Opposition effect of the Moon from LROC WAC data

^a Institute of Astronomy, Kharkiv National University, Sumska St. 35, Kharkiv 61022, Ukraine
^b National Aviation University, Cosmonaut Komarov Ave. 1, Kiev 03680, Ukraine
^c Space Science Institute, 4750 Walnut St. Suite 205, Boulder, CO 80301, USA
^d US Army Research Laboratory RDRL-CIE-S, 2800 Powder Mill Road, Adelphi, MD 20783, USA

Received 12 April 2015, Revised 3 April 2016, Accepted 5 April 2016, Available online 13 April 2016

Highlights

• The coherent-backscatter opposition effect of the Moon is small in visible light.
• The width of the coherent-backscatter opposition surge is 1–4°.
• The maximum amplitude of the surge is near 8%.
• LROC WAC calibration are close to average Earth-based absolute photometry data.

Abstract

LROC WAC images acquired in 5 bands of the visible spectral range were used to study the opposition effect for two mare and two highland regions near the lunar equator. Opposition phase curves were extracted from the images containing the opposition by separating the phase-curve effect from the albedo pattern by comparing WAC images at different phase angles (from 0° to 30°). Akimov's photometric function and the NASA Digital Terrain Model GLD100 were used in the processing. It was found that phase-curve slopes at small phase angles directly correlate with albedo, while at larger phase angles, they are anti-correlated. We suggest a parameter to characterize the coherent-backscattering component of the lunar opposition surge, which is defined as the maximum phase angle for which the opposition-surge slope increases with growing albedo. The width of the coherent-backscattering opposition effect varies from approximately 1.2° for highlands in red light to 3.9° for maria in blue light. The parameter depends on albedo, which is in agreement with the coherent-backscattering theory. The maximum amplitude of the coherent opposition effect is estimated to be near 8%. Maps of albedo and phase-curve slope at phase angles larger than those, at which the coherent-backscattering occurs, were built for the areas under study. Absolute calibration of WAC images was compared with Earth-based observations: the WAC-determined albedo is very close to the mean lunar albedo calculated using available Earth-based observations.

Keywords

Moon, surface;
Photometry;
Image processing