The European Space Agency's Trace Gas Orbiter, which arrived at Mars in 2016, was designed specifically to hunt for methane in the thin Martian atmosphere. After eight years of continuous observations with some of the most sensitive instruments ever built for this purpose, the spacecraft has yet to detect any methane at all. This surprising null result challenges earlier observations and raises big questions about what scientists thought they knew about Mars.

Methane is a colorless, odorless gas made up of one carbon atom bonded to four hydrogen atoms. On Earth, methane comes from many sources: cattle digestion, decomposing plants in wetlands, oil and gas drilling, and countless other biological and geological processes. Scientists became fascinated by methane on Mars because it seemed like a smoking gun. If methane appeared and disappeared in Mars's atmosphere on short timescales, say, seasonally, it could suggest active biological or geological processes happening on the planet right now. A 2003 observation by an instrument on NASA's Mars Express orbiter seemed to show methane spikes in certain regions. Later observations from ground-based telescopes and other orbiters appeared to confirm these findings. The possibility of detecting methane fueled excitement that Mars might still be geologically alive or even harbor microbial life in subsurface reservoirs.

The Trace Gas Orbiter, a joint ESA-Russian mission (though Russia's involvement ended after 2022), was equipped with a sophisticated instrument called the Atmospheric Chemistry Suite and another called NOMAD to measure methane with incredible sensitivity. These instruments can detect individual molecules in parts per trillion, meaning they can spot even vanishingly small amounts of gas. The orbiter circles Mars every two hours, repeatedly scanning the atmosphere over different regions and seasons. Despite this advanced detection capability and years of opportunities, the spacecraft has found no reliable signature of methane. The upper limit of what the orbiter can measure suggests that if methane exists in Mars's atmosphere, there is far less of it than previous observations suggested.

This contradiction has sparked intense debate among planetary scientists. Several possibilities exist to explain the discrepancy. First, earlier observations might have been false positives caused by instrumental errors or misinterpretation of data. Ground-based observations are particularly tricky because Earth's own methane-rich atmosphere can interfere with distant readings. Second, methane on Mars might be concentrated in localized pockets near the surface, destroyed quickly by chemical reactions in the thin upper atmosphere, or hidden in such small quantities that even the Trace Gas Orbiter cannot reliably detect it. Third, the methane-producing processes that scientists hypothesized might not actually be happening, at least not at scales large enough to create measurable atmospheric signatures. Alternatively, if life or active geology does produce methane on Mars, it could be sealed underground in ice or rock, never escaping to the atmosphere in detectable amounts.

The failure to find methane matters because it fundamentally changes how scientists think about Mars's present-day activity. Mars is a geologically ancient planet, with most large volcanic activity having ceased billions of years ago. If no methane floats in its atmosphere today, it suggests the planet is even more geologically dead than previously thought, making current biological life even less likely. However, the finding also refocuses research efforts. Scientists are now investigating whether methane observations were simply mistakes, whether detection methods need improvement, or whether scientists were looking for the wrong sign of Martian activity altogether. The search for life or ongoing geological processes on Mars continues, but researchers must now think beyond methane and consider what other chemical signatures might reveal whether Mars harbors surprises we have not yet imagined.