文档介绍：CLBE001-ESS2E November 11, 2006 4:33
372 Encyclopedia of the Solar System
shadowing among surface particles. The magnitude of this where R is the radius of the satellite, r is the Sun-satellite
surge, known as the opposition effect, is greater for a distance, ε is the emissivity, σ is Stefan–Boltzmann’s con-
more porous surface. Many ary and satellite surfaces stant, A is the Bond albedo, and F is the incident solar
exhibit a large opposition surge at very small solar phase flux (a slowly rotating body would radiate over 2πR2). Typ-
angles that has been attributed to constructive interference ical mean temperatures in Kelvins for the satellites are: the
of sunlight. Earth’s Moon, 280 K; Europa, 103 K; Iapetus, 89 K; the
One measure of how much radiation a satellite reflects Uranian satellites, 60 K; and the Neptunian satellites, 45 K.
is the geometric albedo, p, which is the disk-integrated For thermal equilibrium, measurements as a function of
brightness at “full moon”(or a phase angle of 0◦) compared wavelength yield a blackbody curve characteristic of T:in
to a perfectly reflecting, diffuse disk of the same size. The general, the temperatures of the satellites closely follow the
phase integral, q, defines the angular distribution of radi- blackbody emission values. Some discrepancies are caused
ation over the sky: by a weak greenhouse effect (in the case of Titan), or the
π existence of volcanic activity (in the case of Io).
q = 2 (α)sin α dα Another possible use of radiometric techniques, when
bined with photometric measurements of the reflected
portion of the radiation, is the estimate of the diameter of a
where (α) is the disk-integrated brightness and α is the
= × satellite. A more accurate method of measuring the diam-
phase angle. The Bond albedo, which is given by A p eter of a satellite from Earth involves measuring the light
q, is the ratio of the integrated flux reflected by the satellite from a star as it is occul