After seeing Phobos imaged from the surface of the Red Planet by Mars Curiosity, now we’re lucky to get a close-up treat: here’s a video showing Mars Express images of the Martian moon over the last 10 years. The images reveal mysterious grooves running through the small moon, which is 13.5-miles (22 kilometers) in diameter, and scientists still aren’t sure what’s going on.

“The moon’s parallel sets of grooves are perhaps the most striking feature, along with the giant 9 km-wide Stickney impact crater that dominates one face,” the European Space Agency wrote.

“The origin of the moon’s grooves is a subject of much debate. One idea assumes that the crater chains are associated with impact events on the moon itself. Another idea suggests they result from Phobos moving through streams of debris thrown up from impacts 6000 km away on the surface of Mars, with each ‘family’ of grooves corresponding to a different impact event.”

For more about amazing discoveries from Mars Express, check out our top 10 list from the summer!