Emissivity and Reflectance Spectra of Asteroid Analogs: Their Dependence on Emerging Angle

Very few laboratory experiments have been made up to date to understand the influence of emerging angle on measured emissivity spectra of particulate materials. Most of the existing works are based on measurements of flat, solid surfaces (mainly metals) that are not easily usable for the analysis of remote sensing data. We believe that a systematic study on the influence of emerging angle for emissivity and reflectance laboratory spectra of asteroids analogs is of crucial importance for the interpretation of data from future missions to asteroids (among them, NASA OSIRIS-Rex, and JAXA Hayabusa II) and for the modelling of surface properties of minor bodies in general. At the Planetary Emissivity Laboratory (PEL) of DLR in Berlin we set-up an experiment to measure the emissivity of two asteroid analogs, fine powdered (<25 μm) meteorite Millbillillie and a synthetic enstatite at surface temperature of 373K, with emerging angle (e) from 0° to 60°, with 10° step intervals. Special wedges have been created to incline the samples at the right emission angles: to prevent powder to slipper from the sample cups (always happening for e>30°), the samples have been packed using ethanol. For comparisons, we measured reflectance spectra of the same samples, keeping the incidence angle to the allowed minimum (i=13°) and varying the emerging angle as we did for emissivity measurements. All the data show very small spectral variations up to e=30°, while starting from e=40° the changes in band depth and shape become significant, and increase for each 10° step of increasing emerging angle.