Phase Angle Effects on 3-micron Absorption Band on Ceres: Implications for Dawn Mission

Abstract: Phase angle-induced spectral effects are important to characterize since they affect spectral band parameters such as band depth and band center, and therefore skew mineralogical interpretations of planetary bodies via reflectance spectroscopy. Dwarf planet (1) Ceres is the next target of NASA's Dawn mission, which is expected to arrive in March 2015. The visible and near-infrared mapping spectrometer (VIR) onboard Dawn has the spatial and spectral range to characterize the surface between 0.25-5.0 microns. Ceres has an absorption feature at 3.0 microns due to hydroxyl- and/or water-bearing minerals (e.g. Lebofsky et al. 1981, Rivkin et al. 2003). We analyzed phase angle-induced spectral effects on the 3-micron absorption band on Ceres using spectra measured with the long-wavelength cross-dispersed (LXD: 1.9-4.2 microns) mode of the SpeX spectrograph/imager at the NASA Infrared Telescope Facility (IRTF). Ceres LXD spectra were measured at different phase angles ranging from 0.7o to 22o. We found that the band center slightly increases from 3.06 microns at lower phase angles (0.7o and 6o) to 3.07 microns at higher phase angles (11 o and 22o), the band depth decreases by ~20% from lower phase angles to higher phase angles, and the band area decreases by ~25% from lower phase angles to higher phase angles. Our results will have implications for constraining the abundance of OH on the surface of Ceres from VIR spectral data, which will be acquired by Dawn starting spring 2015.