By definition, lunar permanently shadowed regions (PSRs) do not receive direct sunlight. However, near the poles, some shadowed crater walls and Earth-facing slopes receive direct Earth illumination for an appreciable fraction of the time as a consequence of lunar libration. Although the broadband irradiance from Earth is only ~10-4 that of direct sunlight, Earth illumination can be significant when the only other sources are scattered sunlight and thermal input from nearby sunlit terrain. Direct Earth illumination may add enough energy in some areas to influence the annual-average thermal balance of the surface.
Shackleton crater is a good test-case for assessing the importance of Earth illumination in PSRs, since most of the interior and walls are in permanent shadow, and its simple crater shape is well represented using an azimuthally symmetric height profile. This makes it possible to simulate the visibility of both Sun and Earth on a daily basis from the crater perspective, without the need for a high-resolution digital terrain model. Using realistic models for Earth spectral irradiance, combined with a shadowing code, we examine the relative importance of direct and scattered light contributions from Sun and Earth on the wall of Shackleton crater, spanning a full lunation near equinox. Surface scattering at solar-band wavelengths is approximated using a Hapke rough surface reflectance function. At the Moon, the total irradiance from unocculted Earth at zero phase angle exceeds 0.13 W m-2, and consists of approximately equal contributions at solar reflectance ( 3 microns) wavelengths. The thermal contribution shows virtually no dependence on Earth phase. By comparison, the maximum solar scattered light component in the crater interior over the range of dates in our simulations (05 Mar to 20 Mar, 2008) was found to be ~ 1.4 W m-2, demonstrating that Earth can be a non-negligible radiance contributor. We discuss these simulation results and their implication for the annual-average influence of earthshine in PSRs.