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Mysterious red streaks are painted on Tethys. (NASA/JPL-Caltech/SSI)

The Curious Case of the Blood-Stained Moon

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 called Odysseus, and a lot of fractures. And then there are the streaks, which are a few kilometers wide and hundreds of kilometers long.

“We have these bloody stains on Tethys,” Schenk says.

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Tethys (back) and Enceladus (front) are lined up in front of Cassini’s camera. (NASA/JPL-Caltech/SSI)

The red streaks were faintly visible in early images from NASA’s Cassini spacecraft, which swooped into the Saturn system in 2004. But it wasn’t until April that Cassini got a close look at the extraterrestrial artwork. Now, after a close flyby in November, scientists can peer even more closely at the smudges. And what they’re finding doesn’t make a lot of sense.

“You don’t see any trace of scarps or ridges or depressions of any kind,” Schenk says, meaning there are no obvious landforms associated with the smears – or at least nothing that’s big enough to see at Cassini’s current resolution. Some of the nearby craters have odd, dark material inside them, but it’s not clear what that material is, how it got there, or if it’s associated with the streaks at all.

Instead, it appears as though someone simply painted the moon red.

“If you didn’t have the color, you wouldn’t know they were there,” Schenk says.

Perhaps the best clue about where the streaks are coming from can be found by plotting their locations on the moon. When Schenk mapped the lines onto the moon’s surface, he saw a pattern suggesting the moon is being squeezed or deformed by some kind of global stress – such as irregular rotation, a shifting orbit, or the migration of its poles. But simulations of those processes don’t produce landforms that quite line up with where the streaks are.

One thing is clear, though: The streaks are relatively young. Normally, dust from Saturn’s E ring and charged particles from space would erase the smudges. But they’re still there. And, they’re drawn on top of the Odysseus basin, meaning that the crater came first. Scientists aren’t sure precisely how old Odysseus is, but Schenk suggests it couldn’t have been made more than 2 billion years ago.

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The enormous Odysseus crater on Tethys is 450 kilometers across and nearly 10 kilometers deep. (NASA/JPL-Caltech/SSI)

Schenk’s best guess now is that the streaks are associated with fractures that Cassini can’t quite see, and that those fractures are currently forming or have been reactivated recently, exposing material that might not be water ice like the rest of the surface.

As Cassini’s days of exploring the Saturn system wind to a close, scientists are hoping to solve this little mystery – and spy on a host of otherworldly enigmas associated with the giant, ringed planet and its clutch of moons.