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A.E. Potter (Lunar and Planetary Institute), T.H. Morgan (NASA Headquarters), R.M. Killen (Southwest Research Institute)
We measured lunar sodium emission at altitudes up to 8000 km above the lunar equator every night from June 8 to 16, 1998. These data show two populations of sodium, a high-altitude one with scale heights of the order of 600-800 km and a low-altitude one with scale heights of the order of 100 km. The scale heights of the high-altitude sodium atoms are consistent with those expected for sodium atoms produced by photon stimulated desorption (PSD) (T.E. Madey, et al. JGR 103 5873 (1998)). The atoms in the low scale height population are presumably derived from atoms in the high-altitude population by partial equilibration with the surface. This measurement series included the full Moon, so that we observed sodium emission before and after passage through the magnetotail, inside which the solar wind is blocked. The sodium emission decreased as full Moon was approached, and then increased after full Moon was past. However, the emission intensity reached a minimum at 15-20 degrees past full Moon, at about the time that the Moon reentered the solar wind. This lag suggests that there is a process dependent on the solar wind that supplies sodium to the surface for PSD to eject into the exosphere. In previous work, we explained the latitude dependence of sodium column density by suggesting that the interaction of solar wind protons with the surface produced sodium atoms in the upper layers of the surface. These atoms then diffused to the surface for release into the exosphere (A.E. Potter and T.H. Morgan, GRL 21,2263 (1994)). Thus, we can imagine that solar wind particle impacts provide a reservoir of sodium atoms that diffuse to the surface for ejection into the lunar exosphere by PSD. In support of this concept, W.H. Smyth et al. (in preparation) have shown that both Mercury and Moon image data can be satisfactorily explained by this process. The NASA Planetary Astronomy Program supported this work.
The author(s) of this abstract have provided an email address for comments about the abstract: potter@lpi.jsc.nasa.gov