As the orbiter’s instruments are highly sensitive, if methane is present it must be at an abundance of less than 0.05 ppbv – and more likely less than 0.02 ppbv, say Franck and colleagues. The scientists also hunted for signs of methane around Curiosity’s home, Gale crater, and found nothing, despite the rover reporting the presence of methane there.

“Curiosity measures right at Mars’ surface while the orbiter takes measurements a few kilometres above – so the difference between these two findings could be explained by any methane being trapped to the lower atmosphere or the immediate vicinity of the rover,” adds Franck.

The apparent lack of martian methane reported by Franck and colleagues is supported by a paper using data from the orbiter’s NOMAD (Nadir Occultation MArs Discovery) instrument, again spanning a full martian year and searching for methane and two other biomarkers.

“We also found no sign of methane on Mars, and set an upper limit of 0.06 ppbv, which agrees with TGO’s initial findings using ACS,” says the paper’s lead author Elise Wright Knutsen, previously at NASA’s Goddard Space Flight Center, USA, and now at LATMOS, France. “As well as searching for global methane, we also looked for localised plumes at over 2000 locations on the planet and didn’t detect anything – so if methane is released in this way, it must be sporadic.”