The Surface Composition of Ceres

Abstract

Our understanding of the composition of Ceres is driven by remote sensing of its surface. We review spectral observations of Ceres over wavelengths from the ultraviolet to the radio, as well as non-spectral data such as thermal inertia, photometric properties, radar experiments, and surface variability. We also discuss the closest likely meteorite analogs to Ceres and consider the likelihood that material from Ceres could be delivered to Earth.

The case of the missing Ceres family

“…This idea is not original with this work, and others have broached the idea in a qualitative sense (Li et al, 2006;Rivkin et al, 2012b;Milani et al, 2014) Ceres' shape and moment of inertia have been interpreted as indicating an ice shell above a rocky core (Thomas et al, 2005), in agreement with thermal evolution models (McCord and Sotin, 2005;Castillo-Rogez and McCord, 2010). These thermal evolution models predict the top $10 km of Ceres would remain undifferentiated (McCord and Sotin, 2005), but the density contrast between the undifferentiated crust and the warm ice shell below would likely cause the crust to founder.…”

“…We take the surface of Ceres itself as representative of the sort of lag deposit under discussion. Ceres has a thermal inertia (615 J m À2 K À1 s À1/2 : Spencer et al, 1989) comparable to that of the lunar regolith when the temperature dependences of thermal capacity and heat capacity are taken into account (Rivkin et al, 2012b). The lunar regolith has a typical grain size on the order of 50-100 lm, capable of being held aloft by the gas outflows discussed here.…”

“…The surface composition of Ceres has been interpreted in the past as indicating ammoniated phyllosilicates (King et al, 1992), and the possible presence of NH þ 4 leads naturally to considering an outer Solar System origin. However, other interpretations for the ammonium absorption features exist (indeed, more recent interpretations assign those absorptions to brucite: Milliken and Rivkin (2009)), and mid-IR spectra of Ceres appear inconsistent with ammoniated species (Rivkin et al, 2012b).…”

Extensive water ice within Ceres’ aqueously altered regolith: Evidence from nuclear spectroscopy

“…, the best measurement from Earth-based observatories (49), and a grain density of 2.5 g/cm 3 . The particle size in the surface layer is varied, but must be small (≲100 µm) to be consistent with the low thermal inertia.…”

Asteroid Photometry

“…Rivkin et al, 2011). Ample evidence suggests that water must have played an important role in its evolutionary history and its current status (Rivkin et al, 2011).…”