Geochemistry, Thermal Evolution, and Cryovolcanism on Ceres with a Muddy Ice Mantle

Introduction: Interpretation of data acquired at Ceres by the Dawn spacecraft demands a model for the evolution of Ceres’ structure and composition to date. In a recent paper [1], we presented such a model, consistent with pre-Dawn observations and preliminary data returned by Dawn. Here, we describe this model, compare its physico-chemical outcomes to reported observations, and outline possible tests by ongoing Dawn measurements. Internal structure: Constraints on Ceres’ density and structure come from its mass of 9.4× 10 kg [2], size, shape (assuming Ceres is hydrostatic) of ≈ 482 × 480 × 446 km [3], and gravity measurements [3,4]. These suggest a bulk density near 2150 kg m−3, with a central density concentration [3]. Following [5,6], we assume that Ceres accreted ice and both μmand mm-sized rock particles (mostly silicates and organics), and that micron-sized fines stayed suspended in liquid during differentiation, yielding a core of chondrules and a mantle of mixed ice and fines (“mud”). This assumption reconciles apparently conflicting observations of Ceres’ nearsurface composition: on one hand, it appears icy, exhibiting little large-scale topography [2]; pit craters suggestive of volatile basement material [7]; a low simple-to-complex crater transition diameter [7]; flows, domes, and evidence for glacial mass wasting [8]; and production of water vapor [9]. It also lacks a collisional family, possibly because icy mantle fragments sublimate after ejection [10]. On the other hand, Ceres’ surface is dark and uniform, with mean geometric albedo < 0.1 [11,12] and spectra consistent with hydrated minerals whose unique composition suggest an (in part) endogenic origin [13-15]. Moreover, Ceres’ small-scale topography requires a material stronger than ice [16]. Many two-layer structures can be matched to the above observables by adjusting the bulk rock density (which sets the bulk ice:rock ratio) and fraction of rock in fines. Here, we choose a rock density ρc = 2900 kg m −3, that of grains in CM chondrites [1]. Assuming hydrostaticity, the wide range of reported shapes [2,3,17] can be matched by interiors with a rocky core size up to 360 km. We explore end members with 75% of the rock in chondrules and 25% in fines, yielding a 360-km core and a mantle comprising 74 vol% ice and 26 vol% fines; and with 1% of the rock in chondrules and 99% in fines (85km core with a mantle comprising 62 vol% fines). Early hydration and differentiation: Our 1-D numerical simulations of thermal evolution [1] suggest that following accretion, radionuclide decay heating melts ice in the central layers. Melting can occur quickly throughout the interior if Ceres accretes abundant short-lived radionuclides such as Al (accretion within 4 Myr after Ca-Al-rich inclusions), or within tens of Myr and only at depth otherwise. In the first case, we assume that chondrules and fines are quickly hydrated and emplaced on the surface, overturned by impacts. In the second case, we calculate analytically that in muddy liquid of density ρl and viscosity η, chondrules of radius D fall distances ∆R by Stokes flow on decadal timescales: