SPECTRAL REFLECTANCE OF NEAR-EARTH ASTEROIDS: IMPLICATIONS FOR COMPOSITION, ORIGIN AND EVOLUTION.

摘要

The reflectance spectrum (0.33 - 1.0(mu)m) of 17 near-Earth asteroids were measured telescopically with a photoelectric photometer. Mineralogical intepretation assuming cosmically abundant material is based on crystal field theory. The surface composition of these asteroids (with one exception) contains olivine, pyroxene, phyllosilicates and opaques (which cannot be mineralogically identified). These spectra have stronger absorptions than that of most main belt asteroids. 2201 1947XC cannot be interpreted in terms of common minerals. Cometary emission features may explain the spectrum.;The mineralogical interpretation of near-Earth asteroids addresses three questions: (1) What are their source regions? (2) What is their relation to meteorites? (3) Are they potential extra-terrestrial resources?;Analogues exist in proposed source regions: near Kirkwood gaps, (/(,6) resonance, and the Flora family. Additional data are needed to confirm similarity of surface composition suggested by similar reflectance spectra. Analogues include: 1580 Betulia and 2 Pallas, 1685 Toro and 349 Dembowska, and 1915 Quetzalcoatl and 4 Vesta. These compositions may be common among unmeasured, small asteroids. The existing data suggest all regions proposed on dynamical grounds, except the 2:1 Kirkwood gap and Mars-crossers (which have not been adequately sampled), may be source regions. Two compositions are suggestive of cometary origin. Others do not agree with the assumption that comets are dirty iceballs.;Represented meteoritic analogues are: LL4 ordinary chondrite, shocked-black H chondrite, carbonaceous chondrite type 3, and diogenite. There are no irons, stony-irons, nor carbonaceous chondrites type 1 or 2. Many achondrites are not represented. The low frequency of ordinary chondrite compositions compared to the meteorites is consistent with the hypothesis that a few parent bodies located in dynamically favorable regions supply 90% of meteorite falls. The number of non-meteoritic analogues is consistent with the recently discovered diversity of meteorites. The absence of nickel-irons supports observed cosmic ray exposure age and dynamical models predicting sources with longer mean life times than near-Earth asteroids.;The near-Earth asteroid population consists of material potentially useful for extra-terrestrial activities. Phyllosilicates and opaques can provide water and other volatiles. At least two objects are inferred to have fine-grained metal.