Martian Methane Baffles Scientists: Curiosity Rover’s Surprising Discovery

If that were the whole story, things would be easy. However, SAM has found that methane behaves in unexpected ways in Gale Crater. It appears at night and disappears during the day. It fluctuates seasonally and sometimes spikes to levels 40 times higher than usual. Surprisingly, the methane also isn’t accumulating in the atmosphere: ESA’s (the Why do some science instruments detect methane on the Red Planet while others don’t?

“It’s a story with a lot of plot twists,” said Ashwin Vasavada, Curiosity’s project scientist at NASA’s Jet Propulsion Laboratory in Southern California, which leads Curiosity’s mission.

Methane keeps Mars scientists busy with lab work and computer modeling projects that aim to explain why the gas behaves strangely and is detected only in Gale Crater. A NASA research group recently shared an interesting proposal.

This is a sample of mock Martian regolith, which is “soil” made of broken rock and dust. It’s one of five samples that scientists infused with varying concentrations of a salt called perchlorate that’s widespread on Mars. They exposed each sample to Mars-like conditions in the Mars simulation chamber at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. The brittle clumps in the sample above show that a seal of salt did not form in this sample because the concentration of salt was too low. Credit: NASA/Alexander Pavlov

Reporting in a March paper in the Journal of Geophysical Research: Planets, the group suggested that methane — no matter how it’s produced — could be sealed under solidified salt that might form in Martian regolith, which is “soil” made of broken rock and dust. When temperature rises during warmer seasons or times of day, weakening the seal, the methane could seep out.

Led by Alexander Pavlov, a planetary scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, the researchers suggest the gas also can erupt in puffs when seals crack under the pressure of, say, a rover the size of a small SUV driving over it. The team’s hypothesis may help explain why methane is detected only in Gale Crater, Pavlov said, given that’s it’s one of two places on Mars where a robot is roving and drilling the surface. (The other is Jezero Crater, where NASA’s Perseverance rover is working, though that rover doesn’t have a methane-detecting instrument.)

This image is of another sample of mock Martian “soil” after it was removed from the Mars simulation chamber. The surface is sealed with a solid crust of salt. Alexander Pavlov and his team found that a seal formed after a sample spent three to 13 days under Mars-like conditions, and only if it had 5% to 10% perchlorate salt concentration. The color is lighter in the center where the sample was scratched with a metal pick. The light color indicates a drier soil underneath the top layer, which absorbed moisture from the air as soon as the sample was removed from the simulation chamber, turning brown. Credit: NASA/Alexander Pavlov

Pavlov traces the origin of this hypothesis to an unrelated experiment he led in 2017, which involved growing microorganisms in a simulated Martian permafrost (frozen soil) infused with salt, as much of Martian permafrost is.

Pavlov and his colleagues tested whether bacteria known as halophiles, which live in saltwater lakes and other salt-rich environments on Earth, could thrive in similar conditions on Mars.

The microbe-growing results proved inconclusive, he said, but the researchers noticed something unexpected: The top layer of soil formed a salt crust as salty ice sublimated, turning from a solid to a gas and leaving the salt behind.

Permafrost on Mars and Earth

“We didn’t think much of it at the moment,” Pavlov said, but he remembered the soil crust in 2019, when SAM’s tunable laser spectrometer detected a methane burst no one could explain.

“That’s when it clicked in my mind,” Pavlov said. And that’s when he and a team began testing the conditions that could form and crack hardened salt seals.

Curiosity set out to answer the question: Did Mars ever have the right environmental conditions to support small life forms called microbes? Early in its mission, Curiosity’s scientific tools found chemical and mineral evidence of past habitable environments on Mars. It continues to explore the rock record from a time when Mars could have been home to microbial life. Credit: NASA

Pavlov’s team tested five samples of permafrost infused with varying concentrations of a salt called perchlorate that’s widespread on Mars. (There’s likely no permafrost in Gale Crater today, but the seals could have formed long ago when Gale was colder and icier.) The scientists exposed each sample to different temperatures and air pressure inside a Mars simulation chamber at NASA Goddard.

Periodically, Pavlov’s team injected neon, a methane analog, underneath the soil sample and measured the gas pressure below and above it. Higher pressure beneath the sample implied the gas was trapped. Ultimately, a seal formed under Mars-like conditions within three to 13 days only in samples with 5% to 10% perchlorate concentration.

That’s a much higher salt concentration than Curiosity has measured in Gale Crater. But regolith there is rich in a different type of salt minerals called sulfates, which Pavlov’s team wants to test next to see if they can also form seals.

Curiosity rover has arrived at a region believe to have formed as Mars’ climate was drying.

Improving our understanding of methane generation and destruction processes on Mars is a key recommendation from the 2022 NASA Planetary Mission Senior Review, and theoretical work like Pavlov’s is critical to this effort. However, scientists say they also need more consistent methane measurements.

SAM sniffs for methane only several times a year because it is otherwise busy doing its primary job of drilling samples from the surface and analyzing their chemical makeup.

“Methane experiments are resource intensive, so we have to be very strategic when we decide to do them,” said Goddard’s Charles Malespin, principal investigator for SAM.

Yet, to test how often methane levels spike, for instance, would require a new generation of surface instruments that measure methane continuously from many locations across Mars, scientists say.

“Some of the methane work will have to be left to future surface spacecraft that are more focused on answering these specific questions,” Vasavada said.

Reference: “Formation and Stability of Salty Soil Seals in Mars-Like Conditions. Implications for Methane Variability on Mars” by Alexander A. Pavlov, James Johnson, Raul Garcia-Sanchez, Ariel Siguelnitzky, Chris Johnson, Jeffrey Davis, Scott Guzewich and Prabhakar Misra, 09 March 2024, Journal of Geophysical Research: Planets.
DOI: 10.1029/2023JE007841

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