The Mars Methane Mystery: Life, Geology, or Something Else?

A Gas That Shouldn't Be There

Methane is a short-lived molecule. In the Martian atmosphere, ultraviolet radiation and chemical reactions break it down within a few hundred years — a geological eyeblink. So when scientists detect methane on Mars, they know it must have been produced recently. Something is actively making it. The question is what.

That question has been generating serious debate since 2004, when ESA's Mars Express orbiter reported detecting methane at concentrations around 10 parts per billion (ppb). Subsequent measurements have been inconsistent, tantalizing, and occasionally contradictory — which has only deepened the mystery.

What Curiosity Found — and When

NASA's Curiosity rover has been the most productive on-the-ground methane detector on Mars. Since landing in Gale Crater in 2012, it has measured background methane levels hovering around 0.4 ppb — low, but detectable. More striking were the spikes.

In June 2019, the Curiosity science team reported a methane reading of approximately 21 ppb — the highest ever recorded by the rover. The spike appeared and then largely dissipated within a few days. Frustratingly, ESA's ExoMars Trace Gas Orbiter (TGO), which was designed specifically to detect atmospheric trace gases, observed no methane during the same period. The two instruments were looking at the same planet and getting different answers.

This discrepancy hasn't been resolved. Possible explanations include that methane is being released from localized surface or subsurface sources and disperses rapidly before reaching orbital altitude, or that instrument calibration issues affect one or both measurements. Neither explanation is fully satisfying.

The Three Leading Hypotheses

1. Geological Sources

The leading non-biological explanation is serpentinization — a chemical reaction in which water interacts with iron- and magnesium-rich rocks (olivine and pyroxene) under heat, producing hydrogen and, in the presence of carbon dioxide, methane. This process is well-documented on Earth and requires no life whatsoever. Mars has abundant ultramafic rocks, and liquid water may still exist in subsurface pockets. If serpentinization is occurring today, methane production would be a natural byproduct.

Another geological candidate is UV-triggered breakdown of organic compounds delivered by meteorites. Mars receives a steady rain of carbonaceous material from space. When ultraviolet radiation hits these organics at the surface, it can release small amounts of methane and other gases.

2. Biological Sources

Methane on Earth is overwhelmingly produced by living organisms — primarily methanogenic archaea, single-celled microbes that thrive in oxygen-free environments and exhale methane as a metabolic byproduct. If microbial life survived in Mars's subsurface — insulated from radiation, possibly near liquid water — methanogens are a biologically plausible source.

This hypothesis is far from confirmed, and most planetary scientists treat it as the least likely explanation until other sources are ruled out. But it hasn't been eliminated. That's why the methane question carries such weight: a positive biological attribution would be one of the most significant scientific discoveries in human history.

3. Seasonal and Surface Chemistry

Curiosity data has suggested a seasonal pattern, with methane levels slightly higher in summer and lower in winter — the opposite of what simple atmospheric chemistry would predict. One hypothesis is that methane trapped in subsurface ice (clathrates) is released when surface temperatures rise. Another points to perchlorates in the Martian soil reacting with organics under UV light. The seasonal signal is real but subtle, and interpreting it has proven difficult.

Why the TGO Measurements Matter

The ExoMars Trace Gas Orbiter arrived at Mars in 2016 and began science operations in 2018. It was specifically designed to detect methane and other trace gases at concentrations as low as 0.05 ppb — far more sensitive than previous orbital instruments. Its findings, published in Nature in 2019, placed an upper limit of 0.05 ppb on atmospheric methane — essentially undetectable from orbit.

This doesn't mean surface measurements are wrong. It may mean that methane is being released episodically from small, localized sources and destroyed or absorbed before it can mix into the upper atmosphere. The Martian soil itself may act as a chemical sink, consuming methane faster than expected.

What Comes Next

ESA's Rosalind Franklin rover, long delayed but still planned for launch, carries instruments designed to drill up to two meters below the Martian surface — below the sterilizing reach of UV radiation — and analyze material directly. NASA's ongoing Curiosity and Perseverance missions continue atmospheric monitoring. Pinning down whether the methane is biological, geological, or some unexpected surface chemistry will likely require combining orbital, surface, and subsurface data in ways no single mission can provide alone.

For now, Mars keeps its secret. But every measurement narrows the possibilities — and makes the answer feel a little closer.