Elemental constraints on Ceres’ hydrothermal evolution and crustal processes from data acquired by Dawn’s Gamma Ray and Neutron Detector

Dawn showed that Ceres’ global regolith contains abundant hydrogen in the form of aqueously altered minerals, subsurface water ice, and possibly organic matter. The surface is dotted with bright materials rich in salts and hydrated minerals, the remnants of a subsurface ocean or the products of impact-induced hydrothermal alteration. Initial analyses of the elemental data acquired by Dawn’s Gamma Ray and Neutron Detector (GRaND) indicated that, unlike the smaller parent bodies of carbonaceous chondrites, Ceres underwent ice-rock fractionation and has a partially differentiated interior, consistent with geophysical observations.

Our goal is to test and refine hypotheses for Ceres hydrothermal evolution using elemental data acquired by GRaND, focusing on widely accepted models that support the formation of a volatile-rich crust and subsurface ocean. Estimates of Ceres’ bulk crustal composition are inconsistent with the composition of the regolith, which contains unknown exogenic contributions. Thus, any investigation of Ceres’ interior using near-surface chemistry must be accompanied by studies of the origin and evolution of the crust and regolith. Understanding crustal processes would also help identify events in Ceres history, such as possible early exposure of oceanic materials via crustal overturn.

Regolith composition will be determined by combining published mineralogical data with improved GRaND analyses and mapping of elements such as H, C, K, and Fe, as well as new analyses of elemental signatures and mass ratios, including as Mg/Si and Fe/Si. Mass ratios are less susceptible to bias than absolute concentrations and provide additional constraints on key geochemical processes. Elemental mapping and thermophysical modeling of ice and bound water will be combined to determine the distribution and layering of volatiles, providing a complete picture of Ceres’ shallow subsurface.

We will build on modeling, data reduction, mapping, and analysis pipelines developed during the Dawn mission, utilizing PDS- archived GRaND experimental and reduced data records, ancillary data, and shape models. At Ceres, low-spatial-resolution, global elemental mapping data were acquired along with high-resolution data for selected geologic units, such as Occator crater. A forward modeling approach will be used to analyze GRaND data, fully integrating data acquired at different orbital altitudes and during orbital transitions. For example, this will enable comparison of freshly ejected, crustal materials from the Occator region with the global regolith to support characterization of endogenic and exogenic components.

We will improve elemental analyses by applying spectral unmixing methods demonstrated on Lunar Prospector. A Bayesian Markov chain Monte Carlo approach will be used to address the nonuniqueness of the remote sensing data to determine the plausible range of compositional end members, including exogenic contributions. Spatial deconvolution and forward modeling will be used for focused studies of regional-scale geochemical units. Data products will be archived as supplementary material to two proposed publications.