Density and flux distributions of neutral gases in the lunar atmosphere

Abstract

Using the kinetic theory of gases, we have calculated numerically the density and flux distributions in the lunar atmosphere resulting from variations in the assumed surface gas temperature (T0) and density (N0). Computation of neon density distributions along a 100-km equatorial circular orbit shows that the density on the dayside is about 1 order of magnitude higher than that on the nightside for N0 = constant, but is a factor of 2 smaller for N0 ∼ T0−5/2, as given by Hodges and Johnson (1968). Argon, on the other hand, will be more abundant on the dayside in both cases, by ∼250 for constant N0 and by ∼10 for N0 ∼ T0−5/2. In the 8 × 60 nm elliptic descent orbit of Apollo 15, the relative changes of density for the two values of N0 are ∼50 and ∼25 for neon, and ∼3 × 10³ and ∼7 × 10² for argon. This marked contrast between daytime and nighttime densities is not surprising : the fraction of fast gas molecules that can reach the above high-altitude orbits is most sensitive to temperature. Computation of flux distributions versus altitude over the equatorial terminator shows that a positive net flux from the hot side to the cold side persists at all altitudes if N0 = constant; however, when N0 ∼ T0−5/2, the net flux is positive at higher altitudes but negative at lower altitudes with approximately zero net integrated flux (over altitude). Substantial lateral transport, in addition to the effects of particle loss ensuing from thermal escape, is also evident from the density distribution of hydrogen in a 100-km equatorial orbit, which was computed on the basis of the surface flux distribution given by Vogel (1966).