"Hot" Sodium on Mercury
A.E. Potter (Lunar and Planetary Institute), T.H. Morgan (Southwest Research Institute)
In the course of mapping the sodium emission from Mercury, we found that the sodium exosphere appears to extend to considerable altitudes above the planet (Potter and Morgan, 1997). This suggests that some of the sodium is at a high temperature, but blurring of the data by atmospheric seeing makes it difficult to estimate a temperature from the altitude dependence of the emission. Another way to estimate temperature is to measure the broadening of the emission line caused by thermal motions. We attempted this approach earlier (Potter and Morgan, 1987), but the signal-to-noise in the spectrum was low, and the result was somewhat questionable. We have repeated the measurement,using a modern CCD detector, and obtained a spectrum with excellent signal-to- noise at a spectral resolution of about 600,000. The resulting line profile clearly shows a temperature in excess of a thousand degrees. We are initiating detailed analysis of the line profile, and expect that it will provide new insights into the processes that produce sodium in the exosphere of Mercury.
Potter, A.E. and T.H. Morgan, 1987, Variation of sodium on Mercury with solar radiation pressure. Icarus 71, 472-477
Potter, A.E. and T.H. Morgan, 1997, Evidence for suprathermal sodium
on Mercury. Presented 31st COSPAR meeting, July 14-21, 1996. To
be published, Advances in Space Research.