Of all the discoveries made on Mars, the detection of methane blowing in the air is probably the most compelling. It's a mysterious finding that could have historic implications. You see, on Earth, methane is a well-known byproduct of metabolizing life-forms. On Mars, that could mean – *drumroll please* – aliens!
Or, it might mean nothing at all.
The ongoing Mars methane saga hit the headlines again when NASA announced a measurement made by NASA's Mars rover Curiosity, which has the ability to precisely measure the components of the Martian air. The one-ton wheeled rover is currently trundling up the slopes of Mount Sharp, a 3-mile (nearly 5-kilometer) high mountain in the middle of Gale Crater, where eons of Mars geological history are exposed on its surface for the sophisticated robot to study.
This most recent discovery, however, isn't in the rocks and dust, it's in the atmosphere. Curiosity was able to sniff-out a record-breaking "spike" of methane. Using its Sample Analysis at Mars (SAM) tunable laser spectrometer, Curiosity detected a brief burst of methane with a concentration of 21 parts per billion units per volume (ppbv). That quantity of methane detected may sound minuscule, but on Mars, it's a big deal.
"The methane mystery continues," said Ashwin Vasavada, Curiosity's project scientist at NASA's Jet Propulsion Laboratory in Pasadena, California, in a statement. "We're more motivated than ever to keep measuring and put our brains together to figure out how methane behaves in the Martian atmosphere."
This most recent detection is even more vexing as, when mission scientists carried out a repeat analysis only days after the record-breaking detection, methane concentrations had dove back to just one ppbv, a background level of the gas that Curiosity measures routinely.
So, What's Going on With Mars Methane?
On Earth, which possesses a vast and complex ecosystem of trillions of life-forms, the concentrations of methane are well studied, and, because of human activity, methane levels are increasing – most recently peaking to global methane levels of 1,866 ppbv, the highest it has been for 800,000 years. (This is of concern to the future of our planet, as methane is a potent greenhouse gas.) Although the 21 ppbv Mars measurement may sound anemic in comparison, an accumulation of the stuff, no matter how short-lived, may be an indicator of life.
Before scientists can declare Mars methane being "proof of tiny Martians," they have to work out whether it's methane-parping microbes hiding underground or something less exciting. Unfortunately, although Curiosity can detect very low concentrations of methane, it can't decipher what produced it. (And no, flatulent Mars cows don't exist and therefore cannot be considered as a contributing factor.)
"With our current measurements, we have no way of telling if the methane source is biology or geology, or even ancient or modern," said Paul Mahaffy, in a statement. Mahaffy is the SAM principal investigator at NASA's Goddard Spaceflight Center in Greenbelt, Maryland.
Microbes or Geology?
Methanogens are a group of well-studied single-celled microorganisms on Earth that can thrive in oxygen-deprived environments (in fact, oxygen can be toxic to many of these microbes). They live in wet places (like marshes) and populate the digestive tracts of animals (yes, including cows and humans) generating methane as they metabolize carbon dioxide and molecular hydrogen for energy. There are also nonbiological, geochemical production mechanisms, however. For example, should water react with the minerals in rock, serpentinization may occur, from which methane is a byproduct.
The majority of Earth's methane production comes from organic processes, but what about Mars? Well, simply put, we just don't know. It could be that subsurface water is slowly reacting with the rock beneath Curiosity, which produces methane that leaks to the surface. On the flip side, there might be huge colonies of methanogen-like Mars microbes driving biological methane production that leaks to the surface in bursts – something that might explain the sharp spike in methane concentration when Curiosity made the measurement.
One thing is for certain: Methane is a highly unstable molecule when exposed to ultraviolet light. Without an active production mechanism, methane would be nonexistent in the Martian atmosphere because the sun would have eradicated it, so scientists are confident that something is actively producing the gas, whether that be via geological or biological processes.
Lots of Questions About Mars Methane, Few Answers
Although every Mars methane story makes for exciting "life on Mars" headlines, it's comparatively old news. The gas was first detected by the European Space Agency's Mars Express orbiter in 2004. This initial detection was corroborated in 2011 after Earth-based infrared observations detected small quantities of the gas. Things got really interesting a couple of years after Curiosity touched down on the red planet. The rover had only detected low quantities of methane since landing in 2012, but in 2014, it measured a significant spike in methane concentrations. By 2018, NASA had reported that the background levels of methane would wax and wane seasonally. Currently, scientists are uncertain as to what could be causing these sporadic spikes in methane emissions, though terrestrial methane contamination in Curiosity's instruments has yet to be ruled out.
The upshot is that more work is needed before we can definitively understand where Mars' methane is coming from – let alone whether gassy bacteria are generating it. With the help of the European Trace Gas Orbiter (TGO), which arrived in Mars orbit in late 2016, scientists hope to combine orbital measurements with the detections on the ground to build a better picture of methane emissions. Though, at time of writing, the TGO has yet to detectany methane – a nondetection that only creates more questions than answers around Mars' wonderfully mystifying methane story.