A VARIED HISTORY: An enhanced colour view of Ceres’ Haulani Crater, taken from Dawn’ s High-Altitude Mapping Orbit( HAMO). Named after the Hawaiian goddess of plants, this 34km( 21 mile) diameter crater holds one of Ceres‘ bright spots‘. It is characterised by its central mountain ridge and land-slides along its relatively sharp rim. The colour blue represents here younger material on the cerean surface, and this highlights the rays of ejected material which have partially blanketed surrounding older terrain and craters. Part of this ejecta forms clear flow fronts, probably a result of the melting of subsurface ice at the time of impact. Credit: NASA / JPL-Caltech / UCLA / MPS / DLR / IDA
ice-rich layer below its rocky surface. At least a quarter of Ceres’ s mass is water, a much greater proportion than seen in most asteroids. There is a lack of very large craters, which initially pointed to the idea that most large craters would have‘ relaxed’ into more shallow configurations over long geological time scales. Such‘ relaxation’ would be due to the viscosity of ice-rich sub-surface material. Since larger impactors have become scarcer over time, only the scars of later, smaller impactors would remain.
However, analysis of crater depths from Dawn images suggest that they are too deep to be consistent with the existence of an ice-rich subsurface. Viscous relaxation is only found locally in a few areas. According to mission scientists, this finding would indicate that the dwarf planet’ s subsurface can only be about 30-40 % ice, mixed in with rock and a lower-density material, perhaps hydrated salts and clathrates. This would be supported by the latest analyses of GRaND data, presented by the Dawn team on 15th December, indicating widespread ice just below Ceres’ surface. These studies show that Ceres’ uppermost layer – within about a metre of the surface- is rich in hydrogen, consistent with the presence of water-ice, particularly at mid-to-high latitudes, and that this is likely to be ice contained within a porous mix of rocky materials, rather than pure ice.
The surface of Ceres
The surface composition of Ceres is broadly similar to that of C-type, or carbonaceous, asteroids, which are common in the outer part of the main asteroid belt. Like these it has a typically dark surface, with a low albedo of just 0.09, reflecting just 9 % of the light which falls upon it as compared to 12 % for our Moon and 30 % for the Earth. This is attributed to the presence of organic materials, some of which have been
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