Lateral transport in planetary exospheres

Horizontal temperature variations contribute to horizontal transport of constituents in an exosphere. If gas concentrations are not uniform, their gradients will also give rise to horizontal transport through the exosphere. The sum of these two transport effects plus the transport due to rotation of the planetary body involved, must balance the integrated sources and sinks of gas in the exosphere, mainly by flow up or down through the base of the exosphere. The relative magnitudes of these effects are evaluated for helium in the earth's exosphere and for neon and heavier constituents in the lunar atmosphere. In the absence of significant sources and sinks the temperature and concentration effects tend to set up distributions such that nT5/2 = constant on the base of the exosphere, but rotation of the planetary body tends to establish distributions such that nT = constant.