The sodium and potassium atmosphere of the moon and its interaction with the surface
Abstract
Observations of lunar atmospheric sodium and potassium from May 1988 to July 1991 are reported and analyzed. Densities at 80° north and south are less than equatorial ones by a factor of 2-3. For our observations, which do not reach above 800 km from the limb, the apparent scale heights for the intensity are 119-611 km for Na, and 85 and 154 km for K; most of these are much larger than would be expected for atoms thermalized to the surface temperature. However, the intensity drops off with increasing radius at a much greater rate than would be observed for an atmosphere that is mostly escaping. We interpret our data using both single- and two-component analyses. We amplify an earlier suggestion that source atoms are quickly redistributed into thermal and suprathermal populations by "competing release mechanisms" acting at the surface. The suprathermal distributions are produced by solar radiation releasing atoms adsorbed on the surface ( photodesorption). We present reasons why the energy distribution seems to mimic a Maxwellian. The competing release mechanisms explain an obvious trend of decreasing apparent scale heights toward the subsolar point, where the density in the lowest 100 km appears to be dominated by thermally desorbed atoms. The suprathermal component is expected to appear at greater altitudes, but the early subsolar data do not extend high enough to reveal it. Six of the data sets are tentatively resolved into thermal and suprathermal components. The variation with latitude is naturally explained if a larger fraction of the atoms at large solar zenith angles are adsorbed to the surface, rather than being visible in the atmosphere. Migration to the dark side may also play a role. It is shown that at most a very small fraction of the observed atoms below a few hundred km altitude can be on escape trajectories. We apply these ideas to the budget of atomic oxygen. We suggest that the inventory of oxygen atoms is greatly reduced because they stick to the surface with high efficiency similar to that of the alkalis, and subsequently recombine with each other or with partially reduced oxides of such atoms as Mg, Fe, Al, and Si.
- Publication:
-
Icarus
- Pub Date:
- March 1992
- DOI:
- 10.1016/0019-1035(92)90004-Q
- Bibcode:
- 1992Icar...96...27S
- Keywords:
-
- Atmospheric Composition;
- Lunar Atmosphere;
- Lunar Surface;
- Potassium;
- Sodium;
- Mass Spectrometers;
- Solar Wind;
- Surface Temperature;
- Ultraviolet Spectrometers