Probing asteroid composition using visible and near-infrared spectroscopy.

Abstract

The compositional distribution of asteroids provides clues to understanding solar system formation and evolutionary processes in the asteroid belt. The surface mineralogy and distribution of volatiles on asteroids is determined using visible and near-infrared spectroscopy. A revised asteroid taxonomy is developed which incorporates compositional information from the near-infrared asteroid spectra. A search is conducted for organic compounds on distant asteroids, thought to be most volatile-rich in composition. Our near-infrared spectroscopy of outer belt asteroids shows a trend of increasing red continuum slope with heliocentric distance. I interpret this trend as a compositional change, possibly due to increasing abundance of complex organic compounds on these more distant objects. I do not observe a CN absorption band near 2.2 $\mu$m, and conclude that the organics present are not primarily CN-bearing solids. The 3-$\mu$m water absorption band is not detected on distant D asteroids, but is seen on main belt D asteroids. This observation is consistent with the idea that D asteroids are volatile-rich, but the object must be heated in order to transform the silicates into hydrated minerals. No evidence of clay minerals is seen on any of the distant asteroids observed. The majority of the Jupiter Trojan asteroids have a uniform spectral appearance in spite of collisional processing, which implies that the dark red surface material is not a thin surface coating, but is representative of the bulk composition. Observations of near-Earth asteroids indicates a wide range of compositions, suggesting a variety of source regions. Two objects are detected which may contain hydrated minerals, a valuable resource in space. Three near-Earth asteroids are studied in detail, revealing a range of pyroxene chemistry and olivine content inconsistent with ordinary chondrite composition. The occurrence of spectral variability, and implied compositional inhomogeneity is approximately 20%, and on spatial scales as small as tens of meters. This observation implies that asteroids are geologically complex, not chemically uniform, as is often assumed in meteorite studies.