The Martian Methane Contradiction Has Been Resolved, But We Still Don’t Know the Cause

The Curiosity Rover has been cleared both of recording methane spikes that didn't exist, or releasing the methane itself. NASA/JPL-Caltech/MSSS edited Jeffrey Brief

Planetary scientists have been scratching their heads over apparent discrepancies between methane concentrations recorded by the Curiosity Rover and the ExoMars Trace Gas Orbiter. It was assumed that one must be wrong, but there was fierce disagreement over which. New research shows both readings were right; the differences represented the timing of their measurements.

The news will be a great relief to those responsible for each machine, and is a necessary step towards finding whether the methane is a by-product of life or the result of some geological process.

Curiosity has been recording spikes in methane concentration for years. On Earth methane is often, but not always, a by-product of methanogenic microbes so these have aroused great excitement. However, when the Orbiter didn’t record the same thing, speculation arose the detector was malfunctioning. There was even a theory Curiosity was releasing the methane it was reporting – smelling its own farts, one might say.

However, York University Canada’s Dr John Moores noted Curiosity’s samples were taken in the middle of the night, while ExoMars measures in daylight, and wondered if there was a daily pattern, in addition to the previously identified annual cycle. Moores persuaded the Curiosity team to take readings just before dawn, and demonstrated his hunch was right.

In Geophysical Research Letters Moores and Professor Penny King of the Australian National University fit the observations together. King explained to IFLScience that during the Martian day convection causes air to rise and the atmosphere to expand, before contracting again at night. “Earth’s atmosphere does the same,” she added, “but to a much lesser degree.” This phenomenon was well known, but no one else connected it to the methane measurements.

The night-time atmospheric shrinkage concentrates the small amount of methane present in Mars’ atmosphere near the ground where Curiosity samples it, explaining its higher readings. King told IFLScience Curiosity took its measurements at night because many of its other functions only operate in daytime, so processes that can happen at any time are shunted to the hours of darkness to avoid interfering.

Moores and King used the combined data from the two sets of measurements to calculate that Gale Crater, which Curiosity is exploring, is releasing 2.8 kilograms (6.2 pounds) of methane every Martian day. Given Gale’s 154-kilometer (100-mile) diameter, that’s a tiny but significant amount in the thin Martian atmosphere.

As to what is behind the methane, we still don’t know. “Some microbes on Earth can survive without oxygen, deep underground, and release methane as part of their waste,” King said in a statement. “The methane on Mars has other possible sources, such as water-rock reactions or decomposing materials containing methane.” Now, however, we are much better placed to start looking.

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