Martian methane baffles scientists: Curiosity rover's surprising discovery

A recent research paper may help explain why a mobile chemistry laboratory exists NASAThe Curiosity spacecraft constantly sniffed traces of gas near the surface of Gale Crater.

The most surprising discovery from NASA's Curiosity Mars Rover — that methane is seeping from the surface of Gale Crater — has scientists scratching their heads.

Living organisms produce most of the methane on Earth. But scientists have found no convincing signs of current or ancient life MarsSo I didn't expect to find methane there. However, Curiosity's mobile chemistry laboratory, known as SAM, or Sample Analysis at Mars, constantly smells traces of gas near the surface of Gale Crater, the only place on Mars where methane has been detected so far. Scientists assume that its possible source is geological mechanisms that include water and rocks deep within the Earth.

Filled with salt lakes in South America's Altiplano region, the Quisquiro brine represents the type of landscape that scientists believe might have existed in Gale Crater on Mars, which NASA's Curiosity Rover is exploring. Credit: Maxim Bocharov

If that were the whole story, things would be easy. However, SAM has found that methane is behaving in unexpected ways in Gale Crater. It appears at night and disappears during the day. It fluctuates seasonally, sometimes rising to levels 40 times higher than normal. Surprisingly, methane also does not accumulate in the atmosphere: European Space Agency (ESA). European Space AgencyThe ExoMars Trace Gas Orbiter, which was sent to Mars specifically to study the gas in the atmosphere, did not detect any methane.

Why do some scientific instruments detect methane on the Red Planet while others do not?

“It's a story with a lot of plot twists,” said Ashwin Vasavada, a Curiosity project scientist at NASA's Jet Propulsion Laboratory in Southern California, who is leading the Curiosity mission.

Methane keeps Mars scientists busy with laboratory work and computer modeling projects aimed at explaining why the gas behaves strangely and is only detected in Gale Crater. A NASA research group recently shared an interesting proposal.

This is a sample of imaginary Martian regolith, which is “soil” made of crushed rocks and dust. It is one of five samples that scientists have injected with different concentrations of a salt called perchlorate that is widespread on Mars. They exposed each sample to Mars-like conditions in a Mars simulation chamber at NASA's Goddard Space Flight Center in Greenbelt, Maryland. The brittle masses in the sample above show that a salt seal did not form in this sample because the salt concentration was too low. Image credit: NASA/Alexander Pavlov

Reporting in the March paper on Journal of Geophysical Research: PlanetsThe group suggested that methane — regardless of how it is produced — could be trapped beneath hardened salt that may form in Martian regolith, a “soil” made of crushed rocks and dust. When temperatures rise during warmer seasons or times of day, weakening the seal, methane can leak.

Led by Alexander Pavlov, a planetary scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, the researchers point out that the gas could also explode in jets when the seals crack under the pressure of, say, a rover the size of a small SUV driving over it. . Pavlov said the team's hypothesis may help explain why methane has only been detected in Gale Crater, given that it is one of two places on Mars where the robot roams and drills the surface. (The other is Jezero Crater, where NASA's Perseverance rover operates, although that rover does not have a methane detector.)

This image is of another sample of the mock Martian “soil” after it was removed from the Mars simulation chamber. The surface is sealed with a hard crust of salt. Alexander Pavlov and his team found that the seal forms after the sample has spent three to 13 days under Mars-like conditions, and only if its perchlorate salt concentration is between 5% and 10%. The color is lighter in the center where the sample has been scratched with a metal pick. The lighter color indicates the presence of drier soil beneath the top layer, which absorbed moisture from the air once the sample was removed from the simulation chamber, and turned brown. Image credit: NASA/Alexander Pavlov

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

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

Permanent ice on Mars and Earth

“We didn't think much about it at the moment,” Pavlov said, but he remembers the soil crust in 2019, when SAM's tunable laser spectrometer detected a methane explosion No one can explain.

“That's when the idea came to my mind,” Pavlov said. That's when he and his team began testing the conditions under which hard salt seals could form and break.

Curiosity set out to answer the question: Did Mars have the right environmental conditions to support tiny life forms called microbes? Early in its mission, Curiosity's science instruments found chemical and mineralogical evidence of past habitable environments on Mars. It continues to explore the rocky record from a time when Mars could have been home to microbial life. Credit: NASA

Pavlov's team tested five samples of permafrost mixed with different concentrations of a salt called perchlorate, which is widespread on Mars. (There is likely no permafrost in Gale Crater today, but the seals could have formed long ago when Gale was colder and more icy.) Scientists exposed each sample to different temperatures and air pressures inside NASA Goddard's Mars Simulation Chamber. .

Periodically, Pavlov's team injected neon, an isotope of methane, under the soil sample and measured the pressure of the gas below and above it. High pressure below the sample indicates that gas was trapped. Ultimately, the seal formed under Mars-like conditions in just three to 13 days in samples containing a 5% to 10% concentration of perchlorate.

This is a much higher salt concentration than what Curiosity measured in Gale Crater. But the regolith there is rich in a different type of salt mineral called sulfate, which Pavlov's team next wants to test to see if they can also form seals.

The Curiosity rover has reached an area believed to have formed when Mars' climate was drying out.

Improving our understanding of methane generation and destruction processes on Mars is a key recommendation from NASA's 2022 planetary mission senior reviewTheoretical work such as Pavlov's is crucial to this effort. However, scientists say they also need more consistent measurements of methane.

SAM only smells methane a few times a year because it is busy doing its primary job of drilling samples from the surface and analyzing their chemical composition.

“Methane experiments are resource-intensive, so we have to be very strategic when we decide to conduct them,” said SAM principal investigator Charles Malespin of Goddard University.

However, scientists say that testing how often methane levels rise, for example, will require a new generation of surface instruments that continuously measure methane from many locations across Mars.

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

Reference: “Formation and stability of saline soil seals under Mars-like conditions.” Implications of methane fluctuations on Mars by Alexander A. Pavlov, James Johnson, Raul Garcia Sanchez, Ariel Segelnytsky, Chris Johnson, Jeffrey Davis, Scott Gosiewicz, and Prabhakar Misra, 09 March 2024, Journal of Geophysical Research: Planets.
doi: 10.1029/2023JE007841

Curiosity is built by Jet Propulsion Laboratory, which is administered by the California Institute of Technology in Pasadena, California. JPL is leading the mission on behalf of NASA's Science Mission Directorate in Washington.