Introduction

Until the 1970s, when the Pioneer 10 and Pioneer 11 spacecraft passed by Jupiter, the exploration of the giant planet and its satellites was restricted to remote astronomical observations from the Earth. It was pure speculation whether dust would exist in the environment of Jupiter. Pioneer 10/11 were equipped with in-situ dust detectors which recorded several impacts when the spacecraft flew by Jupiter [1]. Due to the relatively high detection threshold of the penetration detectors, however, only particles larger than several micrometers could be recognized.

The next spacecraft to visit Jupiter were Voyager 1 and Voyager 2. Although they did not carry dedicated dust detectors on board, they drastically changed our knowledge of dust in the Jovian system. Jupiter's rings were discovered by remote sensing with Voyager 1, although earlier hints that this faint dusty ring might exist came from a dip in the density of charged particles measured near Pioneer ll's closest approach to Jupiter [2,3], as well as from the impact events recorded by the Pioneer dust detectors. Typical grain sizes derived from the Voyager images were a few micrometers for the faint gossamer ring, whereas the main ring turned out to be composed of macroscopic rocky material. Another discovery by Voyager was tidally driven active volcanism on Io, Jupiter's innermost Galilean moon. At the time it was speculated that small dust grains entrained in Io's plumes may get accelerated away from Io by electromagnetic forces [4,5].

Distance from Jupiter [AU] 1.5 1.0 0.5 0 0.5 1.0 1.5 2.0

10* 103

10"' 1er2

1991.5 1992.0 1992.5 1993.

Time [years]

Figure 1. Impact rate of dust particles observed by Ulysses around Jupiter fly-by. The curves show all impacts recorded (upper curve) and impacts of dust particles with masses greater than 10~15 kg (lower curve). The impact rates are means always including six impacts. The distance from Jupiter is indicated at the top. Note that after Jupiter fly-by the craft receded from the planet at about 35° jovi-graphic latitude.

Distance from Jupiter [AU] 1.5 1.0 0.5 0 0.5 1.0 1.5 2.0

Figure 1. Impact rate of dust particles observed by Ulysses around Jupiter fly-by. The curves show all impacts recorded (upper curve) and impacts of dust particles with masses greater than 10~15 kg (lower curve). The impact rates are means always including six impacts. The distance from Jupiter is indicated at the top. Note that after Jupiter fly-by the craft receded from the planet at about 35° jovi-graphic latitude.

The next major step forward in the investigation of Jovian dust came from the Ulysses spacecraft which flew by the planet in 1992. Ulysses is equipped with a highly sensitive impact-ionization dust detector capable of measuring dust grains down to sizes of 0.1/im [6]. With Ulysses, periodic collimated streams of dust particles with up to 2000 impacts per day were discovered while the spacecraft was within 2 AU from the giant planet [7,8] (Figure 1). The streams occurred at approximately monthly intervals (28 ± 3 days) and their impact directions implied that the grains originated from the Jovian system. No periodic phenomenon for small dust grains in interplanetary space was known before.

Confirmation of the Jupiter dust streams came from the Galileo spacecraft which carries a twin of the Ulysses dust detector on board [9]: dust 'storms' with up to 10,000 impacts per day were recorded while Galileo was within 0.5 AU from the planet. [10,11]. Since December 1995, Galileo has been the first man-made spacecraft in orbit about a giant planet of our Solar System. It explores Jupiter, its satellites and its huge magnetosphere. With the Galileo dust detector the dust streams seen in interplanetary space were also detected within the planet's magnetosphere. The grains showed a strong electromagnetic interaction with the Jovian magnetic field (see Sect. 2).

In December 2000 the Cassini spacecraft flew by Jupiter on its way to Saturn and provided a unique opportunity for simultaneous two-spacecraft measurements of the Jovian dust streams. The Cassini cosmic dust analyser [12] measured the chemical composition of dust stream particles in-situ for the first time.

Apart from the Jovian dust streams, Galileo allowed for studies of impact-generated dust clouds surrounding the Galilean satellites [13] (Sect. 3), a tenuous dust ring in the region between the Galilean satellites [14,15] and further out from the satellites [16] as well as interplanetary and interstellar particles captured by the Jovian magnetosphere [17,18] (Sect. 4). The detection of most of the observed features was unexpected and their discovery has greatly expanded our knowledge about dust in the Jovian magnetosphere.

Comprehensive reviews of more than 10 years of dust measurements with Ulysses and Galileo focussing on Jovian dust as well as interplanetary and interstellar dust have also been given by Grün et al. [19] and Krüger et al. [20].

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