NIR Phase Slopes of C-complex Asteroids as an Indicator for the Presence of Organics and Volatiles: A Preliminary Study

C-complex asteroids are scientifically interesting due to their association with carbonaceous chondrite meteorites and because they are water rich planetesimals and thus potentially represent possible sources of volatile and organic material for the inner solar system. Further study of C-complex asteroid families can provide insight into the role such asteroids may have played in early solar system evolution. Compounds of particular interest include water ice, organic species, and ammoniated materials (which have outer solar system origins) that have spectral features in the 3-4μm region, wavelengths covered by the W1 band of the Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE) mission. To explore the presence of such materials, we investigate the near-infrared (NIR) photometry provided by the NEOWISE mission as a function of phase angle for low-albedo, outer main belt asteroids that display 3μm spectral features, most of which fall within the C-complex. Previous research indicates a correlation between the shape of an asteroid’s phase curve and its taxonomic type, suggesting a link between the phase curve and surface composition. While most available asteroid phase curves use brightness measurements made at visible wavelengths, we extend this to NIR wavelengths to search for correlations between the shape of the NIR phase curve and the presence of compounds with spectral features within the W1 bandpass. This work represents a pilot study intended to demonstrate the efficacy of NIR phase slope as an indicator for the presence of organic and volatile material, with the goal of extending this analysis to a large number of C-complex asteroid family members in order to explore the potential contribution of this class of bodies to the inner solar system’s water and organics. Here, we present NIR phase slopes of asteroids with detected 3µm absorption features, as well as NIR phase slopes for select featureless objects for comparison. To put these measurements in context, we also present spectral mixture modeling of available NIR spectra of asteroids with detected 3µm features, resulting in abundance estimates for the substances of interest (water ice, ammoniated materials, and selected organics).