Solar System Dust 631
and the extended E ring have been identified as sources.
The ejection mechanism is very similar to that acting at
Jupiter. Freshly generated nanometer-sized dust grains get
charged and—if the charge is positive—thrown out by
Saturn’s magnetic field. In some parts of the magneto-
sphere, dust particles become negatively charged; these
particles remain bound to the magnetic field and stay in
the vicinity of Saturn. The stream particles primarily con-
sist of silicate materials that imply that the particles are the
contamination of icy ring material rather than the ice par-
ticles themselves. [SeePlanetaryRings.]
2.5.3 INTERSTELLAR DUST IN THE HELIOSPHERE
The solar system is currently passing through a region
of low-density, weakly ionized interstellar material in our
galaxy, which shows a larger abundance of heavy refractory
elements in the gas phase such as iron, magnesium, and
silicon than in cold dense interstellar clouds. Interstellar
dust is part of the interstellar medium, although it has not
been directly observed by astronomical means in the ten-
uous local interstellar cloud. Interstellar dust is formed as
stardust in the cool atmospheres of giant stars and in nova
and supernova explosions.
More than a decade ago, interstellar dust was positively
identified inside the planetary system. At the distance of
Jupiter, the dust detector on board theUlyssesspacecraft
detected impacts predominantly from a direction that was
opposite to the expected impact direction of interplane-
tary dust grains. The impact velocities exceeded the local
solar system escape velocity, even if radiation pressure ef-
fects were considered. The motion of interstellar grains
through the solar system is parallel to the flow of neutral
interstellar hydrogen and helium gas, both traveling at a
speed of 26 km/s with respect to the Sun. The interstel-
lar dust flow persisted at higher latitudes above the ecliptic
plane, even over the poles of the Sun, whereas interplane-
tary dust is strongly concentrated toward the ecliptic plane
(Fig. 10).
Since that time,Ulyssesmonitored the stream of inter-
stellar dust grains through the solar system at higher lati-
tudes. It was found that the flux of small interstellar grains
showed some variation with the period of the solar cycle,
which indicates a coupling of the flux to the solar wind
magnetic field. Interstellar dust has initially been identi-
fied outside 3 AU out to Jupiter’s distance. However, refined
analyses showed that bothCassiniandGalileorecorded sev-
eral hundred interstellar grains in the region between 0.7
and 3 AU from the Sun. Even in theHeliosdust data inter-
stellar grains were identified down to 0.3 AU distance from
the Sun.
The radii of clearly identified interstellar grains range
from 0.1μm to above 1μm with a maximum at about
0.3μm. Even bigger interstellar particles have been reliably
FIGURE 10 Ulyssesdust impact rate observed around the time
of its ecliptic plane crossing (ECL). ECL occurred on 12 March
1995 at a distance of 1.3 AU from the Sun. The boxes indicate
the mean impact rates and their uncertainties. The top scales
give the spacecraft latitude. Model calculations of the impact
rate duringUlysses’ south to north traverse through the ecliptic
plane are shown by the lines. Contributions from interplanetary
dust on bound orbits and interstellar dust on hyperbolic
trajectories and the sum of both are displayed. From these
measurements, it is concluded that interstellar dust is not
depleted to a distance of 1.3 AU from the Sun.identified by their hyperbolic speeds in radar meteor ob-
servations. The flow direction of these bigger particles
varies over a much wider angular range than that of small
(submicrometer-sized) grains observed by spacecraft. The
deficiency of small grain masses (<0.3μm) compared to as-
tronomically observed interstellar dust indicates a depletion
of small interstellar grains in the heliosphere.
There are significant differences in the particle sizes that
were recorded at different heliocentric distances. Measure-
ments of the interstellar particle mass distribution revealed
a lack of small grains inside 3 AU heliocentric distance.
Measurements byCassiniandGalileoin the distance range
between 0.7 and 3 AU showed that interstellar particles
were bigger than 0.5μm with increasing masses closer to
the Sun. The flux of these bigger particles did not exhibit
temporal variations due to the solar wind magnetic field like
the flux of smaller particles observed byUlysses. The trend
of increasing masses of particles continues as demonstrated
byHeliosmeasurements, which recorded particles of about
1 μm radius down to 0.3 AU. These facts support the idea
that the interstellar dust stream is filtered by both radiation
pressure and electromagnetic forces. It is concluded that
interstellar particles with optical properties of grains con-
sisting of astronomical silicates or organic refractory ma-
terials are consistent with the observed radiation pressure
effect.