A persistent, lopsided cloud of dust is hanging around our moon, scientists have discovered. While any sort of cloud hovering around an airless body is strange enough, the culprits creating that dusty moon-shroud are even more exotic: passing comets.

As they zoom through our solar system, comets shed small particles that eventually smash into the moon, temporarily catapulting dust from the lunar surface into space, the researchers report Wednesday in Nature.

“We continue to learn new surprises about our nearest neighbor in space,” says Richard Vondrak of NASA’s Goddard Spaceflight Center. “Previously, asteroidal particles were expected to be the main external source of orbital dust, with dust lofted from the lunar surface by electrical fields also suspected to be another