NASA’s Curiosity Mars rover recently found a surprising result: the largest amount of methane ever measured during the mission — about 21 parts per billion units by volume (ppbv). One ppbv means that if you take a volume of air on Mars, one billionth of the volume of air is methane.

The finding came from the rover’s Sample Analysis at Mars (SAM) tunable laser spectrometer. It is exciting because microbial life is an important source of methane on Earth, but methane can also be created through interactions between rocks and water.

Curiosity does not have instruments that can definitively say what the source of the methane is, or even if it is coming from a local source within Gale Crater or elsewhere on the planet.

“With our current measurements, we have no way of telling if the methane source is biology or geology, or even ancient or modern,” said SAM Principal Investigator Paul Mahaffy of NASA’s Goddard Spaceflight Center in Greenbelt, Maryland.

The Curiosity team has detected methane many times over the course of the mission. Previous papers have documented how background levels of the gas seem to rise and fall seasonally. They have also noted sudden spikes of methane, but the science team knows very little about how long these transient plumes last or why they are different from the seasonal patterns.

A follow-up experiment a few days later revealed methane levels had sharply decreased, with less than 1 ppbv detected. That is a value close to the background levels Curiosity sees all the time.

The finding suggests that the earlier methane detection — the largest amount of the gas Curiosity has ever found — was one of the transient methane plumes that have been observed previously. While scientists have observed the background levels rise and fall seasonally, they have not found a pattern in the occurrence of these transient plumes.

“The methane mystery continues,” said Ashwin Vasavada, Curiosity’s project scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California. “We’re more motivated than ever to keep measuring and put our brains together to figure out how methane behaves in the Martian atmosphere.”

Curiosity does not have instruments that can definitively say whether the source of the methane is biological or geological. A clearer understanding of these plumes, combined with coordinated measurements from other missions, could help scientists determine where they are located on Mars.

For more about the Curiosity mission, please visit: https://mars.nasa.gov/msl/.