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Where are all the large impact craters on Ceres?

Think of an asteroid. Doesn’t matter which asteroid. Odds are good you thought of something that looks like a lumpy, craterous potato. Astronomers agree with you — most of the rocks in our asteroid belt look pretty careworn. So they expected that when NASA’s Dawn spacecraft got to Ceres, they would see similarly many large impact craters marring the dwarf planet’s surface, just like every other rocky thing in our entire solar system. But that’s not what they found. Ceres is strangely smooth.

Scientists working on the problem have just published some ideas about why this is so. One hypothesis is that Ceres’ own internal structure is responsible. Recent studies, including several detailed analyses of the dwarf planet’s bright spots, points to a layer of ice and salt just beneath the surface. Over geologic time, a process known as “viscous relaxation” could have smudged out the details of the surface. It’s not unlike how the freeze-thaw process breaks down rocks over time. Co-author Michael Bland of the USGS said viscous relaxation flattens out big craters faster than small craters.

The other major hypothesis is cryovolcanism. “We have these bright spots all over the surface – clearly, that’s stuff that came out of the interior,” said Simone Marchi, another co-author. It’s possible that an early period of intense cryovolcanic eruptions dramatically altered the planet’s surface, changing a much different early surface into what we see today. Pearlescent, sparkling cryolavas consisting of phyllosilicated ammonia slush could have obliterated craters, filling them in or abrading them away.

Data from Dawn could offer us answers. The spacecraft is currently in a low-altitude orbit, snapping high-res photos that NASA puts online in an enormous, daily updated gallery. “With this high-resolution data, we can look more specifically at sites on the surface that may have evidence of large-scale cryolavas,” Marchi said.

Dawn has high-res imaging capabilities in both the visible and the IR bands. It also has instrumentation collecting data on the gravity field surrounding Ceres, and that will give us a clearer understanding of Ceres’ interior.

Speaking of what the spacecraft can do: Dawn’s original goal was to orbit Ceres and Vesta, two protoplanets in the asteroid belt between Mars and Jupiter. This latest barrage of data marks the successful conclusion of its mission. But, looking at the stalwart little spacecraft’s persistent good health, NASA decided to extend the mission and keep Dawn in orbit around Ceres. The extended mission will continue to extract data from the icy protoplanet, doing science from the asteroid belt until 2019.

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