ORIGIN OF BRIGHT MATERIALS AT OCCATOR CRATER, CERES, FROM A MORPHOLOGIC PERSPECTIVE

The most intriguing geologic feature on the dwarf planet Ceres is the bright material at the center of the 92-km-wide very young crater Occator. Global mapping by Dawn has provided morphologic, spectral, and topographic insights into this enigmatic feature. The material forms a discretely bounded but amoeboid-shaped patch coincident with a partly rimmed central depression, or “pit.” The 9-km-wide, 700-m-deep pit strongly resembles central pits observed on Ganymede and Callisto and is similar to others central pits observed in large craters on Ceres. Curiously none are observed on similarly-sized icy bodies orbiting Saturn. The pit is partly occupied by a 600-m-high 3-km-wide dome, with a fractured surface suggestive of uplift or possible inflation from below. Much of the bright material appears painted on the surface but in some locations clearly avoids high standing topography and “flows” downhill, suggesting partial topographic control of emplacement. The bright material has a different color than the knobby crater floor material (likely impact melt) surrounding it, but the fractures on the dome expose a different color yet again. Carbonates may comprise 50% or more of the bright material. The presence of central pit/domes on Ceres and Ganymede/Callisto but not the smaller icy moons suggests water ice plays a leading role in formation but that this is moderated or suppressed by the colder temperatures at Saturn. The mechanism of formation is enigmatic and not directly revealed by imaging, but the central location, partial control by local topography, and composition suggest that hydrothermal fluids may have been driven off by residual impact heat, precipitating solids on the surface and leading to dome formation in the center.