An EUVE Spectrum of the Moon

During its all-sky survey, the Extreme Ultraviolet Explorer (EUVE) satellite observed the Moon several times at first and last quarters, and once near the December 10, 1992, lunar eclipse. We present here results for the EUV albedo of the Moon, and examine for the first time an EUVE spectrum of the Moon. Extreme ultraviolet observations of the Moon are of considerable interest, since it has been speculated that lunar EUV emissions are primarily due to L- and M-shell X-ray fluorescence and may provide a useful diagnostic of surface elemental abundances. Using the EUVE photometer observations, we have determined average lunar geometric albedos of 0.15% (+/- 0.02%), 3.1% (+/- 0.2%), and 3.5% (+/- 0.2%), over wavelength intervals of 150--240\ang, 400--580\ang, and 550--650\ang, respectively. An upper limit geometric albedo of 0.13% is obtained for the wavelength interval 75--180\ang. Also, using previously published ROSAT data we estimate a lunar geometric albedo of 0.0097% (+/- 0.0012%) over the wavelength interval 50--80\ang. These EUV albedos, and previously published far-ultraviolet albedos from Apollo 17 and Mariner 10, are well fit by the scaled reflectivities of SiO_2 and Al_2O_3. Over the wavelength ranges of the EUVE photometers, the observed brightness of the Moon seems to be largely consistent with reflected sunlight rather than X-ray fluorescence. Since the L- and M-shell fluorescence signal is expected to be carried by only a few emission lines, however, EUV observations with higher spectral resolution are required to determine their exact contribution, if any, to the lunar EUV spectrum. Using a 1/2 hour spectrum obtained by the EUVE spectrometers on December 10, 1992, we will determine the level of the X-ray fluorescence contribution to the lunar EUV spectrum. This work has been supported by NASA contract NAS5-30180.