The surface composition of Ceres: Using new IRTF spectral measurements Andrew Rivkin (JHU/APL) Eric Volquardsen (UH/IRTF) Beth Clark (Ithaca College) Icarus, press.

HST image of Ceres (J. Parker PI) Ceres is the largest object in the asteroid belt, and a target of the Dawn mission. Recent work suggests Ceres is a complex object with a rocky core over an icy mantle, perhaps with a primitive crust, and the prospect for profound chemical evolution having occurred over its history. Despite 30+ years of observations, interpretations of its surface composition still disagree on the presence / absence of water ice, ammonium (NH4+), and whether any of the meteorites in our collections are good analogs for the composition of Ceres. HST image of Ceres (J. Parker PI)

Observations The spectrum of Ceres contains few diagnostic absorption features at visible wavelengths, so the 3-m region, where OH and H2O produce diagnostic absorptions in minerals, is critical for understanding the composition of Ceres. Due to its superior atmospheric transmission, Mauna Kea is the only northern hemisphere site where the 3-m wavelength region can be regularly observed, and the IRTF is one of the few telescopes in the world with the instrumentation necessary for this spectroscopic observation. Atmospheric transmission vs. wavelength (bottom), compared to meteorite spectra (top)

Modeling We modeled the 2-4 m spectrum of Ceres averaged over an entire night using a Hapke theory based mixing code. Models with ~5% carbonates (solid lines), an amount similar to what is seen in CI meteorites, are a much better fit than models with no carbonates (dashed). When considered in combination with archival KAO data leads to preference for iron-rich clays over ammonium-rich clays. Furthermore, water ice appears to be excluded at a meaningful level over a large fraction of Ceres’ surface. Ceres spectra (black and red) compared to CM meteorite (green) and model spectra (other lines)

Implications Along with Earth and Mars, Ceres is only the third solar system object with evidence for carbonates, supporting models of Ceres’ chemical evolution that predict release of CO2 during aqueous alteration. The preference for iron-rich clays (also found in CI meteorites) suggest a relatively oxidized precursor material for Ceres. Other objects are also observed (with the IRTF) to have Ceres-like band shapes, suggesting the possible use of 3-m band shapes to map out oxidation state of the early solar system. Objects with Ceres-type bands (squares) appear in restricted parts of the asteroid belt, compared to Pallas-type bands (stars)