Methane on Mars

Summary

NASA's Curiosity rover has reported methane in the air of Gale crater on Mars, at a low background with occasional spikes, while the ExoMars Trace Gas Orbiter has found none anywhere on the planet. The two results differ by a factor of ten or more and have not been reconciled.

Methane can be made by microbes, which is why the question draws attention, and also by reactions between water and rock. On Mars it is not yet agreed that the gas is there at all.

What is documented

Early reports. In 2004 the European Mars Express orbiter reported methane at about 10 parts per billion by volume (ppbv). Telescopes on Earth have reported up to about 45 ppbv. These figures are as the European Space Agency summarised them in 2019.

Curiosity's background and spikes. Curiosity carries a tunable laser spectrometer in its Sample Analysis at Mars (SAM) laboratory. In December 2014 NASA reported twelve air samples over twenty months: a background of about 0.7 ppbv, and a tenfold rise to about 7 ppbv over two months in late 2013 and early 2014. Chris Webster of the Jet Propulsion Laboratory led the study, published in Science. Sushil Atreya of the University of Michigan said the rise pointed to "some relatively localized source".

A cross-check from orbit. On 16 June 2013, a day after Curiosity recorded about 6 ppbv, the Planetary Fourier Spectrometer on Mars Express recorded about 15 ppbv. Marco Giuranna and colleagues published this in 2019 as independent confirmation of the spike, and traced a possible source region east of Gale crater.

The seasonal cycle. On 7 June 2018 NASA announced that the background, measured over nearly three Mars years, rose in the warm season and fell in the cold; the European Space Agency later summarised the range as about 0.2 to 0.7 ppbv. "This is the first time we've seen something repeatable in the methane story," Webster said. In 2020 Edward Gillen, Paul Rimmer and David Catling reanalysed the same data statistically and found that, taken as a whole, they "do not indicate seasonal variability."

The orbiter. The ESA-Roscosmos ExoMars Trace Gas Orbiter reached Mars in 2016 and carries two spectrometers used in the methane search, NOMAD and ACS. On 10 April 2019 ESA reported the first results: no methane, with an upper limit of 0.05 ppbv, 10 to 100 times lower than every earlier detection. In 2021 Franck Montmessin and colleagues lowered the limit to 20 parts per trillion (0.02 ppbv), from 1.4 Mars years of observations, including searches around Gale crater.

The largest spike. In June 2019 Curiosity measured about 21 ppbv, the most of the mission. When the team repeated the measurement a few days later, the level had fallen below 1 ppbv. Ashwin Vasavada, the project scientist, said: "The methane mystery continues."

Day and night. Curiosity's spectrometer has taken most of its air samples at night. John Moores proposed that methane released from the ground builds up in the still night air near the surface and is mixed away when the day's heat stirs the atmosphere, while the orbiter, which needs sunlight, looks at air about 5 kilometres up. The rover then took two daytime measurements in a single Martian day, and methane was effectively zero. "John predicted that methane should effectively go down to zero during the day, and our two daytime measurements confirmed that," said Paul Mahaffy, principal investigator of SAM. Webster and colleagues published the day and night results in 2021. NASA noted that methane should survive about 300 years before sunlight destroys it, so the question of how it could stay out of the global atmosphere remained.

The instrument question. In April 2025 Sébastien Viscardy, David Catling and Kevin Zahnle re-examined the spectrometer's own records. They reported that a separate compartment of the instrument, its foreoptics chamber, typically held methane at abundances three to four orders of magnitude higher than those reported for the sample cell, and argued that small leaks, together with systematic errors in retrieving the spectra, could account for the reported detections. They proposed experiments to test this.

Leading explanations

Local, episodic seepage at Gale, trapped near the ground at night (the Curiosity team's working explanation; peer-reviewed, not established). It fits the night-time background, the near-zero daytime readings and the orbiter's silence at higher altitude. It does not yet explain how methane with a lifetime of centuries avoids building up in the global atmosphere. That requires either a fast loss process near the surface, which has not been identified, or methane confined to the immediate vicinity of the rover, a possibility the orbiter team has raised.

Methane from geology or from life (hypotheses about the source, both untested). On Earth, methane is made by microbes and by reactions between water and rock. Mahaffy said in 2019 that the measurements gave no way of telling whether the source was biology or geology, or ancient or modern. In ESA's account of the Mars Express study, Giuseppe Etiope suggested that ice east of Gale could trap methane below ground and release it episodically along faults.

No Martian methane: an instrumental artefact (peer-reviewed 2025; not yet tested). Viscardy and colleagues argue that terrestrial methane leaking inside the instrument, with retrieval errors, could explain what Curiosity reported. The Mars Express reading of 16 June 2013 came from a different instrument on a different spacecraft.

A strong seasonal cycle (reported 2018, disputed). Gillen and colleagues found no statistical support for it in the data taken as a whole.

Current status

Unsolved. Curiosity's measurements and the Trace Gas Orbiter's limits disagree: the rover's reported background of about 0.2 to 0.7 ppbv is 10 to 35 times the orbiter's upper limit of 0.02 ppbv. Three kinds of explanation are on the table, none established: methane that is real but confined to the ground at Gale and destroyed or trapped by an unknown process; a source too small and local for the orbiter to see; or methane that belongs to the instrument rather than to Mars.

What would change this: the experiments Viscardy and colleagues proposed to test the instrument for leaks, more daytime measurements by the rover, a detection from orbit, or a second instrument on the surface.

Last reviewed: September 2026. Records are provisional. Where the evidence changes, the entry changes. Found an error? Tell us.