Mercury: Sodium Atmospheric Enhancements, Radar-Bright Spots, and Visible Surface Features

Abstract

Telescopic observations of Mercury's sodium atmosphere were made to test the theory that two mid-latitude radar-bright spots centered near ∼345° longitude and ∼55° N and ∼25° 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 was 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 × 1011and 8.0 × 1011Na atoms cm−2for the 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 × 1012Na atoms cm−2) when Mercury's morning terminator was in view and another radar-bright region centered near ∼240° longitude and ∼15° N was near the noon-point. Both sunspot index and solar F10.7 flux (a proxy for EUV) were close to the solar minimum.