o ues with decreasing intensity toward the planet and vertically [487]. The halo ring .£ O

extends from ~92,000 to ~122,500 km outward from Jupiter [488] (Fig. 6.49). The halo is caused by interaction with Jupiter's magnetic field, and is thought to be composed of fine dust grains that have been dragged inward from the main ring until they are excited vertically. It has a height of ~20,000 km [489-493].

Fig. 6.49. Two images of Jupiter's rings, taken by the Galileo spacecraft. Different brightness scales accent different parts of the ring system. The ring system has three main parts - a flat main ring; a halo inside the main ring shaped like a double-convex lens; and the gossamer ring outside the main ring. In the top view, a faint mist of particles is seen above and below the main ring. This "halo" is unusual in planetary rings, and is caused by electromagnetic forces pushing the smallest grains, which carry electric charges, out of the ring plane. (Credit: NASA/JPL/Cornell University)

The main ring is the brightest and easiest ring to detect. It was seen clearly in Voyager images and later in ground based images [494]. According to Brooks et al., the main ring is thought to extend 6,500-7,000 km [495]. Ockert-Bell et al., suggest a width of ~6,440 km, spanning a distance of 122,500-128,940 km [496]. Cassini observations placed an upper limit on the rings thickness at 80 km [497]. The main ring possesses a sharp outer boundary and a diffuse inner one. The Galileo spacecraft also revealed that the ring displays a phenomenon known as the Metis notch, which is located near the outer boundary of the ring. The location of the Metis notch suggests a relationship with that moon, although the relationship is not fully understood. Perhaps it has something to do with the manner in which the moon shepherds material in the ring. The Metis notch appears as a significant decrease in brightness of the ring in the vicinity of Metis' orbit, which is bounded on its outer edge by a bright annu-£ lus of ring material. Galileo also found bright patches in the main ring whose

£ origin is not understood. These bright patches may be debris generated by c smaller, less energetic collisions [498, 499]. The Cassini spacecraft saw possible

O ^ 1,000 km-scale azimuthal clumps within the ring, and ruled out the possibility

•> V that they could be previously undetected moons. A possible explanation is that

,0 they are simply slight density variations at that vicinity of the ring, a simple q © clumping of material. Or they may be no more than a natural consequence of

O a long line-of-sight [500]. The Galileo spacecraft also observed a diffuse and vertically extended cloud of material, reminiscent of the halo, above and below the main ring [501]. It is believed that particles for the main ring come from the small moons Adrastea and Metis, with Adrastea thought to be the primary source of material. Adrastea's orbit lies just outside the outer edge of the main ring. The orbit of Metis lies just 1,000 km inside Adrastea. Outside of Metis, the brightness of the main ring drops off very quickly.

There is some further asymmetry in the ring brightness that is not fully explained. Among the possible explanations for this is that asymmetry may arise from elongated ring particles oriented along a particular axis. Another is that it may be related to the magnetic field. Or, it may be due to localized enhancement in the number of ring particles. Another explanation may be the generation of debris from collisions or from an impact into a ring parent body by an external impactor. However, because of the expected effect of shearing in the rings, the origin for this material would require a more recent impact event and thus, this asymmetry might be a localized short term material-producing impact event. Light-scattering studies suggest that the ring parent-bodies are concentrated near the outer edge of the main ring, in the outer most ~2,000 km of the ring [502]. Parent bodies probably make up a majority of the ring system's mass, but a much smaller fraction of it surface area. Dynamical arguments suggest that parent-bodies are the collisional remnants of a fragmented satellite [503]. In November 2002, the Galileo spacecraft observed a series of flashes near the moon Amalthea. It is believed that these flashes represent sunlight reflected from 7 to 9 small moonlets located within ~3,000 km of Amalthea. Analysis indicates that these small bodies are between 0.5 m to several tens of km in diameter. In September 2003 Galileo detected a single additional body. It is suggested that these small bodies are a part of a discrete rocky ring embedded within Jupiter's gossamer ring [504].

The gossamer ring is the outer most member of the ring system. The gossamer ring has two components: one extends from the exterior of the main ring 128,940 km to just inside Amalthea's orbit at 181,366 km and the other extends

Fig. 6.50. A schematic depicting the structure of Jupiter's main and gossamer rings. The top panel shows that the main ring (red) is formed mostly from meteoroid impact debris kicked up from the innermost moons, Metis (m) and Adrastea (a). Since both satellites orbit in paths not inclined to Jupiter's equator, the main ring appears as a narrow line. The Middle panel shows the effect of dust ejected from the satellite Amalthea (A), responsible for producing one of the two moon components of the gossamer ring. Amalthea's orbit is inclined to Jupiter's plane, and at different times the satellite's vertical position can range anywhere between the two extreme limits shown. Dust ejected from Amalthea (orange) produces a ring whose thickness equals Amalthea's vertical positions beyond Jupiter's equatorial plane. The lower panel shows the additional effect of dust ejected from Thebe (T), which makes up the second component (shown in green) of the gossamer ring. The two positions shown represent the maximum projections of Thebe from Jupiter's equatorial plane. This component of the gossamer ring is thicker than the Amalthea dust component because Thebe's orbit is more inclined than that of Amalthea. (Credit: NASA/JPL/Cornell University)

from the exterior of the main ring out to just inside Thebes orbit at 221,888 km (Fig. 6.50). Very faint material can be detected out beyond the orbit of Thebes where it finally blends into the background at 250,000 km [505].

The moons Thebes and Amalthea are believed to be the source of the material in the translucent gossamer ring. These inner moons are heavily cratered by interplanetary meteor impacts and it is the dust from these impacts that sustain the rings. There are many clues that support this conclusion. Among them is the fact that the structure of the outer gossamer ring comprises two rings, one embedded within the other! The orbits of Amalthea and Thebes are slightly inclined to Jupiter's equatorial plane. The outer gossamer rings are also slightly inclined to Jupiter's equatorial plane and the two ring components are even tilted at an angle to one another!

Ockert-Bell et al., refer to the innermost gossamer ring as the Amalthea ring, and the other as the Thebe ring. The outermost material beyond Thebe is simply referred to as the gossamer extension. The thickness of the Amalthea ring is thought to be < 4,000 km, and the Thebe ring is thought to be > 8,000 km across vertically £ [506]. The thickness of these rings seems to be connected to their corresponding

£ moon and the elevation each moon reaches above and below the planet's equato-

c rial plane. Amalthea and Thebes exhibit epicyclic latitudinal oscillations that carry

O ^ them above and below Jupiter's equatorial plane and, as material is lost from the

•> V surface of these moons, it is deposited in the gossamer rings [507].

J5 IjS In addition to observations by the Voyager, Galileo, and Cassini spacecraft, q © Earth based observations have been carried out by the Palomar 5-m telescope

0 and the IRTF telescope, the Keck 10-m telescope, and the Hubble Space Telescope.

These observations indicate that the material in the rings is very red in color. This red color comes from both the intrinsic color of the large bodies, and the red light preferentially scattered by the dust. The observations also indicate that the particles in the ring are both non-spherical small particles of sizes up to tens of microns and perhaps mm- to km-size large bodies. This non-spherical aspect fits with the effects of impacts and collisions between bodies [508].

The rings have been observed from Earth with professional size instruments in visible, near infrared, and infrared wavelengths. To my knowledge no amateur with amateur equipment has yet managed to image Jupiter's ring system. However, with the advancement in equipment and telescopes that are becoming available to amateurs, how long will it be before this happens? My own astronomical society possesses an advanced 0.6-m instrument and I know of other organizations that possess larger ones. Surely, this observation will not continue to be the domain of professionals forever.

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