Albedo
Contents
Albedo
(glossary entry)Description
The reflectance of a surface. In lunar studies this generally refers to the intensity of light reflected back towards a detector (eye or camera) expressed as a fraction relative to an idealized perfect reflector dispersing light "equally" in all directions. Albedos can be for a specific wavelength or averaged over a range of wavelengths. They can also refer to individual features or be averaged over larger areas. Variations in albedo cause areas to appear lighter (high albedo) or darker (low albedo).
Additional Information
- The overall albedo of the Moon is frequently quoted as being about 7%. This is actually the so-called Bond albedo at visible wavelengths, which refers to the fraction of the total energy impinging on a surface that is reflected in all directions. It is a concept which is useful in studies of planetary enegy balance, but has little relevance to perceived brightness, which depend entirely on the intensity reflected in a specific direction. The NASA Moon Fact Sheet gives the Bond albedo of the Moon (presumably averaged over the entire solar spectrum, including non-visible wavelengths) as 0.11. However the CERES Earth orbiting satellite climate radiometers have measured the value to be higher and somewhere between 0.136 and 0.137, at a lunar phase angle of seven degrees.
- A concept more closely related to the variations in the perceived brightness of planets due to variations in surface properties is the geometric albedo, or sometimes called the visual geometric albedo when referring specifically to the band of visible wavelengths. Geometric albedo is determined by comparing the light received from an entire spherical planet to that expected from an idealized "perfect" reflectance diffusing disk of the same same cross section, with a light source directly behind the detector. This is an indication, for example, of the total intensity of moonlight reflected back towards the Earth near Full Moon. The NASA Moon Fact Sheet gives the visual geometric albedo of the Moon as 0.12. However this number probably doesn't include the opposition surge which can increase the reflectance of the lunar surface by 50% or more when the light source and detector are precisely aligned.
- A third concept is the normal albedo (or visual normal albedo) which is still more closely to the variation in in the perceived brightness of individual surface features due to variations in their reflectances. The word "normal" is used here to mean "with illumination perpendicular to the surface". Normal albedo is obtained by comparing a small sections of a planetary surface to what would be expected from a perfect diffusing reflector of the same area. Such a concept can be used to differentiate bright (high albedo) features, like the lunar highlands, from dark (low albedo) features, like the maria. However again, the retroreflectivity of many lunar materials makes it difficult to assign universally meaningful numbers. Unlike an ideal diffuser of a given reflectance, two lunar features that have equal reflectances when observed with the detector exactly aligned with the light source may be less similar when viewed at a different angle. Also the amount of brightening observed as the phase angle decreases varies with wavelength. A list of what are presumably visual normal albedo measurements can be found under Brightness of Selected Features, although it might be noted that only features near disk center are actually being evaluated at normal incidence near Full Moon.
- The IAU nomenclature scheme includes a category for lunar Albedo Features, although at present it contains only one member.
Variations in Lunar Brightnesses
The Moon is a place of strong brightness variations, sometimes seen in one view while looking at a brilliant peak sticking up from a black shadow; at other times from variations in surface reflectance under conditions of uniform illumination. See the list of Brightnesses of Selected Features.
LPOD Articles
Bibliography
- Albedo -- Wikipedia article
- Matthews, Grant. 2008. Celestial body irradiance determination from an underfilled satellite radiometer: application to albedo and thermal emission measurements of the Moon using CERES. Applied Optics, vol. 47, issue 27, p. 4981. "The average broadband Moon albedo is measured by CERES at a value of 0.1362 (±2-3%) when normalized to a static lunar phase angle of 7° using the U.S. Geological Survey lunar irradiance Robotic Lunar Observatory model."
- Very, Frank W. 1917. Lunar and terrestrial albedoes. Boston : The Four seas company.
