BELLEVUE, Wash. — NASA says the record-setting belch of Martian methane that its Curiosity rover detected last week has faded away, leaving some big questions hanging in the air: Where did the gas come from, and what were its origins?

Much of the methane on Earth is produced biologically, from sources ranging from microbes to the digestive tracts of cows and humans. But methane can also be produced through geological, completely non-biological processes. For example, methane makes up about 5 percent of the atmosphere of the Saturnian moon Titan, which is so cold that methane and other hydrocarbons pool up in lakes and rivers.

Curiosity’s onboard chemistry lab — known as Sample Analysis at Mars, or SAM — has an instrument that can sense methane levels in the Red Planet’s atmosphere, and those levels usually amount to less than 1 part per billion by volume. But SAM has registered several curious methane spikes during its seven years of surface operations — including a rise to 6 parts per billion in 2013 that got NASA’s attention, and another detection that rose even higher during the following Martian year.

Last week, methane levels spiked to the highest levels ever detected by Curiosity: 21 parts per billion. That caused the SAM science team to change their plans for the weekend and make follow-up measurements.

Those measurements were sent back to the science team this morning, and they showed that methane levels were back to their usual level.

“A plume came, and a plume went,” SAM’s principal investigator, Paul Mahaffy of NASA’s Goddard Space Flight Center, reported here at the Astrobiology Science Conference. “We’re very confident of the measurement.”

The quick rise and fall suggests that last week’s spike was similar to the previously observed phenomena, albeit on a significantly larger scale.

Last year, a team of scientists analyzed the 2013 readings from Curiosity as well as the European Space Agency’s Mars Express orbiter, and suggested that such spikes occur when seismic shocks on Mars free up reserves of methane that are trapped beneath the Martian surface. Such a mechanism could well have been behind last week’s spike.

Mars Express hasn’t reported any methane spikes lately. Nor has ESA’s Trace Gas Orbiter, which is designed to make high-resolution measurements of methane and other gases in Mars’ atmosphere. “We’re coordinating with them, and we’re very happy to make coordinated measurements,” Mahaffy said.

Such measurements offer the best hope for tracking down the sources of Martian methane, be they biological or purely geological.

“The methane mystery continues,” Ashwin Vasavada, Curiosity’s project scientist at NASA’s Jet Propulsion Laboratory, said in NASA’s latest online update. “We’re more motivated than ever to keep measuring and put our brains together to figure out how methane behaves in the Martian atmosphere.”