Albedo is a reflection coefficient ranging from zero to one.
Albedo, or reflection coefficient, derived from Latin albedo “whiteness” (or reflected sunlight) in turn from albus “white,” is the diffuse reflectivity or reflecting power of a surface.
It is the ratio of reflected radiation from the surface to incident radiation upon it.
Its dimensionless nature lets it be expressed as a percentage and is measured on a scale from zero for no reflection of a perfectly black surface to 1 for perfect reflection of a white surface.
Asteroid 433 Eros, for example, is quite dark with an albedo of 0.25.
The Moon is darker than 433 Eros with an albedo of around 0.12 or 0.136 [depending upon source].
The overall albedo of the Moon is around 0.12, but it is strongly directional and non-Lambertian, displaying also a strong opposition effect.
While such reflectance properties are different from those of any terrestrial terrains, they are typical of the regolith surfaces of airless solar system bodies.
Celestial body irradiance determination from an underfilled satellite radiometer: application to albedo and thermal emission measurements of the Moon using CERES.
Matthews, Grant (2008). Applied Optics 47 (27): 4981–93.
Therefore, in theory, a Full Moon should look a little darker than the “moon” in the following illustration which has an albedo 0.15.
However, a Full Moon [even on a hazy night] shines down on Earth with an albedo of 1.0.
Some people attribute this additional [and anomalous] Lunar Luminosity to Earthshine.
Unfortunately, as can be seen in the picture below, Earthshine is only dimly reflected by the Moon while the lunar crescent that is illuminated directly by the Sun shines brilliant white.
Earthshine is reflected earthlight visible on the Moon’s night side.
It is also known as the Moon’s ashen glow or as the old Moon in the new Moon’s arms.
Earthshine is used to help determine the current albedo of the Earth.
The data are used to analyze global cloud cover, a climate factor.
Oceans reflect the least amount of light, roughly 10%.
Land reflects anywhere from 10–25% of the Sun’s light, and clouds reflect around 50%.
So, the part of the Earth where it is daytime and from which the Moon is visible determines how bright the Moon’s earthshine appears at any given time.
The Earthshine explanation is imaginative but [none the less] it’s pure moonshine.
Luckily, there is an explanation for this visual anomaly [lurking in the dark recesses of Wikipedia].
The Compton Gamma Ray Observatory detected gamma rays from the Moon
The Moon as seen by the Compton Gamma Ray Observatory, in gamma rays of greater than 20 MeV.
The Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory has detected gamma rays from the Moon as it passed through the instrument field of view several times between 1991 and 1994.
And [as we have seen] Gamma Rays trigger electromagnetic cascades in the Earth’s atmosphere which generate fluorescent light.
This is why Cherenkov telescopes [that are used for detecting cosmic rays] “can only function well on clear nights without the Moon shining”.
The first detection method is called the air Cherenkov telescope, designed to detect low-energy (<200 GeV) cosmic rays by means of analyzing their Cherenkov radiation, which for cosmic rays are gamma rays emitted as they travel faster than the speed of light in their medium, the atmosphere.
While these telescopes are extremely good at distinguishing between background radiation and that of cosmic-ray origin, they can only function well on clear nights without the Moon shining, and have very small fields of view and are only active for a few percent of the time.
The full effect of the atmospheric fluorescence can be clearly identified in the following lunar image.
The Lunar Gamma Rays are produced by Cosmic Ray bombardment of the Moon’s surface.
The average flux, and the energy spectrum of the lunar gamma radiation are consistent with a model of gamma ray production by cosmic ray interactions with the lunar surface, and the flux varies as expected with the solar cycle.
The specific process is called sputtering.
Sputtering is a process whereby atoms are ejected from a solid target material due to bombardment of the target by energetic particles.
It only happens when the kinetic energy of the incoming particles is much higher than conventional thermal energies (≫ 1 eV).
This process can lead, during prolonged ion or plasma bombardment of a material, to significant erosion of materials, and can thus be harmful.
The sputtering process is initiated when the incident ion triggers a collision cascade.
The thick line illustrates the position of the surface, and the thinner lines the ballistic movement paths of the atoms from beginning until they stop in the material.
The purple circle is the incoming ion.
Red, blue, green and yellow circles illustrate primary, secondary, tertiary and quaternary recoils, respectively.
Two of the atoms happen to move out from the sample, i.e. be sputtered.
The Cosmic Rays triggering the Lunar sputtering include Solar Cosmic Rays.
The sun as seen in gamma rays by COMPTEL during a June 15, 1991, solar flare.
Cataloging the gamma-ray universe, weighing black holes, and a hat trick
Overall, the mainstream have been very efficient when it comes to losing low energy Gamma Rays [aka Solar Cosmic Rays] down the back of the sofa.
The Gamma Ray image of the Moon used in this posting, for example, was produced by the Energetic Gamma Ray Experiment Telescope [EGRET] on board NASA’s Compton Gamma Ray Observatory satellite.
The Energetic Gamma Ray Experiment Telescope (EGRET) was one of four instruments outfitted on NASA’s Compton Gamma Ray Observatory satellite.
Since lower energy gamma rays cannot be accurately detected on Earth’s surface, EGRET was built to detect gamma rays while in space.
EGRET was created for the purpose of detecting and collecting data on gamma rays ranging in energy level from 30 MeV to 30 GeV.
EGRET was designed to detect gamma rays ranging from 30 MeV to 30 Gev [roughly 107 eV through to 1010 eV] and ignore the lower energy gamma rays [aka Solar Cosmic Rays] above 105.
Radiation below 100 keV is classified as X-rays and is the subject of X-ray astronomy
However, the situation deteriorates in the published EGRET observations paper because only gamma rays above 100 MeV [108 eV] are included in the results.
Energetic gamma ray experiment telescope
high-energy gamma ray observations of the Moon and quiet Sun
Thompson, Bertsch, Morris and Mukherjee.
Journal of Geophysical Research A7 102 (1997): 14730-4740. Print.
Therefore, EGRET was very successful at not observing Solar Cosmic Rays.