Curving across the surface of Saturn’s moon Tethys are crimson streaks – and scientists have no idea what the material is or how it got there.

“It’s clearly painted on the surface in some way that we do not as yet understand,” says Paul Schenk of the Lunar and Planetary Institute, who presented the observations Tuesday at the American Geophysical Union’s annual meeting. “We basically have a little mystery.”

At just a bit more than 1,000 kilometers across, Tethys is a medium-sized moon and is made almost entirely of water ice. Aside from the bloody arcs, its surface is pretty normal as far as outer solar system moons go: There are a bunch of craters, including a 450-kilometer-wide behemoth