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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