Earthshine as an Illumination Source at the Moon

TL;DR

Earthshine provides a significant natural illumination source on the Moon, enabling exploration of shadowed regions and influencing volatile transport, with spectral modeling showing its stability and potential for optical lunar surface measurements.

Contribution

This study models Earth's spectral radiance at the Moon using advanced 3D spectral models, quantifies earthshine's brightness, and presents a formulation for estimating its irradiance for lunar exploration.

Findings

Earthshine's broadband irradiance at the Moon is about 0.15 W/m².

Earth's thermal irradiance varies minimally over two lunar months.

Earthshine's spectral characteristics include diurnal modulation and a red edge feature.

Abstract

Earthshine is the dominant source of natural illumination on the surface of the Moon during lunar night, and at locations within permanently shadowed regions that never receive direct sunlight. As such, earthshine may enable the exploration of areas of the Moon that are hidden from solar illumination. The heat flux from earthshine may also influence the transport and cold trapping of volatiles present in the very coldest areas. In this study, Earth's spectral radiance at the Moon is examined using a suite of Earth spectral models created using the Virtual Planetary Laboratory (VPL) three dimensional modeling capability. At the Moon, the broadband, hemispherical irradiance from Earth near 0 phase is approximately 0.15 watts per square meter, with comparable contributions from solar reflectance and thermal emission. Over the simulation timeframe, spanning two lunations, Earth's thermal irradiance changes less than a few mW per square meter as a result of cloud variability and the south-to-north motion of sub-observer position. In solar band, Earth's diurnally averaged light curve at phase angles < 60 degrees is well fit using a Henyey Greenstein integral phase function. At wavelengths > 0.7 microns, near the well known vegetation "red edge", Earth's reflected solar radiance shows significant diurnal modulation as a result of the longitudinal asymmetry in projected landmass, as well as from the distribution of clouds. A simple formulation with adjustable coefficients is presented for estimating Earth's hemispherical irradiance at the Moon as a function of wavelength, phase angle and sub-observer coordinates. It is demonstrated that earthshine is sufficiently bright to serve as a natural illumination source for optical measurements from the lunar surface.