The Extraterrestrial Spectrum for Planetary Mapping: a Look at Options for Chandrayaan- 1’s Moon Mineralogy Mapper

Introduction: The Extraterrestrial Spectrum (ETS) is the Solar Spectral Irradiance (SSI) on a surface at Air Mass Zero (AM0). It is the incident solar spectrum prior to distortion by terrestrial or atmospheric conditions. Its precise determination is of primary importance for converting radiance data from space-borne remote sensing instruments to accurate reflectance values necessary for geological interpretations. Historically, the ETS has been determined by modeling, and removing, atmospheric interferences on a solar spectrum obtained by telescopic observations [1,2], aircraft [3,4], and ground measurements [5]. Over several years these models have been refined, and compiled [6,7,8], and are the basis for the ETS reference standards of the American Society for Testing and Materials (ASTM) [9,10]. Modeling an atmosphere is incredibly complex. Variations in local pressures, humidity, and particulate compositions present a many-bodied problem, and preclude a truly accurate representation of atmospheric conditions at any given time. This is somewhat apparent when one considers the reliability of the local weatherman’s daily predictions. Although the ultraviolet and visible portion of the spectrum is virtually unhindered, several wavelengths in the petrologically significant infrared is replete with absorptions caused by the molecular vibrations of O2, O3, H2O, and CO2. The SSI measured from the ground, telescope, and even aircraft, all exhibit these spectral absorptions, so an ETS calculated from any of these must make some general assumptions about atmospheric conditions at the time of acquisition in order to compensate for their effects.