Why Does Methane on Mars Appear and Disappear?

Background

Since 2012, NASA's Curiosity rover has repeatedly detected methane in Gale Crater's atmosphere using its onboard Tunable Laser Spectrometer, part of the Sample Analysis at Mars instrument suite. Background levels typically sit below 0.5 to 1 part per billion by volume, but the rover has recorded episodic spikes considerably higher, with one measurement in the mission's data reaching roughly 20 parts per billion, an increase of more than twenty times the surrounding background level. The pattern is not a one-off: elevated readings have recurred at irregular intervals across more than a decade of measurements, with some research suggesting a possible seasonal component layered on top of the shorter-term spikes.

The puzzle deepened in 2016, when the European Space Agency and Russia's Roscosmos launched the ExoMars Trace Gas Orbiter specifically to map atmospheric methane across the entire planet with instruments roughly a thousand times more sensitive than Curiosity's. Despite that sensitivity advantage, the orbiter has never detected methane anywhere on Mars, in Gale Crater or elsewhere, a flat contradiction of what a surface rover was directly measuring below it. For several years, this left two capable, well-calibrated instruments producing genuinely incompatible results about the same planet's atmosphere.

The Day-Night Reconciliation

A 2021 study published in Astronomy & Astrophysics, led by planetary scientist John E. Moores, proposed a resolution rooted in measurement timing rather than instrument error. Curiosity's Tunable Laser Spectrometer draws significant power, so its methane measurements are taken predominantly at night, when Mars's atmosphere is calm and largely still. Moores's team proposed that methane seeping from the ground accumulates and pools close to the surface overnight, precisely where Curiosity's inlet can detect it in concentrated form. During the Martian day, solar heating drives convection that mixes the lower atmosphere thoroughly, diluting that same methane across a much larger volume of air, below the threshold the orbiter, which observes from altitude, can detect.

Curiosity's team tested this proposal directly, taking two daytime measurements bracketing a nighttime one. The daytime readings fell to effectively zero, while the nighttime reading matched the mission's previously recorded background level, exactly as the day-night hypothesis predicted. SAM principal investigator Paul Mahaffy and TLS lead Chris Webster both treated the result as confirming that the discrepancy between the two missions was a genuine, previously unaccounted-for atmospheric transport effect rather than a fault in either instrument.

What Might Be Producing the Methane

Resolving the timing discrepancy explained why the two instruments disagreed; it did not explain what is generating the methane in the first place, and that question remains open. The leading non-biological explanation is serpentinization, a chemical reaction between certain iron- and magnesium-rich rocks and liquid water that produces hydrogen gas, which can be further converted into methane through additional geochemical reactions. This mechanism requires liquid water interacting with rock at some depth, connecting the methane question directly to the broader, separately studied question of subsurface water on Mars. More recent analysis, including 2021 and 2024 studies modelling back-trajectories and barometric transport of the detected gas, has pointed toward an active emission source somewhere to the northwest or west of Curiosity's position within Gale Crater, though the exact location and mechanism have not been pinned down.

A biological source, methane-producing microorganisms living in the Martian subsurface, remains a possibility current evidence cannot exclude, since Earth's own methane-producing archaea thrive in comparably extreme, oxygen-free environments. It is not, however, the explanation favoured by most published research, which has concentrated on identifying and characterising a plausible geological source rather than treating biology as the leading hypothesis.

Evidence For and Against

The case for an active, ongoing source is built on the methane's own limited atmospheric lifetime: solar ultraviolet radiation breaks methane down within roughly 300 years, so any methane measured today must come from a source still producing it, rather than a reservoir accumulated over Mars's billions-of-years history. The recurring, if irregular, pattern of spikes across more than a decade further supports an active rather than one-off release. Against a firm conclusion either way stands the field's central limitation: no experiment to date has directly sampled the proposed source region, and both the geological and biological hypotheses remain consistent with the same surface-level atmospheric measurements, which cannot by themselves distinguish a chemical reaction from a biological process happening below the rover's reach.

Current Consensus

The apparent contradiction between Curiosity's methane detections and the Trace Gas Orbiter's null results is now considered resolved as a measurement-timing effect rather than a genuine disagreement between the two missions, following the 2021 day-night study and its subsequent direct confirmation. The methane's underlying source remains an open scientific question. Current research leans toward a geological explanation, most plausibly serpentinization or a related water-rock reaction in a specific subsurface region of Gale Crater, while treating a biological source as a possibility that has not been ruled out but also has not been positively supported by any evidence collected so far.

Why This Mystery Endures

The Mars methane mystery endures because it sits in an unusually productive scientific position: it is neither a debunked claim nor a confirmed discovery, but a genuinely open, actively narrowing question, with each new study eliminating some explanations while leaving the core puzzle, an unidentified active source producing an atmospheric gas that is, at least on Earth, overwhelmingly biological, intact. It also endures because the day-night resolution, while real progress, illustrates how much of planetary science still comes down to measurement conditions rather than the underlying phenomenon itself, a reminder that an apparent contradiction between two good instruments does not automatically mean either one is wrong.

The mystery sits naturally alongside two other actively disputed potential biosignatures this site covers: phosphine in the clouds of Venus and the disputed dimethyl sulfide detection on K2-18b both involve a candidate biosignature gas whose detection itself is scientifically contested and whose ultimate source, geological, chemical, or biological, remains unsettled. Water on Mars is the closest sibling case of all, since the leading geological explanation for the methane, serpentinization, depends directly on the same subsurface liquid water that page's own evidence concerns. ALH84001 offers a useful point of contrast rather than similarity: unlike methane's still-open source question, each of that meteorite's proposed biosignatures has since been given a specific, well-supported non-biological explanation, a reminder that a genuinely open Mars mystery and a settled one can look superficially similar from the outside. This page is part of this site's search for extraterrestrial life coverage, within the wider space mysteries cluster.