Bond albedo
From Halbeeg, the open encyclopedia · Af-Soomaali
19 languages
Bond albedo is a measure (a number between 0 and 1) of the fraction of incoming light energy from a star that a celestial body—such as a planet, moon, or asteroid—reflects back into space, compared to the total energy received from the star. It accounts for light reflected in all directions and across all wavelengths emitted by the star. It is therefore a bolometric (total energy) measurement that indicates how much energy an object absorbs and how much it reflects.
The name derives from American astronomer George Phillips Bond. This measurement is important for calculating the equilibrium temperature of a celestial body: absorbed energy is re-radiated to space as infrared radiation. A planet with high bond albedo absorbs less energy and therefore has a lower equilibrium temperature, all other things being equal.
Distinction from other albedos
Bond albedo differs from geometric albedo, which compares light reflected by an object when illuminated perpendicularly (the apparent brightness of a surface lit uniformly from directly behind the observer). There are also spectral albedos, which measure reflectivity at each wavelength separately. Bond albedo can be derived from geometric albedo by multiplying by the phase integral—a number indicating how light is distributed across different angles.
Measurement
True measurement requires observing an object from different phase angles, which cannot be done from Earth for distant planets. For this reason, uncrewed space probes—such as those that have orbited Jupiter, Saturn, and other planets—are the primary source of data. Calculation involves integrating measurements across all wavelengths and all angles.
Application
Bond albedo is fundamental to the science of planetary energy balance. It is used to estimate surface temperature, to determine whether an object emits heat from internal sources (when the energy it radiates exceeds the energy it absorbs), and to model the effects of clouds and ice cover. It is also one of the measures used to characterize exoplanets outside our solar system.