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Figure 5. Number density of dust as a function of altitude above the surface of Ganymede (data from 4 fly-bys), Europa (8 flybs) and Callisto (3 fly-bys). The altitude is shown in units of the satellite radius Rsat = 1560,2634,2409 km in the case of Europa, Ganymede and Callisto, respectively. Vertical error bars reflect statistical uncertainty due to the small number of impacts. The solid lines are least squares fits to the measured number densities.

the close vicinities of Europa, Ganymede and Callisto. No dust cloud could be measured close to Io because the spacecraft orientation prevented the detection of dust particles during all fly-bys at this satellite.

Analysis of the impact directions and impact speeds showed that the grains belonged to steady-state dust clouds surrounding the satellites [13,39]. The measured radial density profiles of the dust clouds (Figure 5) together with detailed modelling of the impact-ejection process implied that the particles had been kicked up by hypervelocity impacts of micrometeoroids onto the satellite's surface [16]. The projectiles were most likely interplanetary dust particles.

The measured mass distribution of the grains was consistent with such an ejection mechanism with grain sizes being mostly in the range 0.5 fim < s < 1.0 fim. It implied that the particle dynamics was dominated by gravitational forces, whereas non-gravitational, especially electromagnetic forces were negligible. Most ejected grains follow ballistic trajectories and fall back to the surface within minutes after they have been released. Only a small fraction of the ejecta has sufficient energy to remain at high altitudes for several hours to a few days. Although they eventually strike the satellite's surface, these short-lived but continuously replenished particles form a tenuous steady-state dust cloud which entirely envelopes the satellite. The total amount of debris contained in such a steady-state cloud is roughly 10 tons.

The optical thickness of the cloud is by far too low to be detectable with imaging

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