Much like Star Trek's Starship Enterprise without its shields, when you're a planet without an atmosphere, space can be a dangerous place – especially in the earlier days of our Solar System's formation. With lots of asteroids floating around, chances are good that some doozies are going to smack into you. That is likely the case with dwarf planet Ceres, but its surface doesn't show any craters larger than 175 miles (280 kilometers) across. So what happened to the evidence? It looks like Ceres has been erasing it.
Scientists got their first truly up-close view of Ceres in December of last year as the Dawn spacecraft made its closest approach to the mini-planet that's the smallest dwarf in our solar system, but the largest object in the asteroid belt.
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Using data from Dawn, researchers were surprised to see the relatively small size of Ceres' craters. Using modeling, by their calculations, the dwarf planet should have about 10 to 15 craters on its surface larger than 250 mi (400 km) in diameter. In fact, before arriving at Ceres, Dawn visited the protoplanet Vesta and showed it to contain a crater that measures 300 mi (500 km) wide.
A new study in the journal Nature Communications posits that the lack of large craters on Ceres comes from its unique composition.
Previously, Dawn revealed that Ceres' upper layers probably hold ice and that its famed bright spots are likely salt deposits left over from water under the surface that swelled upwards. A subsurface of ice or water could not only erupt and change the dwarf planet's surface, it could also allow the rocky surface itself to smooth out as it moves on the less-dense material below, thus erasing the craters.
"If Ceres had widespread cryovolcanic activity in the past – the eruption of volatiles such as water – these cryogenic materials also could have flowed across the surface, possibly burying pre-existing large craters," says NASA.
Vesta isn't believed to have ever had a liquid core, so that would explain why its large crater is still visible.
"The ability to compare these two very different worlds in the asteroid belt – Vesta and Ceres – is one of the great strengths of the Dawn mission," said lead investigator Simone Marchi, a senior research scientist at the Southwest Research Institute.
"Whatever the process or processes were, this obliteration of large craters must have occurred over several hundred millions of years," he added.
This brief video offers a bit more insight into the study.