Scientists are now a step closer to solving the mystery of methane gas detected by Nasa’s Curiosity rover on Mars. On Earth, a significant amount of methane is produced by microbes that help livestock digest their food and the process ends with cattle releasing the gas into the air while exhaling or burping.
Curiosity rover’s repeated detections of methane right above the surface of Gale Crater has captivated scientists and non-scientists alike as it may imply that microbes were, or are, present on the Red planet. It is also, however, possible that the gas was produced due to the geological processes that involve the interaction of rocks, water, and heat.
But the mystery of methane gas intensified after ExoMars Trace Gas Orbiter sent by the European Space Agency (ESA) failed to detect any methane higher in the Martian atmosphere.
Chris Webster, a senior research scientist at Nasa’s Jet Propulsion Laboratory (JPL), said he was fully expecting the Trace Gas Orbiter team to report that there’s a small amount of methane everywhere on Mars when the orbiter came onboard in 2017.
“But when the European team announced that it saw no methane, I was definitely shocked,” said Webster, lead of the Tunable Laser Spectrometer (TLS) instrument in the Sample Analysis at Mars (SAM) chemistry lab aboard the Curiosity rover.
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The TLS has measured less than one-half part per billion (in volume) of methane on average in Gale Crater, punctuated by spikes of up to 20 parts per billion in volume. The contradictory data from the high precision of Curiosity’s TLS and the European orbiter, designed to be the gold standard for measuring methane and other gases over the whole planet, baffled the scientists.
John E Moores, a planetary scientist from York University in Toronto, published an intriguing prediction in 2019 asking “what if Curiosity and the Trace Gas Orbiter both are right?” The hypothesis suggested that the discrepancy between methane measurements comes down to the time of day they’re taken.
TLS operates mostly at night when no other Curiosity instruments are working since it needs a lot of power. Moores noted that the Martian atmosphere is calm at night, so the methane seeping from the ground builds up near the surface where Curiosity can detect it. On the other hand, the European orbiter requires sunlight to pinpoint methane about 5 kilometres above the surface.
During the daytime, warm air rises and cool air sinks. The methane gas detected near the Martian surface at night gets mixed into the atmosphere during the day, getting diluted to undetectable levels.
The Curiosity team decided to test Moores’ prediction and powered TLS over the course of one Martian day, bracketing one night-time measurement with two daytime ones. SAM sucked in Martian air for two hours, leaving a concentrated sample of methane that TLS could easily measure by passing an infrared laser beam through it many times
“John predicted that methane should effectively go down to zero during the day, and our two daytime measurements confirmed that,” said Paul Mahaffy, the principal investigator of SAM.
While the scientists were able to solve the mystery behind variations, they are yet to solve the global methane puzzle at Mars. According to Nasa, methane is a stable molecule that is expected to last on Mars for about 300 years before getting torn apart by solar radiation. But scientists suspect that something is destroying methane on Mars in less than 300 years, given the constant seeping of methane from similar craters should have accumulated enough in the atmosphere for the Trace Gas Orbiter to detect.
