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R. M. Killen (Southwest Research Institute), A. E. Potter (Lunar and Planetary Institute), A. Fitzsimmons (Queens Univ., Belfast), T. H. Morgan (SwRI)
High resolution line profiles of the Na D2 emission from Mercury's atmosphere were obtained at the Anglo-Australian Telescope June 6, 1998, at approximately 3 mA resolution. The spatial resolution (not including atmospheric smearing) was 1"x1.5", or about 1/3 planetary radius. Line profiles were obtained at high N, high S and equatorial latitudes, respectively. If interpreted in terms of a gas in thermal equilibrium, the gas temperature would vary from 1500 K at the equator to 750 K and 550 K at high N and high S latitudes, respectively. In addition, at high latitudes 1 - 10 hot component as source atoms ejected by an energetic process such as meteoritic vaporization or sputtering. The gas is nowhere in thermal equilibrium with the surface but remains hotter than the surface temperature. This implies that an energetic process such as chemistry or photon-stimulated desorption dominates the surface interaction. The difference in derived temperature at the N and S polar regions may be real or due to differences in viewing geometry relative to the magnetic field, or in slit placement. These line profiles are in general agreement with those obtained at McDonald Observatory 29 - 30 May, 1997, which were fit with an ambient gas at 750 K and a source gas at 6500 K.