From: hal@finney.org, Wed, 29 Nov 2000
>For a flexible ring, I think the problem is that if the orbit is
>slightly elliptical, as any true orbit will be, the particles of the
>ring want to move faster during the lower part of the orbit. This
>will compress the ring in some portions and stretch it elsewhere. I
>suspect that this energy transfer will cause the ring to become
>deformed and to collapse into the atmosphere eventually.
More specifically: the energy loss causes the orbital decay. The
angular momentum loss changes the orbital eccentricity.
Some words about a "solid" ring. We have in our history books and in
our observational databases a few hundred years of studies of one
example of a "ring", that is, Saturn's rings. Much of the following
words came from a nice little book: _Rings: Discoveries from Galileo
to Voyager_ by James Elliot and Richard Kerr, Cambridge University
Press, 1984. Here is some history.
The Dutch astronomer Christian Hugens observed Saturn in the
mid-1600s with the telescopes of the day, which had only been in use
for several decades. He discovered Titan, observed Saturn's rings
again and worked out orbital periods of objects around Saturn. He
was the first to hypothesize that Saturn had a "thin flat ring,
nowhere touching, and inclined to the eliptic". Then soon later,
Jean Dominque Cassini announced in 1675 of the dark line that seemed
to divide the ring in two, raising questions among the physicists of
the day of how a solid ring could stand up under the strain of
rotating so close to Saturn, and Cassini's Division just confused
the issue more. Cassini thought that a swarm of small satellites
would give the appearance of solidity when viewed from so far away
as Earth, but would require no new dynamical explanations. Each
particle in the ring would orbit the planet just as the Moon orbited
Earth, Galileo's moons orbited Jupiter, and Titan orbited Saturn.
Cassini's division would make no difference at all. So then towards
the end of the 1600s, the concept of particulate rings gained favor.
Then science took a step backward, when William Herschel, in the
1700s pronounced the ring to be solid. He resisted the division of
his solid ring by Cassini's division, preferring to see it as a
marking of only one side of the ring. Herschel had some support
about his solid ring through Pierre Simon de Laplace.
The rings, to Laplace, represented an arrested, petrified stage in
his nebular formation of the solar system, during which the ball of
material that would become Saturn was "shedding rings" that would
coalesce into solid satellites. The present rings of Saturn,
according to Laplace, never broke up to form satellites, but
solidified as they were, forming many solid rings. Unlike Herschel,
he found that a single ring, or even two rings to be impossibly unstable-
only at a single across the width of a rotating ring would the
outward push of centrifugal force balance the inward pull of
gravity. The imbalance of forces elsewhere on a wide ring would tear
it apart, no matter how thick it was, Laplace said. His solution was
to subdivide the two known rings (divided by Cassini's division),
into a myriad of narrow rings nested inside one another, thus
reducing the strain across any one ring. By concentrating the mass
of each ring off center, he further reduced the strain. He actually
made a ring behave as much like an orbiting particle as possible,
while remaining a solid ring. This is how things stood with regards
to the physical perception of Saturn's rings, for the next 50 years.
Observations in the 1800s found a bewildering array of ringlets.
However a few scientists during the next 50 years were unconvinced
on the concept of solid rings. George Bond in 1850, from the seeming
mutability of Saturn's rings, proposed that the rings must not be
solid or rigid, but somehow fluid. He reevaluated Laplace's
calculation, and concluded the solid rings would not work.
The concept of solid rings began to change then. The C ring was
discovered and observers noted that one could see the planet right
through the new ring, finally rejecting the idea of solid rings.
James Clerk Maxwell won Cambridge University's Adams Prize in 1857
by demonstrating that Newtonian physics would not tolerate solid
rings, no matter how finely subdived. The irregularities demanded by
Laplace would be so huge as to be visible from Earth, Maxwell said,
even stabilized by such moutainous terrain, the slightest
disturbance would shatter the rings. Liquid rings, on the other
hand, would break up and form visible satellites. Maxwell reached
the "new" conclusion that "the only system of rings which can exist
is one composed of an indefinite number of unconnected particles
revolving around the planet with different velocities according to
their respective distances". Maxwell actually was in error,
regarding the liquid rings concept, because they were physically
possible, but that error was not discovered for another 100 years.
Hope this provides some insight.
Amara
P.S. In 30 days, the spacecraft named after Cassini on the spacecraft's
journey toward Saturn flies by Jupiter in a close encounter. Both the
Galileo and Cassini spacecraft are now in dual-spacecraft measurements
mode, observing the planet Jupiter.
**********************************************************************
Amara Graps | Max-Planck-Institut fuer Kernphysik
Interplanetary Dust Group | Saupfercheckweg 1
+49-6221-516-543 | 69117 Heidelberg, GERMANY
Amara.Graps@mpi-hd.mpg.de * http://www.mpi-hd.mpg.de/dustgroup/~graps
**********************************************************************
"Never fight an inanimate object." - P. J. O'Rourke
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