| アブストラクト |
Spacecraft investigations during the last 15 years have vastly improved our knowledge about dust around Jupiter. In-situ measurements with the dust detectors on board the Ulysses and Galileo spacecraft have detected, for the first time, i) the electromagnetic escape of tiny dust grains from Jupiter, ii) the production of impact ejecta from large moons, and iii) previously unseen structures in Jupiter's ring system.
i) The majority of the escaping 10 nanometer dust grains condense in the volcanic plumes of Jupiter's moon Io. They collect a positive electric charge in the Io plasma torus and are accelerated away from Jupiter by electromagnetic forces in bursts or streams that are then detected by the spacecraft.
ii) All of the Galilean moons are surrounded by tenuous clouds of mostly sub-micrometer ejecta grains generated by collisions of interplanetary micrometeoroids with the moons' surfaces. A tiny fraction of the ejecta gets sufficient energy to leave the moons' gravity and is dispersed in circum-jovian space. Very tenuous rings have been detected in-situ in the region between the Galilean moons and further beyond out to ~ 250 RJ from the planet (Jupiter radius RJ = 71492 km), showing that impact-ejecta derived from hypervelocity impacts onto moons are a major constituent of dusty planetary rings.
iii) Galileo dust measurements in Jupiter's gossamer ring have for the first time provided in-situ measurements in a dust ring also accessible with imaging techniques. They allow the first actual comparison of in-situ data with the ring structure and grain size distributions inferred from inverting optical images. Our results show that electromagnetic effects caused by variable dust charging on the day and night side of Jupiter are crucial in shaping the dusty jovian ring.
The Galileo dust investigations showed that the jovian system is a natural laboratory to study fundamental 'dusty' processes that are of primary importance in many astrophysical contexts far beyond our own solar system. |
