EUVE Science Highlights -
January 31, 1998
EUVE Science Highlights -
January 31, 1998
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Slide 1 of 2
1. On November 14 1997, a 10,000 sec DDT observation with EUVE provided the first high signal-to-noise-ratio (SNR) images of the Moon at extreme ultraviolet (EUV) wavelengths. Previous EUV images of the Moon consist of several EUVE all-sky survey detections of the quarter Moon in 1992 with total exposure times of only 41 and 18 sec for the Al/C and Tin filter bands, respectively, precluding any detection of surface albedo variations. The November 1997 Scanner images, shown here along with a visible-light image, have effective exposure times of 9,200 and 3,000 sec, allowing the detection of albedo contrasts of as little as 2% in each image. 2. Apollo 17 far ultraviolet (FUV) spectrographic observations of a limited portion of the Moon first indicated the existence of an ``albedo reversal'' at wavelengths shortward of the visible and UV bands. That is, mare appear brighter than highland regions, the opposite of the case at optical and UV wavelengths. The EUVE Al/C band image shows for the first time that this effect extends to the entire Earth-facing hemisphere of the Moon and that it also occurs at EUV wavelengths. Brightness contrasts of 30-40% between mare and highlands in the EUVE Al/C image are comparable to those seen at optical wavelengths. 3. Surprisingly, the EUVE Tin band image, which lies between the Al/C image and the Apollo 17 FUV observations in wavelength, is completely devoid of albedo features larger than a resolution element (100-200 km in diameter on the Moon) down to the detection level of about 2% contrast. Furthermore, the Tin image indicates the presence of limb darkening, which is absent at optical wavelengths. 4. Two possible explanations for the differences between the Al/C and Tin images are being explored. First, the blandness of the Tin image may indicate that mare and highlands have the same albedo down to the 2% level in the 52-75 nm wavelength region, compared with the 30-40% contrast seen in the 14-36 nm Al/C band. In this case the images would point to previously unknown complexity in the spectral reflectance of the Moon in the EUV, due mainly to compositional differences between mare and highland regions. A second, less likely, explanation is that the Tin band image may be detecting emission from the lunar atmosphere, hence the uniform nature of its appearance. The strongest emission lines of neutral He and Ne lie within the Tin bandpass. He has been previously detected at the lunar nighttime surface by Apollo 17 instruments and Ne is thought to be a likely constituent of the atmosphere, however the expected abundances of these elements in the lunar dayside atmosphere appear to be roughly 1% or less of that needed to account for the observed brightness.