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A.L. Sprague, W.J. Schmitt, R.E. Hill (LPL, U of AZ)
Especially designed observations were made on Oct. 4, 1996 to see if the radar-bright regions on Mercury's unimaged side were associated with high atmospheric Na abundance compared with abundances over other locations on the planet. We did find very high Na abundances (8.6 x 1011 and 8.0 x 1011 Na atoms cm-2, respectively) for the northern and southern spot. These are the highest Na abundances ever reported with only one exception (May, 1988). These data show that exceptionally high Na abundances are present at periods of low solar activity because both the sunspot index and solar F10.7 flux were close to the solar minimum. Large column abundances have also been observed during periods of high solar activity. We also found that it was necessary to include two very bright visible albedo features (at the locations of the mid-latitude radar-bright spots) in our Hapke surface model to get a good match to the observed reflected light from Mercury's surface. We also illustrate how atmospheric "seeing" can cause Na enhancements that are actually close to the limb appear to be located well inside the apparent disk of the planet.
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