THE “MISSING MANTLE” OF THE ASTEROID BELT: PHYSICAL AND GEOCHEMICAL PROPERTIES OF TAXONOMIC A-CLASS ASTEROIDS

A puzzling question in planetary geology is the apparent rarity of olivine-dominated A-class asteroids in the main belt; they represent no more than 17 known objects. As surveys record smaller objects, the abundance of olivine-dominated asteroids is not increasing. These results suggest several scenarios for their scarcity; these asteroids either: (1) have been shattered to small sizes (<5 km) over the collisional lifetime of the asteroid belt, (2) are abundant but their spectra have been altered in some way masking their presence, or (3) differentiated asteroids did not form thick olivine-rich, metal-poor mantles, and differentiation on these bodies is not understood.

We are seeking to address these questions on two fronts: a) collecting new observational data on recognized A-class asteroids, and b) characterizing olivine-bearing meteorite classes for comparison to spectral results. We present here initial photometric observations of three A-class asteroids: 246 Asporina (d ~ 60.1 km), 446 Aeternitas (~ 45.4 km), and the small Near Earth Asteroid (NEA) 3352 McAuliffe (~ 2.0 km). Results for 246 and 446 were obtained using the 0.4 m telescope at Egan Observatory during April 2009. Results for 3352 were obtained using the 0.8 m telescope at Tenagra Observatories, Ltd. during June 2010 and reveal a partial lightcurve with colorimetric reductions forthcoming. We derive refined rotational periods for: 246 Asporina (P_syn = 16.234 ± 0.001 h, amplitude ~ 0.25 mag), and 446 Aeternitas (P_syn = 15.740 ± 0.003 h, amplitude of ~ 0.40 mag).

The olivine-rich achondrite meteorites have been well characterized geochemically; however, silicates in olivine-rich stony-irons (pallasites) have been less well studied. Ureilites are posited as being shallow mantle residues, but high concentrations of Ru, Re, Os, and Ir would seem to preclude the segregation of a metallic core in their parent planet. In contrast, siderophile and PGE abundances in pallasites corroborate a model of formation during crystallization at a parent body core-mantle boundary. The highly siderophile PGEs - Ru, Re, Os, and Pt - correlate positively with Ir in ureilites; however, correlations with Ir in pallasites, are far more variable. What is clear is that these different meteorite classes are distinguishable via their PGE signatures.