A. L. Sprague, W. J. Schmitt, and R.E. Hill

Lunar and Planetary Laboratory
University of Arizona, Tucson, AZ 85721
fax 520-621-4933


(ICARUS,136, 60 - 68, (1998)

Telescopic observations of Mercury's sodium atmosphere were made to test the theory that two mid-latitude radar bright spots centered near ~ 345o longitude and ~55o N and ~25o S are sources of sodium in Mercury's atmosphere. Three important results came from these observations. 1. When the spectrograph slit was placed over, or near the location of the radar- bright spots, notable sodium enhancements were seen. When the spectrograph is placed at a greater distance from the spots and beyond the seeing disk, no sodium enhancements were seen. We conclude that these two locations on Mercury's surface are Na-rich and are two sources of the Na atmosphere. 2. To fit the surface continuum traces from the data, it was necessary to include two highly reflective regions at the locations of the radar-bright spots in our Hapke rough-surface reflectance model. This indicates the mid-latitude radar-bright spots are also brighter than surrounding terrain at visible wavelengths. 3. High Na atmospheric abundances occur during periods of low solar activity as well as high solar activity. Average zenith column abundances above the spots are 8.6 x 1011 and 8.0 x 1011 Na atoms cm-2 for th e northern and southern spot respectively. These Na abundances are among the highest ever reported, exceeded only by measurements of May, 1988 (maximum value 1.5 x 1012 Na atoms cm-2 ) when Mercury's morning terminator was in view and another radar-bright region centered ~ 240o longitude and ~15o N was near the noon-point. Both sunspot index and solar F10.7 flux (a proxy for EUV) were close to the solar minimum.


Figure 6. (a) radar image: spots A and B of Slade et al. (1992). (b - i) All published images and spectra showing bright Na emission spots. Overlying longitude/latitude grid s show the longitude of the central meridian and the terminator. Latitude increments are 30o . Also shown are spots A, B, Caloris Basin (CB) and the Kuiper-Murasaki crater complex (K) for easy reference. (b) Diagram of geometry for slit spectroscopy measurements of Na enhancements over spots A and B, after Sprague et al. (1997) ( c ) Adapted from Potter and Morgan (1990) Na enhancements over spot K, A, and a region from the unimaged part of the planet. Notice that a rotation of the planetary grid and terminator would align B with the bright emission that is anomalously partially over the dark side of the planet. (d) Na enhancements from spectroscopic measurements shown in Fig.3 interpolated to portray an "image" (e) Another case of Na enhancements over K, combined with B and A (adapted from Potter and Morgan 1990). (f)Spectroscopic measurements from Sprague et al. (1997) when the slit was placed o ver Caloris Basin (CB) and unidentified source in the unimaged portion of the southern hemisphere. (g) Na enhancement over Caloris Basin (from Potter and Morgan 1990). (h) Na enhancements in this image from Potter and Morgan (1990) are seen over the regions of bright albedo features at 155o longitude, 65N; 125o longitude, 0 lat.; and 105o longitude, 9S. (i) shows a large enhancement of Na spreading from ~43 - 73o longitude and ~10 - 60o S latitude and a slight Na enhancement centered near ~70o longitude and ~40o N latitude, a region not imaged by Mariner 10 (Potter and Morgan 1997).