630 Encyclopedia of the Solar System
FIGURE 8 Flux of meteoroids with masses> 8 × 10 −^10 kg
(about 10μm in size) in the outer solar system measured by the
Pioneer 10penetration detector. At 18 AU from the Sun, the
instrument quit operation. The measurements are in agreement
with a model of constant spatial dust density in the outer
planetary system. [From D. H. Humes (1980),J. Geophys. Res.
85 , 5841–5852.]
spacecraft moves in the same direction (outward) than in
the opposite case when the spacecraft moves inward. The
spatial dust density follows roughly an inverse radial dis-
tance dependence. Close passages of the asteroids Gaspra
and Ida did not exhibit increased dust impact rates.
In the outer solar system, the dust detectors on board
Pioneers 10/11,Galileo,Ulysses, and recentlyCassinimea-
sured the flux of interplanetary dust particles. The flux of
micrometer-sized particles decreased from 1 AU going out-
ward. No sign of a flux enhancement in the Asteroid Belt
was detected. Outside Jupiter’s orbit,Pioneer 10recorded
a flat flux profile (Fig. 8), which indicates a constant spatial
density of micrometer-sized dust in the outer solar system.
This observation has been interpreted to be due to the com-
bined input of dust from the Kuiper Belt and comets like
Halley and Schwassmann-Wachmann 1.
In interplanetary space, the highest dust fluxes have been
observed near comets. So far, four comets were visited by
spacecraft that carried dust detectors: Comets P/Giacobini-
Zinner, P/Halley, P/Grigg-Skjellerup, and Wild 2. Specially
optimized dust analyzers have been used to study Comet
Halley’s dust. Chemical analyses showed that, in addition
to the expected dust particles consisting of silicates, a large
fraction of cometary dust consists of carbonaceous materi-
als. Extreme isotopic anomalies have been found to exist in
some of these particles. Similar compositions are expected
for interplanetary dust. [SeePhysics and Chemistry of
Comets;Meteorites.)
FIGURE 9 Dust impact rate observed by theUlyssesdust
detector during the 400 days around the closest approach to
Jupiter (CA, 8 February 1992). At the beginning and end of the
period shown, Ulysses was 240 million km (1.6 AU) from Jupiter,
while at CA the distance was only 450,000 km. Except for the
flux peak at CA, when bigger particles were detected, the peaks
at other times consisted of submicrometer-sized dust particles.2.5.2 PLANETARY DUST STREAMS
Inside a distance of about 3 AU from Jupiter, bothUlysses
and Galileospacecraft detected unexpected swarms of
submicrometer-sized dust particles arriving from the direc-
tion of Jupiter. Figure 9 shows the strongly time-variable
dust flux observed byUlyssesduring its flyby of Jupiter.
About one month after its closest approach to Jupiter,
Ulyssesencountered the most intense dust burst at about
40 million km from Jupiter. For about 10 hours, the impact
rate of submicrometer-sized particles increased by a fac-
tor 1000 above the background rate. The similarity of the
impact signals and the sensor-pointing directions indicated
that the particles in the burst were moving in collimated
streams at speeds of several 100 km/s. Even stronger and
longer lasting dust streams were observed in 1995 by the
Galileodust detector during its approach to Jupiter. Dust
measurements inside the jovian magnetosphere showed a
modulation of the small particle impact rate with a period of
10 hours, which is the rotation period of Jupiter and its mag-
netic field. Positively charged dust particles in the 10-nm
size range coupled to the magnetic field and are thrown
out of Jupiter’s magnetosphere in the form of a warped
dust sheet. Sources of these dust particles are the volca-
noes on Jupiter’s moon Io and to a smaller extent Jupiter’s
ring. DuringCassini’s flyby of Jupiter, this phenomenon
was also observed, and mass spectra of the particles were
obtained. Both sodium chloride and sulfurous compo-
nents were identified in the mass spectra, which is con-
sistent with spectral measurements of Io’s volcano-induced
environment.
At Saturn,Cassiniobserved dust streams emanating
from this system as well. In this case, Saturn’s dense A ring