Saturn’s Dark Materials

by Paul Gilster on February 21, 2008

What exactly is that dark material spread so widely over Saturn’s various moons? From Hyperion to Iapetus, Dione and Phoebe, we find a black substance coating a wide range of objects, suggesting that whatever the stuff may be, there must be a common mechanism for moving it from one moon to another. A series of papers on Saturn’s moons appears in the February issue of Icarus, where these interactions are now under study.

Just what the material is remains a mystery. But Roger Clark (US Geological Survey) notes that as the Cassini data build, we’re beginning to track down some of its components, including bound water and, possibly, ammonia. Studying Dione, Clark’s team noted the fine-grained nature of the dark material there. Its distribution and composition indicate the dark material is not native to the moon, and indeed, the same signature appears not only among other moons but also in Saturn’s F-ring. From the abstract to the study by Clarke and colleagues:

Multiple lines of evidence point to an external origin for the dark material on Dione, including the global spatial pattern of dark material, local patterns including crater and cliff walls shielding implantation on slopes facing away from the trailing side, exposing clean ice, and slopes facing the trailing direction which show higher abundances of dark material.

Dione and Saturn

Image: Speeding toward pale, icy Dione, Cassini’s view is enriched by the tranquil gold and blue hues of Saturn in the distance. The horizontal stripes near the bottom of the image are Saturn’s rings. Images taken on Oct. 11, 2005, with blue, green and infrared (centered at 752 nanometers) spectral filters were used to create this color view, which approximates the scene as it would appear to the human eye. We have much to learn about this moon’s dark material and its interactions with the rest of the Saturn system. Credit: NASA/JPL/Space Science Institute.

So while much of our attention around Saturn has focused on Enceladus because of its icy plumes (one paper in the Icarus group discusses traces of organic compounds or silicate materials within Saturn’s E-ring, believed to be fed by those very plumes), we are now learning that the planet’s entire ‘system’ is more dynamic than believed. Dione itself, as Clark goes on to report in Icarus, shows some evidence of being geologically active.

Saturn is coming to be seen in terms of a unique ecology, as Cassini scientist Bonnie Buratti notes:

“Ecology is about your entire environment — not just one body, but how they all interact. The Saturn system is really interesting, and if you look at the surfaces of the moons, they seem to be altered in ways that aren’t intrinsic to them. There seems to be some transport in this system.”

A cometary origin for Saturn’s dark materials? Perhaps, but the forces moving these materials between the various moons remain ripe for investigation. Two papers from the special section on Saturn are particularly relevant here, the first being Lopes, Buratti et al., “The Saturn system’s icy satellites: New results from Cassini,” Icarus Volume 193, Issue 2 (February 2008), pp. 305-308 (abstract). Also see Clark et al., “Compositional mapping of Saturn’s satellite Dione with Cassini VIMS and implications of dark material in the Saturn system,” Icarus Vol. 193, Issue 2 (February 2008), pp. 372-386 (abstract).

James M. Essig February 22, 2008 at 5:54

Hi Folks;

It occurred to me that this dark material may be of use if it could somehow be mined. Could it be some form of dark frozen hydrocarbon or other dark carbonacious material? This material, depending on what it is, might make excellent fuel for future manned space craft plying the outer depths of the solar system. Perhaps electromagnetic mass drivers could launch pods of this material to transiting space craft wherein the transiting space craft are chemical rocket powered craft already doing 10 to 20 kilometers/second relative to Earth. Gravity assists combined with in flight chemical fuel sequestation from mass drivers located around Saturn might be an excellent way to boost the craft’s velocity to as much as 100 kilometers/second or more while replensihing such space craft with vital goods for living purposes. Matching the projectile speed with the outgoing space craft might be much less risky then trying to reflect back a momentum transfering lump of mass with incident velocity on the order of 10 to 20 or more kilometers/sec. These mass driven fuel pods and supply pods might have chemical rocket means for final velocity correction and docking to the out going space craft.

If a space craft traveling at 100 kilometers/sec were to recieve one of these pods which could power a 10,000,000 newton thrust rocket for 10 minutes, that would entail a pretty good constant accelleration with respect to the ships reference frame experiencing the 10 EXP 7 newton thrust for 10 minutes. If enough pods could be fired out of a mass driver that could be amped up to highly relativistic velocities, I see no reason why a chemical rocket powered space craft could not reach high gamma factors in theory.

A nuclear fusion rocket powered space craft should definately be able to reach very high gamma factors wherein the effiiciency of the propulsion scheme would rely on the craft not carrying all of its fuel at once. The fuel could keep on arriving as need. Such a space craft could reach gamma factors many times greater than that afforded by fusion rockets that would carry all of their fuel on board from the beginning.

A matter antimatter fueled space craft should be able in theory to reach exteme gamma factors, factors many times greater than than available for matter antimatter space craft that would have to carry all of their fuel from the beginning.

Now all we need is one big, powerful, adjustable, superhigh muzzle velocity capable electromagnetic gun. A 10 million kilometer long gun with the repulsive force of the one tested by the U.S. NAVY recently which achieved a muzzle velocity of about 2.5 kilometers second should be able to accellerate a 10 pound slug to about 1/3 to 1/2 C assuming a barrel length of the NAVY gun at about 5 to 10 meters. Improved technology in terms of magnetic or other electrodynamic field strength should afford much greater rates of accellerations. For the sake of efficiency, the electromagnetic drive mechanism could take the form of a cascading electromagnet solenoidal drive wherein only the portions of the barrel would be activated that were adjacent to and/or ahead of the projectille as it screamed down the barrel.

In theory, given a powerful enough electrodynamic repulsive field and a long enough barrel, and electromagnetic gun muzzle velocities only limit is C itself.
Although this NAVY gun is perhaps unfortunately intended to be weaponized in the near future, the technology developed by it and improved upon might afford us a practical way to get to the stars, and perhaps ply the depths of the Milky Way.

Thanks;

Jim

Hans Bausewein February 23, 2008 at 12:07

Maybe Saturn’s magnetic field and the sun’s radiation cooperate to create a giant mass spectrometer delivering different mass particles to different rings from a single source.

ljk March 6, 2008 at 19:47

Saturn’s Moon Rhea Also May Have Rings

NASA’s Cassini spacecraft has found evidence of material orbiting
Rhea, Saturn’s second largest moon. This is the first time rings may
have been found around a moon.

A broad debris disk and at least one ring appear to have been detected
by a suite of six instruments on Cassini specifically designed to
study the atmospheres and particles around Saturn and its moons.

“Until now, only planets were known to have rings, but now Rhea seems
to have some family ties to its ringed parent Saturn,” said Geraint
Jones, Cassini scientist, and lead author on a paper that appears in
the March 7 issue of the journal Science. Jones began this work while
at the Max Planck Institute for Solar System Research,
Katlenburg-Lindau, Germany, and is now at the Mullard Space Science
Laboratory, University College, London.

Rhea is roughly 950 miles in diameter. The apparent debris disk
measures several thousand miles from end to end. The particles that
make up the disk and any embedded rings probably range from the size
of small pebbles to boulders. An additional dust cloud may extend up
to 3,000 miles from the moon’s center, almost eight times the radius
of Rhea.

“Like finding planets around other stars, and moons around asteroids,
these findings are opening a new field of rings around moons,” said
Norbert Krupp, a scientist on Cassini’s Magnetospheric Imaging
Instrument from the Max Planck Institute for Solar System Research.

Since the discovery, Cassini scientists have carried out numerical
simulations to determine if Rhea can maintain rings. The models show
that Rhea’s gravity field, in combination with its orbit around
Saturn, could allow rings that form to remain in place for a very long
time.

The discovery was a result of a Cassini close flyby of Rhea in
November 2005, when instruments on the spacecraft observed the
environment around the moon. Three instruments sampled the dust
directly. The existence of some debris was expected because a rain of
dust constantly hits Saturn’s moons, including Rhea, knocking
particles into space around them. Other instruments’ observations
showed how the moon was interacting with Saturn’s magnetosphere, and
ruled out the possibility of an atmosphere.

Evidence for a debris disk in addition to this tenuous dust cloud came
from a gradual drop on either side of Rhea in the number of electrons
detected by two of Cassini’s instruments. Material near Rhea appeared
to be shielding Cassini from the usual rain of electrons. Cassini’s
Magnetospheric Imaging Instrument also detected sharp, brief drops in
electrons on both sides of the moon, suggesting the presence of rings
within the disk of debris. The rings of Uranus were found in a similar
fashion, by NASA’s Kuiper Airborne Observatory in 1977, when light
from a star blinked on and off as it passed behind Uranus’ rings.

“Seeing almost the same signatures on either side of Rhea was the
clincher,” added Jones. “After ruling out many other possibilities, we
said these are most likely rings. No one was expecting rings around a
moon.”

One possible explanation for these rings is that they are remnants
from an asteroid or comet collision in Rhea’s distant past. Such a
collision may have pitched large quantities of gas and solid particles
around Rhea. Once the gas dissipated, all that remained were the ring
particles. Other moons of Saturn, such as Mimas, show evidence of a
catastrophic collision that almost tore the moon apart.

“The diversity in our solar system never fails to amaze us,” said
Candy Hansen, co-author and Cassini scientist on the Ultraviolet
Imaging Spectrograph at NASA’s Jet Propulsion Laboratory, Pasadena,
Calif. JPL manages Cassini for NASA. “Many years ago we thought Saturn
was the only planet with rings. Now we may have a moon of Saturn that
is a miniature version of its even more elaborately decorated parent.”

These ring findings make Rhea a prime candidate for further study.
Initial observations by the imaging team when Rhea was near the sun in
the sky did not detect dust near the moon remotely. Additional
observations are planned to look for the larger particles.

The Cassini-Huygens mission is a cooperative project of NASA, the
European Space Agency and the Italian Space Agency. The Magnetospheric
Imaging Instrument was designed, built and is operated by an
international team led by the Applied Physics Laboratory, Johns
Hopkins University, Laurel, Md. For information on the Cassini
mission, visit: http://www.nasa.gov/cassini

ljk March 8, 2008 at 17:30

The Secular Evolution of a Close Ring-Satellite System: The Excitation of Spiral Density Waves at a Nearby Gap Edge

Authors: Joseph M. Hahn

(Submitted on 5 Mar 2008)

Abstract: The Lagrange planetary equations are used to study to secular evolution of a small, eccentric satellite that orbits within a narrow gap in a broad, self-gravitating planetary ring. These equations show that the satellite’s secular perturbations of the ring will excite a very long-wavelength spiral density wave that propagates away from the gap’s outer edge. The amplitude of these waves, as well as their dispersion relation, are derived here.

That dispersion relation reveals that a planetary ring can sustain two types of density waves: long waves that, in Saturn’s A ring, would have wavelengths of order 100 km, and short waves that tend to be very nonlinear and are expected to quickly damp. The excitation of these waves also transports angular momentum from the ring to the satellite in a way that damps the satellite’s eccentricity e, which also tends to reduce the amplitude of subsequent waves. The rate of eccentricity damping due to this wave action is then compared to the rates at which the satellite’s Lindblad and corotation resonances alter the satellite’s e. These results are then applied to the gap-embedded Saturnian satellites Pan and Daphnis, and the long-term stability of their eccentricities is assessed.

Comments: Accepted for publication in the Astrophysical Journal

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0803.0576v1 [astro-ph]

Submission history

From: Joseph Hahn [view email]

[v1] Wed, 5 Mar 2008 01:07:48 GMT (145kb)

http://arxiv.org/abs/0803.0576

ljk April 2, 2008 at 9:40

Dense planetary rings and the viscous overstability

Authors: Henrik N. Latter, Gordon I. Ogilvie

(Submitted on 28 Mar 2008)

Abstract: This paper examines the onset of the viscous overstability in dense particulate rings. First, we formulate a dense gas kinetic theory that is applicable to the Saturnian system. Our model is essentially that of Araki and Tremaine (1986), which we show can be both simplified and generalised. Second, we put this model to work computing the equilibrium properties of dense planetary rings, which we subsequently compare with the results of N-body simulations, namely those of Salo (1991).

Finally, we present the linear stability analyses of these equilibrium states, and derive criteria for the onset of viscous overstability in the self-gravitating and non-self-gravitating cases. These are framed in terms of particle size, orbital frequency, optical depth, and the parameters of the collision law. Our results compare favourably with the simulations of Salo et al. (2001). The accuracy and practicality of the continuum model we develop encourages its general use in future investigations of nonlinear phenomena.

Comments: Accepted in Icarus; 58 pages; 15 figures

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0803.4123v1 [astro-ph]

Submission history

From: Henrik Latter [view email]

[v1] Fri, 28 Mar 2008 14:14:36 GMT (101kb)

http://arxiv.org/abs/0803.4123

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