Jupiter's ring system consists of at least three components: the inner main ring, the vertically extended halo and the gossamer ring(s) further out. The small moons Thebe and Amalthea orbit Jupiter within the gossamer ring and are believed to be the sources of gossamer ring material. A very faint ring extension has also been observed beyond Thebe's orbit. On 5 November 2002 the Galileo spacecraft traversed Jupiter's gossamer ring system for the first time. High-resolution dust data were obtained with the dust detector on board down to 2.33 R_J , i.e. well inside Amalthea's orbit. A second ring passage occurred on 21 September 2003, a few hours before Galileo impacted Jupiter. This time, dust data were successfully received down to Amalthea's orbit at 2.5 R_J , however, with much reduced time-resolution. Several thousand dust impacts were counted during both ring passages, and the full data sets (impact charges, rise times, impact directions, etc.) of about 90 dust impacts were transmitted to Earth. In-situ dust measurements provide information about the physical properties of the dust environment not accessible with imaging techniques. They directly measure dust spatial densities along the spacecraft trajectory as well as grain sizes and impact speeds. Our as yet preliminary analysis %of the gossamer ring data implies particle sizes in the sub-micron and micron range. The size distribution -- increasing towards smaller particles -- is similar in the Thebe ring and the ring's outer extension, whereas in the Amalthea ring it is steeper. Dust number densities are about 104 - 106 km-3 . Our dust data allow for the first time to compare in-situ measurements with the results optical obtained from the inversion of optical images. It appears that small sub-micron grains dominate the number density whereas larger particles with at least a few micron radii contribute most to the optical depth. The dust density shows previously unrecognised fine-structure in the ring between Thebe's and Amalthea's orbit. This structure as well as the Thebe ring extension may be explained by variable photoelectric grain charging on the day- and night-sides of Jupiter and the resulting variable susceptibility of the grains to electromagnetic forces. The Galileo measurements in Jupiter's gossamer ring pave the way towards the in-situ investigation of Saturn's E ring with Cassini beginning in July 2004.
35th COSPAR Scientific Assembly
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