Where do the meteorites come from? A re-evaluation of the earth-crossing apollo objects as sources of chondritic meteorites

Abstract

The total number of Earth-crossing Apollo objects larger than 500 m in radius is estimated to be 600, based on failure of chance rediscovery, lunar crater frequency and completeness-of-search results of Shoemaker, Helin and Gillett. The number of Amor objects (perihelion between 1.0 and 1.3 A.U.) is estimated to be about 500. These estimates are about an order of magnitude higher than those given by previous workers, and these objects appear sufficiently numerous to dominate post-mare lunar and terrestrial cratering ( d ≥ 10 km). The terrestrial meteorite and meteorite yield of 100-10 6 g bodies derived from fragmentation of Apollo objects is re-evaluated using this estimate, together with more recent data on asteroid albedos and on hypervelocity impact. Terrestrial rate of impacts of these fragments at sufficiently low velocities to penetrate the atmosphere is estimated to be ~2 × 10 8g/yr. This is in the middle of the range of the actual extraterrestrial impact rate based on photographic fireball surveys (Prairie Network), lunar seismometry, and recovery of meteorites. It is likely that most ordinary chondrites are fragments of Apollo objects, provided that these fragments are sufficiently strong to survive atmospheric entry. Possible asteroidal and cometary sources of Apollo objects are reviewed. Several mechanisms for the removal of asteroids into Earth-crossing orbit are qualitatively acceptable, but appear inadequate by at least an order of magnitude to supply the required number. Most Apollo objects are probably the cores of comets which have lost their volatile material by repeated solar evaporation, as proposed by Öpik. The distribution of the component of the Apollo objects' angular momentum perpendicular to the plane of the solar system is tabulated. It is found that considerable non-random clustering of these values exists, for which no adequate explanation is known.