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Oooo, I’ve been waiting for this: The Dawn spacecraft is now close enough to the asteroid Ceres to get nice close-ups of the battered surface!
That shot was taken on May 23, from a distance of about 5,100 kilometers. The moment I saw it, I thought, “Oh wow, those grooves aren’t grooves, they’re secondary craters!”
In the more distant shots from Dawn, long linear features can be seen across the surface. We see the same sort of thing on other bodies (most notably Mars’ moon Phobos and the asteroid Vesta) and I thought they might be stress fractures. That’s weird, since Ceres is way far away from any other large object, so it doesn’t get massaged by tides or gravity.
But this close-up shows them for what they are: long chains of small craters, certainly formed when a bigger impact splashed out debris which then fell back down onto the surface. This ejected material tends to form plumes, so when they fall back you get these radial chains of secondary craters. We see this on essentially every cratered body in the solar system.
I love when a single higher-resolution picture clears up a question. Now, hopefully, we’ll soon see the zoomed shots everyone is really waiting for, showing those white spots in the big crater. I doubt a single close-up will crack that particular nut, but I suspect we’ll know pretty soon what’s what. Are they ice? If so, how did they get there? Is Ceres leaking water from its interior, or are those impact-released ice pockets like we see on Mars?
Patience. It took Dawn a long time to get to Ceres, but now we’re there. Things’ll only get cooler from now on.