Nasa scientists have detected a methane "burp" which could indicate life on Mars.
It has been determined the gas spike, which had been disputed, was picked up by a rover from the space agency in 2013 by Curiosity, then a day later by the European Space Agency orbiter.
It had previously been suggested the gas came from the rover itself, though this theory was rejected by the Curiosity team.
Writing in the journal Nature Geoscientist, Mars Express researchers did not address exactly what produced the methane "spike", despite offering the clarification on its occurrence.
The gas can be generated by geological processes though much of it is released by micro-organisms known as methanogens, some of which live in the guts of certain mammals.
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."
It was indicated that the gas burst out of ice cracks near Gale Crater, thought to be the site of an ancient lake.
According to computer simulations, up to 4,000 tonnes of methane in total may have been released.
The Mars Express detection itself is linked to 46 tonnes of methane out-gassing from an area covering 49,000 square kilometres.
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 and the instrument did not confirm a later methane discovery by Curiosity.
A second European spacecraft, the ExoMars Trace Gas Orbiter, is set to look into more details of the mystery of Martian methane.
Looking at the strain of carbon in the methane may help scientists determine whether or not the gas is sign of life.
