* Astronomy

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RE: Saturn's rings

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Mystery solved behind birth of Saturn's rings

A team of researchers has presented a new model for the origin of Saturn's rings based on results of computer simulations. The results of the simulations are also applicable to rings of other giant planets and explain the compositional differences between the rings of Saturn and Uranus. The findings were published on October 6 in the online version of Icarus.
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Saturn's rings may be from the whirl of a passing icy rock

Saturn's rings might have formed when it ate a rotating icy rock that passed too close. This scenario could explain why Saturn's rings are made of different stuff from those of other gas giants.
Existing theories assume that rings form when objects such as asteroids or comets are pulverised by the gravity of a planet like Saturn. But they fail to explain why Saturn's rings are mostly water ice, while other gas giants are rocky, says Ryuki Hyodo at Kobe University in Japan.
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Icy Tendrils Reaching into Saturn Ring Traced to Their Source

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Centaurs galloped into Saturn's rings

Saturn's rings were the victim of a hit-and-run - and the culprits may be centaurs, strange objects that are part icy comet, part rocky asteroid.
In earlier work with images from the Cassini spacecraft, Matthew Hedman, now at the University of Idaho, noticed that two of the planet's rings sported alternating bright and dark bands. He argued they were low hills and valleys caused by a small comet that had smacked into the rings. The features could be seen getting smaller over time, suggesting the impact occurred in 1983.
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Saturn's vibrations create spirals in rings

Astronomers know that gravity from Saturn's various moons tug at the planet's rings and make spirals in them. But the catalyst for certain spiral patterns has been difficult to pin down. Now, two Cornell astronomers have determined the source: Saturn itself.
The entire planet can vibrate like a bell within periods of a few hours, and these oscillations cause gravitational tugs that, in turn, create the spiral patterns in the rings. The cause of the vibrations remains unknown.
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NASA Probe Observes Meteors Colliding with Saturn's Rings

NASA's Cassini spacecraft has provided the first direct evidence of small meteoroids breaking into streams of rubble and crashing into Saturn's rings.
These observations make Saturn's rings the only location besides Earth, the moon and Jupiter where scientists and amateur astronomers have been able to observe impacts as they occur. Studying the impact rate of meteoroids from outside the Saturnian system helps scientists understand how different planet systems in our solar system formed.
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Blame it on the Rain (from Saturn's Rings)

A new study tracks the "rain" of charged water particles into the atmosphere of Saturn and finds there is more of it and it falls across larger areas of the planet than previously thought. The study, whose observations were funded by NASA and whose analysis was led by the University of Leicester, England, reveals that the rain influences the composition and temperature structure of parts of Saturn's upper atmosphere. The paper appears in this week's issue of the journal Nature.
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Title: An Aggregate Model for the Particle Size Distribution in Saturn's Rings
Authors: Nikolai Brilliantov, Pavel Krapivsky, Hisao Hayakawa, Anna Bodrova, Frank Spahn, Juergen Schmidt

Saturn's rings are known to consist of a large number of water ice particles. They form a flat disk, as the result of an interplay of angular momentum conservation and the steady loss of energy in dissipative particle collisions. For particles in the size range from a few centimetres to about a few meters a power law distribution of radii r^(-q), with q = 3, is implied by the light scattering properties of the rings. In contrast, for larger sizes the distribution drops steeply with increasing r. It has been suggested that this size distribution may arise from a balance between aggregation and fragmentation of ring particles, but to date neither the power-law dependence, nor the upper size-cutoff have been explained or quantified within a unique theory. Here we present a new kinetic model for the collisional evolution of the size distribution and show that the exponent q is expected to be constrained to the interval 2.75 < q < 3.5. An exponential cutoff towards larger particle sizes establishes naturally, the cutoff-radius being set by the relative frequency of aggregating and disruptive collisions. This cutoff is much smaller than the typical scale of micro-structure seen in Saturn's rings (100 m for self-gravity wakes) and our theory represents values averaged over these structures.

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Title: Migration of small moons in Saturn's rings
Authors: Benjamin C. Bromley (University of Utah), Scott J. Kenyon (SAO)

The motions of small moons through Saturn's rings provide excellent tests of radial migration models. In theory, torque exchange between these moons and ring particles leads to radial drift. We predict that moons with Hill radii r_H ~ 2-24 km should migrate through the A ring in 1000 yr. In this size range, moons orbiting in an empty gap or in a full ring eventually migrate at the same rate. Smaller moons or moonlets -- such as the propellers (e.g., Tiscareno et al. 2006) -- are trapped by diffusion of disk material into corotating orbits, creating inertial drag. Larger moons -- such as Pan or Atlas -- do not migrate because of their own inertia. Fast migration of 2-24 km moons should eliminate intermediate-size bodies from the A ring and may be responsible for the observed large-radius cutoff of r_H ~ 1-2 km in the size distribution of the A ring's propeller moonlets. Although the presence of Daphnis (r_H ~ 5 km) inside the Keeler gap challenges this scenario, numerical simulations demonstrate that orbital resonances and stirring by distant, larger moons (e.g., Mimas) may be important factors. For Daphnis, stirring by distant moons seems the most promising mechanism to halt fast migration. Alternatively, Daphnis may be a recent addition to the ring that is settling into a low inclination orbit in ~10^3 yr prior to a phase of rapid migration. We provide predictions of observational constraints required to discriminate among possible scenarios for Daphnis.

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