Planetary scientists from Rice University, NASA’s Johnson Space Center and the California Institute of Technology have an answer to a mystery that’s puzzled the Mars research community since NASA’s Curiosity rover discovered a mineral called tridymite in Gale Crater in 2016.

NASA’s Curiosity Mars rover snapped this low-angle self-portrait at the site where it drilled into a rock July 30, 2015, producing a powder (visible in foreground) that was later confirmed to contain the rare mineral tridymite. (Image courtesy of NASA/JPL-Caltech/MSSS)

Tridymite is a high-temperature, low-pressure form of quartz that is extremely rare on Earth, and it wasn’t immediately clear how a concentrated chunk of it ended up in the crater. Gale Crater was chosen as Curiosity’s landing site due to the likelihood that it once held liquid water, and Curiosity found evidence that confirmed Gale Crater was a lake as recently as 1 billion years ago.

If a volcanic eruption like the one in the scenario did occur when Gale Crater contained a lake, it would mean explosive volcanism occurred more than 3 billion years ago, while Mars was transitioning from a wetter and perhaps warmer world to the dry and barren planet it is today.