Spectral Reflectance Properties of Underdense Asteroid Regoliths Simulated by Pressed Pellets

English Abstract:

A number of upcoming spacecraft missions are targeting low albedo asteroids, particularly the NASA-CSA OSIRIS-REx and JAXA Hayabusa-2 asteroid sample return missions. The target asteroids of these missions are a few hundred meters in diameter, and as a result their surfaces have microgravity environments. Due to the effects of microgravity and vacuum, the regoliths on the surfaces of these asteroids will likely be very loosely packed (termed underdense regoliths). Prior to sample acquisition, the target asteroids will be subjected to comprehensive mapping campaigns which will include a number of imaging spectrometers. This research aims to examine the spectral properties of dark asteroids by simulating loosely packed regolith in the laboratory, with the goal of aiding in the identification and remote characterization of scientifically interesting targets.

Underdense regolith samples were simulated by mixing potassium bromide (KBr, an optically transparent material) with serpentine. A spectral reflectance study (0.35-2.5 µm) was conducted on various ratios of KBr to serpentine, to simulate varying densities of underdense regolith. We measured the KBr-serpentine concentrations in two forms: pressed pellets (smooth and roughened surface) and loosely packed poured powder. This allows for a comparative analysis of the two forms as well as gaining a better understanding of how and whether properties of the mixtures we produced in the lab differ from properties of underdense regoliths. The spectral data has been analyzed in order to look for patterns in band depth, albedo and slope of the various sample concentrations. Preliminary findings show an increasing blueness of slope with an increase in the KBr:serpentine ratio, and this is qualitatively similar to the blue slope observed in Bennu spectra. Quantitative analysis of band depth in the 1 µm region indicates that a slight deepening occurs with increasing KBr percentage, however band positions do not appear to be affected. This data helps us to determine whether optically robust underdense regoliths can be simulated using pressed pellets and whether spacecraft measurements can be used to identify loosely packed regolith for suitable sampling regions.