The gas breaks down in sunlight, so its presence on Mars indicated that either biological activity or a recent geologic event was responsible for its release.
The gas, which lasts about 300 years in Earth's atmosphere, could be expected to stick around for 200 years on Mars. But Curiosity's findings, compiled over eight months, indicate that the methane may have virtually disappeared in a matter of years.
Based on the previous observations, scientists had expected to find about six times more methane in the atmosphere than the negligible amounts Curiosity found.
"There's a discrepancy," lead research Christopher Webster, with NASA's Jet Propulsion Laboratory in Pasadena, California, told Reuters. "Suddenly the whole interpretation of earlier observations is stuck."
Webster said it is possible but unlikely that the lack of methane is particular to Curiosity's landing site, a giant basin near the planet's equator.
Once methane is released from the surface, scientists believe it would spread fairly quickly through the planet's thin atmosphere.
"It's disappointing, of course," Webster said. "We would have liked to get there and find lots of methane."
The search is not over. Curiosity will continue to take air samples and test for methane as it continues its geology mission.
Scientists also plan another round of observations with Earth-based telescopes next year.
The research appears this week in the journal Science.
(Editing by Kevin Gray and Xavier Briand)
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