Evidence for Ice Meteoroids Beyond 2 AU

Abstract

Humes et al. (1974) deduced from the P-10 (Pioneer 10) meteoroid penetration data that the spatial density of 2 × 10−9 g and larger meteoroids was nearly constant (or possibly increasing) with increasing heliocentric distances between 2 and 5 AU from the sun. With an assumed mass density of 0.5 g/cm3, P-10 particles (particles whose mass is ⪞ 2 × 10−9 g) would have a particle radius in excess of about 10 μm. The observation of a constant, or increasing, spatial density leads to some interesting conclusions regarding the processes that control the population of P-10 particles between 2 and 5 AU from the sun. We shall explore some of these processes below and shall obtain the result that the P-10 meteoroid data can be best understood if many of the penetrating meteoroids are made of ice.