Pockmarked body NASA/JPL

The bright spots of Ceres, a dwarf planet in the main asteroid belt, have provoked curiosity and speculation ever since NASA’s Dawn spacecraft spotted them in 2015. Now it seems they might all have formed the same way, even though they are made of different materials.

Ceres is speckled with hundreds of bright splotches. An international team led by Ernesto Palomba at the National Institute for Astrophysics in Rome is analysing the light reflected by them – as observed by Dawn, presently in orbit around Ceres – to identify any differences between them.

“The bright spots are only bright relative to Ceres’ already-dark surface,” says Nathanial Stein, a collaborator at the California Institute of Technology. “If you saw those spots on Earth or even on [the asteroid] Vesta you would consider them to be dark spots.”


While the biggest and brightest spots are in Occator crater, more exist elsewhere on the dwarf planet. “Almost all of them are associated with impact craters,” says Stein. The team found 90 per cent of the bright spots are in craters or are debris ejected from a crater.

Researchers theorise that the spots are the result of the heat of an impact melting subsurface materials, which then well up to the surface to create the bright spots.

“As of 20 years ago we would have said that Ceres was just a big, round rock that was the same the whole way through,” explains Andy Rivkin, a planetary astronomer at the Johns Hopkins University Applied Physics Laboratory in Maryland. The jagged mountains and craters led researchers to theorise that Ceres might have an icy core with a rock-ice mantle.

But these spots are telling a story of a younger, more geologically active Ceres than researchers expected. That’s because we would expect material ejected by impacts to mix eventually and create a uniform surface. “Mixing hasn’t had time to occur yet, which means these spots must be young.”

Changing its spots

Over time, the bright spots could be darkening as exposure to the harsh conditions of space scours their surface, or because darker material is being tossed around by subsequent impacts.

Most of the spots are made from the same basic material as the rest of Ceres’s surface: calcium or magnesium carbonates mixed with ammonia-rich clays. But a handful of the spots in the youngest craters – including the exceptionally bright spots in Occator crater – are made of sodium carbonates without nearly as much ammonium clay.

These are small variations, but they are enough to point to new avenues of enquiry.

“Does this mean that there are different formation mechanisms that account for the different compositions?” asks Stein, “or is it just that they’re end-members of the same process?” The compositions of the bright spots seem similar enough that they could be explained by the impact-melt and upwelling theory, just modified by local conditions on the surface.

“Carbonates are interesting because they’re made via reactions between water and minerals with carbon in them,” says Rivkin. While researchers don’t expect to find life on Ceres, understanding the processes that created carbonates in the bright spots will help them think about the types of reactions necessary for life to occur in such a hostile environment.

The next step is to build a computer model of Ceres based on everything scientists have learned about its composition, then start pelting it with simulated rocks to try to match the craters and bright spots we see there today. “We have all the information we need,” says Stein. Now researchers just need to figure out how to piece it together.

And after that? “These bright spots are almost certainly where you would want to send the next mission,” says Rivkin, dreaming of future exploration with robotic landers or rovers on Ceres.

Journal reference: Icarus, DOI: 10.1016/j.icarus.2017.09.020