Mars gave a good burp last week, but the gas has come and gone, leaving scientists no closer to knowing whether there is life on or beneath the red planet.

Last Wednesday, scientists grew excited when NASA’s Mars Curiosity rover, while testing the air of Gale Crater, detected high levels of methane, which on Earth can be produced by microbes or other living things.

Curiosity’s keepers at the Jet Propulsion Laboratory , in Pasadena, canceled the rover’s weekend plans to repeat the experiment, hoping for clues that might point to the existence of life on the planet. But the second reading indicated that the methane had fallen to its normal level in the Martian air — less than one part per billion, all but nonexistent.

Only a few days earlier, the methane level had spiked to 21 parts per billion, according to the rover’s Sample Analysis at Mars experiment.