The Bond albedo (also called spheric albedo, planetary albedo, and bolometric albedo), named after the American astronomer
George Phillips Bond (1825–1865), who originally proposed it, is the fraction of
power in the total
electromagnetic radiation
In physics, electromagnetic radiation (EMR) is a self-propagating wave of the electromagnetic field that carries momentum and radiant energy through space. It encompasses a broad spectrum, classified by frequency or its inverse, wavelength ...
incident on an astronomical body that is scattered back out into space.
Because the Bond albedo accounts for all of the light scattered from a body at all
wavelength
In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats.
In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...
s and all
phase angles, it is a necessary quantity for determining how much energy a body absorbs. This, in turn, is crucial for determining the
equilibrium temperature of a body.
Because bodies in the outer Solar System are always observed at very low phase angles from the Earth, the only reliable data for measuring their Bond albedo comes from spacecraft.
Phase integral
The Bond albedo (''A'') is related to the
geometric albedo
In astronomy, the geometric albedo of a celestial body is the ratio of its actual brightness as seen from the light source (i.e. at zero phase angle (astronomy), phase angle) to that of an ''idealized'' flat, fully reflecting, diffuse reflection, d ...
(''p'') by the expression
:
where ''q'' is termed the ''phase integral'' and is given in terms of the directional scattered flux ''I''(''α'') into phase angle ''α'' (averaged over all wavelengths and azimuthal angles) as
:
The phase
angle
In Euclidean geometry, an angle can refer to a number of concepts relating to the intersection of two straight Line (geometry), lines at a Point (geometry), point. Formally, an angle is a figure lying in a Euclidean plane, plane formed by two R ...
''α'' is the angle between the source of the radiation (usually the Sun) and the observing direction, and varies from zero for light scattered back towards the source, to 180° for observations looking towards the source. For example, during opposition or looking at the full moon, α is very small, while backlit objects or the new moon have α close to 180°.
Examples
The Bond albedo is a value strictly between 0 and 1, as it includes all possible scattered light (but not radiation from the body itself). This is in contrast to other definitions of
albedo such as the geometric albedo, which can be above 1. In general, though, the Bond albedo may be greater or smaller than the geometric albedo, depending on the surface and atmospheric properties of the body in question.
Some examples:
Albedo of the Earth
/ref>
See also
* Albedo
* Geometric albedo
In astronomy, the geometric albedo of a celestial body is the ratio of its actual brightness as seen from the light source (i.e. at zero phase angle (astronomy), phase angle) to that of an ''idealized'' flat, fully reflecting, diffuse reflection, d ...
References
{{reflist
External links
discussion of Lunar albedo
Concepts in astrophysics
Electromagnetic radiation
Radiometry
Scattering, absorption and radiative transfer (optics)