The Moon as a Spectral Calibration Standard Enabled by Lunar Samples: The Clementine Example

Spectral calibration of Clementine data rely on the Apollo 16 site and laboratory measurements of mature soil 62231. The process produces calibrated spectral reflectance factors. What color is the Moon? A reflectance spectrum is essentially a measure of how much radiation incident on a surface (solar radiation) is reflected and how much is absorbed at each wavelength. To the eye the Moon is gray-white, but to photoelectric instruments it is various shades of red, that is, exhibiting an increase in brightness with wavelength. In the near-infrared there are absorptions diagnostic of minerals superimposed on the Moon's redness. For the Moon, and other rocky bodies such as asteroids, most of the detectable absorptions arise from ferrous iron in various crystallographic sites. The wavelength, shape, and strength of these absorptions identify the minerals present, and allow their abundances to be estimated. To accurately measure these diagnostic mineral absorptions with remote detectors requires not only a quality instrument, but also excellent electronic calibration and either direct measurement of the light source (the sun), or a proxy, or a well known reference standard illuminated by the same light source. In the laboratory a white reference such as halon is used (or commercial Spectralon), which in turn has been extensively calibrated relative to a known radiance. In space, or at the telescope, a separate reference must be found to mimic solar radiation and to eliminate instrumental and atmospheric effects. Radiation from stars to a first order follow a black body spectral curve with multiple emission and absorption lines superimposed. Because stellar lines vary with spectral type and very few stars are really solar-like, it is actually quite difficult to use stars as spectral standards. The Moon is a nearby atmosphere-less body that reflects solar radiation. Because the Moon's surface itself does not change with time (at least within our lifetimes), it provides an excellent reference standard. The calibration challenge then reduces to identifying an area on the Moon whose measurable properties are exceptionally well known. The return of lunar samples allows their properties to be measured accurately in earth-based laboratories. Since the samples were collected from known areas on the surface of the Moon, carefully selected samples can be used to represent the properties of that area. For Clementine data, the Apollo 16 site was chosen as a calibration target because it is a relatively homogeneous area with no nearby units of a significantly different material. Since all remote data are acquired as bidirectional reflectance, we use i=30°, e=0° as the standard geometry. The calibration steps used and the assumptions made are discussed briefly below.