An infrared reflectance study of water in outer belt asteroids: Clues to composition and origin.

• Main Author: Jones, Thomas David.
• Other Authors: Lewis, John S.
• Language: en
• Published: The University of Arizona. 1988
• Subjects: Asteroids > Analysis.; Water.
• Online Access: http://hdl.handle.net/10150/184487

This study consisted of a comprehensive laboratory and telescopic investigation of H₂O distribution among the low-albedo, outer belt asteroids (2.5-5.2 AU). The water distribution was determined by surveying asteroids for the 3-μm molecular H₂O and structural OH ion absorption, the spectral signature of meteorite and asteroid hydrated silicates. Survey results were interpreted using the mid-infrared (2.5-25 μm) reflectance spectra of 16 carbonaceous chondrites and other likely asteroidal materials. The 19 asteroids observed in this program, augmented by earlier reflectance data, yielded 3-μm band depth measurements that reflect the present outer belt H₂O distribution. Of the 32 C-class asteroids in this sample, 66% have hydrated silicate surfaces, indicating a mild aqueous alteration episode early in solar system history. Strictly speaking, the C class is thus not a primitive asteroid group, but the anhydrous objects appear little altered, as do the P and D asteroid classes beyond 3.5 AU. In addition to this pronounced difference in hydration state among the outer belt classes, the C region shows a gradual decline in hydrated silicate abundance from 2.5 to 3.5 AU. This trend, coupled with the apparently anhydrous P and D surfaces, is consistent with an original outer belt asteroid composition of anhydrous silicates, water ice, and complex organic material. Early solar wind induction heating of proto-asteroids declined in intensity with heliocentric distance, and produced the observed radial decrease in hydrated silicate abundance. The mild thermal processing of the outer belt is a continuation of the intense heating and differentiation episode that occurred sunward of 2.5 AU, and both events support the induction heating mechanism. The larger outer belt inventory of volatile-rich objects may have been a significant contributor to the atmospheres of the terrestrial planets.