[ITAL]ORFEUS II[/ITAL] Far-Ultraviolet Observations of the Lunar Atmosphere

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Abstract
In 1996 November and December, remote FUV observations of the lunar atmosphere were conducted using the Berkeley spectrograph aboard the ORFEUS-SPAS II satellite. The main goal of the observations was to measure the distribution of Ar above the lunar dayside for the first time and to search for another predicted atmospheric constituent, Ne. During a 33 minute observation, the 20'' diameter spectrograph aperture was scanned repeatedly across the lunar dayside limb to obtain data over the range ±90'' from the lunar limb. The data reveal a 3 σ emission feature at 1048.2 Å, which is interpreted as resonance scattering from lunar atmospheric Ar. The detected emission level of 1.3±0.4×10−3 photons cm-2 s-1 yields a daytime atmospheric abundance at the surface of 8±3×105 cm-3, assuming a spherically symmetric exospheric distribution. A search for atmospheric Ne I 736 Å emission resulted in a 2 σ upper limit of 6×10−4 photons cm-2 s-1 or a daytime surface density upper limit of 2×106 cm-3.