AAPG Annual Convention and Exhibition

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Water in the Asteroid Belt: Dawn Mapping of Vesta and Ceres


The largest asteroid in the Asteroid Belt, Ceres, is also distinctive for its low density, ~2 g/cc. This density has long been known and been interpreted to indicate a significant abundance of low-density material, namely water ice. The Dawn mission, launched in 2006, was designed to orbit and map the morphology, composition, topography and internal gravity field of two of the largest asteroids, Vesta and Ceres. These two bodies were chosen mostly to investigate the relatively dry basaltic parent body of the HED meteorite suite and the potentially water-rich world Ceres. Cratering and mapping of Vesta indicates a dry basaltic body, with only very limited evidence for volatiles, in the form of small pits on crater floors. Mapping of Ceres is now complete and reveals a volatile-rich transitional yet enigmatic body. Water-ice is observed at only one small location, the wall of the small crater Oxo, indicating that water ice is hidden by an outer refractory layer. All morphometric measurements and morphologies of the many craters on Ceres match exactly those for icy moons of the Outer Solar System, in sharp contrast to those observed on water-poor Vesta. Despite this, the prediction of extensive crater relaxation in an ice-rich Ceres failed, as almost none were found, and the shape-gravity model of Ceres appears to restrict water ice content in the outer layers to <30% (depending on model assumptions). This paradox is potentially resolved if it is recalled that crustal materials respond very differently under the extreme strain rates experienced during impact and during epochal-scale creep. Here we will overview the observations and findings of the Dawn mission and their implications for our understanding of the physical state of these two large asteroids.