Methane on Mars seems to have gone missing – mystifying the scientists who hoped to study it.
Researchers had hoped the Trace Gas Orbiter (TGO), currently floating above Mars, would be able to sniff out the methane and further study it. The gas could be an indication that there is life on the planet, since on Earth it is produced by living organisms.
Methane has been spotted on numerous occasions, both on the surface of the planet and in its atmosphere.
Researchers hoped that the TGO would be able to confirm those observations and tell us more, by using the on-board sensors that allow it to find methane at levels up to 100 times lower than previously. It is also able to examine the make-up of that methane, identifying the type of carbon and so giving a clue to whether it is arising out of the geology of Mars or coming from living things beneath the surface.
But the TGO did not smell any of that methane. And scientists aren't sure about how it could have disappeared.
Dr Manish Patel, from the Open University, who is in charge of the Nomad chemical analysis instrument on TGO, said: "The measurements we have made are very surprising. The methane previously detected by ground-based telescopes, the ESA (European Space Agency) Mars Express spacecraft and the Nasa Curiosity rover seems to have disappeared.
"Mars continues to confound us. The only way these results make sense with previous observations is if there is a new mechanism in the atmosphere, removing the methane at a rate far faster than thought possible.
"As always, Mars provides us with another mystery to solve."
The new findings were published in the journal Nature and presented at the European Geosciences Union meeting in Vienna.
A "spike" of methane detected on the Martian surface by the Curiosity rover on June 16, 2013, was picked up the next day by the Mars Express orbiter.
Scientists traced the methane "burp" to a region near Gale Crater, a 96-mile-wide bowl where Curiosity landed in 2012 and which may be the site of an ancient lake.
Writing in the journal Nature Geoscience earlier this month, they said it was likely the methane had bubbled up through cracks in surface ice.
The discovery left open the question of whether the gas was produced by geological or biological processes.
Confirmation that methane is spouting from the Martian surface in at least one location makes it all the more extraordinary that none was found by TGO.
ExoMars is a two-part mission. The TGO will be joined in 2021 by the British-built Rosalind Franklin rover, now under construction by Airbus in Stevenage.
The rover will be able to probe under the Martian surface for signs of past or present life.
Sue Horne, head of space exploration at the UK Space Agency, said: "ExoMars embodies the best of UK and European space science and I'm delighted that Britain is one of the biggest supporters.
"This data release is the first of many on our mission to unearth the mysteries of the Red Planet. The results both answer, and raise new questions, paving the way for more exciting discoveries from the Rosalind Franklin rover which is due to launch next year."
Other results from TGO, also published in Nature, describe the impact of a massive global dust storm that has enveloped the Red Planet.
The spacecraft monitored the storm from the moment it began and studied how the dust affected water vapour in the atmosphere.
Britain is a major contributor to ExoMars mission. The UK Space Agency has invested £248 million to the overall ExoMars mission and £12 million to its instruments.
Additional reporting by Press Association
