Velocity Structure in Mercury's Exosphere

R.M. Killen (Southwest Research Institute), A.E. Potter (Lunar and Planetary Institute), T.H. Morgan (Southwest Research Institute)

A high resolution line profile of the Na D2 emission from Mercury was obtained on May 29, 1997, at the McDonald Observatory 107" telescope with the echelle spectrograph in double pass (Potter and Morgan 1997, BAAS, in press). This line profile indicates that the bulk of the atmosphere is hot, in agreement with evidence from the image data which indicates a gas at approximately 6000 K (Potter and Morgan 1997 Adv. Space Res.in press). A line profile measured in 1986 (Potter and Morgan 1987, Icarus 71, 472) indicated that the bulk of the sodium was cooler, <1100 K, indicative of possible temporal or spatial variation. Previous estimates if the gas temperature were based on a Chamberlain exosphere model, assuming optically thin conditions. We have used a Monte Carlo radiative transfer program to investigate the velocity distribution in the exosphere. Models will be presented for line profiles simulated for various source processes and will be compared with the line profile and image data.