Encyclopedia of the Solar System 2nd ed

372 Encyclopedia of the Solar System

shadowing among surface particles. The magnitude of this
surge, known as theopposition effect, is greater for a
more porous surface. Many planetary and satellite surfaces
exhibit a large opposition surge at very small solar phase
angles that has been attributed to constructive interference
of sunlight.
One measure of how much radiation a satellite reflects
is thegeometric albedo,p, which is the disk-integrated
brightness at “full moon” (or a phase angle of 0◦) compared
to a perfectly reflecting, diffuse disk of the same size. The
phase integral,q, defines the angular distribution of radi-
ation over the sky:

q= 2

∫π

0

(α)sinαdα

where(α) is the disk-integrated brightness andαis the
phase angle. TheBond albedo, which is given byA=p×
q, is the ratio of the integrated flux reflected by the satellite
to the integrated flux received. The geometric albedo and
phase integral are wavelength dependent, whereas a true (or
bolometric) Bond albedo is integrated over all wavelengths.
Another ground-based photometric measurement that
has yielded important information on the satellites’ surfaces
is the integrated brightness of a satellite as a function of
orbital angle. For a satellite in synchronous rotation with
its primary, the subobserver geographical longitude of the
satellite is equal to the longitude of the satellite in its orbit.
Observations showing significant albedo and color variega-
tions for Io, Europa, Rhea, Dione, and especially Iapetus
suggest that diverse geologic terrains coexist on these satel-
lites. This view was confirmed by images obtained by the
Voyagerspacecraft.
Another important photometric technique is the mea-
surement of reflected light as one celestial body occults, or
blocks, another body. Time-resolved observations of occul-
tations yield the light reflected from successive regions of
the eclipsed body. This technique has been used to map
albedo variations on Pluto and its satellite Charon and to
map the distribution of infrared emission—and thus vol-
canic activity—on Io. Stellar occultations have been used
to probe the diameters and atmospheres of many satellites,
including Iapetus, Titan, and Triton.

3.1.3 RADIOMETRY

Satellite radiometry is the measurement of radiation that is
absorbed and reemitted at thermal wavelengths. The dis-
tance of each satellite from the Sun determines the mean
temperature for the equilibrium condition that the ab-
sorbed radiation is equal to the emitted radiation:

πR^2 (F/r^2 )(1−A)= 4 πR^2 εσT^4

T=

(

(1−A)F

4 σεr^2

) 1 / 4

whereRis the radius of the satellite,ris the Sun-satellite

distance,εis the emissivity,σis Stefan–Boltzmann’s con-

stant,Ais the Bond albedo, andFis the incident solar

flux (a slowly rotating body would radiate over 2πR^2 ). Typ-

ical mean temperatures in Kelvins for the satellites are: the

Earth’s Moon, 280 K; Europa, 103 K; Iapetus, 89 K; the

Uranian satellites, 60 K; and the Neptunian satellites, 45 K.

For thermal equilibrium, measurements as a function of

wavelength yield a blackbody curve characteristic ofT:in

general, the temperatures of the satellites closely follow the

blackbody emission values. Some discrepancies are caused

by a weakgreenhouse effect(in the case of Titan), or the

existence of volcanic activity (in the case of Io).

Another possible use of radiometric techniques, when

combined with photometric measurements of the reflected

portion of the radiation, is the estimate of the diameter of a

satellite. A more accurate method of measuring the diam-

eter of a satellite from Earth involves measuring the light

from a star as it is occulted by the satellite. The time the

starlight is dimmed is proportional to the satellite’s diame-

ter.

A third radiometric technique is the measurement of the

thermal response of a satellite’s surface as it is being eclipsed

by its primary. The rapid loss of heat from a satellite’s surface

indicates a thermal conductivity consistent with a porous

upper surface. Eclipse radiometry of Phobos, Callisto, and

Ganymede suggests that these objects all lose heat rapidly

and thus have porous regoliths created from eons of mete-

oritic bombardment.

3.1.4 POLARIMETRY

Polarimetry is the measurement of the degree of polariza-

tion of radiation reflected from a satellite’s surface. The

polarization characteristics depend on the shape, size, and

optical properties of the surface particles. Generally, the ra-

diation is linearly polarized and is said to be negatively polar-

ized if it lies in the scattering plane and positively polarized

if it is perpendicular to the scattering plane. Polarization

measurements as a function of solar phase angle for atmo-

sphereless bodies are negative at small phase angles; com-

parisons with laboratory measurements indicate that this

is characteristic of complex, porous surfaces consisting of

multisized particles. In 1970, ground-based polarimetry of

Titan that showed it lacked a region of negative polarization

led to the correct conclusion that it has a thick atmosphere.

3.1.5 RADAR

Planetary radar is a set of techniques that involve the trans-

mittance of radio waves to a remote surface and the analysis

of the echoed signal. Among the outer planets’ satellites,

the Galilean satellites, Titan, and several other Saturnian

satellites have been observed with radar. [SeePlanetary

Radar.]