In 2014, water vapour was detected around Ceres, a dwarf planet in the asteroid belt. NASA's Dawn spacecraft, in orbit around Ceres since March 2015, subsequently found water ice on the dwarf planet, in a small, mid-latitude crater named Oxo. Now, writing in Nature Astronomy, Platz et al. show that polar regions of Ceres that lie in perpetual darkness trap and preserve water ice, providing valuable information about the processes that create water reservoirs on planetary bodies (T. Platz et al. Nature Astron. http://dx.doi.org/10.1038/s41550-016-0007; 2016).
Permanently shadowed areas on airless planetary bodies, usually found in craters at polar latitudes, have favourable conditions for retaining water ice. Without sunshine, these regions are extremely cold (a few tens of kelvin), and can therefore efficiently trap water molecules and retain them for geological timescales. This cold-trapping process is known to be active on the Moon and on Mercury.
Platz and colleagues analysed images obtained by Dawn's Framing Camera to reconstruct complete maps of the shadowed areas of the northern polar region of Ceres (pictured). They find more than 600 craters in perennial shadow, 10 of which exhibit bright features. One of these features has a (partially) illuminated portion, allowing Platz et al. to perform a spectral analysis using Dawn's imaging spectrometer. The spectra show clear signatures of water ice, providing definitive evidence for the nature of these bright deposits.
The authors also constrain the timescale of water-ice production on Ceres using the fact that ice deposits evolve through two competing mechanisms: slow accumulation by cold-trapping and destruction by impact 'gardening', the overturning of planetary soil by impacts. The authors estimate that the deposits are young (not more than a few hundred thousand years old), which implies that ice delivery is continuously active. In addition, the deposits are relatively scarce (compared with, for example, those on Mercury), suggesting that Ceres is unable to retain much water or didn't have much to begin with.