1 Introduction Lunar mare basalts represent the products of partial remelting of deep mantle sources and provide windows into the compositions of lunar interior.Nine Apollo andLuna missions returned large amounts of mare basaltic samples,while remote sensing suggests that sampled basalts may cover only a small number of the lunar basalt
The imaging interferometer(IIM)aboard the Chang’E-1 lunar orbiter is the first multispectral imaging spectrometer for Chinese lunar missions.Before science applications(e.g.,FeO and TiO2mapping)of the IIM raw data,the radiance variation due to changes in illumination and viewing geometry has to be removed from the radiometrically calibrated IIM Level 2A images.To achieve this,we fit the IIM Level 2A radiance data with a Lommel-Seeliger photometric model consisting of an exponential term and a fourth order polynomial in the phase function,without distinguishing between lunar maria and highlands.The exponential and the fourth order polynomial parameters are derived separately by fitting to two datasets divided at a solar phase angle threshold,avoiding a decrease in the phase function close to zero phase angle.Different phase angle thresholds result in coincident fitting curves between 20°and 75°,while large discrepancies occur at other phase angles.Then the derived photometric model is used to normalize the IIM Level 2A data to radiance values at an incidence and phase angle of 30°and emission angle of 0°.Our photometric model is validated by comparing two photometrically normalized IIM radiance spectra covering the same areas,showing a relative deviation consistent with the IIM preflight calibration.
