Compositional Variegation of Large-Diameter Low-Albedo Asteroids
F. Vilas (NASA JSC), K.S. Jarvis (LMSMSS), T.D. Anz-Meador (VSTI), C.A. Thibault (Denison U.), S.R. Sawyer (NASA Ames), A. Fitzsimmons (Queen's U., Belfast)
Asteroids showing signs of aqueous alteration and thermal metamorphism
in visible/near IR spectroscopy and photometry (C, G, F, B, and P classes)
ranging from 0.37 - 0.90 m dominate the asteroid population at
heliocentric distances of 2.6 - 3.5 AU. Age dating of meteorites
indicates that the Solar System was subjected to a major heating event
4.5 Gyr ago. Recent meteoritic research has produced evidence of a
carbonaceous chondrite subjected to two separate aqueous alteration events
with a metamorphic heating inbetween (Krot et al., 1997, submitted).
Models of the effects of heating by electromagnetic induction or decay of
short-lived radionuclides combined with models of the early collisional
history of the Solar System after Jupiter's formation indicate that
asteroids observed today can be divided into two groups by diameter.
Those asteroids having diameters greater than 100 km were mixed by
multiple collisions but remain as gravitationally bound rubble piles.
Asteroids with diameters less than 100 km should show more compositional
diversity. Vilas and Sykes (1996, Icarus, v. 124, 483) have shown using
ECAS photometry that this compositional difference exists. Those
asteroids having diameters greater than 100 km should be individually
homogeneous, with spectral differences showing the combined effects of a
primordial compositional gradient in the asteroid belt with thermal
metamorphism. We address the significance of spatially-resolved spectra
of 42 asteroids to the collective origin of these asteroids.