The 2013 plume observed by Curiosity and Mars Express was even more striking, about 100 times as much methane as what the Trace Gas Orbiter data says is not there.

“So what’s going on?” said Dorothy Z. Oehler, a senior scientist at the Planetary Science Institute, based in Tucson. “Nobody really knows.”

Dr. Oehler was involved with the Mars Express research but not the Trace Gas Orbiter findings.

Researchers first reported the detection of methane in the Martian atmosphere a decade and a half ago based on measurements by Mars Express and telescopes on Earth. But because those claims were at the edge of what the instruments were capable of measuring, many scientists remained skeptical.

The Trace Gas Orbiter uses a different technique. From behind Mars, it looks at sunlight passing through the planet’s atmosphere. Specific colors of light absorbed by the air provide fingerprints that identify specific molecules.

So far, the fingerprint for methane is missing.

However, the orbiter and Curiosity are looking at different places. The orbiter has yet to make a measurement directly over Gale Crater. Because clouds and dust block sunlight, the Trace Gas Orbiter is peering at least several miles above the surface, while Curiosity is measuring what is at the surface.

In addition, Mars was enveloped by a giant dust storm much of last year — the same one that ended the mission of NASA’s Opportunity rover. In 2018, the Curiosity rover only made a single methane measurement, in June. Some scientists think that the methane in the atmosphere at that time could have stuck to the storm’s dust particles, speeding up its cleansing from the air.

Still, scientists had expected that any releases of methane would mix throughout the Martian atmosphere, much as a drop of food coloring diffuses in a glass of water.