Planetary scientists perk up whenever methane is mentioned. Methane is produced by living things on Earth, so it’s considered to be a potential biosignature elsewhere. In recent years, MSL Curiosity detected methane coming from the surface of Gale Crater on Mars. So far, nobody’s successfully explained where it’s coming from.
NASA scientists have some new ideas.
Ever since Curiosity landed on Mars in 2012, it’s been sensing methane. But the methane displays some odd characteristics. It only comes out at night, it fluctuates with the seasons, and sometimes, the amount of methane jumps to 40 times more than the regular level.
The ESA’s ExoMars Trace Gas Orbiter entered a science orbit around Mars in 2018, and scientists fully expected it to detect methane in the planet’s atmosphere. But it didn’t, and it has never been detected elsewhere on Mars’ surface.
If life was producing the methane, it appears to be restricted to the subsurface under Gale Crater.
There’s no convincing evidence that life exists on Mars. It may have in the past, and it’s possible that some extant life clings to a tenuous existence in subsurface brines or something. But we lack evidence, so life is basically ruled out as the methane source. Especially since the evidence shows life would have to be under Gale Crater and nowhere else.
Scientists have been trying to determine the source of methane, but so far, they haven’t come up with a specific answer. It has something to do with subsurface geological processes involving water, most likely.
“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.
Alexander Pavlov is a planetary scientist at NASA’s Goddard Space Flight Center who leads a group of NASA scientists studying the Martian Methane Mystery. In recent research, they suggested that the methane is stored underground. They didn’t explain what produced it, but they showed that methane can be sealed underground by salt solidified in the Martian regolith.
They suggested that the methane could be released from its subsurface reservoir by the weight of the Curiosity rover itself. The rover’s weight could break the salt seal and release methane in puffs. That’s an interesting proposition, but it doesn’t explain the seasonal and diurnal fluctuations. That makes sense since the Gale Crater is one of only two regions where a rover is working. The other is Jezero Crater, where the Perseverance Rover is working, but it doesn’t have a methane detector. (Neither will the ESA’s Rosalind Franklin rover, which is scheduled to land on Mars in 2029.)
The research group addressed those fluctuations by suggesting that seasonal and daily heating could also break the seal and release methane.
Their potential explanations stem from research Pavlov conducted in 2017. He grew bacteria called halophiles, which grow in salty conditions, in simulated Martian permafrost. The simulated soil was infused with salt, replicating conditions on much of Mars. The microbe growth was inconclusive, but the researchers noticed something else. As the salty ice sublimated, a layer of solidified salt remained, forming a crust.
“We didn’t think much of it at the moment,” Pavlov said.
But he remembered it when MSL Curiosity detected an unexplained burst of methane on Mars in 2019.
“That’s when it clicked in my mind,” Pavlov said. Then, he and a team of researchers began testing conditions that could form the hardened salt seals and then break them open.
Perchlorate is a chemical salt that’s widespread on Mars. Pavlov and his fellow researchers recreated different simulated Martian permafrosts with varying amounts of perchlorate. Inside a Mars simulation chamber, they subjected the samples to different temperatures and atmospheric pressures to see if they would form seals.
In their experiments, they used neon as a methane analog and injected it under the soil. Then, they measured the gas pressure below and above the soil. They found that the pressure was higher under the soil, meaning the gas was being trapped by the salty permafrost. Furthermore, they found that seals formed in samples containing as little as 5% or 10% perchlorate, and they formed within 3 to 13 days. Those are compelling results.
While 5-10% perchlorate doesn’t sound like much, it’s actually a higher concentration than in Gale Crater, where the methane has been detected. But perchlorate isn’t the only salt in Martian regolith. It also contains sulphates, another type of salt mineral. Pavlov says he and his team will test sulphates next for their ability to form a seal.
The Martian Methane Mystery is commanding a lot of attention. It’s a juicy mystery, and once it’s solved, our understanding of methane as a biosignature or false positive will be much improved. NASA’s 2022 Planetary Mission Senior Review recommended that the issue of methane production and destruction at Mars be investigated further.
The type of work that Pavlov and his colleagues are doing is important, but it’s being held back. Pavlov says that they need more consistent methane measurements. The problem is that Curiosity’s SAM (Sample Analysis at Mars) instrument, which senses the methane, is busy with other tasks. It only checks for methane a few times per year. It’s mostly occupied with drilling samples and testing them, a critical and time-consuming part of the rover’s mission.
“Methane experiments are resource intensive, so we have to be very strategic when we decide to do them,” said Goddard’s Charles Malespin, SAM’s principal investigator.
Curiosity’s mission wasn’t designed to measure methane fluctuations. In 2017, NASA said its SAM instrument only sampled the atmosphere 10 times in 20 months. That’s a very inconsistent sample that leaves lots of unanswered questions.
Scientists think another mission is needed to advance their understanding of Martian methane. Rather than one sensor taking irregular methane readings from one location, we need multiple testing stations on the surface that regularly monitor the atmosphere. Nothing like it is in the works.
“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.