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キーワード:
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要旨:
H2O-rich materials are locally exposed at the surface of Ceres as discovered from infrared reflectance spectra of the Visible and InfraRed mapping spectrometer (VIR) of the Dawn mission. Nine locations on Ceres exhibit diagnostic absorption bands of the H2O molecule at 2.00, 1.65 and 1.28 µm. The detections are all consistent with H2O ice mixed with low-albedo components. All the reported H2O exposures occur at latitudes poleward of 30° in fresh craters near rim shadows, have a surface area < 7 km2, and are associated with one or more surface features such as a morphological flow or landslide, fractures, high albedo, or a pole-facing slope (one case is confirmed to be adjacent to persistent shadow). In four occurrences, these detections are associated with small (< 0.1 km2) high-albedo areas that can be recognized in high-resolution imagery (∼35 m/pixel) from the Framing Camera (FC). Since all these observations are compatible with an H2O-rich subsurface, the replenishment of surficial H2O likely comes from the ice that is present underneath. In four other occurrences, H2O is detected on walls and floors of fresh impact craters, either in the shadow or adjacent to shadows, which suggests that local thermodynamical conditions may also favor the concentration of H2O in these areas.
