The composition and origin of the C, P, and D asteroids: Water as a tracer of thermal evolution in the outer belt

We present results of an extensive 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 in that region for the 3-ωm reflectance absorption of molecular H₂O and structural OH ions; the 3-ωm band is the spectral signature of meteorite and asteroid hydrated silicates. The 19 asteroids observed in the survey, augmented by earlier reflectance data, yielded 3-ωm 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, evidence of a mild aqueous alteration episode early in solar system history. The C class is not a primitive asteroid group in the mineralogical sense, but its anhydrous members appear little altered, as do the P and D classes beyond 3.5 AU. In addition to this pronounced hydration state difference between Cs and the more distant Ps and Ds, hydrated silicate abundance declines gradually among the Cs from 2.5-3.5 AU. Coupled with the apparently anhydrous P and D surfaces, this points to 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 outer belt asteroids is a continuation of the intense heating and differentiation that occurred sunward of 2.5 AU, and both events are consistent with the induction heating mechanism. Water ice may be preserved in the interiors of P and D objects, and in some larger Cs.