CERES’ OCCATOR CRATER AND ITS FACULAE REVEALED THROUGH GEOLOGIC MAPPING

When the Dawn spacecraft arrived at dwarf planet Ceres in March 2015, bright regions were observed on the mostly dark surface. The brightest regions are found in the 92-km-diameter Occator crater (Nathues et al., 2015; Russell et al., 2016; Schenk et al., 2017), which is ~20-30 million years old (Nathues et al., 2017; Neesemann et al., 2017). The central bright region is Cerealia Facula and the ancillary bright regions in the eastern floor are the Vinalia Faculae. The faculae are mostly composed of sodium carbonate, are distinct from Ceres’ average surface composition and are proposed to be the solid residues of crystallized brines (De Sanctis et al., 2016). The faculae’s emplacement mechanism is intriguing: did the Occator-forming impact trigger the formation of the faculae, or was an endogenic process involved? Here we present a detailed geologic map of Occator crater, which we use in conjunction with a variety of studies by the Dawn Science Team to decipher Occator’s geologic history. The crater is surrounded by an asymmetric ejecta blanket, which indicates that the impactor came from the northwest at ~30-45°. Terraces encircle the crater’s interior and are interpreted to form via crater-wall collapse shortly after the impact. At a similar time, mass wasting likely formed the hummocky crater floor materials that dominate the northwestern and northern floor. Lobate materials fill the floor from the northeast to the west, and are subdivided on the basis of their surface textures (smooth, knobby and hummocky), which grade into one another and are likely indicative of variations in emplacement process. The lobate materials superpose, and appear to have flowed onto, the crater terrace and hummocky crater floor materials. We are currently investigating whether the lobate materials are impact melt with enhanced mobility/volume due to impact-heated volatiles that were pre-existing within the target material. Cerealia Facula is located within the crater’s central pit, and both faculae are associated with fractures (Buczkowski et al., 2017). The Cerealia Facula coats the majority of the central pit, which also contains a dome. The Vinalia Faculae are diffuse deposits located within the lobate materials, while Cerealia Facula is a more continuous deposit. Recent mass wasting formed talus deposits on the crater walls.