Ringparticle Properties

The beginning of our understanding of the sizes of the ring particles came as a result of the discovery in 1973 that Saturn's rings are strong radar reflectors [36]. Unless the ring particles were at least as large as the radar wavelength (12.6 cm), there would have been no reflected radar signal detected. The rings were expected to be transparent to radar on the basis of failure to observe any passive radio signals from Saturn's rings prior to that date. The transmitted radar signal was polarized, but the reflected signal was largely depolarized. That meant that the ring particles reflecting the radar signal were moderately irregular [37]. The same radar measurements showed a reflectivity for the rings that was inconsistent with silicate (rocky) composition for the rings. Such high reflectivity was consistent only with metallic composition (which was highly unlikely) or with relatively pure water-ice composition.

There is evidence in the Voyager and Earth-based data for Saturn ring particle sizes ranging from dust-sized particles of a micrometer or less in radius to embedded satellites akin to Pan with a radius of about 10 km. The primary pre-Cassini constraints on ring particle size are provided by the radio occultation and stellar occultation data, the observed light-scattering characteristics of the individual rings, analysis of data indicating particle hits on the Voyager spacecraft, and theoretical considerations.

The radio occultation data on Voyager 1 [38] will be discussed in detail later (Section 9.12). The difference in transparency of the rings at the 13-cm and 3.6-cm radio wavelengths provides a measure of the relative abundance of particles in the size range from 1- to 4-cm radius. That relative transparency varies greatly with location in the rings.

There is almost no difference between the X-band and S-band radio transparencies of the Cassini Division; this ring segment must have a large population of ring particles greater than 10 cm in radius. The C ring, on the other hand, shows significant differences and must therefore have many particles in the 1- to 4-cm size range. The Bring noise levels are higher, and except for its innermost part, little useful data on particle sizes was obtained. Interpretation of radio occultation data, especially those of the B ring, was also made more difficult by the low tilt angle of the rings as viewed from the Earth in 1980. It appears that the greatest differences between S- and X-band transparencies occur in the outer A ring, implying that it possesses an abundance of particles between 1 and 4 cm in radius. It is also noteworthy that the greatest differences between optical and radio depths also occur in the outer half of the A ring, which must therefore also have a large fraction of particles under a centimeter in radius. It is possible that at least part of the difference is due to the low tilt angle of the rings during the radio occultation measurements, combined with the likelihood that larger particles that block the radio signal occupy a smaller fraction of the total vertical thickness of the outer A ring.

The D, G, and E rings were detected in neither the radio nor the stellar occultation experiments. The implication is that the D, G, and E rings are dominated by dust-sized particles. The core of the F ring was detected at X-band, S-band, and optical wavelengths. The radio detections implied a much narrower ring than seen in visible-light images. The F-ring core must therefore contain large particles, but the more extended F-ring structure mainly comprises dust-sized particles, perhaps with a small population of larger (but still sub-centimeter) particles. Of course, the data do rule out even larger bodies (moonlets) widely separated in ring longitude.

Now, lest our readers get the false impression that the particle size distribution in the rings is well determined, consider a recent paper by French and Nicholson [39], based on a re-analysis of the stellar occultation data by the Voyager photopolarimeter at three wavelengths: 3.9, 2.1, and 0.9 micrometers. In this analysis, instead of analyzing the directly transmitted starlight, they processed the data to calculate that portion of the signal due to the directly transmitted light and subtracted it from the total signal. The remainder was due to light scattered from the ring particles; the sharpness of the angular distribution is set by the largest particles and the breadth of the angular distribution reflects the abundance of smaller, centimeter-sized particles. By assuming uniformity in particle size distribution across major ring regions, they found characteristic particle sizes in the A and C rings that are fairly consistent with those derived from the radio occultation data. Their analysis of the B-ring data seems to indicate minimum characteristic diameters of about 30 cm and maximum characteristic diameters of about 20 m. Hopefully, Cassini data will provide better information on particle size distributions within all of Saturn's rings.

Both Earth-based and Voyager measurements of color differences within the main rings make it clear that there are at least small amounts of contaminant mixed in with the water ice, especially in (but not limited to) the C ring and the Cassini Division. However, it doesn't take much contaminant to cause the observed coloration, certainly no more than 10% of the ring material and perhaps as low as 1% [40].

The D ring has not been seen in backscattered light (i.e., with the observer between the Sun and the D ring). It was imaged by both Voyager 1 and Voyager 2 in forward-scattered light (with the D ring between the observer and the Sun). The Voyager 2 image is reproduced in Figure 9.11. The D ring is the only one of the tenuous rings of Saturn (D, G, and E rings) with narrow ring structure, reminiscent of the Uranus dust disk observed by Voyager 2. The source of the radial structure is unknown. From the greatly enhanced visibility which occurs at large phase angles when the ring particles are comparable in size with the wavelength of the light, the particles are thought to be about a micrometer in radius.

The F ring is still somewhat of an enigma. It may be a recent, unusually large collision product [41]. Detection of its extended width in stellar occultation measurements and of only its core in radio occultation measurements seems to imply that there are few centimeter-sized and larger particles away from its central core, but that larger particles exist within that core. The F ring is known to be highly variable on a range of timescales [42]. In particular, the brighter "knots" seen in Voyager 1 imaging are not correlated with those seen by Voyager 2, and one bright region in a series of Voyager 2 images generated several new clumps which orbited at different rates from the bright region itself [43]. Information on its composition comes primarily from visual imaging; its redder color relative to the A ring suggests a larger fraction of silicate material, although that conclusion is somewhat tentative. A better understanding of the F ring needs repeated detailed measurements by an orbital spacecraft like Cassini (see Chapter 10).

The G ring does not have the apparent fine structure of the F ring and it is much wider and more diffuse in character (see Figure 9.12). Showalter and Cuzzi initially concluded that the G ring was composed of a broad (5,000 km) band of fine dust grains, which might contain a narrower core of large objects, as had earlier been suggested by Van Allen [44], which served as the source of the G-ring material. In the past, the closest counterpart to the G ring was thought to be the E ring. Several more recent (but still pre-Cassini) papers challenge this view. The G-ring material is confined to a narrow vertical layer across its entire width, unlike the vertically extended E ring [45]. It also has a reddish hue, in contrast to the distinct bluish color of the E ring [46]. Impacts on the spacecraft during passage through the edges of the G ring are also incompatible with the narrow size distribution which characterizes the E ring [47]. These results imply that there are larger ring particles across the entire G-ring width and that their composition is distinctly different from the water-ice grains that make up the E ring.

Figure 9.11. Saturn's D ring is best viewed in forward-scattered light; like dust on an automobile windshield is much more visible when driving toward the Sun. This image is from Voyager 2 from a distance of 195,400 km. The phase angle (angle between the Sun and the observer as seen from the target) is 166°, or only 14° from the direction of the Sun. The dark band at the right edge of the image is Saturn's shadow across the D ring. (PIA01388)

Figure 9.11. Saturn's D ring is best viewed in forward-scattered light; like dust on an automobile windshield is much more visible when driving toward the Sun. This image is from Voyager 2 from a distance of 195,400 km. The phase angle (angle between the Sun and the observer as seen from the target) is 166°, or only 14° from the direction of the Sun. The dark band at the right edge of the image is Saturn's shadow across the D ring. (PIA01388)

The E ring extends from the orbit of Mimas (which orbits at a mean distance of 185,520 km) to a radial distance of at least 483,000 km from Saturn's center and perhaps nearly to the orbit of Titan (orbital radius of 1,221,830 km), as derived from Cassini orbiter data and depicted in Figure 2.14. The thickness of the E ring is about 1,000 km near the orbit of Enceladus (orbital radius of 238,020 km), where it is also brightest. The physical thickness increases to more than 15,000 km in its outer portions, but its optical thickness decreases with distance beyond the orbit of Ence-ladus until it becomes essentially optically undetectable. The particle size, as determined from analysis of particle impacts with the two Voyager spacecraft and from

Figure 9.12. Saturn's G ring is seen as a faint, diffuse stripe in this long-exposure image taken by Voyager 2's narrow-angle camera from a range of 305,000 km on August 26, 1981. The overexposed F and A rings are seen in the right-hand part of the image. The white dots seen in and near the F and A rings are incompletely removed reseau marks on the camera face that are used for geometric reconstruction of the image. (PIA01964)

Figure 9.12. Saturn's G ring is seen as a faint, diffuse stripe in this long-exposure image taken by Voyager 2's narrow-angle camera from a range of 305,000 km on August 26, 1981. The overexposed F and A rings are seen in the right-hand part of the image. The white dots seen in and near the F and A rings are incompletely removed reseau marks on the camera face that are used for geometric reconstruction of the image. (PIA01964)

Earth-based observations, is very narrowly centered near a radius of one micrometer. The particle composition is probably water ice, and even prior to Cassini, it was suspected that the source of those water-ice grains was Enceladus itself, although the mechanism for generation of those grains was not well understood. This is discussed in more detail in Chapter 10.

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