Solar System Dust 627
solar radii; others have not seen a sharp edge. Perhaps the
inner edge of the zodiacal cloud may change in time.
The large-scale distribution of the zodiacal dust cloud is
obtained from zodiacal light measurements onboard inter-
planetary spacecraft spanning a distance ranging from 0.3
to approx. 3 AU from the Sun. Even though the intensity
decreases over this distance by a factor 150, the spatial den-
sity of dust needs only to decrease by a factor 15. The radial
dependence of the number density is slightly steeper than
an inverse distance dependence. From zodiacal light mea-
surements, a slight inclination of about 3◦of the symmetry
plane of zodiacal light with respect to the ecliptic plane has
been determined.
At visible wavelengths, the spectrum of the zodiacal light
closely follows the spectrum of the Sun. A slight redden-
ing (i.e., the ratio of red and blue intensities is larger for
zodiacal light than for the Sun) indicates that the majority
of particles are larger than the mean visible wavelength of
0.54μm. In fact, most of the zodiacal light is scattered by 10-
to 100-μm-sized particles. Therefore, the dust seen by zo-
diacal light is only a subset of the interplanetary dust cloud.
Submicrometer- and micrometer-sized particles, as well as
millimeter and bigger particles, are not well represented by
the zodiacal light at optical wavelengths.
Above about 1μm in wavelength, the intensities in the
solar spectrum rapidly decrease. The zodiacal light spec-
trum follows this decrease until about 5μm, above which
the thermal emission of the dust particles prevails. Because
of the low albedo (fraction of incident sunlight reflected
back and scattered in all directions is smaller than 10%) of
interplanetary dust particles, most visible radiation (>90%)
is absorbed and emitted at infrared wavelengths. The max-
imum of the thermal infrared emission from the zodiacal
dust cloud lies between 10 and 20μm. From the thermal
emission observed by theIRASand Cosmic Background
Explorer (COBE) satellites, an average dust temperature
at 1 AU distance from the Sun between 0◦C and 20◦C has
been derived. Some spatial structure has been observed at
thermal infrared wavelengths. Asteroid bands mark several
asteroids families as significant sources of solar system dust
just as comet trails identify dust emitted from individual
comets.
Optical and infrared observations of other extraterres-
trial dusty phenomena have also provided important in-
sights into the zodiacal complex. Cometary and asteroidal
dust is considered to be an important source of the zodia-
cal cloud. The study of circumplanetary dust and rings has
stimulated much research in the dynamics of dust clouds.
Interstellar dust is believed to be the ultimate source of
all refractory material in the solar system. Circumstellar
dust clouds like the one aroundβ-Pictoris are “zodiacal
clouds” of their own right. The study of which may eventu-
ally give information on extra solar planetary systems. [See
InfraredViews of theSolarSystem fromSpace;
PlanetaryRings;Extra-SolarPlanets.]
FIGURE 5 Microcraters on the glassy surface of a lunar sample.
Bright spallation zones surround circular central pits.2.4 Lunar Microcraters and the Near-Earth
Dust Environment
The size distribution of interplanetary dust particles is rep-
resented by the lunar microcrater record. Microcraters on
lunar rocks have been found ranging from 0.02μm to mil-
limeters in diameter (Fig. 5). Laboratory simulations of
high-velocity impacts on lunar-like materials have been per-
formed to calibrate crater sizes with projectile sizes and im-
pact speeds. Submicrometer- to centimeter-sized projec-
tiles have been used with speeds above several kilometers
per second. The typical impact speed of interplanetary me-
teoroids on the Moon is about 20 km/s. For the low-mass
particles, electrostatic dust accelerators that reach projec-
tile speeds of up to 100 km/s were used. The high-mass pro-
jectiles were accelerated with light-gas guns, which reached
speeds up to about 10 km/s. For the intermediate mass
range, plasma drag accelerators reached impact speeds of
20 km/s. The crater diameter to projectile diameter ratio
varies from 2 for the smallest microcrater to about 10 for
centimeter-sized projectiles.
The difficulty in deriving the impact rate from a crater
count on the Moon is that the degree to which rocks shield
other rocks and thus the exposure time of any surface is
generally unknown. Therefore, the crater size or meteoroid
distribution has to be normalized with the help of an im-
pact rate or meteoroid flux measurement obtained by other
means. In situ detectors or recent analyses of impact plates
that were exposed on NASA’sLong Duration Exposure Fa-
cilityto the meteoroid flux for several years provided this
flux calibration (Fig. 6). Flux of the smallest particles dom-
inates, and the mass flux of meteoroids peaks at 10−^5 g.
The total mass density of interplanetary dust at 1 AU is
10 −^16 g/m^3 and the total mass of the zodiacal cloud inside
Earth’s orbit is between 10^16 and 10^17 kg, which corresponds