Infrared Views of the Solar System from Space 687
FIGURE 8 As hypothetical interplanetary dust particles
originating in the Eos family torus migrate in towards the Sun,
they contribute to the overall zodiacal cloud. The density
contours of their contribution is shown, with darker regions
corresponding to increasing particle number densities. The x axis
is heliocentric distance in AU, and the y axis is roughly the
distance above the ecliptic plane. As the particles evolve to
smaller heliocentric distances, the number density increases, and
the extrema near the upper and lower edges of the cloud
component is maintained. Viewed from the Earth, as we scan
from the pole to the ecliptic, the column density of particles
increases as we approach an angle near their average orbital
inclination resulting in the appearance of a pair of parallel dust
bands. (Coutesy of W. Reach).
and generating dust, then it would follow that the most
dust would be generated in the regions of greatest aster-
oid concentration—the largest asteroid families. Assuming
this dust to be the main source of the zodiacal cloud, the
cloud itself would be something expected to change slowly
over much of the age of the solar system. An alternative
hypothesis proposed that the dust bands arose from more
recent collisions of smaller asteroids and that the zodiacal
cloud was highly variable over time. David Nesvorny and
colleagues identified the sources of the two most prominent
pairs of dust bands as the Karin and Veritas families and de-
termined that the collisions forming these families occurred
within the past 10 million years. This demonstrates that the
zodiacal cloud, once assumed to be in relative steady state,
may vary substantially over time as dust production in a
given family slowly declines as more and more of its mass is
ground up and removed by radiation forces and a new ran-
dom collision creates a family of debris that generates more
dust. It is interesting that a faint inner pair of bands is still
associated with the very ancient Themis family, the largest
asteroid family, which was formed by the catastrophic
disruption of a 240 km diameter asteroid billions of years
ago and may still be producing dust today.
3. A Ring of Dust Around the Earth’s Orbit
As the sky was being mapped for the first time in the ther-
mal infrared byIRAS, something odd was noticed: The sky
always seemed to be a few percent brighter in the direction
opposite the Earth’s motion about the Sun than in the direc-
tion of the Earth’s motion. Since the satellite orbited above
the terminator of the Earth and was facing different parts
of the sky as the Earth orbited the Sun, if it was a difference
in the actual sky brightness, eventually the satellite would
see that difference flip when viewed from the other side of
Earth’s orbit. That did not happen. The “trailing” sky was
always brighter. It made no sense that the Earth could be
tracked by a large orbiting cloud—such a cloud would not
be stable and disperse. Unable to come up with a satisfac-
tory explanation, it was thought to be a strange calibration
problem.
In 1993, a graduate student, Sumita Jayaraman, calcu-
lated that particles evolving from the Asteroid Belt past the
Earth under Poynting–Robertson drag would have that or-
bital decay interrupted as a consequence of resonance inter-
actions with the Earth (where the ratio of particle and Earth
orbital periods is a ratio of integers). This dust would pile up
for a while before continuing its sunward spiral, forming a
ring around the Earth’s orbit. The resonant ring has a clump
(about 10% enhancement in density over the background
zodiacal cloud) always trailing the Earth by about 0.2 AU
in its orbit (Fig. 9). This resonant structure represents a
volume through which particles are circulating around the
Sun, to be distinguished from a cloud of self-attracting par-
ticles. This explained theIRASmystery. The existence of
the resonance ring structure was confirmed byCOBEob-
servations.
TheSpitzer Space Telescopeis in a heliocentric orbit
slowly trailing further and further behind the Earth. At
the end of its nominal 5 year mission,Spitzerwill end up
0.6 AU behind the Earth and will have completely traveled
through the trailing cloud, allowing for its 3-dimensional
structure to be probed in detail. These observations will
place tight constraints on the production and evolution of
particles from the Asteroid Belt in the size range sensitive
to resonance with Earth motion, and provide insights into
how such structures in dust disks about other stars may
provide details about planets imbedded in those disks. [See
Extra-SolarPlanets.]4. Comets and Their Nature
Comets are members of the solar system that have been
known since ancient times. At visible wavelengths they are
characterized by distinctive tails of micrometer-sized dust
particles ejected from the nucleus and pushed away under
radiation pressure, a coma of gas, ice, and dust surround-
ing the nucleus, and sometimes an ion tail of gas molecules