The lunar atmosphere

Abstract

Optical and radio estimates of the upper limit of density of the lunar atmosphere are reviewed. Properties of the lunar ionosphere in contact with the surface are analyzed theoretically and applied to an estimate of its composition and density. Considering the balance between injection and escape, the daytime average probable number density at the surface is rated at 3 to 5 × 10 5 cm −3 , with an uncertainty ratio of about 2 and with 50–70 per cent CO 2 , 45-27 per cent H 2 O anf 4-2 per cent H 2 . The electron density, in equilibrium with contact recombination at the surface, is then 200–300 cm −3 . The sources of the atmosphere are solar wind, its interaction with, and sputtering of, the surface, meteor impact degassing, and somewhat dubious “volcanic” sources. The surface electric charge is slightly negative or neutral; only when the total density falls below 3 × 10 5 , and the electron density below 200, will there be a positive charge. The photoelectric efficiency of silicates is estimated to be of the order of 1 80 , and the electron emission as depending on surface potential is estimated accordingly from the solar ultraviolet emission. Mechanisms of escape—thermal, collisional with solar wind, and ionic—are evaluated quantitatively. Thermal escape prevails for hydrogen; the other two processes for the heavier molecules. The longest escape time scale is about 2 years for xenon, provided the layer is thin (exospheric). No permanent atmosphere can exist on the moon. The conditions of static and escaping plasma equilibrium are reviewed. Conservation of space charge in an escaping plasma stream requires the establishment of turbulent electric fields which force the electrons to oscillate with thermal velocities around the mean motion determined by the momentum of the ions. The fields prevent direct escape of the photo-electrons from the lunar surface, and play a decisive role in the structure of the lunar ionosphere. From cometary phenomena and optical data for the zodiacal light the persistent and principal component of solar wind is estimated to correspond to N 6 = 50 cm −3 , v = 200 km sec , a relative electron temperature of about 400–500°K at the earth's orbit, and a translational energy of 210 eV for the protons and ∼1000°K for the electrons. Whereas other constituents of the lunar atmosphere are escaping to space, water molecules created chemically by solar wind chiefly sublimate into the cold spots of permanent shadow where lunar glaciers up to 100 meters equivalent thickness may have accumulated over the ages.