First You See It, Then You Don’t: Scientists Closer to Explaining Mars Methane Mystery

Reports of methane detections at Mars have captivated scientists and non-scientists alike. On Earth, a significant amount of methane is produced by microbes that help most livestock digest plants. This digestion process ends with livestock exhaling or burping the gas into the air.

While there are no cattle, sheep, or goats on Mars, finding methane there is exciting because it may imply that microbes were, or are, living on the Red Planet. Methane could have nothing to do with microbes or any other biology, however; geological processes that involve the interaction of rocks, water, and heat can also produce it.

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Before identifying the sources of methane on Mars, scientists must settle a question that’s been gnawing at them: Why do some instruments detect the gas while others don’t? NASA’s Curiosity rover, for instance, has repeatedly detected methane right above the surface of Gale Crater.

NASA’s Curiosity Rover Captures

But ESA’s (the European Space Agency) ExoMars Trace Gas Orbiter hasn’t detected any methane higher in the Martian atmosphere.

“When the Trace Gas Orbiter came on board in 2016, I was fully expecting the orbiter team to report that there’s a small amount of methane everywhere on Mars,” said Chris Webster, lead of the Tunable Laser Spectrometer (TLS) instrument in the Sample Analysis at Mars (SAM) chemistry lab aboard the Curiosity rover.

The TLS has measured less than one-half part per billion in the volume of methane on average in Gale Crater. That’s equivalent to about a pinch of salt diluted in an Olympic-size swimming pool. These measurements have been punctuated by baffling spikes of up to 20 parts per billion in volume.

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“But when the European team announced that it saw no methane, I was definitely shocked,” said Webster, who’s based at NASA’s Jet Propulsion Laboratory in Southern California.

The European orbiter was designed to be the gold standard for measuring methane and other gases over the whole planet. At the same time, Curiosity’s TLS is so precise, it will be used for early warning fire detection on the International Space Station and for tracking oxygen levels in astronaut suits. It’s also been licensed for use at power plants, on oil pipelines, and in fighter aircraft, where pilots can monitor the oxygen and carbon dioxide levels in their face masks.

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Still, Webster and the SAM team were jolted by the European orbiter findings and immediately set out to scrutinize the TLS measurements on Mars.

Some experts suggested that the rover itself was releasing the gas. “So we looked at correlations with the pointing of the rover, the ground, the crushing of rocks, the wheel degradation—you name it,” Webster said. “I cannot overstate the effort the team has put into looking at every little detail to make sure those measurements are correct, and they are.”

As the SAM team worked to confirm its methane detections, another member of Curiosity’s science team, planetary scientist John E. Moores from York University in Toronto, published an intriguing prediction in 2019. “I took what some of my colleagues are calling a very Canadian view of this, in the sense that I asked the question: ‘What if Curiosity and the Trace Gas Orbiter are both right?’” Moores said.

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Moores, as well as other Curiosity team members studying wind patterns in Gale Crater, hypothesized that the discrepancy between methane measurements comes down to the time of day they’re taken. Because it needs a lot of power, TLS operates mostly at night when no other Curiosity instruments are working. The Martian atmosphere is calm at night, Moores noted, so the methane seeping from the ground builds up near the surface where Curiosity can detect it.

The Trace Gas Orbiter, on the other hand, requires sunlight to pinpoint methane about 3 miles, or 5 kilometers, above the surface. “Any atmosphere near a planet’s surface goes through a cycle during the day,” Moores said. The heat from the Sun churns the atmosphere as warm air rises and cool air sinks.

Thus, the methane that is confined near the surface at night is mixed into the broader atmosphere during the day, which dilutes it to undetectable levels. “So I realized no instrument, especially an orbiting one, would see anything,” Moores said.

Immediately, the Curiosity team decided to test Moores’ prediction by collecting the first high-precision daytime measurements. TLS measured methane consecutively over the course of one Martian day, bracketing one nighttime measurement with two daytime ones.

With each experiment, SAM sucked in Martian air for two hours, continuously removing the carbon dioxide, which makes up 95% of the planet’s atmosphere. This left a concentrated sample of methane that TLS could easily measure by passing an infrared laser beam through it many times, one that’s tuned to use a precise wavelength of light that is absorbed by methane.

“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, who’s based at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

TLS’s nighttime measurement fit neatly within the average the team had already established. “So that’s one way of putting to bed this big discrepancy,” Mahaffy said.

While this study suggests that methane concentrations rise and fall throughout the day at the surface of Gale Crater, scientists have yet to solve the global methane puzzle at Mars. Methane is a stable molecule that is expected to last on Mars for about 300 years before getting torn apart by solar radiation.

If methane is constantly seeping from all similar craters, which scientists suspect is likely given that Gale doesn’t seem to be geologically unique, enough of it should have accumulated in the atmosphere for the Trace Gas Orbiter to detect. Scientists suspect that something is destroying methane in less than 300 years.

Experiments are underway to test whether very low-level electric discharges induced by dust in the Martian atmosphere could destroy methane, or whether abundant oxygen at the Martian surface quickly destroys methane before it can reach the upper atmosphere.  

“We need to determine whether there’s a faster destruction mechanism than normal to fully reconcile the data sets from the rover and the orbiter,” Webster said.

Banner image: This photo was taken on March 19, 2017, by the Mars Hand Lens Imager camera on the arm of NASA’s Curiosity rover. The image helped mission team members inspect the condition of Curiosity’s six wheels during the 1,641st Martian day, or sol, of the rover’s mission on Mars. Credits: NASA/JPL-Caltech/MSSS. 

Break in Raised Tread on Curiosity Wheel

Two of the raised treads, called grousers, on the left middle wheel of NASA’s Curiosity Mars rover broke during the first quarter of 2017, including the one has seen partially detached at the top of the wheel in this image from the Mars Hand Lens Imager (MAHLI) camera on the rover’s arm.

This image was taken on March 19, 2017, as part of a set used by rover team members to inspect the condition of the rover’s six wheels during the 1,641st Martian day, or sol, of Curiosity’s work on Mars.

Holes and tears in the wheels worsened significantly during 2013 as Curiosity was crossing terrain studded with sharp rocks on the route from near its 2012 landing site to the base of Mount Sharp.

Team members have used MAHLI systematically since then to watch for when any of the zig-zag-shaped grousers begin to break. The last prior set of wheel-inspection images from before Sol 1641 was taken on Jan. 27, 2017, (Sol 1591) and revealed no broken grousers.

Longevity testing with identical aluminum wheels on Earth indicates that when three grousers on a given wheel have broken, that wheel has reached about 60 percent of its useful life.

Curiosity has driven well over 60 percent of the amount needed for reaching all the geological layers planned as the mission’s science destinations, so the start of seeing broken grousers is not expected to affect the mission’s operations.

As with other images from Curiosity’s cameras, all of the wheel-inspection exposures are available in the raw images collections at http://mars.nasa.gov/msl/multimedia/raw/.

Curiosity’s six aluminum wheels are about 20 inches (50 centimeters) in diameter and 16 inches (40 centimeters) wide. Each of the six wheels has its own drive motor, and the four corner wheels also have steering motors.

MAHLI was built by Malin Space Science Systems, San Diego. NASA’s Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Mars Science Laboratory Project for the NASA Science Mission Directorate, Washington. JPL designed and built the project’s Curiosity rover. More information about Curiosity is online at http://www.nasa.gov/msl and http://mars.nasa.gov/msl/.