Lunar and Planetary Science XLVIII (2017) 1374.pdf

Regional analysis of Ceres’ gravity anomalies. Anton I. Ermakov1, Ryan S. Park1, Maria T. Zuber2, David E. Smith2,3, Roger R. Fu4, Michael M. Sori5, Carol A. Raymond1, Christopher T. Russell6. 1Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, 91109, USA ([email protected]); 2Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA; 3NASA/Goddard Space Flight Center, Greenbelt, MD, 20771, USA; 4Lamont-Doherty Earth Observatory, Earth Institute, Columbia University, Palisades, NY 10964, USA; Lunar and Planetary Laboratory, University of Arizona, Tuscon, AZ, 85721, USA; 6University of California Los Angeles, IGPP/EPSS, Los Angeles, CA, 90095, USA.

Introduction: Ahuna Mons: The current gravity model of Ceres is accurate up to Ahuna Mons is a pyramid-shaped mountain of possible degree and order 14 [1], high enough to resolve the cryovolcanic origin located near the equator in the gravitational signatures of several largest impact basins western hemisphere of Ceres (Fig. 2). This feature has a [2]. unique morphology that cannot be found anywhere else Hanami Planum and Occator: on Ceres. It was proposed that hydrated salts with low Hanami planum is the region of high standing terrain eutectic temperatures and low thermal conductivities around the Occator crater (Fig. 1). The Occator crater enabled the presence of cryomagmatic liquids within corresponds approximately to the location where the Ceres, which facilitated the formation of Ahuna Mons [6]. elevations are the highest within the Hanami planum. This Ahuna Mons is associated with a strong positive Bouguer region is associated with the strongest negative Bouguer and isostatic anomaly. However, since the feature is only anomaly on Ceres. Hanami planum is large enough (~600 17 km wide, it is not possible to associate the anomaly km) to be resolved in the current gravity model. Since this region has positive topography and negative gravity with the feature itself, but only with the general anomaly, it is expected that, at least partially, the negative surrounding area. Positive anomaly in the Ahuna Mons anomaly is a consequence of isostatic compensation. region can be explained by an extrusion of high-density However, even after computing the isostatic correction, brines to the surface. Alternatively, we note the possibility the gravitational anomaly is still negative, indicating of a thin crust being the source of, or at least contributing supercompensation. A possible explanation for this to, the positive gravity anomaly at Ahuna Mons, observation could be a combination of a buoyancy-driven analogous to the correlation between thin crust and maria anomaly with a high-rigidity/thick shell at the time scales on the Moon. But why is there only one Ahuna Mons relevant to the buoyant process. Alternatively, a relatively observed on Ceres? It was proposed that cryovolcanic low regional density can create such a negative isostatic domes might have local ice enrichment such that their anomaly. topography quickly relaxes [7]. However, this ice enrichment is at odds with the observed strong positive gravity anomaly. We model the gravitational contribution of Ahuna Mons and the surrounding tholus and produce constraints on the density contrast and composition of these features.

Figure 1: Isostatic anomaly map plotted over projected Dawn clear filter mosaic for Hanami Planum and Occator Figure 2: Ahuna Mons isostatic anomaly map plotted over projected Dawn clear filter mosaic.

Lunar and Planetary Science XLVIII (2017) 1374.pdf

Kerwan: features are located at higher latitudes than Kerwan or With a diameter of 281 km, Kerwan is the biggest lower ice content. However, similar to Kerwan, the two unambiguous impact basin on Ceres (Fig. 3). It is located basins have negative isostatic anomalies indicating near the equator in Ceres' eastern hemisphere. Its subdued subisostasy. topography could indicate that this feature has Conclusions: experienced viscous relaxation. The negative isostatic Regional deviations from isostasy provide a useful insight anomaly within the basin indicates that the basin is about the crustal structure of Ceres. Hanami Planum and subisostatic. [8] argued that viscous relaxation of Kerwan Ahuna Mons – the two regions proposed to have been can only be achieved if the ice content in the subsurface is shaped by cryovolcanic activity – have strong gravity anomalies, however, of opposite signs, which could enhanced relative to the rest of Ceres, which would be indicate different formation mechanisms. Kerwan, Urvara consistent with a negative isostatic gravity anomaly. and Yalode are associated with a negative isostatic anomaly despite having different morphology. The negative anomaly for Kerwan could be interpreted as due to higher ice content in the subsurface, which would facilitate viscous relaxation and explain its subdued topography. For Urvara and Yalode, the negative isostatic anomalies could manifest the fact that their impact features happened relatively recently and the state of isostatic compensation has not yet been achieved leaving the basins subisostatic. References: [1] Konopliv, A.S. et al., (2017) in prep for Icarus [2] Ermakov A.I. et al. (2017) in prep for JGR; [3] Park et al., (2016) Nature 537,515–517; [4] Fu, R.R. et al., (2017) in prep for EPSL; [5] Bland, M.T., (2013) Icarus, 226,1,510-521; [6] Ruesch, O., et al., (2016) Science, 02 Sep 2016, 254,6303; [7] Sori, M.M. et al., (2017) in review for GRL; [8] Bland M.T. (2016) Nature Geoscience, 9,538–542. Figure 3: Isostatic anomaly plotted over the height with respect to the geoid for the Kerwan basin.

Figure 4: Isostatic anomaly plotted over the height with respect to the geoid for Urvara and Yalode.

Urvara and Yalode: Urvara and Yalode are the two adjacent impact features in the southern hemisphere (Fig 4). Their diameters are 163 and 271 km, respectively. Unlike Kerwan, these two impact features have well-defined rims and unrelaxed morphology. This pristine morphology could be explained by either their younger age or their lower rate of viscous relaxation due to lower temperatures since the two