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Figure 1-10e GEOLOGIC LUNAR RESEARCH SELENOLOGY TODAY # 7 5. Mare Tranquillitatis spectra divided by MS-2 Spectra Figure 1-11a and 1-11b M. Tranquillitatis/MS-2 Site Linear Regression Fit page 35
GEOLOGIC LUNAR RESEARCH SELENOLOGY TODAY # 7 When plotting mare basalt spectra, the usual procedure is to plot a relative reflectance of the mare region of interest as compared to a standard reference mare area. The most commonly used reference area for this purpose is in southeastern Mare Serenitatis and is referred to as the MS-2 site (18.7 degrees north latitude, 21.5 degrees east longitude). Figures 1-11a and 1-11b respectively show spectra recorded for Mare Tranquillitatis divided by spectra taken from the MS-2 area. This method has been used to compare the titanium and iron content of mare sites of interest relative to central Mare Serenitatis (Pieters, 1978). This is done by assessing the 400 nm/560 nm ratio which is given by the continuum slope (Figure 1-11b). Larger ratios indicate higher titanium content (but in a non-linear fashion). In recent years, many complicating factors have been recognized which have a significant bearing on reflectance results including basalt grain size and iron content. Increasing lunar phase angle causes a shift of the ratio toward redder wavelengths and for this reason spectra are usually only obtained at phase angles of between 20 and 50 degrees where this effect is felt to be less of a problem. Taking a least squares linear fit through the curve out to 650 nm (Figure 1-11b), the 400 nm to 560 nm ratio is about 1.09 in each case which agrees closely with the value determined by Pieters. This was felt to correspond to a titanium content of about 8 percent (Pieters, 1978). Appendix 1-2: Wavelength Calibration Using Fraunhofer Lines Fraunhofer lines are thin absorption lines found at specific wavelength locations along the solar spectrum. The solar spectrum below, taken with the spectroscope described in this paper using a 25 micron slit, a 200 nm transmission grating, and a Lumenera 075M digital camera is shown below in Figure 1-12. The Fraunhofer lines are labelled. Figure 1-12 page 36
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