The distribution of mantle material among main-belt asteroids

International audienceWe expect there to have been many bodies in the Main Asteroid Belt (MB) sufficiently heated at the time of solar system formation to allow their interiors to differentiate into an iron core and silicate-rich crust and mantle. Evidence for early solar system differentiation includes the diversity of iron meteorites that imply the existence of over 60 distinct parent bodies (Mittlefehldt et al. 2006). Searches have been performed to identify silicate-rich basaltic crust material (spectral V-type asteroids) in the outer MB have been successful (e.g., Roig et al. 2006, Masi et al. 2008, Moskovitz et al. 2008, Solontoi et al. 2012). The olivine-rich mantles of differentiated asteroids should have produced substantially greater volumes (and therefore substantially greater numbers) of remnant asteroids compared with basaltic and iron samples. Yet olivine-rich asteroids (A-types) are one of the rarest asteroid types (Bus & Binzel 2002, DeMeo et al. 2009). An alternative way to search for differentiated bodies that have been heavily or completely disrupted is to identify these spectral A-type asteroids, characterized by a very wide and deep 1 micron absorption indicative of large amounts (> 80%) of olivine. Burbine et al. (1996) proposed that these asteroids are only found among the largest because most were ``battered to bits'' due to collisions, so smaller A-types were below our detection limit. Using the Sloan Digital Sky Survey Moving Object Catalog to select A-type asteroid candidates, we have conducted a near-infrared spectral survey of asteroids over 12 nights in the near-infrared in an effort to determine the distribution and abundance of crustal and mantle material across the Main Asteroid Belt (MB). From three decades of asteroid spectral observations only ~10 A-type asteroids have been discovered. In our survey we have detected >20 A-type asteroids thus far throughout the belt, tripling the number of known A-types. We present these spectra and their distribution throughout the MB. We estimate the total mass of mantle material present in the belt today and discuss the implications