A methane “burp” on Mars that has divided scientists but could be evidence of life on the Red Planet really was detected by a Nasa rover in 2013, say scientists.
Findings from the orbiting Mars Express spacecraft indicate that the gas burst out of ice cracks near Gale Crater, thought to be the site of an ancient lake.
The initial discovery of the methane emission by the Curiosity rover in June 2013 had been questioned by some experts.
It was even suggested that the gas came from the rover itself – a claim rejected by the Curiosity team.
But now it’s been confirmed that an eruption of methane did occur and was picked up by Curiosity and, a day later, the European Space Agency (Esa) orbiter.
Writing in the journal Nature Geoscientist, Mars Express researchers do not address the thorny question of what produced the methane “spike”.
On Earth, the gas can be generated by geological processes, but much of it is released by micro-organisms known as methanogens, some of which live in the guts of ruminant animals.
Methane in the Martian atmosphere has long been considered a “smoking gun” that might point to the presence of life.
The “spike” observed by the rover and orbiter is thought to have come from a reservoir of the gas trapped under ice.
It could have been produced by a non-biological process, such as a chemical reaction involving water, carbon dioxide and the mineral olivine, or microbial bugs.
In either case, the methane would have punched its way through cracks in the surface ice caused by pressure build up, seismic stresses or meteor impacts.
According to computer simulations, up to 4,000 tonnes of methane in total may have been released from a region less than 800 kilometres east of Gale Crater.
The Mars Express detection itself corresponds to 46 tonnes of methane out-gassing from an area covering 49,000 square kilometres.
The scientists led by Dr Marco Giuranna, from the National Institute of Astrophysics in Rome, wrote: “The results presented in this work not only corroborate previous detections by Curiosity but, in a broader perspective, might change our view of methane occurrence on Mars.
“Rather than by large emissions and a global presence, our data suggest that the presence of methane on Mars might be characterised by small, short emissions and transient events.”
They add: “We do not address the ultimate origin of the detected Martian methane. Many abiotic (non-biological) and biotic (biological) processes can generate methane on Mars.
“However, the first step to understanding the origin of any Martian methane is to determine its release location.”
The methane was spotted by a “chemical sniffer” instrument on Mars Express called the Planetary Fourier Spectrometer.
Only one detection was made during two years of searching. The instrument did not confirm a later methane discovery by Curiosity.
A second European spacecraft, the ExoMars Trace Gas Orbiter, is expected to shed more light on the mystery of Martian methane.
Analysing the isotopic signature or atomic “strain” of carbon in the methane may help scientists determine whether or not the gas is sign of life.