To Martian methane, there is a season.
NASA’s Curiosity rover has found evidence that methane in Mars’ thin atmosphere varies during the year. Higher concentrations appear in late summer and early autumn in the northern hemisphere and lower concentrations in the winter and spring, researchers report in the June 8 Science.
What’s more, Curiosity also spotted organic molecules previously unseen on Mars preserved in mudstone, some of the same researchers report in another study in the same issue of Science. Although neither methane nor organics alone are signs of life, the implications for astrobiology are “potentially huge,” says planetary scientist Michael Mumma of NASA’s Goddard Space Flight Center in Greenbelt, Md., who was not involved in the studies.
In 2004, Mumma and colleagues reported the first observation of huge plumes of methane spewing into Mars’ atmosphere (SN: 2/14/09, p. 10). These plumes, detected with Earth-based telescopes, had methane concentrations as high as 45 parts per billion.
That finding was exciting, because methane doesn’t last long in the Martian atmosphere before ultraviolet radiation from the sun destroys it. Something must have been creating or releasing the gas as astronomers watched. On Earth, most methane is produced by living creatures, so the plumes raised hopes that Mars supports life.
When Curiosity landed on the Red Planet in 2012, however, the rover initially found no methane to speak of (SN: 10/19/13, p. 7). “A lot of people were disappointed and upset,” says Christopher Webster, a planetary scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., and a coauthor of the new methane study. But in 2014, after more searching, the Curiosity team found traces of methane, though much less than what was expected based on the earlier results (SN: 1/10/15, p. 11).
Now after two full Martian years (five Earth years) of observing, the team reports that the annual average concentration of methane in Mars’ atmosphere is 0.41 ppb. But methane levels seem to rise and fall with the seasons, ranging from 0.24 ppb in winter to 0.65 ppb in summer. The researchers also saw relatively large methane spikes, up to about 7 ppb, at apparently random intervals.
Slow seepage from an underground reservoir could explain both the seasonal cycle and the spikes, Webster says. Surface rocks could mostly hold on to the methane in winter and release it when warmed by the summer sun. Occasionally, something in the rocks could break loose, releasing larger spurts. Similar scenarios are found on Earth.
Scientists can’t say what produced the stored methane in the first place. “The existence and behavior of methane on Mars remains puzzling,” Webster says. “While we think it likely that it’s produced abiologically [by a geologic process], we cannot rule out the possibility of a biological or microbial source.”
Though lower, the concentrations of methane in the spikes that Curiosity sees are still consistent with the huge plumes seen from Earth, Mumma says, if Curiosity is located at the edge of a plume. But he’s not sure if a seasonal cycle is the only explanation for the data. A flat, constant methane level could fit within the errors of the measurements, too, he argues.
Webster disagrees. “Even to the untrained eye,” he says of the results, “there is a clear, repeatable rise in the summertime…. The seasonal cycle is real.”
In the other new paper, astrobiologist Jennifer Eigenbrode of NASA Goddard and colleagues analyzed samples collected from 3.5-billion-year-old mudstone that was once part of an ancient lake and found chemical evidence that plenty of organic molecules had been preserved in the lake bed.
In 2014, Curiosity had detected organic molecules in rocks from one location in Gale crater. The new finding, from samples drilled at the base of a mountain in the crater’s center, shows signs of larger and more complex organic molecules than had been seen before, including some that are similar to coal and black shale found on Earth.
“There were a lot of people who didn’t think we were going to find organic matter using the drill on the Curiosity rover, because it only goes down five centimeters,” Eigenbrode says. The Martian surface is bombarded with radiation that can break up organic molecules. The fact that organics survive on the surface means digging deeper may yield even more.
The European Space Agency’s ExoMars rover, slated to launch in 2020, will drill two meters into the surface. “This opens up the prospect that [the rover] might find better preserved organic material, and maybe find biosignatures” of life, Eigenbrode says.
Curiosity isn’t done drilling yet, though. The rover’s drill broke in 2016. But engineers successfully hacked the drill, which dug out a sample on May 20.