Laboratory Measurement of Bi-Directional Reflectance of Candidate Planetary Regolith Materials

We report here on an expanded range of bi-directional reflectance (BDR) measurements of a suite of particulate candidate regolith materials. This expanded set combines the results of our two Goniometric Photopolarimeters (GPPs): one is able to measure, with great angular resolution, phase angles from 0.05˚ to approximately 20˚, and the other measures at coarser angular resolution but with wider range of phase angle, from 5˚ to 80˚. We include in our investigation measurements of mixtures and well-sorted, mineralogically pure materials contrived to simulate regolith components of the Moon and other Atmosphereless Solar System Bodies (ASSBs).

For half a millennium, remotely-sensed reflectance and polarization measurements from photonic detectors located on Earth-based telescopes, observatories in Earth orbit, or on interplanetary spacecraft have been the principal gateway to understanding the physical and chemical properties of the surfaces of ASSBs. These observations of reflected electromagnetic radiation at many different angles of incidence, emission and solar phase (the angle defined by the sun, the object and the observer) are collectively called angular scattering measurements. We replicate these remotely-sensed observational conditions and measure the BDR in the laboratory with our GPPs.

The results of BDR measurements like ours have been used to better understand the textural properties of planetary regolith and the processes that modify regolith over time, as well as the working physical processes in clouds, aerosols, and planetary rings (Nelson et al., 2018). Another use for our controlled BDR experiments is in determining the single-scattering albedo and single-scattering phase function of individual particles (Hapke, 2012), making our measurements relevant to understanding, e.g., the potential use of atmospheric aerosols in Solar Radiation Management (geoengineering).

References

Hapke, B.W., 2012, Icarus 221: 1079-1083

Nelson, R.M. et al., 2018, Icarus 302: 483-498