Optical observation by visible and near-infrared light of the moon is very useful in the exploration of the mineral composition of the lunar surface. However, photometric correctionEis necessary for a detailed analysis of the observation data. This article introduces research on a photometric correction method in the Spectral Profiler (SP) onboard the lunar orbiting explorer KAGUYA.
Photometric correction for visible and near-infrared light data
The wavelength of light we see with our eyes (visible light) is 0.4 to 0.75µm (1µm=1/1,000mm). Light of longer wavelength than visible light is called infrared. In the infrared range, light of a little longer wavelength than visible light (i.e., up to approx. 3µm) is called near-infrared. Most minerals on the lunar surface have reflection spectra (colorEin a broad sense) unique to each mineral in this wavelength range. Therefore, by exploring in detail the colorEof the sunlight reflected on the lunar surface, we can identify the type of materials on the moon. The brightness (intensity of light) of observed reflection varies depending on the angle condition of the three elements, i.e. Sun, lunar surface and observer. Brightness does not always change evenly in all wavelengths, so color trends vary. To compare multiple data observed from different angles, we need to correct the effect of the angle condition. The objectives of photometric correction are to eliminate the effect by the angle condition and to draw the information such as mineral composition.
A model formula indicating the relation between reflection intensity and angle condition of observation is called photometric function. In particular, the part showing phase angle dependence in the photometric function is called phase function or phase curve. The phase angle is one given by the Sun, lunar surface and observer. The phase curve is affected by the light-scattering property of the surface particles or their degree of density. The curve is also influenced by observing wavelength. To determine multiple coefficients contained in the phase function based on observation, we need to conduct, ideally, multiple observations of the same place on the lunar surface from different angles. To determine coefficients without any assumption, observations must be performed repeatedly at least more than the number of coefficients. For example, a well-known model in this field, Hapke model,Ehas six coefficients, so we need to conduct observations more than six times. It is difficult, however, to perform multiple observations of the whole lunar surface in expected condition with explorer having various restrictions. For this reason, in practice the standard phase curve, which is commonly applicable to a wide range of lunar-surface types, is used.
There are roughly two types of terrain on the moon, one called seaEwhich is blackish and another called highland (lunarite)Ewhich is whitish. It has been pointed out for a long time that the phase curve differs between sea and highland. Therefore, we need to conduct our research by separating the phase curve into at least two types, sea and highland.