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Earthshine as an illumination source at the Moon
•Major features of Earth's solar reflectance and thermal spectra are reviewed.•Earthshine at the Moon provides illumination for optical measurements in shadowed lunar terrain.•The total energy of Earthshine is sufficient to influence some volatiles on the lunar surface.•Earth's visible and...
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Published in: | Icarus (New York, N.Y. 1962) N.Y. 1962), 2019-03, Vol.321, p.841-856 |
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Main Authors: | , , , , |
Format: | Article |
Language: | English |
Subjects: | |
Citations: | Items that this one cites Items that cite this one |
Online Access: | Get full text |
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Summary: | •Major features of Earth's solar reflectance and thermal spectra are reviewed.•Earthshine at the Moon provides illumination for optical measurements in shadowed lunar terrain.•The total energy of Earthshine is sufficient to influence some volatiles on the lunar surface.•Earth's visible and near-IR radiance exhibits large (up to ∼30%) diurnal fluctuations.•A semi-empirical approximation for Earth's radiance at the Moon is presented.
Earthshine is the dominant source of natural illumination on the surface of the Moon during lunar night, and at some locations within permanently shadowed regions (PSRs) near the poles that never receive direct sunlight. As such, earthshine has the potential to enable the scientific investigation and exploration of conditions in areas of the Moon that are either temporarily or permanently hidden from the Sun. Earthshine has also been used to refer to Earthlight reflected from the lunar surface, but in this study we use it to refer specifically to Earthlight incident at the Moon. Under certain circumstances, the heat flux from earthshine could also influence the transport and cold-trapping of volatiles present in the very coldest areas within PSRs. In this study, Earth's spectral irradiance, as it would appear at the Moon in the solar reflectance band (0.3–3.0 µm) and at thermal emission wavelengths (3–50 µm), is examined with a suite of model image cubes and whole-disk spectra created using the Virtual Planetary Laboratory (VPL) three-dimensional (latitude, longitude and altitude) modeling capability. At the Moon, the broadband, hemispherical irradiance from Earth at full-phase is approximately 0.15 W m−2 with comparable contributions from solar reflectance and thermal emission; for context, this about 0.01% that of solar irradiance and has an equivalent temperature of around 40 K. Over the simulated timeframe, spanning two lunations, Earth's thermal irradiance shows very little net change (less than a few mW m−2 resulting from cloud variability and the south-to-north motion of the sub-observer latitude on Earth). In the solar band, Earth's diurnally averaged light curve at phase angles g ≤ 60° is well-fit using a Henyey–Greenstein integral phase function. At wavelengths longward of about 0.7 µm, near the well-known vegetation “red edge”, Earth's reflected solar radiance shows significant diurnal modulation as a result of the broad maximum in projected landmass at terrestrial longitudes between 60°W and 0°, as well as from the d |
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ISSN: | 0019-1035 1090-2643 |
DOI: | 10.1016/j.icarus.2018.12.025 |