Evidence for Supra-Thermal Sodium on Mercury

Details Evidence for Supra-Thermal Sodium on Mercury Evidence for Supra-Thermal Sodium on Mercury

Sep 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996dps....28.1305p&link_type=abstract

American Astronomical Society, DPS meeting #28, #13.05; Bulletin of the American Astronomical Society, Vol. 28, p.1116

Astronomy and Astrophysics

Astronomy

Scientific paper

The temperature of sodium vapor on Mercury is an important clue to the processes that originate and maintain its sodium exosphere. Different source processes will lead to different temperatures for the sodium. For example, physical sputtering should lead to temperatures of the order of tens of thousands of degrees, while other processes, such as chemical sputtering, meteoroid impact, and diffusion from subsurface reservoirs should lead to much lower temperatures. High spectral resolution measurements of the line profile of the sodium emission yielded a Doppler temperature of about 500 K, close to the surface temperature (Potter and Morgan, 1987). However, the measurement was noisy, and the presence of a high temperature component in the wings of the emission line could not be ruled out. Measurements of the sodium distribution over the planet during the period of high solar activity from 1988 to 1993 showed most, if not all, sodium emission closely confined to the planet surface, as expected for 500K sodium. However, recent measurements during the current time of low solar activity showed sodium well above the planet surface, indicating a high temperature. During July 1994, strong sodium emission was detected almost a planetary radius above the poles. During October 1995, a detailed survey of sodium around the planet revealed weak sodium emission extending almost two planetary diameters above the subsolar point, and even further in the anti-sunward direction. These results suggest that there exists both a "cold " sodium population from sodium that has partly or wholly accommodated to the surface, and a "hot" sodium population from sodium atoms freshly formed by the source process. The mean velocity required for sodium atoms to populate space up to two planetary diameters corresponds to temperatures of the order of 3000 K. We acknowledge support from the NASA Planetary Astronomy program and the McMath-Pierce Solar Telescope of the National Solar Observatory Reference: Potter, A.E. and T.H. Morgan, Icarus 71, 472-477 (1987)

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