CERES: THE DWARF PLANET NEXT DOOR

Introduction: Pluto, Charon, and the other bodies in that system are typically compared to transneptunian objects (TNOs) due to similar histories and compositions. However, as we learn more about the largest asteroids and the overlap between the asteroidal and cometary populations, it seems clear that the asteroids can provide interesting context for the exploration of the Pluto system. Here, I look specifically at Ceres, the current target of the Dawn spacecraft.

Some Darkness Before Dawn: A decade of observations and modeling of Ceres suggested that Ceres would be differentiated into a rocky core beneath an icy shell. The temperature of Ceres’ surface was calculated to be too warm to maintain ice except at very high latitudes, but it was thought that ice would be stable in the shallow subsurface. The relatively high temperature of this subsurface ice was expected to lead to viscous relaxation of most of Ceres’ topography.

Disk-integrated spectra of Ceres were consistent from one observer to another, but the composition interpreted from those spectra varied. Carbonates had been identified in 2006, but an absorption near 3.06 µm was variously assigned to organic material, iron-rich clays, brucite, and ammoniated phyllosilicates prior to Dawn’s arrival, generally pointing to heavily aqueously-altered products. The possibility of ammoniated material gave rise to speculation that Ceres was formed among the TNOs. It also led to the possibility that liquid water might still exist today deep in Ceres’ interior, kept from freezing by the presence of ammonia.

A Bit of What We’ve Learned: Dawn’s encounter has led us to revisit many of our previous conclusions about Ceres. Initial views showed that it maintained plenty of topography, and it is no longer thought to be fully differentiated. The Dawn team now interprets Ceres’ spectrum as indicating ammoniated minerals. Finally, Occator crater, one of the famous “bright spots” first seen in HST data, seems to point to recent or even ongoing activity. All together, the Dawn data seems to indicate Ceres is a much more complex and interesting body than even enthusiastic boosters had hoped.

What is Ceres? That’s a good question. I will present a summary of current thought about Ceres, with an eye to properties that may be of interest to the Pluto community.