On July 28, China’s Yutu-2 lunar rover prepared to power down for its daily nap to protect itself from the midday sun during its mission to study the far side of the moon. Before it could finally shut down, one of its handlers here on Earth noticed something strange in one of the panoramic images the moon robot had taken. Inside a small crater there appeared to be a strangely colored substance with a “gel-like” consistency.

The rover team scrapped their plans for rest to investigate the crater, examining the lustrous spot with Yutu’s Visible and Near-Infrared Spectrometer (VNIS), reports Andrew Jones at Space.com. The results of that analysis, however, have not been released nor have images of the mystery substance. News of the substance appeared on the Chinese-language Yutu-2 “drive diary” on the website Our Space and was tweeted out by the state-run newspaper People’s Daily.

In the absence of details, the announcement has led to speculation. The most likely explanation, Jones reports, is that the lustrous spot isn’t really a gel, but is some form of shiny melted glass created when a meteorite struck the moon.

Mahesh Anand, planetary scientist at the Open University in the United Kingdom, tells Hannah Osborne at Newsweek that it’s hard to say for certain what the material is with so little info to go on. But he agrees the material could be a type of glass.

“The fact that it has been observed associated with a small impact crater, this finding could be extremely exciting as it would indicate that a very different material could just be hiding underneath the very top surface,” he says. “This would assume even a greater significance if these material turn out to have experienced interaction with water-ice (as the possibility of existence of water-ice in the top few meters of the lunar South polar region is predicted on the basis of recent remote sensing dataset).”

Walter Freeman, a physicist at Syracuse University, also tells Osborne it’s possible the substance is lunar dust that was turned into glass by a meteorite impact. While there are a lot of processes—like waves, volcanoes and wind—on Earth that can create interesting geology, meteorites are pretty much the only thing that can reshape the surface of the moon.

“There’s a bit of precedent for this on Earth: at the site where the first nuclear bomb was tested in New Mexico, there is a glassy mineral called ‘trinitite’ formed from the heat of the explosion,” he says. “The same thing happens around meteorite impacts here.” (You can read more about that piece of trinitite here.)

This is not the first odd patch of color found on the moon. Most of the moon’s surface is covered by a fine gray dust called lunar regolith. However, in 1972 during the Apollo 17 mission, the final manned mission to the moon, geologist Harrison “Jack” Schmitt scuffed the surface of Shorty Crater revealing orange soil. The crew brought a sample back to Earth, and researchers discovered that the soil was formed when molten droplets sprayed out of a volcanic eruption 3.64 billion years ago.

The Yutu-2 rover is set to continue its mission, heading west of the crater. Launched in December 2018 aboard China’s Chang'e-4 lander, Yutu is the first rover to explore the far side of the moon.