. It also can be produced by geological processes, but either way, solar radiation acts to destroy it over relatively short timescales. As a result, any detections indicate recent activity. That’s true even if the methane was generated millions to billions of years ago and remained trapped in sub-surface reservoirs until now.

On Earth, 95 percent of all the methane in the atmosphere was produced by biological processes. The concentration is about 18 parts per billion by volume, or ppbv, meaning that out of every one billion molecules, 1,800 are methane.

Previous methane detections in the martian atmosphere have been difficult to interpret. ESA’s Mars Express orbiter found levels as high as 10 ppbv while Earth-based instruments captured transient events with readings as high as 45 ppbv. Data from NASA’s Curiosity Mars rover suggests background levels that vary with the martian seasons between about 0.2 and 0.7 ppbv.

Measurements from earlier spacecraft and Earth-based instruments showed a wide variety in the amount of methane suspended in the atmosphere of Mars. ESA’s ExoMars orbiter indicates a surprising absence. Click on the graphic for a larger view. Image: ESA

The new results from TGO indicate an upper limit of just 0.05 ppbv, or 10 to 100 times less methane than all previous detections. That still means up to 500 tons of methane is present, but it is distributed through the entire martian atmosphere.

“We can only report a modest upper limit that suggests a global absence of methane,” said ACS principal investigator Oleg Korablev from the Space Research Institute at the Russian Academy of Sciences, Moscow.

“The TGO’s high-precision measurements seem to be at odds with previous detections. To reconcile the various datasets and match the fast transition from previously reported plumes to the apparently very low background levels, we need to find a method that efficiently destroys methane close to the surface of the planet.”

Håkan Svedhem, ESA’s TGO project scientist, said the latest observations mean “we don’t have all the pieces of the puzzle or see the full picture yet, but that is why we are there with TGO, making a detailed analysis of the atmosphere with the best instruments we have, to better understand how active this planet is – whether geologically or biologically.”