Chang’ E-1 Imaging Interferometer Data Processing And Landing Sites Selection In The Aristarchus Region

The planned Chinese Lunar Exploration Program (CLEP) consists of three stages: orbital missions, soft-landings, and sample returns. The first lunar orbiter Chang’E-1 for the first exploration stage was launched in October,2007. The Imaging Interferom-eter (IIM), one of the eight payloads aboard the Chang’E-1, is the first Sagnac-based imaging spectrometer flown on lunar missions, designed to identify and map miner-alogical compositions, and to analyze FeO and TiO2 abundance distributions across the surface of the Moon. Through the Chang’E-1 mission duration, the IIM obtained 32-band multispectral observations covering 84% of the lunar surface between 70°S and 70°N at a spatial resolution of 200 m/pixel in the spectral range 480-960 nm and with a solar phase, incidence and emission angle range 0-80°,0-7° and 0-80° respec-tively. The IIM raw data are calibrated to Level 2A radiance images through data preprocessing pipeline. In order to compare the IIM data with other lunar observations and with laboratory measurements, and to reduce the errors in mineral identifications and FeO and TiO2 abundance analyses caused by illumination-viewing geometry vari-ations and by solar spectral features, we photometrically normalize the IIM Level 2A data to a standard illumination-viewing geometry, and then convert them to reflectance based on the IIM characteristics and data release status. Lunar lander and rover mis-sions for the second and the third CLEP stages are planned to conduct in situ surface investigations and sample returns. Therefore, we consider compositional data for the Aristarchus region, and argue that it is a scientifically appealing potential landing site for future lunar surface explorations.To photometrically normalize the IIM Level 2A data, we collect Level 2A radi-ance spectra by binning each images into blocks and their corresponding illumination-viewing geometries, fit the collected spectra with Lommel-Seeliger and Hapke’s pho-tometric functions, and photometrically normalize the IIM Level 2A radiance data to the standard illumination-viewing geometry of the Brown University Keck/NASA Re- flectance Experiment Laboratory(RELAB) measurements. The phase function for the Lommel-Seeliger model is a fourth order polynomial, added with an exponential term to account for the lunar opposition effect. The exponential and the fourth order polyno-mial parameters are derived separately by fitting two datasets screened at a solar phase angle threshold using the fitting package MPFTT based on the Levenberg-Marquardt algorithm, avoiding a decrease in the phase function close to zero phase angle. Based on the spectral and solar phase angle ranges of the DM data, the Hapke’s function is simplified as follows:multiple scattering and macroscopic roughness are ignored; one-term Henyey-Greenstein function describing backward scattering is used as phase function; only shadow-hiding opposition effect is taken into account. The Differential Evolution algorithm is used to find the best fit to this simplified Hapke’s function by searching the constrained parameter space, independent of initial parameter values.To convert the photometrically normalized DM Level 2A radiance data to re-flectance, we select a flat and homogenous area in the IIM Orbit 2225 image as the calibration standard, as the earth-based telescopic and Clementine UWIS observa-tions suggest that its optical properties can be represented by the mature Apollo 16 lunar soil 62231. Then the ratio between the photometrically normalized IIM Level 2A radiances for each pixel and the calibration standard is mutiplied by the Apollo 16 62231 lunar soil reflectance to derive reflectance data.The Aristarchus region lies between 70°W and 30°W, and 10°N and 48°N in the northwestern part of the lunar nearside, consisting of Aristarchus Crater and Plateau, Lichtenberg Crater, and Gruithuisen Domes. We analyze the FeO-Th mixing trends in this region, arguing that nonmare rocks exposed by Aristarchus Crater represent compositional endmembers for the lunar crust related to late-stage intrusive activity within the Procellarum KREEP Terrane. Previous works showed that diverse volcanic features, such as pyroclastic deposits, sinus rilles, and domes, occur in this region. The dome featues are consisten with the rare silici volcanic materials found in lunar samples.The collected IIM Level 2A radiance data are divided by the Lommel-Seeliger factor to correct limb-darkening, showing two distinct classes in 2D phase angle- radiance histogram, corresponding to lunar maria and highlands. The Lommel-Seeliger function fit to the IIM data varies with a change in solar phase angle threshold; how-ever, the fitting curves between 20° and 75° are barely affected. Our photometric normalization based on the Lommel-Seeliger function is validated by comparing two photometrically normalized IIM Level 2A radiance spectra for overlapping flat and ho-mogenous areas, indicating a relative deviation within the uncertainty given by the IIM preflight calibration experiment. The photometric normalization factors derived from the Hapke’s function fit to the IIM Level 2A data are consistent with the results from the most recent Lommel-Seeliger function fit to the USGS Robotic Lunar Observato-ry(ROLO) data. Our IIM reflectance data are validated with the Clementine UVVIS 750 and 900 nm reflectances, showing relative deviations within 10%. The diverse volcanic features of the Aristarchus Plateau, the age of basalts at Lichtenberg Crater and the nature of volcanism at the Gruithuisen Domes make the Aristarchus region scientifically appealing for surface exploration; the extensive excavation of the most chemically fractionated lunar crustal materials makes the Aristarchus Crater/Plateau region a prime scientific target for understanding the petrologic evolution of the most thermally active part of the Moon.