Martian Methane Mystery
At Gale Crater, NASA’s Curiosity rover has repeatedly sniffed traces of methane that appear at night, fade by day, change with the seasons, and sometimes spike far above the background, yet ESA’s ExoMars Trace Gas Orbiter, built to search the planet’s atmosphere for the same gas, has largely found nothing from orbit; methane can be made by water-rock chemistry or by microbes on Earth, but recent doubts about measurement reliability and theories involving sealed subsurface gas pockets leave Mars with a maddening question: is the planet breathing from underground, fooling our instruments, or hiding a chemistry we have not yet understood?
The Martian methane mystery is one of the most careful modern examples of a scientific mystery: a faint chemical signal, measured near the edge of instrument capability, with consequences far larger than the gas itself. Methane is simple, one carbon atom and four hydrogen atoms, but on Earth it is strongly associated with living systems, hydrothermal activity, petroleum reservoirs, geological alteration, and chemical cycling. On Mars, where the surface is cold, dry, oxidizing, and bombarded by ultraviolet radiation, even tiny amounts of methane raise a question that cuts across geology, atmospheric chemistry, and astrobiology. If the gas is present today, something must have produced it, stored it, released it, or preserved it against destruction.
The modern story began with telescopic and orbital reports in the early 2000s. Ground-based teams used high-resolution infrared spectroscopy through powerful telescopes such as NASA's Infrared Telescope Facility and the W. M. Keck Observatory, while ESA's Mars Express spacecraft used its Planetary Fourier Spectrometer from orbit. The reported amounts were small, measured in parts per billion by volume, and the observations were difficult because Earth-based measurements had to separate possible Martian methane from the much stronger methane and water signatures in Earth's own atmosphere. Still, the reports suggested localized plumes and seasonal behavior, especially around northern summer 2003, making Mars appear less chemically dead than many had assumed.
Curiosity changed the case because it carried the Sample Analysis at Mars instrument suite directly to the surface of Gale Crater. Instead of looking across space at a whole planet, Curiosity sampled the air around itself. Its Tunable Laser Spectrometer initially reported little or no methane, but later detected a tenfold spike during part of the mission and, in a 2018 Science report, a low background level that appeared to vary seasonally. In June 2019 the rover measured an unusually high transient reading of about 21 parts per billion by volume, the highest methane value reported by the mission, only for the signal to fall back close to background levels in a follow-up experiment days later. These episodes gave the mystery a dramatic rhythm: a whiff of gas, then disappearance.
The problem became stranger when ESA's ExoMars Trace Gas Orbiter began its science campaign. TGO was designed specifically to inventory trace gases in the Martian atmosphere with far greater sensitivity than earlier missions. If methane were spread through the atmosphere and had the expected chemical lifetime of hundreds of years, TGO should have seen at least some global background. Instead, early observations reported no methane above very low upper limits. ESA's Mars Express later reanalysis did independently match one Curiosity spike from June 2013, but ESA also reported that Mars Express and TGO did not see the June 2019 Curiosity burst despite observing nearby in time or location. The instruments were not merely disagreeing in a simple way. They were describing a gas that may be local, brief, trapped near the surface at night, destroyed or absorbed quickly, or sometimes not present at all.
Several conventional explanations remain under serious discussion. Methane could be generated by serpentinization, a geological reaction involving water, olivine-rich rocks, and heat. It could be ancient methane trapped in clathrates, zeolites, icy pockets, salt-sealed regolith, or other subsurface reservoirs, then released when temperature, pressure, impacts, cracks, or rover disturbance change the seal. It could be produced by ultraviolet-driven reactions involving surface organics or delivered cometary dust. It could even, in the most astrobiologically interesting possibility, be generated by subsurface microbes, but no instrument has found evidence that requires that explanation. At the same time, skeptics and atmospheric chemists emphasize that some detections may sit close to instrument limits, that Earth-based observations are vulnerable to telluric contamination, and that known photochemistry alone does not easily reconcile Curiosity's low-altitude readings with TGO's upper-atmosphere non-detections.
The unresolved core is therefore not a single missing witness or lost object. It is a conflict between measurements, lifetimes, altitudes, seasons, and scales. Curiosity can sniff the air near Gale Crater only occasionally, while TGO views the atmosphere from orbit and may not sample the lowest surface layer at the right time of day. Mars itself may be releasing methane in rare puffs that collapse before dawn, or it may be teaching scientists that the chemistry of its dust, regolith, oxidants, salts, and boundary-layer atmosphere is not yet understood. As of the latest NASA and ESA summaries, the methane question remains open: Mars has not yielded proof of life, but neither has it given a simple chemical answer.