GEOLOGIC MAPPING OF FRACTURES AND SECONDARY CRATER CHAINS UNCOVERS THE INTERIOR AND SURFACE EVOLUTION OF CERES

In 2015 Dawn became the first spacecraft to visit and orbit Ceres, a dwarf planet and the largest body in the asteroid belt (radius ~470 km) (Russell et al., 2016). Before Dawn’s arrival, telescopic observations and thermal evolution modeling indicated Ceres was differentiated, with an average density of 2,100 kg/m³ (e.g. McCord & Sotin, 2005; Castillo-Rogez & McCord, 2010). Moreover, pervasive viscous relaxation in a water-ice-rich outer layer was predicted to erase most features on Ceres’ surface (Bland, 2013). However, a full understanding of Ceres’ surface and interior evolution remained elusive until Dawn’s exploration of Ceres. Here we present a global geologic map, based on images from Dawn’s Framing Camera, that highlights the ≥1 km wide linear features on Ceres’ surface. We interpret that these linear features form: 1) as the surface expression of subsurface fractures, and 2) as material ejected during impact-crater formation impacts and scours the surface, forming secondary crater chains. The formation and preservation of these linear features indicates Ceres’ outer layer is relatively strong, and is not dominated by viscous relaxation as predicted. The fractures, called the Samhain Catenae, give us insights into Ceres’ interior: their spacing indicates the fractured layer is ~30 km thick, and we interpret that the Samhain Catenae formed because of uplift and extension induced by an upwelling region, consistent with geodynamic modeling (King et al., 2016). In addition, we find that many secondary crater chains are arranged radially around their source impact craters. However, the Junina Catenae secondary crater chains are not radial to their source impact crater(s). On account of Ceres’ fast rotation (period of ~9 hours) and relatively small radius, we interpret that the Junina Catenae originated from the Urvara and/or Yalode impact craters, which are located in a different hemisphere than the Junina Catenae. Our results show that Ceres has different surface and interior characteristics than predicted, and that Ceres underwent a complex surface and interior evolution. On account of Ceres’ place as the largest body in the asteroid belt, our fuller understanding of Ceres’ evolution, based on Dawn data, gives us important insights into both the evolution of Ceres and the evolution of the entire asteroid belt.