We've been wondering about those bright, white spots on the dwarf planet Ceres ever since the Dawn spacecraft arrived there in March. Speculation was all over the map: many people's gut reaction said water ice, others fantasized about ice volcanoes (or Death Stars). With each passing month (and subsequent image release), the answer seemed to be some sort of salt. But scientists working with the spacecraft's measurements have remained relatively tight-lipped on the subject, waiting for Dawn to inch closer and closer to the surface of Ceres before making a call.
Today, two different papers were published in the journal Nature. One, led by Andreas Nathues from the Max Planck Institute for Solar System Research, claims that the bright areas (of which there are more than 100) contain a magnesium sulfate called hexahydrite. Over time, asteroids impacted Ceres' surface and exposed a subsurface layer made of briny water-ice, Nathues' team claims. The hexahydrite, a salty substance that's of the same ilk as Epsom salt, is just the residue that remains after the exposed water-ice evaporates.
The other paper sheds light on Ceres' origins. In it, the Dawn spacecraft's own science team make the case for the presence of ammonia-rich clays on Ceres. But unlike the bright spots being salt, this bit is more counterintuitive, and raises new questions.
Ceres lives between Mars and Jupiter, but it may have formed in the outer Solar System
Ceres currently orbits the Sun in between Jupiter and Mars, but the presence of ammonia-rich clays hints that the 584-mile wide body might have formed in the outer solar system. This is because the ammonia ice that's found in the clay would have evaporated too quickly and probably wouldn't still be here had Ceres formed in the inner solar system. So it's likely that Ceres took a wrong turn somewhere and simply wound up stuck in the orbit we found it occupying. (And if that's not the case, then it may have formed nearby and accreted the ammonia-rich material as it drifted in from the outer solar system, the team says.)
One of the reasons why so many people guessed that the bright spots were made of ice was their apparent reflectiveness. But these spots (especially the now famous ones in the Occator crater, seen above in enhanced color) appear to be so bright in part because the rest of Ceres is very dark, similar in reflectiveness to asphalt. As of now, there is no concrete evidence of water-ice, but the scientists continue to look. Dawn is now in its closest orbit around Ceres, ringing the dwarf planet at an altitude of 240 miles, so more definite answers to these questions could come soon after the next set of observations, scheduled for later this month.