A Planetary Gas Leak
Inside the 154-kilometer-wide Gale Crater, NASA's Curiosity rover sniffs the thin Martian air. Its Sample Analysis at Mars (SAM) instrument suite contains a Tunable Laser Spectrometer, a device capable of detecting minute quantities of specific gases. Since 2012, SAM has been measuring methane, a molecule that on Earth is overwhelmingly a product of life.
The rover found that methane in Gale Crater behaves strangely. There is a consistent background level that cycles with the seasons, ranging from about 0.24 parts per billion by volume (ppbv) in the winter to 0.65 ppbv in the late summer. More perplexing are the sudden spikes. On several occasions, the methane concentration has surged dramatically. In June 2019, Curiosity measured the largest spike to date: about 21 ppbv. A few days later, the levels dropped back to the usual background hum, suggesting the plume was transient and local.
The Case of the Missing Methane
This ground-level story is contradicted by observations from orbit. The ESA/Roscosmos ExoMars Trace Gas Orbiter (TGO), designed specifically to map trace gases with extreme sensitivity, has detected almost no methane in the wider Martian atmosphere. Its instruments, which are 10 to 100 times more sensitive than previous missions, have established an upper limit of just 0.05 ppbv.
This discrepancy creates a scientific puzzle: how can Curiosity detect methane near the surface if a highly sensitive orbiter sees none from above? One theory suggests that methane released from the ground at night is trapped near the cold surface in a shallow atmospheric layer. When the sun rises, this layer mixes with the broader atmosphere, diluting the methane to concentrations too low for the orbiter to detect. Another possibility is an unknown and highly efficient destruction mechanism that scrubs methane from the lower atmosphere before it can spread. The known photochemical process, where ultraviolet sunlight breaks down methane, takes hundreds of years, which is too slow to explain the gas's rapid disappearance.
The source of the methane is an open question. Geological processes, such as the interaction of water with olivine rock (a process called serpentinization), could produce the gas without life. Alternatively, the methane could be a biosignature, produced by subsurface microbes called methanogens. Curiosity's instruments cannot definitively determine the origin, leaving the 21 parts-per-billion question unanswered.