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Ceres is the largest object in the asteroid belt, a world more than 900 kilometers in diameter. It’s so big that planetary scientists tend to refer to it as a protoplanet rather than an asteroid. The latter group consists of pulverized rubble left over from the planetary formation process billions of years ago, but Ceres is different. It got big enough that it was well on its way to being a planet before it ran out of material to build with.
When we look at objects in the asteroid belt (and many moons of large planets), we see them covered in craters. Saturated, actually, with so many craters that a new impactor is likely to erase a few older ones when it hits.
The smaller the crater, the more of them there are. This makes sense; there are only a few objects big enough to create really big craters, but gazillions of smaller ones that can make smaller craters.
Still, pretty much every object we look at has a handful of really monster impact craters, some approaching the diameter of the object itself—remember, a crater will follow the curvature of the surface; Vesta, smaller than Ceres at 525 km across, has a crater 500 km across on it. That crater stretches over about a third (well, about 1/pi) of the surface.
So we expect to see a few very large craters on every object we study.
… except with Ceres we don’t. For some reason, Ceres has a definite paucity of really big craters.
The Dawn mission has been orbiting Ceres since March 2015, taking high-resolution images of the surface (as well as lots of other data). Using these images, scientists basically counted up the craters above a certain size. The biggest craters, named Keran and Yalode, are 280 and 270 km in size. That’s big, but Ceres is 940 km across. Where are the big craters?
And it’s not just the very biggest; the numbers start to drop off at craters wider than about 100 km in diameter. It’s weird, and that’s not just intuition. The numbers and sizes of craters can be predicted using impact models based on the numbers and sizes of asteroids in the main belt. They find that there should be six or so craters bigger than 280 km, but none is found. The chance of that happening is less than one percent! They also expect roughly 40 craters bigger than 100 km, but only 16 are seen. The odds of that occurring are essentially zero.
So what’s going on?
Most likely, Ceres did have huge craters long ago, but something happened to erase them. Either lots of subsequent impacts erased the evidence, or Ceres itself did. By that I mean perhaps the composition of Ceres itself makes it such that huge craters fill in, the material surrounding the crater flowing back into it ad “patching” it.
Under the huge pressure of impact rock can flow pretty well, and Ceres also has a lot of water ice under the surface, so this idea has merit. In fact, the authors of the research indicate this is the most likely solution; the process happens to big craters but not smaller ones, so it seems to be connected to Ceres itself, and not subsequent impacts.
I’ll note that there are three very large (>800 km wide) basins, or depressions, on the surface of Ceres. Those might be impact-related, but it’s difficult to be certain. The authors discuss those, and the idea that they’re so difficult to identify lends credence to the idea that something happened to resculpt them.
As I’ve written about before, looking at craters is a great way to understand what big bodies in the solar system have gone through over the eons. It helps establish a timeline of events across the solar system, and can be used to see how objects compare with one another. We already know Ceres is a bit weird—it has a mantle of briny water ice under the surface that oozes up, sublimates, and leaves behind brighter salty deposits, as one example of its odd behavior—so it’s likely that if we see other unusual things going on, they’re tied together. In this case, it’s due to the internal structure of Ceres.
Ceres is a midway point between the stuff used to make planets and the planets themselves. It’s a frozen remnant straddling that line from 4.5 billion years ago, and studying it tells us more about how our own planet came to be.