688 Encyclopedia of the Solar System
FIGURE 9 A simulated image of the Earth’s resonant ring (left) showing a close-up of the cloud of dust that trails the Earth in its
orbit through the year. The ellipse shows the orbit of the Earth in a rotating reference frame. The resolution of the image is 0.01
AU in the X and Y directions. The cloud is modeled using 12μm spherical particles of astrophysical silicate. The proximity of the
clump of dust behind the Earth near its orbit explains why scans of the sky behind the Earth were always brighter than scans in
front of the Earth at the same solar elongation angle. Over the course of its mission, Spitzer will travel through the dust cloud
trailing Earth (right). The ‘loops’ of Spitzer’s orbital path and the oval of Earth’s motion arises from the small eccentricity of the
orbits. (Figure courtesy of S. Jayaraman.)carried away by the solar wind (its typically blue color arising
from electron recombination events) (Fig. 10). In the 1950s,
Fred Whipple developed the standard model of comet nu-
clei as bodies largely of ice with a mixture of some dust—a
“dirty snowball,” which explained the nongravitational com-
ponents of their motion. He also linked their activity to the
maintenance of the zodiacal cloud, which required constant
replenishment as its constituent particles spiraled into the
Sun under Poynting–Robertson drag. [SeePhysics and
Chemistry ofComets.]
Comets are known to eject large particles from their as-
sociation with many meteor streams. These particles spread
over a comet’s orbit and are scattered within its plane. If the
comet orbit happens to extend inside the Earth’s, these par-
ticles will be seen as meteors as the Earth passes through
their orbital plane. Because they are striking the Earth’s at-
mosphere from the same direction, meteor streams seem to
come from a particular location in the sky. This is called the
radiant. Analysis of meteors as they burn up in the Earth’s
atmosphere indicates that particles associated with known
comets have low (<1 g/cm^3 ) to modest (<2.5 g/cm^3 ) mass
densities.
FIGURE 10 Comet West on March 9, 1976, less than half an
AU from the Sun after perihelion, exhibiting classic dust (white)
and ion (blue) tails. (Image by J. Laborde.)