One of the most important science questions today may be “is there life on Mars?” Early images from Mariner 4, 6-7, and 9 made it clear that Mars is a modern-day desert, and no cities or seas dot its surface. Subsequent experiments by the Viking landers are generally interpreted as indicating there isn’t microbial life on Mars, but the results of those experiments are still actively debated more than forty years later. 

Since those early experiments, we’ve learned several important facts about Mars: It had a watery surface in the past, it has subsurface ice and possibly subsurface saltwater, and microbial life on Earth has figured out how to survive in Mars-like environments. 

There have also been confusing observations of methane made by the Mars Curiosity Rover. Methane is destroyed by sunlight over time, and any methane that is observed has to be freshly produced. Methane has two major sources, biology and geology, and we had thought Mars was both biologically dead and geologically dead. This methane detection indicates that either life, geologic activity related to magma and volcanic, or both exist. I personally am rooting for the “or both.” 

The reason these results are confusing is that there are extremely sensitive atmospheric sensors orbiting Mars that haven’t been able to replicate these detections. There is every reason to believe both the orbiter and rover are working perfectly. So how can both be right? 

It turns out that measuring these gases is an energy-intensive process, so the orbiting ExoMars Trace Gas Orbiter, with its solar panels, was making its measurements during the day. Mars Curiosity, with its radio-thermal generator, was taking its measurements at night when other instruments weren’t active and the detector could have all its power to itself. 

In new models from a team led by Christopher Webster, scientists have shown that in the cool Martian night, the air is still, and methane can build up near the surface, where Curiosity is sitting. During the day, the circulation of the atmosphere scatters that methane to undetectable levels. To prove this theory, the team had Curiosity do something new: it looked for methane during the Martian day, and it didn’t find it. 

Over two days and one night, Curiosity repeatedly sampled for methane, and during daylight, it wasn’t there in sufficient amounts to be detected. This not only explains ExoMars’ lack of detection, but it also reminds us that methane is, somehow, seeping to Mars’ surface regularly. Now, we just need to sort out how.

More Information

NASA JPL press release

“Day-night differences in Mars methane suggest nighttime containment at Gale crater,” Christopher R. Webster et al., 2021 April 29, Astronomy & Astrophysics