Cometesimals in the inner Solar System

Abstract

A large flux of small comets in the inner Solar System¹ might produce a recognizable Lyman-α signature near 1 AU (ref. 2). We have now examined spectra obtained by the ultraviolet spectrometer (UVS)³ on the Voyager 2 spacecraft between 1 and 2.5 AU and have found evidence for a very large number of 'cometesimals' with radii between a few metres and a few tens of metres in the neighbourhood of the Earth. The evidence consists of a component in the interplanetary Lyman-α radiation that decreases rapidly (between r⁻³ and r⁻⁴) with heliocentric distance. The Lyman-α emission rate in a direction normal to the Sun-spacecraft line and downwind (the anti-apex direction) in the interstellar wind at 1 AU is 640 rayleighs, of which our analysis attributes 477 R to the interstellar medium (ISM) and 163 R to another source. Despite this result we propose that this source consists of comets of a different sort than those proposed by Frank et al.¹ and with a flux seven orders of magnitude smaller. We propose that these cometesimals are ice-coated, porous, low density refractory boulders that may be the building blocks of ordinary comet nuclei⁴. We show that the cometesimals required to produce the observed Lyman-α emission can also account for all the lunar craters with diameters between 200 m and 1,500 m produced during the past 3,200 million years at sites such as Mare Tranquillitatis.