The Lunar Sodium Exosphere: 2011 - 2013

A program of high-spectral resolution, ground-based observations of lunar exospheric sodium emission was conducted over a period of several years. The observations reported here were taken during the pre-solar maximum period for solar cycle number 24 from March 2011 to December 2013.

Potter and Morgan [3] discovery of the lunar sodium cloud, in 1988, provided an invaluable observational tool to probe the sources and sinks of the lunar atmosphere. Sarantos et al. [5] and Kuruppuaratchi et al. [2] argue while photon-stimulated desorption (PSD) is a dominant source other mechanisms like impact vaporization, sputtering, or nonthermal PSD also play a role. Measurements by the Ultraviolet Spectrometer on the Lunar Atmosphere Dust and Environment Explorer (LADEE) indicate a monthly (lunation) sodium variation and show increased brightness due to meteor streams [1].

The observations were taken with a Fabry-Perot spectrometer at the NSO McMath-Pierce telescope. The spectral resolution of 180,000 (1.7 km s^-1) allowed for line profile measurements of sodium emissions (D2 5889.9509 Å). The field of view (FOV) was 3 arcmin (~336 km at the mean lunar distance of 384,400 km) and was positioned tangent to the sunlit limb. The FOVs are shown in top Figure for the different reference craters used for offset pointing. Offset pointing was limited to moving in cardinal directions, so there are slight selenographic latitudinal variations in the FOV as the moon moves its orbit and changes its declination. The lunar phase and day of year coverage is shown in bottom Figure.

The effective temperatures (T_eff) determined from the measured line width as a function of lunar phase are in general agreement with Kuruppuaratchi et al. where the T_eff increases with decreasing phase angle. However, temperature near full moon (phase angles < 30°) in February 2013, are at the lower end of their reported temperatures (~2000 ± 300 K).

The authors thank the staff of the Kitt Peak National Solar Observatory and the Kitt Peak mountain staff. This work was supported by NASA through the CRESST II Coop Agreement, the ROSES Solar System Observations program, and the Goddard EIMM program.

[1] Colaprete, A., Sarantos, M., Wooden, D.H., Stubbs, T.J., Cook, A.M., M. (2016), How Surface Composition and Meteoroid Impacts Mediate Sodium and Potassium in the Lunar Exosphere. Science, 351, 249-252. doi: 10.1126/science.aad2380.

[2] Kuruppuaratchi, D.C.P., Mierkiewicz, E.J., Oliversen R.J., Sarantos, M. et al. (2018): High-resolution, ground-based observations of the lunar sodium exosphere during the Lunar Atmosphere and Dust Environment Explorer (LADEE) mission. Journal of Geophysical Research: Planets, 123, 2430- 2444, doi:10.1029/2018JE005717.

[3] Potter, A.E, and Morgan, T.H. (1988), Discovery of sodium and potassium vapor in the atmosphere of the Moon. Science, 241 (4866), 675-680.

[4] Rosborough, S.A., Oliversen, R.J., Mierkiewicz, E.J., Sarantos, M., et al. (2019), High Resolution Potassium Observations of the Lunar Exosphere, Geophys Res Letters, accepted.

[5] Sarantos, M., Killen, R.M., Surjalal, S.A., Slavin, J.A. (2010), Sources of Sodium in the Lunar Exosphere: Modeling Using Ground-Based Observations of Sodium Emission and Spacecraft Data of the Plasma. Icarus, 205, 364-374. doi: 10.1016/j.icarus.2009.07.039.

Top: Overview of our observational geometry. The location of the offset craters used for positioning the FOV off the sunlit limb (orange boxes). The dotted circles represent the approximate FOVs for each of their adjacent craters. Figure adapted from Rosborough et al. [4].

Bottom: Day of year coverage of exospheric sodium emission off the sunlit limb of the moon. Negative (positive) modified phase angles refer to waxing (waning) lunar phase angles.