Mankindís Explanation: Saturn's Rings
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ďSaturnís ring system
ranks among the most spectacular phenomena in the solar system.
With a diameter of 270,000 kilometers, the system is an enormous object,
yet its thickness does not exceed 100 metros, and its total mass comprises only
about 3 x 10.22 grams, similar to the mass of Mimas.
Like the rings of the other
giant planets Jupiter, Uranus, and Neptune, those of Saturn lie for the most
part within the classical Roche limit. This
limit, which for the idealized case is at 2.44 Saturn radii, represents the
closest distance at which a small satellite can approach a massive primary
before it is torn apart by tidal forces. Conversely,
small bodies within the Roche limit are prevented from aggregating into larger
objects by tidal forces. The limit
applies only to objects held together by gravitational attraction and thus does
not restrict the stability of relatively small bodies for which molecular
cohesion is important. Thus, small
moons with sizes in the range of tens of kilometres or less can persist
indefinitely within the Roche limit.
Although the individual
particles that make up Saturnís rings cannot be seen directly, their size
distribution can be deduced from their effect on the scattering of signals
propagated through the rings from spacecraft and stars.
This analysis shows a broad and continuous spectrum of particle sizes,
ranging from centimetres to several metres, with larger objects being
significantly fewer in number than smaller ones.
This spectrum is consistent with the distribution that might be expected
from repeated collision and shattering of initially larger objects. In some parts of the rings, where collisions are apparently
more frequent, even smaller (dust-sized) grains are present, but these have
short lifetimes owing to a variety of loss mechanisms. Clouds of the smaller grains apparently acquire electrical
charges, interact with the magnetic field, and manifest themselves in the form
of moving, wedge-shaped spokes that extend radially. Much larger ring moons--those of several kilometres--may
exist within the rings but are apparently quite rare. Spectra of sunlight reflected from the rings show absorption
by water ice, and the rings are highly reflective to visual light.
Thus, it is conceivable that the rings were produced by the disruption of
a satellite of the size and composition of Mimas.
The ring system shows
structures on many scales, ranging from the broad divisions into the classical
A, B, and C rings down to a myriad of individual ringlets with radial scales on
the order of kilometres. The
structures have provided a fertile field for investigating gravitational
resonance and the collective effects of many small particles orbiting in close
proximity. Although many of the
structures can be understood theoretically, a large number remain enigmatic, and
a complete synthesis of the system is still lacking.
Because the Saturn ring system may be an analogue of the original
disk-shaped system of particles out of which the solid planets formed, an
understanding of its dynamics and evolution has implications for the origin of
the solar system itself.
The structure of the rings is
broadly described by the optical depth, t, as a function of radial distance from
the centre of Saturn. The optical
depth, which is a measure of the average density of the rings, is defined as the
natural logarithm of the ratio of the incident intensity of light of a specified
wavelength to the emergent intensity, where the light is assumed to propagate in
a direction perpendicular to the ring plane. Radio signals with wavelengths of several centimetres and
greater are largely unaffected by the smallest ring particles and thus encounter
smaller optical depths than signals with wavelengths in the visible region of
the electromagnetic spectrum and shorter.
The B ring is the thickest and
broadest of the rings, extending from 1.52 to 1.95 Saturn radii, with optical
depths between 1.2 and 1.8. It is
separated from the outer major ring, the A ring, by the Cassini division. The Cassini division (1.95 to 2.02 Saturn radii) is not
devoid of particles but exhibits
complicated variations in t, with an average value of 0.12. The A ring extends from 2.02 to 2.27 Saturn radii with a t
value of about 0.7 to 0.6. Interior
to the B ring lies the C ring (sometimes known as the Crepe ring), at 1.23 to
1.52 Saturn radii, with optical depths about 0.1.
Interior to the C ring lies the extremely tenuous D ring (1.11 to 1.23
Saturn radii), visible only in reflected light. Exterior to the A ring lies the narrow, shepherded F ring at
2.33 Saturn radii. The tenuous G
ring, at 2.8 Saturn radii, was originally detected by its influence on charged
particles in Saturnís magnetosphere and is faintly discernible in Voyager
images. The E ring extends from 3
to 8 Saturn radii.
Numerous gaps are seen in the
distribution of optical depth in the major ring regions.
Some of the major gaps have been named after famous astronomers who were
associated with studies of Saturn. In
addition to the Cassini division, they include the Maxwell gap (1.45 Saturn
radii), the Huygens gap (1.95 Saturn radii), the Encke gap (2.21 Saturn radii),
and the Keeler gap (2.26 Saturn radii). Of
the latter four gaps, only the Encke gap was known prior to the existence of
spacecraft images of Saturn.
Particles can be cleared from
a region to form a gap by the gravitational effects of a moon about 10
kilometres in size orbiting within the gap region; such a moon (S18) was found
within the Encke gap. The
corresponding moon within the Keeler gap has not yet been found but is believed
to exist. Gaps can also be cleared in ring regions that are in orbital
resonance with satellites whose orbits are substantially interior or exterior to
the rings. The condition for
resonance is that orbital periods of the satellite and ring particle be in the
ration l:(m - 1), where l and m are integers.
When this condition is satisfied, the satellite (if external to the ring)
receives angular momentum from the resonant ring particles, launching a tightly
wound spiral density wave in the ring and ultimately clearing a gap if the
resonance is strong enough. The
outer edge of the B ring (inner edge of the Cassini division) is in 2:1
resonance with Mimas and shows the two-lobed excursions in radius predicted by
such a resonance. Similarly, the
outer edge of the A ring, and of the main ring system itself, is in 7:6
resonance with the co-orbital satellites Janus and Epimetheus and is scalloped
with seven lobes. Effects of other
resonances with various orbital frequencies of external satellites are seen
throughout the ring system, but many similar features have no such
Information acquired within the quotes is from: (Encyclopedia Britannica copyright 1997 15th edition macropaedia volume 27 pages 513-514)
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