Overnight changes in Mars’ atmosphere could solve a methane mystery

Methane released in Gale crater stays in Gale crater. An overnight change in the Martian atmosphere could hold the gas close to the ground until morning, explaining why the Curiosity rover caught a whiff of methane while an overhead orbiter found none.

The theory offers “a way for the two measurements to live in harmony with each other,” says planetary scientist John Moores of York University in Toronto. He and his colleagues lay out the theory’s details online August 20 in Geophysical Research Letters.

Since 2003, several spacecraft have detected varying amounts of methane on Mars (SN: 1/15/09). NASA’s Curiosity rover, which landed in Gale crater in 2012, has found that amounts of the gas rise and fall in a seasonal cycle (SN: 6/7/18).

Methane should last no more than about 300 years in the Martian atmosphere before sunlight breaks it down. “To see a seasonal cycle tells you that something is actively producing or destroying methane in the present time,” Moores says. Microbes produce methane on Earth, so finding the gas on the Red Planet has been seen as a possible sign of life — although not a definitive one.

Methane “can be produced by abiotic processes,” says Dorothy Oehler, a planetary geologist and astrobiologist with the Planetary Science Institute who is based in Houston. “But even if it’s not directly related to biology, it can enhance habitability for other kinds of microbes. So it’s an important thing to be seeking,” says Oehler, who was not involved in the new study.

Curiosity measured average methane concentrations of 0.41 parts per billion inside Gale crater, a 154-kilometer-wide depression near Mars’ equator. So it was a surprise when the European Space Agency’s Trace Gas Orbiter, part of the ExoMars mission which arrived at Mars in 2016 (SN: 10/18/16), flew over Gale crater and found no methane at all. There could still be minuscule methane concentrations below 0.05 parts per billion in the atmosphere that the Trace Gas Orbiter can’t smell, the satellite team reported in Nature April 10.

Even so, “it is hard to reconcile those” different findings, Moores says. If Mars is oozing enough methane that Curiosity would sense so much, there should be enough methane in the atmosphere for the orbiter to detect.

But Moores’ team noticed a coincidence: Curiosity took all its methane measurements at night, when the rover is standing still and charging its batteries. Night could also mark a time when gases mix differently in the Martian atmosphere than they do in the daytime, the team realized.

During the day, sunlight warms the air, creating currents and convection that mix different molecules together. So, methane in the daytime atmosphere can get mixed up and diluted. But overnight, the air calms and methane could build up near the surface, where Curiosity can sniff it. At sunrise, the methane would get diluted again.

The idea is plausible, and the argument in the paper is convincing, says planetary scientist Sébastien Viscardy of the Royal Belgian Institute for Space Aeronomy in Brussels, a member of the Trace Gas Orbiter team. But the theory doesn’t explain everything, he says.

For one thing, Moores and colleagues calculated that, to be consistent with both measurements, only 27,000 square kilometers of Mars’ surface should emit methane at a constant rate. That’s an area equivalent to 1½ Gale craters.

And “it’s difficult to imagine that only Gale emits methane,” Moores says. “Either Gale is even more special than we imagine, or there’s something we’re missing in the chemistry of the atmosphere.”

The bigger problem, Viscardy says, is that the different nighttime methane concentrations detected don’t explain the periodic spikes in methane that Curiosity has also observed (SN: 4/28/15). Those spikes showed huge methane increases occurring at random intervals throughout Curiosity’s mission. The most recent spike, in June 2019, was the largest plume yet observed, with methane levels of about 20 parts per billion, about 50 times above the seasonal average. Within a few days, though, detected methane levels returned to normal.

So the daily change in atmospheric mixing “solves a very small problem, but not the global methane story,” Viscardy says.