Why do scientists still argue about methane on Mars?

Because a handful of instruments disagree, and each side has a technical reason. Methane is chemically unstable in Mars's oxidizing atmosphere and would quickly break down, with a lifetime of about 400 years, so any persistent methane implies a source continually replenishing it. Microbes called methanogens could potentially live in warm, wet places underground on Mars, and water-rock reactions could make methane too. That is why people care.

The reports go back and forth. In 1969 the Mariner 7 team announced methane at a press conference, then retracted the claim when the signals turned out to be carbon dioxide ice. In 2003, three telescope teams reported plumes, and critics argued the lines sat too close to those of Earth's own atmosphere. In 2013-2014 the Curiosity rover saw methane rise from below 1 to 7±2 ppbv at Gale Crater, then fall back.

Then, from 2019, the ExoMars Trace Gas Orbiter reported non-detections, with a detection limit of 0.05 parts per billion, contradicting Curiosity. Reported measurements now range from 60 ppbv to below that limit, and there is no scientific consensus on whether the detections are real. A 2025 suggestion that terrestrial methane could leak into Curiosity's instrument adds one more possibility. It could be microbes, water-rock chemistry, instrument contamination or something not understood. The data do not yet choose.