Scientists finally seem to know what’s going on with methane on Mars

In 2019, NASA’s Mars rover Curiosity discovered methane on Mars. But how it gets there and why it behaves so strangely was unclear. More is now known about this.

On Earth, almost all methane is produced by living things, such as cows. But as far as we know, little life has been found on Mars, let alone that it can also produce methane. So it was quite surprising that methane was found at the Gale crater on the red planet, where Curiosity set foot more than a decade ago. The NASA researchers assumed that there must be some mechanisms involving water and rocks deep underground that could release methane.

But that’s not the whole story. The Mars rover has discovered that the methane behaves in a strange way. It appears at night and disappears during the day. It also fluctuates seasonally, sometimes peaking to levels 40 times higher than normal. Moreover, the methane does not accumulate in the atmosphere, as it does on Earth.

The solution
Scientists are working to find an explanation for the gas’s unusual behavior and why it only occurs in Gale Crater. Recently they came up with an interesting idea. The methane may be trapped under hardened salt that forms in Mars’ regolith. This is the surface, which consists of loose stones, dust and other grit. As temperatures rise in the warmer seasons or at certain times of the day, the salt softens and allows methane to seep through.

The gas may also be released through cracks in the ground, which develop under pressure from, for example, a Mars rover. That would also explain why gas has only been detected at Gale Crater. It is one of two places on Mars where a robot drills into the surface. This also happens at the Jezero crater, but that rover does not have an instrument on board to detect methane.

An old experiment
The theory stems from a 2017 experiment in which scientists grew microorganisms in simulated Martian permafrost to which salt had been added, as is the case on Mars itself. It was then tested whether certain bacteria that live in salt water on Earth could also exist on Mars. The outcome remained unclear, but the researchers did discover something else: the top layer formed a salt crust, as salt ice sublimes, or changes from solid to gaseous, leaving salt behind.

“We didn’t think much of it at the time,” says NASA’s Alexander Pavlov, but when the Mars rover detected a methane burst in 2019 that no one could explain, it came back to his mind. He and his team then began testing the conditions that could form and crack hard salt crusts. They added varying amounts of perchlorate, a salt common on Mars, to the permafrost.

Different salt concentrations
Today, there is no longer any permafrost in Gale Crater, but the salt crust may have formed long ago when it was colder and icier. The scientists then exposed the samples to different temperatures and air pressure to see what happened. Periodically, the scientists injected neon, a methane analog, under the crust and measured the gas pressure above and below. Higher pressure under the sample implied that the gas was trapped. And what the researchers expected also happened: in Mars-like conditions, a salt crust formed within three to thirteen days, but only in samples with 5 to 10 percent perchlorate.

That is a much higher salt concentration than Curiosity measured in Gale Crater. But the soil there is rich in a different type of salt minerals called sulfates. The researchers now want to test whether sulphate can also form such salt crusts.

Waiting for the future
But in addition to these theoretical exercises, field research is also needed. However, Curiosity only searches specifically for methane twice a year, because the rest of the time it spends drilling into the surface of Mars to analyze its chemical composition. “Methane experiments are very intensive, so we have to think carefully about when we want to do them,” responds a researcher.

The technology is also not yet developed far enough. For example, to test how often methane levels peak, a new generation of instruments will be needed that can continuously measure methane at multiple locations on Mars. “Some of the methane work is for future spacecraft that are more focused on answering these specific questions,” it said.