Where are all of Ceres’ large impact craters? That was a question that stumped scientists when the Dawn spacecraft arrived in orbit in March 2015. Now, we may be closer to an answer thanks to a team of Dawn scientists.
Ceres is notable for having no large craters formed by impacts on its surface. Its largest, the Kerwan Basin, is a “measly” 285 kilometers (177 miles) in diameter. In fact, simulations suggest it should have 40 craters larger than 100 kilometers (60 miles) wide, and 10 to 15 larger than 400 kilometers (250 miles). But it has just 16 of the former, and none of the latter.
On other similar worlds, like protoplanet Vesta, also in the asteroid belt, we see plenty of evidence for large impacts. Ceres, being located in the asteroid belt and having formed 4.55 billion years ago, should certainly have them too.
But the reason why it doesn’t may be due to one of the key differences between Vesta and Ceres. Whereas the former is mostly rocky, Ceres seems to contain large amounts of ice. And it is this ice that may be key to its less pummeled appearance.
Publishing their findings in Nature Communications, the team suggests that if the crust of Ceres contains a significant amount of ice and salts, it could be weak enough that large craters may smooth out over time, or even disappear. This could be exacerbated by internal heat from Ceres, driven by the radioactive decay of elements – similar to what we think is happening on Pluto.
The Vendimia Planitia depression may be the remnant of an ancient large impact. Southwest Research Institute/Simone Marchi
“If Ceres were highly rocky, we’d expect impact craters of all sizes to be preserved,” said David Williams from Arizona State University, a member of the Dawn team behind this study, in a statement. “Remote sensing from Earth, however, told us even before Dawn arrived that the crust of Ceres holds a significant fraction of ice in some form.”
Evidence for this activity is supported by the presence of the famous bright spots on the surface, which are thought to be salts left on the surface, but may be linked to cryovolcanism.
It would seem likely, then, that Ceres did have large impacts in the past – but the dwarf planet has hidden them from our view. Indeed, we can sort of see the remnants of such impacts, large depressions up to 800 kilometers (500 miles) wide.
Following a recent decision by NASA to keep Dawn in orbit around Ceres, rather than sending it to study a third body in the asteroid belt, the spacecraft will continue observing the dwarf planet for the next few years. In April 2018, it will make its closest approach to the Sun, and the increased warmth could spur activity like this.