NASA's Curiosity rover has detected methane at Mars's Gale crater (which is shown here in false colour).Credit: ASU/JPL-Caltech/NASA

A spacecraft that was supposed to solve the mystery of methane on Mars has instead compounded scientists’ confusion. The European–Russian Trace Gas Orbiter (TGO), which began looking for the gas last year, has yet to find any whiffs of it in Mars’s atmosphere, says a study published on 10 Ap﻿ril in Nature1.

“It’s a huge surprise,” says Dorothy Oehler, a planetary geologist at the Planetary Science Institute in Houston, Texas.

Earlier Mars missions have detected hints of methane wafting through the atmosphere. The TGO’s failure so far to find the gas suggests that an unknown process is scrubbing much of it from the atmosphere soon after it appears, says Oleg Korablev, a physicist at the Space Research Institute in Moscow and lead author of the study.

Planetary scientists are keen to understand where Martian methane comes from because most methane in Earth’s atmosphere is produced by living organisms. Pinning down how much methane is in Mars’s atmosphere, and where, would help researchers to determine whether the gas comes from geological sources, such as chemical reactions in rocks, or has a more exciting origin — potentially, Martian life2.

Scientists have repeatedly spotted methane on Mars over the past 15 years. The reports include telescopic views of a plume of methane3 in 2003 and occasional detections by NASA’s Curiosity rover since it landed at Gale crater4 in 2012. The European Space Agency’s Mars Express spacecraft has spotted the gas at multiple sites5 — including a plume near Gale6 in 2013.

Sniffing the skies

Aiming to answer the question more definitively, the TGO arrived at Mars in 2016 and began collecting atmospheric data in April 2018. Between April and August 2018, it found no signs of methane, even though its instruments can detect the gas at concentrations below 50 parts per trillion.

Curiosity sniffed about 500 parts per trillion of methane in mid-June 2018 — at the same time that the TGO flew overhead without seeing any, says Christopher Webster of the Jet Propulsion Laboratory in Pasadena, California, who runs the rover’s methane-measuring instrument. Models suggest that any methane plumes should drift upward and mix well into the planet’s atmosphere within a few months.

“Why do they disappear so quickly?” asks John Moores, a planetary scientist at York University in Toronto, Canada. “There’s some piece of the puzzle we are missing.”

Researchers are looking for answers in the gap between Curiosity, which hunts for methane 1 metre above Mars’s surface, and the TGO, which takes its best measurements at least 5 kilometres above the planet. The scientists are trying to determine how methane could be destroyed relatively close to Mars’s surface.

One possibility is that methane seeping out of the ground is removed by some kind of low-altitude chemical reaction — perhaps involving dust — before it can drift higher into the air, says Michael Mumma, a planetary scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. A team based at Aarhus University in Denmark, which has studied how dust particles could deplete Martian methane7, will describe its ideas on 11 April at the European Geosciences Union meeting in Vienna.

In June 2018, just a few months after the TGO began monitoring Mars's atmosphere, a giant dust storm spread across the planet. It obscured Mars's atmosphere and temporarily forced the spacecraft to focus its search for methane on high elevations and latitudes.

Outlook hazy

Still, some researchers are sceptical that the TGO will ever find the gas. “I’ve never seen a convincing detection of indigenous methane on Mars, and I don’t believe I ever will,” says Kevin Zahnle, a planetary scientist at NASA's Ames Research Center in Moffett Field, California, who has long argued that the reports of Martian methane are observational errors.

But Oehler says that methane probably wafts erratically out of geologically active regions on Mars, such as those riddled with faults. With the only on-ground measurements of the gas coming from Curiosity, scientists simply haven’t had the chance to observe how it might be seeping from different parts of the planet’s surface, or how methane might be destroyed as it drifts upward8. Sushil Atreya, a planetary scientist at the University of Michigan in Ann Arbor, would like to see a series of methane-measuring drones, dirigibles or balloons float over many areas of the Martian surface.

In the meantime, the TGO will continue to monitor Mars’s atmosphere through at least 2022. So far, it has observed only a fraction of a Martian year, which lasts nearly two Earth years. Hints of methane might yet emerge as the seasons pass.

“One thing about Mars is it’s never boring,” says Oehler.