The Solar System and Its Place in the Galaxy 19
FIGURE 8 The ring systems of the jovian planets: Jupiter’s single ring photographed in forward
scattered light while theGalileospacecraft was in eclipse behind the giant planet: the lit circle is
sunlight filtering through the atmosphere of Jupiter (top); Saturn’s rings break up into hundreds of
ringlets when viewed at high resolution, as in thisCassinimosaic (middle); Uranus’ system of narrow
rings as viewed in forward scattered light byVoyager 2as it passed behind the planet (bottom left);
two of Neptune’s rings showing the unusual azimuthal concentrations, as photographed byVoyager 2
as it passed behind the planet; the greatly overexposed crescent of Neptune is visible at lower right in
the image (bottom right).are eventually incorporated into terrestrial sediments. In
the 1970s, NASA began experimenting with collecting in-
terplanetary dust particles (IDPs, also known as Brownlee
particles because of the pioneering work of D. Brownlee)
using high altitude U2 reconnaissance aircraft. Terrestrial
sources of particulates in the stratosphere are rare and con-
sist largely of volcanic aerosols and aluminum oxide particles
from solid rocket fuel exhausts, each of which are readily
distinguishable from extraterrestrial materials.
The composition of the IDPs reflects the range of source
bodies that produce them and include ordinary and car-
bonaceous chondritic material and suspected cometary par-
ticles. Because the degree of heating during atmospheric
deceleration is a function of the encounter velocity, recov-
ered IDPs are strongly biased toward asteroidal particles
from the main belt, which approach the Earth in lower
eccentricity orbits. Nevertheless, suspected cometary par-
ticles are included in the IDPs. The cometary IDPs show
a random, “botryoidal” (cluster-of-grapes) arrangement of
submicron silicate grains similar in size to interstellar dust
grains, intimately mixed in a carbonaceous matrix. Voids in
IDPs may have once been filled by cometary ices. In 2006,
theStardustspacecraft returned samples of cometary dust
collected during a flyby of comet Wild 2; these will provide