626 Encyclopedia of the Solar System
TABLE 2 Average Elemental Composition (All Major and Selected Minor and Trace Elements) of Several
Chondritic IDPs Is Compared with C1 Chondrite Compositiona
Element C1 IDP Variation TcMg 1,071,000 0.9 0.6–1.1 1067
Si 1,000,000 1.2 0.8–1.7 1311
Fe 900,000 1 1 1336
S 515,000 0.8 0.6–1.1 648
Al 84,900 1.4 0.8–2.3 1650
Ca 61,100 0.4 0.3–0.6 1518
Ni 49,300 1.3 1.0–1.7 1354
Cr 13,500 1.1 0.9–1.4 1277
Mn 9,550 1.1 0.8–1.6 1190
Cl 5,240 3.6 2.8–4.6 863
K 3,770 2.2 2.0–2.5 1000
Ti 2,400 1.5 1.3–1.7 1549
Co 2,250 1.9 1.2–2.9 1351
Zn 1,260 1.4 1.1–1.8 660
Cu 522 2.8 1.9–4.2 1037
Ge 119 2.3 1.6–3.4 825
Se 62 2.2 1.6–3.0 684
Ga 38 2.9 2.1–3.9 918
Br 12 34 23–50 690aThe IDP abundances are normalized to iron (Fe) and to C1. C1 abundance is normalized to Si=1,000,000 condensation temperaturesTc(◦C). From E. K.
Jessberger et al. (1992),Earth Planet. Sci. Lett. 112 , 91.iron–sulfur–nickel, 30%; and mafic silicates (iron–
magnesium–rich silicates, i.e., olivine and pyroxene), 10%.
Most chondritic IDPs are porous aggregates, but some
smooth chondritic particles are found as well. Chondritic
aggregates may contain varying amounts of carbonaceous
material of unspecified composition. Table 2 shows a signifi-
cant enrichment in volatile (low condensation temperature)
elements when compared to C1 chondrites. This observa-
tion is being used to support the argument that these par-
ticles consist of some very primitive solar system material
that had never seen temperatures above about 500◦C, as
is the case for some cometary material. This and compo-
sitional similarity with comets argue for a genetic relation
between comets and IDPs.
A remarkable feature of IDPs is their large variability
in isotopic composition. Extreme isotopic anomalies have
been found in some IDPs. Under typical solar system con-
ditions, only fractions of a percent of isotopic variations can
occur. These huge isotopic variations indicate that some
grains are not homogenized with other solar system mate-
rial but have preserved much of their presolar character.
Submicrometer-sized grains known as GEMS (glass with
embedded metal and sulfides) are major constituents of
the chondritic porous class of IDPs. Several GEMS with
nonsolar oxygen isotopic compositions were identified, con-
firming that at least some are indeed presolar grains. These
amorphous interstellar silicates are considered one of the
fundamental building blocks of the solar system.
2.3 Zodiacal Light
The wedge-shaped appearance of the zodiacal light (see
Fig. 1) demonstrates its concentration in the ecliptic
plane. For an observer on Earth, the zodiacal light extends
in the ecliptic all the way around to the antisolar direction,
however, at strongly reduced intensities. In the direction
opposite to the Sun, this light forms a hazy area of a few
degrees in dimension known as the gegenschein, or coun-
terglow. If seen from outside the solar system, the zodiacal
dust cloud would have a flattened, lenticular shape that ex-
tends along the ecliptic plane about seven times farther
from the Sun than perpendicular to the ecliptic plane.
The brightness of zodiacal light is the result of light scat-
tered by a huge number of particles in the direction of
observation. The observed zodiacal brightness is a mean
value, averaged over all sizes, compositions, and structures
of particles along the line of sight. Zodiacal light brightness
can be traced clearly into the solar corona. However, most
of this dust is foreground dust close to the observer because
of a favorable scattering function. Nevertheless, the vicinity
of the Sun is of considerable interest for zodiacal light mea-
surements because it is expected that close to the Sun the
temperature of the dust rises, and the dust particle starts
to sublimate, first the more volatile components and closer
to the Sun even the refractory ones. Inside about four solar
radii distance, dust should completely sublimate. Some ob-
servers have found a sharp edge of a dust-free zone at four