Three basaltic earth-approaching asteroids and the source of the basaltic meteorites

Abstract

We report five-color photometry, near-infrared spectra (0.8–2.5 μm), and thermal observations of the Earth-approaching asteroids (3551) 1983 RD, (3908) 1980 PA, and (4055) 1985 DO2. We derive their diameters as 1.2, 1.0, and 3.4 km, respectively. Apart from greater absorption band depths, the spectra of these three small asteroids are nearly identical to that of Vesta, which is known to have a differentiated basaltic surface. The deeper pyroxene bands (0.9 and 1.9 μm) relative to Vesta are consistent with more pyroxene-rich surfaces, larger mineral grain sizes in their optically immature regoliths, more bare rock surfaces, or all of these factors. These small Earth-approaching asteroids have similar orbits, with perihelia near Earth's orbit in July August. They are probably not fragments of Vesta, but may be fragments of one or more Vesta-like parent bodies. The spectra of Vesta and the three Earth-approachers resemble those of the basaltic meteorites, i.e., eucrites, howardites, and diogenites (HED meteorites). We find that HED fall times show a weak peak in July/August, suggesting that 3551, 3908, and 4055 may be fragments of the source bodies of the HED meteorites, or perhaps of a single common HED parent body. The kilometer-size asteroids have regoliths with significant insulating properties, but these regoliths differ from that of the Moon in that the particle sizes are larger than on the Moon, and lunar-like glasses and agglutinates are largely absent. The causes of these differences are related to impact velocities, impact shaking, and debris retention on small bodies. Differentiated basalt or mantle-like asteroids may have impacted Earth and left discontinuities in the geologic/climatic record without leaving the chemical signatures associated with the KT boundary event.