NASA’s Mars missions have provided mankind with a wealth of knowledge about our dusty orange neighbor, but signs of life have been hard to come by. We know that Mars was one probably very, very wet, and that it likely had a more robust atmosphere long ago, but has life ever called Mars home? We simply don’t know, but a new discovery made by the Curiosity rover offers what might be a tantalizing clue.

In a new blog post, NASA’s Jet Propulsion Laboratory describes the discovery of what it calls “surprising” levels of methane in the Martian air. The measurement, which was roughly 21 parts per billion by volume, isn’t proof of current or past life, but it’s exciting nonetheless.

You might be wondering why methane on Mars would matter if we’re searching for life. Here on Earth, methane is often a byproduct generated by microbial organisms, but it can also come from various geological mechanisms. Put simply, it’s not a smoking gun.

“With our current measurements, we have no way of telling if the methane source is biology or geology, or even ancient or modern,” Paul Mahaffy, principal investigator of the Sample Analysis at Mars laser instrument built into the rover, said in a statement.

Before scientists can determine what the surprising methane reading means, if anything, they need to know if the measurement was a fluke. This is hardly the first time Curiosity has detected methane, but it is a significant spike in the concentration of the gas in the Martian air. Researchers will now try to determine if the methane levels are linked to seasonal changes, and perhaps even measure how long the elevated gas levels last. These data points could provide new clues as to the source of the gas and whether it is linked to biological or geological activity.