Dusty Jovian Rings

Apart from Io dust streams (Sect. 2) and circum-satellite ejecta-clouds (Sect. 3) the in-situ Galileo measurements have revealed additional populations of Jovian dust (Table 1): since the beginning of Galileo's orbital tour about Jupiter the dust detector has measured more than 400 impacts of mostly micrometer-sized grains widely distributed in circum-jovian space. Although the highest fluxes of grains occurred in the region between Io's and Callisto's orbit (~ 6 to 26 Rj from Jupiter, [24,25], Figure 6) impacts were also detected out to 200 Rj and beyond. These grains form a tenuous dust ring around Jupiter with a number density of ss 2 • 102 km-3 at Europa's orbit. The spatial locations where these grains were detected, the impact directions and the charge signals imply that these are actually two populations: besides a population of particles on prograde orbits about Jupiter, another population on retrograde orbits must exist as well [14]. The grains on retrograde orbits are most likely interplanetary or interstellar grains captured by the Jovian magnetosphere [17,18]. Numerical models show that a tiny fraction of the impact debris released from the surface of the satellites by hypervelocity impacts (Sect. 3) is ejected at speeds sufficient to escape from the satellites entirely [15] (an amount of 10 g sec-1 has been estimated to leave Ganymede). The ejected material goes into orbit about Jupiter and forms a tenuous ring of dust particles mostly on prograde orbits. This ring extends at least from Io's orbit (5.9Rj from Jupiter) out to Callisto's orbit (26Rj) but the dust detections indicate that it continues further out and further in (see below).

In the outer region of the Jovian system, between 50 and 300 Rj, about 100 dust impacts were detected. Their orbits are compatible with prograde and retrograde jovicentric orbits with a wide range of inclinations [16]. The number densities of ~ 10 km-3 are more than an order of magnitude lower than those found in the region between the Galilean satellites but, on the other hand, by about an order of magnitude larger than the interplanetary background. Sources for these grains are Jupiter's outer regular and irregular moons.

Indications for the existence of the ring can already be found in earlier measurements by the Pioneer 10/11 and Ulysses spacecraft: 12 meteoroid penetrations have been recorded with Pioneer within 45 Rj (Jupiter radius, Rj = 71,492 km) from Jupiter [1] and Ulysses has recorded 9 impacts of micrometer-sized dust grains in this spatial region. Two-third of the Ulysses impacts were detected at ~ 35° jovigraphic latitude after Jupiter fly-by.

Between Io's orbit at 5.9 Rj and the outer extension of the gossamer ring at about 3.1 Rj, extremely little is presently known about the dust environment. Although Galileo has traversed part of this region during orbit insertion in December 1995, dust measurements were very patchy because the instrument had to be saved from the hazards of Jupiter's radiation environment. However, a few probably micrometer-sized dust impacts were detected within Io's orbit [42,11],

Still closer to Jupiter lies the region of Jupiter's prominent ring system which consists of three components: the main ring, the halo and the tenuous gossamer rings. Here, the dust densities are so large that dust investigations habe been performed with remote sensing techniques. The vertical extension and density profiles of the rings imply that a significant fraction if not all of the dust forming the rings is impact-ejecta derived from the inner moons Adrastea and Metis (in the case of the main ring), and Amalthea and Thebe (in the case of the gossamer rings [43]). These satellites orbit Jupiter inside the ring system. The motion of the dust grains in a certain size range contained in the gossamer ring is most probably dominated by the Poynting-Robertson drag force, indicating that the plasma density in this region is much lower than previously thought [44].

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