The composition of near-earth objects

Abstract

Thesis (Ph. D.)--University of Washington, 1998I present reflectance spectra of 34 near-Earth objects (NEOs), 6 main-belt asteroids, and four non-NEO cometary candidates, all obtained with the Apache Point Observatory 3.5m telescope + Double Imaging Spectrograph. The spectra cover the wavelength range 3800-10,000 A, encompassing regions of mineralogically important absorption features. Nearly all of the NEOs observed display ultraviolet and near-infrared absorptions characteristic of rock-forming silicate minerals. Of the 27 NEOs belonging to the S or Q taxonomic classes observed in this study, 15 are spectrally indistinguishable from ordinary chondrite meteorites. I perform extensive Monte Carlo simulations of the NEO and main belt populations aimed at quantifying the severe biases affecting observed taxonomic distributions. The bias-corrected NEO population in the 1-10 km diameter range is composed of 67 $\pm$ 13% S- or Q-type, objects, and 30 $\pm$ 7% C-types objects, with the remainder being primarily of the spectrally degenerate X-Class. The NEO population resembles most closely that of the inner main belt near the 3:1 mean motion resonance, and is consistent with that region being the sole source for NEOs. If extinct comet nuclei resemble the primitive taxonomic classes C, P, or D, the cometary component of the NEOs is constrained at \sbsp{\sim}{<}30%. I investigate trends of S-type spectral characteristics with size. The strength of the 1 $\mu$m absorption increases with decreasing size. There is a possible trend towards shorter-wavelength band centers with decreasing size, possibly reflecting a decreasing olivine abundance in the optically active surface fraction. For sizes below about 6 km, the spectral continuum reddens with increasing size. Above 6 km, this trend reverses, and larger objects have on average bluer continua. For the smaller objects, these trends are all consistent with predictions of the "space weathering" hypothesis. The bluer continua of the larger objects remains unexplained. All of these trends appear to be systematically dependent on size; specifically, no distinct separation exists between the larger S-type objects and the smaller ordinary chondrite-like bodies