New analyses of data collected by NASA’s Dawn spacecraft during its visit to the dwarf planet Ceres, which lies in the asteroid belt between the orbits of Mars and Jupiter, are published in Nature and Nature Geoscience this week. The papers shed light on the composition of the dwarf planet’s mysterious bright spots and its subsurface, and suggest that some subsurface fluids existed, perhaps transiently, and may still exist.

The mostly dark surface of Ceres is dotted with more than 130 bright spots, with the most prominent situated within the Occator crater. Previous studies have suggested that these small bright areas may consist of large amounts of hydrated magnesium sulfate.

In the Nature paper, Maria Cristina De Sanctis and colleagues analyse data collected by the Visible and InfraRed Mapping Spectrometer on board the Dawn spacecraft at a distance of 1,400 kilometres from Ceres. They report that spectra of the bright material at Occator’s base are consistent with large amounts of sodium carbonates that are mixed with a dark component, small amounts of silicate minerals known as phyllosilicates, and ammonium carbonate or ammonium chloride. The authors suggest that these compounds were transported from within Ceres to the surface in an aqueous process after the impact crater was formed.

Writing in Nature Geoscience, Michael Bland and colleagues show that most of Ceres’ largest craters are too deep for the subsurface below the dwarf planet’s rocky outer layer to be mostly made up of ice. They conclude that the subsurface is more likely to be composed of only 30 to 40 per cent ice, with the remaining 60 to 70 percent a combination of rock and a strong, low-density material, perhaps made up of hydrated salts and clathrates.