ORFEUS-II FAR-ULTRAVIOLET OBSERVATIONS OF THE LUNAR ATMOSPHERE

In 1996 November and December, remote FUV observations of the lunar at mosphere were conducted using the Berkeley spectrograph aboard the ORF EUS-SPAS II satellite. The main goal of the observations was to measur e the distribution of Ar above the lunar dayside for the first time an d to search for another predicted atmospheric constituent, Ne. During a 33 minute observation, the 20'' diameter spectrograph aperture was s canned repeatedly across the lunar dayside limb to obtain data over th e range +/- 90'' from the lunar limb. The data reveal a 3 sigma emissi on feature at 1048.2 Angstrom, which is interpreted as resonance scatt ering from lunar atmospheric Ar. The detected emission level of 1.3 +/ - 0.4 x 10(-3) photons cm(-2) s(-1) yields a daytime atmospheric abund ance at the surface of 8 +/- 3 x 10(5) cm(-3), assuming a spherically symmetric exospheric distribution. A search for atmospheric Ne I 736 A ngstrom emission resulted in a 2 sigma upper limit of 6 x 10(-4) photo ns cm(-2) s(-1) or a daytime surface density upper limit of 2 x 10(6) cm(-3).