The Hydrated Mineralogies of the Largest Asteroids

Introduction: There is evidence that the largest asteroids are different from the smaller ones. Collisional evolution models suggest that objects above 100 km or so are overwhelmingly likely to remain intact through solar system history, while those below roughly 50 km are likely to be a fragment of a once larger object [1]. Recent planetesimal creation and evolution models suggest that asteroids were “born big”, going straight from mm-scale particles to objects 100 km in size or larger [2]. Only 28 objects in the present-day asteroid belt have diameters of 200 km or larger, with two additional large objects disrupted to form the Themis and Eos families. The list of the largest asteroids is dominated by low-albedo objects. Only 6 of the 28 objects have albedos higher than 0.10, and one of them is 2 Pallas (which has an albedo very close to that threshold and belongs to a taxonomic class typically included among the low-albedo groups). The remaining 22 objects are split fairly evenly in terms of taxonomic class between the Ch/Cgh class (6 objects), the B or Cb class (5 objects), the C class (6 objects), and classes in the Xcomplex (5 objects). We report new observations in the 3-μm region that, together with published work, provide insight into the hydrated mineralogy of all 28 objects in the present-day asteroid belt larger than 200 km.