Lunar Phase Function in the Near-Infrared from the Lunar Orbiter Laser Altimeter

Abstract:

The reflectance of the lunar surface as a function of wavelength and viewing geometry is a fundamental measurement related to the scattering properties of the regolith particles and the structure of the surface. In this study, we report preliminary results on the near-infrared phase function observed with the Lunar Orbiter Laser Altimeter (LOLA) onboard the Lunar Reconnaissance Orbiter. Since December 2013, LOLA has been collecting passive radiometry (reflected sunlight) in the northern hemisphere where the spacecraft altitude is too high for normal altimetric ranging. In this mode, LOLA acts as a 4-pixel radiometer with pixel size ~60 m, integration time of 1/28th sec (every ~60 m along-track), and signal-to-noise ratio ~50 per pixel in a single “exposure” at low latitudes. We report on the passive radiometric calibration, and compare the LOLA near-infrared phase function to that at similar and smaller wavelengths measured with other instruments. The unique capability of LOLA to also actively measure the normal albedo from the received pulse energies during altimetric ranging provides a crucial anchor point for the passively-derived phase function that is not easily obtained with typical imagers. Finally, we explore what constraints can be placed on the parameters of physically-motivated phase function models. This work will ultimately provide insight on the wavelength dependence of the phase function, for which the theoretical understanding is presently incomplete.