Cometesimals in the inner solar system

Abstract

Spectra obtained by the ultraviolet spectrometer on the Voyager 2 spacecraft between 1 and 2.5 AU are shown to provide evidence for a very large number of 'cometesimals' with radii between a few meters and a few tens of meters in the neighborhood of the earth. The evidence consists of a component in the interplanetary Lyman-alpha radiation that decreases rapidly with heliocentric distance. It is proposed that the source consists of cometesimals which are ice-coated, porous, low-density refractory boulders that may be the building blocks of ordinary comet nuclei. It is shown that the cometesimals required to produce the observed Lyman-alpha emission can also account for all the lunar craters with diameters between 200 m and 1500 m produced during the phase 3200 million years at sites such as Mare Tranquillitatis.