It was just a couple of days ago that NASA announced something pretty exciting about Mars. Its Curiosity rover had detected shockingly high levels of methane, suggesting an unseen geological or even biological process was at work on the planet. Signs of past or present life? Perhaps, but a follow-up reading has now generated more questions than answers.

Along with its original announcement of Curiosity’s methane findings, NASA said it would conduct another reading in short order to determine whether or not the elevated methane level was consistent. Well, its second reading revealed a dramatic drop in the amount of methane in the air, leaving scientists to wonder what is causing methane “plumes” on Mars.

The data that caused the initial stir came from a Curiosity air sample reading that showed methane levels of around 21 parts per billion by volume. That’s rather high for Mars, and NASA suggested a number of potential causes, such as past or present biological activity or even geological mechanisms at work on Mars today.

This latest reading, however, saw the methane level drop to less than one part per billion by volume of methane. That’s roughly the same as the background levels of the gas that Curiosity has observed consistently during its stay on the Red Planet.

“The methane mystery continues,” Curiosity project scientist Ashwin Vasavada said in a statement. “We’re more motivated than ever to keep measuring and put our brains together to figure out how methane behaves in the Martian atmosphere.”

Curiosity isn’t equipped with the tools to offer potential clues as to the source of the methane, leaving researchers to offer their best guesses until further investigations can be conducted during future missions. Wherever the gas is coming from, and whether it’s still being created today or is merely the remnants of past activity on Mars, we likely won’t know for some time.