Imaging in Shadows: A Comparison of Craters Observed in Primary and Secondary Illumination with the Lunar Reconnaissance Orbiter Camera

Abstract

By definition, permanently shadowed regions (PSRs) never receive primary illumination from the Sun. However, most receive secondary illumination reflected from crater walls and nearby massifs. The nature of that secondary illumination, diffuse lighting that can vary significantly across small distances, complicates interpretations of geologic features. To better understand secondary illumination and aid in interpreting images of PSRs, we analyzed a collection of long-exposure Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Camera (NAC) images of five equatorial craters collected when the crater interiors were mainly in shadow, using these as analog images for PSR illumination conditions but enabling comparisons with LROC NAC images of the same features under direct primary illumination. Using illumination models, we quantified the range of secondary photometric angles contributing to illuminating each pixel in the shadowed terrain and measured reflectance under secondary illumination. The phase angles of secondary illumination (secondary phase angles) contributing to a pixel can be well represented by the median value of contributing illumination weighted by magnitude. We show that features observed to have approximately constant albedo in primary illumination show substantial variations in reflectance across the scene in secondary illumination depending on the median secondary phase angle. Approximately 30% of craters with high-reflectance ejecta deposits are detectable in secondary illumination; only when the reflectance contrast is high (>1.3 times higher than the background reflectance in primary illumination) are a majority (>70%) of these deposits detectable in secondary illumination, meaning that small albedo differences will be hard to detect in images of PSRs.

Keywords

• The Moon
• Lunar craters
• Surface photometry