Lunar Reflectance and Polarization Revisited – A Quarter Century Retrospective

Abstract:

A century of astronomical observation has established that the reflectance and polarization of planetary surfaces increases sharply as solar phase angle (α) approaches opposition (Seeliger; Umov). A half century of laboratory observations shows that lunar particulates behave similarly (Zellner et al.). This brightening is the Opposition Effect (OE; Gehrels).

A quarter century ago laboratory measurements of Apollo samples provided evidence to explain the OE in particulate materials via two distinct radiative scattering processes. These are called ‘shadow hiding’ (SH) and ‘coherent backscattering’ (CB) (Hapke et al.). The shape of the reflectance and polarization phase curves is influenced by the microproperties of the particulate regolith candidate material. This suggests that we might better understand the surface characteristics of distant solar system objects by measuring their near opposition reflectance and polarization behavior from Earth or near Earth orbit (Nelson et al.).

The distinction between SH and CB is determined by how these particulate materials behave when presented with circularly polarized light. CB is characterized by a pronounced increase in the Circular Polarization Ratio (CPR) as α decreases. This is due to increased multiple scattering in the medium (Nelson et al.; Shkuratov et al.). On the other hand, SH is indicated by a decrease in the CPR.

As we enter a new era of Lunar sample acquisition and Lunar surface observations from the Chang’e and other sample return missions, a re-analysis of the foundational work of Hapke et al. is warranted. Profound advances in laboratory techniques have occurred since these original measurements. Foremost among these is the utilization of the reciprocity principle, established by Helmholtz in 1856, which significantly improves signal to noise as demonstrated in our advanced Goniometic Photopolarimeter (GPP) design (Nelson et al.).

Hapke et al. measured the photometric properties of eight Apollo samples. We have been approved by NASA to re-measure the same samples with our modernized GPP. This additional foundational basis will enhance the value of the samples and data returned by the Chang’e and other missions. It also opens the possibility of understanding regolith microtextures of distant solar system objects.

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