Lunar Dust and the Gegenschein

Abstract

OF the four possible explanations of the gegenschein which have been proposed, not one so far has explained satisfactorily the meagre observational data concerning this phenomenon. The Glyden–Moulton hypothesis1, which suggests a concentration of meteoric material at a libration point of the Sun–Earth system, seems incapable of explaining all the luminosity of the gegenschein. The zodiacal light hypothesis2, which claims that the glow opposite the Sun results from a phase-function for the interplanetary dust layer which produces such a brightening, fails to explain the fact that the gegenschein usually lies approximately 3° west of the anti-solar point3. The gaseous tail hypothesis4,5, in explaining the light as the result of excitation of Earth-escaped gases by the interplanetary plasma, does not account for the fact that the colour of the gegenschein is slightly redder than that of the Sun6. Finally, the dust tail hypothesis7, although explaining the facts in a qualitative way, at least, requires a large and continuous source of dust particles in Earth's neighbourhood.