Infrared Views of the Solar System from Space 685
FIGURE 4 The zodiacal light from Mauna Kea, Hawaii. It is
seen most prominently after sunset in the spring and before dawn
in autumn at northern latitudes. (Courtesy M. Ishiguro, ISAS.)
FIGURE 5 The zodiacal cloud (top) is seen extending from 0◦to 360◦in ecliptic latitude from right to left, constructed
from scans of the ecliptic plane by IRAS. Ecliptic latitudes between 30◦and− 30 ◦are shown. The diagonal structure
crossing the ecliptic plane near 90◦and 270◦longitude is the galactic plane. Where the cloud is bright and wide (in
latitude), the sky is being scanned at lower solar elongations, picking up the brighter thermal emissions of the warmer
dust that lies closer to the Sun. As the satellite scans further away from the Sun at higher solar elongations, it is looking
through less dust near the Earth and seeing a greater fraction of colder fainter dust. When filtered to remove its broad
component (bottom), the zodiacal cloud reveals dust bands, located out in the asteroid belt and surrounding the inner
solar system. Parallax results in their separation being smaller at lower solar elongations, where they are seen at a
greater distance. Other solar system structures include dust trails.when the geometry is optimal. Comets were long thought
to be the origin of the zodiacal cloud. However, estimates
of dust production by short-period comets fell far short of
that needed to maintain the cloud in steady state against
losses from particles spiraling into the Sun. This mecha-
nism, where the absorbtion and reemission of solar radiation
continually decreases particle velocity, is called Poynting–
Robertson drag.
A cometary cloud would have to be replenished by the
occasional capture of “new,” highly active comets into short-
period orbits. Comet Encke was suggested as one such pos-
sible source in the past. Asteroid collisions have also been
considered to be a source of interplanetary dust, and a sig-
nificant fraction of interplanetary dust particles (IDPs) col-
lected by high-altitude aircraft are thought to be consistent
with such an origin, but there were few observational con-
straints on estimates of their relative contribution to the
cloud as a whole.
At thermal wavelengths, interplanetary dust is seen
around the sky, peaking about the ecliptic plane (Fig. 5).
It appears brighter as we look closer to the Sun (where it
is warmer and more dense, hence giving off more thermal
radiation). Within this broad band of dust, there are struc-
tures related to dust-producing processes not seen before
the advent of space-based infrared telescopes. The most
prominent of these structures are the dust bands—parallel
rings of dust straddling the plane of the ecliptic (Fig. 5).
These bands arise from collisions in the Asteroid Belt. When
asteroids collide, the resultant fragments are ejected with
velocities that are small compared to the orbital velocity of