Behavior and Stability of Ground Ice on Ceres: Initial Clues from Dawn

Abstract:

Models of the historical evolution of Ceres [1] and recent observations of surface geomorphology by the Dawn spacecraft [2] suggest a crust with a substantial fraction of water ice. However, clear spectral detections of ice are absent indicating that the uppermost material is dry. Further constraints on near surface ice come from detections of water vapor around Ceres by the Herschel telescope [3], which indicate production rates of 10²⁶molecules per second.

Here we build on the pioneering work of [4] and examine ground ice stability with the benefit of these new observations as well as accurate pole-vector determinations by Dawn. We model surface and subsurface temperatures on Ceres by balancing surface insolation, thermal emission and conduction to the subsurface. We estimate ice loss rates for both surface ice and ice covered with a low-thermal-inertia sublimation lag.

Dawn observations show no large high-albedo regions indicating pure surface ice. In the case where pore-filling ice (50% ice by volume) extends to the surface, average loss rates range from almost zero (mm/Gyr) at the poles to several decimeters per year at the equator. These loss rates are suppressed by 2-3 orders of magnitude when this pore-filling ice is covered by even a few centimeters of dry sublimation lag, which is expected to form on geologically short timescales. Assuming negligible internal heat flux, we estimate the outgassing of water molecules expected from buried ice at all latitudes and in combination with previous work [5,6] find that the observations of [3] cannot be due to global sublimation of buried ice.

Dawn observations also show the presence of small-scale high-albedo spots. Our models show that, if icy, the most prominent of these (in Occator crater) loses ~2cm/yr. Suggestively, the area and sublimation rates of the Occator bright spots match the observed vapor production rates of [3]. However, these high loss rates are difficult to reconcile with long-term spot persistence without a source of resupply. Continued monitoring by Dawn over the coming months in the high- and low-altitude mapping orbits will test for changes.

[1] McCord and Sotin, J. Geophys. Res., 2005. [2] Schmidt et al., this conference. [3] Kuppers et al., Nature, 2014. [4] Fanale and Salvail, Icarus, 1989. [5] Schorghofer, Ast. J. 2008. [6] Schorghofer, LPSC 2015.