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Saturn’s Rings: Soaking Up Plasma

Saturn’s rings turn out to be more dynamic than expected, and it’s clear that what Cassini has to tell us about them — and about the rest of the Saturnian system — is only beginning. Throw Enceladus into the picture and things get even more complicated, and interesting. Geysers on the moon have already been found to supply content to the so-called E-ring, while material flowing from it in the form of the gas of electrically charged particles called plasma is now known to influence Saturn’s magnetosphere. The latest discovery is that this plasma is, in turn, being drawn into Saturn’s A-ring, where it is being absorbed.

Enceladus and the Rings

Image: Enceladus seen across the un-illuminated side of Saturn’s rings. A hint of the moon’s active south polar region can be seen as a just slightly dark area at bottom. This view was obtained from about 1 degree above the ringplane. Enceladus is 505 kilometers (314 miles) across. Credit: NASA/JPL/Space Science Institute.

Unlike Jupiter, then, Saturn seems to have acquired a way to soak up low and high-energy particles from the plasma cloud that surrounds it. How to detect the phenomenon? Cassini’s arrival at Saturn in 2004 included a close flyby over the A-ring. The spacecraft was able to determine that radio signals were being emitted from that part of the A-ring in collision with the plasma. From the signals, the density of the plasma could be inferred, as could the change in density over time. William Farrell (NASA GSFC) describes what is happening:

“As we approached the A-ring, the frequency dropped, implying that the plasma density was going down because it was being absorbed by the ring,” said Farrell. “What really drove this home was what happened to the signal when we passed over a gap in the rings, called the Cassini division. There, the frequency went higher, implying that the plasma density was going up because plasma was leaking through the gap.”

It’s an interesting result given the distance between the A-ring and Enceladus: about 100,000 kilometers. Now we’re seeing that a portion of Enceladus’ mass is being delivered to the outer edge of the ring. Good work by Cassini’s Radio and Plasma Wave instrument, whose data is studied by Farrell et al. in “Mass unloading along the inner edge of the Enceladus plasma torus,” Geophysical Research Letters Vol. 35 (January 23, 2008), L02203 (abstract). A NASA news release is here.

Comments on this entry are closed.

  • James M. Essig February 8, 2008, 21:25

    Hi Folks;

    This amazing, what may seem to be trivial planetary dynamics to some, is quite profound. Studying such phenomenon such as plasma dissipation/absorption has implications for life on moons orbiting planets around stars simmilar to the Sun. There is a complex interplay of phenomenon here that will help keep our multi-PetaFlops capable supercomputers that are able to do complex calculations buzy. At a time when some folks say “How much more computer power do we need?”. It is good to note that the ever increasing complexity of phenomenon that we are able to model has largely come about through supercomputer operations. Other complex phenomena involve hypernova, supermassive blackhole accretion disks, terrestrial planet developmment and evolution, cold dark matter distribution, galaxy formation and evolution, stellar evolution, gamma ray burst, cosmic galactic jets, interstellar gas cloud evolution, supernova shock wave fronts, the formation and evolution of our Big Bang and also that of other possible existent Big Bangs with computational adjustments of basic physical parameters such as fundamental physical constants, number and level of curvature of spacetime dimensions, numbers and types of forces, particles and fields etc., perhaps even the evolution of the multiverse or onmiverse as hypothesized to exist in the theory of chaotic inflation, and the very cosmically remote evolution of our universe including any specific predictions of the risk of natural and/or accidental manmade or ETI made runaway or global phase changes in the vaccuum energy state of the universe and the list goes on and on.

    In short, we will always have a need for ever increasing computational speed and the contunued development of such will always result in good and new numerical models and applications. I encourage any one who is contemplating a career in the physical sciences and who wants to study computational informatics or computer science to consider carefully the usefulness of your desired careers. If you have children or younger siblings, encourage them as well where appropriate. Although I am not a computational scientist nor a code writter, I encourage any folks aspiring to be such to consider how your efforts can benefit astronomy, astrophysics, cosmology, planetary atmospheric and planetary geology sciences and the like. There will always be room for imployment for you. In addition, your work can help us get to the stars all the sooner.


    Your Friend Jim

  • ljk February 19, 2008, 13:56

    Despite the incredible diversity of Saturn’s icy moons,
    theirs is a story of great interaction. Some are pock-marked,
    some seemingly dirty, others pristine, one spongy, one two-
    faced, some still spewing with activity and some seeming to
    be captured from the far reaches of the solar system. Yet
    many of them have a common thread – black ‘stuff’ coating
    their surfaces.

    More at:


  • ljk February 22, 2008, 13:24


    UK and German scientists explain intriguing phenomenon on Saturn’s moon

    An enormous plume of dust and water spurts violently into space from the south
    pole of Enceladus, Saturn’s sixth-largest moon. This raging eruption has
    intrigued scientists ever since the Cassini spacecraft provided dramatic
    images of the phenomenon.

    Now, physicist Nikolai Brilliantov, at the University of Leicester, and
    colleagues in Germany, have revealed why the dust particles in the plume
    emerge more slowly than the water vapour escaping from the moon’s icy crust.

    Enceladus orbits in Saturn’s outermost “E” ring. It is one of only three outer
    solar system bodies that produce active eruptions of dust and water vapour.
    Moreover, aside from the Earth, Mars, and Jupiter’s moon Europa, it is
    one of the only places in the solar system for which astronomers have direct
    evidence of the presence of water.

    The erupting plume on Enceladus is ejected by geyser-like volcanic eruptions
    from deep, “tiger-stripe” cracks within the moon’s south pole. Some astronomers
    have suggested that the myriad tiny grains of dust from these eruptions could
    be the actual source of Saturn’s E-ring. However, the dynamics and the
    origin of the plume itself have remained a mystery.

    Now, Brilliantov, who is also on the faculty at the University of Potsdam,
    Germany and Moscow State University, working with Juergen Schmidt and Frank
    Spahn of Potsdam and Sascha Kempf of the Max Planck Institute for Nuclear
    Physics in Heidelberg, and the Technical University of Braunschweig, Germany
    have developed a new theory to explain the formation of these dust particles
    and to explain why they are ejected into space.

    The researchers point out that once ejected the dust particles, which are in
    fact icy grains, and water vapour are too dilute to interact with each other
    and so the water vapour cannot be the cause of the dusty slowdown. Instead,
    the team suggests that the shift in speed must occur below the moon’s
    surface before ejection.

    The numerous cracks through which the plume material escapes from the moon’s
    icy surface, and which can be hundreds of metres deep, are narrower at some
    points along their length. At these points temperature and pressure of vapour
    drop drastically down, causing condensation of vapour into icy grains
    and hence to formation of the dust-vapour mixture. The required vapour density
    to accelerate the grains to the observed speeds implies temperatures where
    liquid water can exist in equilibrium with solid ice and water vapour
    within the moon’s frozen crust.

    These peculiar conditions allow the water vapour to erupt rapidly carrying
    with it the dust particles. However, these particles undergo countless frequent
    collisions with the inside of the channel walls which causes friction
    that slows them down before final ejection. The larger the particle the slower
    the ejection speed. This effect, quantified by the new theory, explains
    the structure of the plume and eventually the particle size distribution of
    the E-ring of Saturn.

    The existence of liquid water is a prerequisite for life and, while not
    suggesting there is life on Enceladus, offers another extraterrestrial place
    that might be searched.

    The scientists published details of their findings in the journal Nature.


    For More information, please contact Nikolai Brilliantov on 0116 252 2521
    Email: nb144@le.ac.uk

    The paper appeared in:

    Nature, Vol 451, p. 685-688, |7 February 2008|doi:10.1038/nature06491

    Images: available from pressoffice@le.ac.uk must
    be accompanied with the following credit in full

    “Geyser on Enceladus” -Michael Carroll, http://www.stock-space-images.com

  • ljk January 6, 2009, 0:42

    Physical collisions of moonlets and clumps with the Saturn’s F-ring core

    Authors: Sebastien Charnoz

    (Submitted on 5 Jan 2009)

    Abstract: Since 2004, observations of Saturn’s F ring have revealed that the ring’s core is surrounded by structures with radial scales of hundreds of kilometers, called “spirals” and “jets”. Gravitational scattering by nearby moons was suggested as a potential production mechanism; however, it remained doubtful because a population of Prometheus-mass moons is needed and, obviously, such a population does not exist in the F ring region.

    We investigate here another mechanism: dissipative physical collisions of kilometer-size moonlets (or clumps) with the F-ring core. We show that it is a viable and efficient mechanism for producing spirals and jets, provided that massive moonlets are embedded in the F-ring core and that they are impacted by loose clumps orbiting in the F ring region, which could be consistent with recent data from ISS, VIMS and UVIS.

    We show also that coefficients of restitution as low as ~0.1 are needed to reproduce the radial extent of spirals and jets, suggesting that collisions are very dissipative in the F ring region. In conclusion, spirals and jets would be the direct manifestation the ongoing collisional activity of the F ring region.

    Comments: Accepted for publication in ICARUS 17 pages, 6 figures, 1 table

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0901.0482v1 [astro-ph]

    Submission history

    From: Sebastien Charnoz [view email]

    [v1] Mon, 5 Jan 2009 13:53:42 GMT (509kb)


  • ljk September 9, 2009, 12:30

    A model of force balance in Saturn’s magnetodisc

    Authors: N. Achilleos (1 and 2), P. Guio (1 and 2), C. S. Arridge (3 and 2) ((1) Physics and Astronomy, University College London, United Kingdom (2) Centre for Planetary Sciences, UCL/Birkbeck, UK (3) Mullard Space Science Laboratory, Department of Space and Climate Physics, UCL, UK)

    (Submitted on 8 Sep 2009)

    Abstract: We present calculations of magnetic potential associated with the perturbation of Saturn’s magnetic field by a rotating, equatorially-situated disc of plasma. Such structures are central to the dynamics of the rapidly rotating magnetospheres of Saturn and Jupiter. They are `fed’ internally by sources of plasma from moons such as Enceladus (Saturn) and Io (Jupiter).

    We use a scaled form of Euler potentials for the Jovian magnetodisc field (Caudal, 1986). In this formalism, the magnetic field is assumed to be azimuthally symmetric about the planet’s axis of rotation, and plasma temperature is constant along a field line. We perturb the dipole potential by using simplified distributions of plasma pressure and angular velocity for both planets, based on observations by Cassini (Saturn) and Voyager (Jupiter).

    Our results quantify the degree of radial `stretching’ exerted on the dipolar field lines through the plasma’s rotational motion and pressure. A simplified version of the field model, the `homogeneous disc’, can be used to easily estimate the distance of transition in the outer magnetosphere between pressure-dominated and centrifugally-dominated disc structure.

    We comment on the degree of equatorial confinement as represented by the scale height associated with disc ions of varying mass and temperature. For Saturn, we identify the principal forces which contribute to the magnetodisc current and make comparisons between the field structure predicted by the model and magnetic field measurements from Cassini.

    For Jupiter, we reproduce Caudal’s original calculation in order to validate our model implementation. We also show that compared to Saturn, where plasma pressure gradient is, on average, weaker than centrifugal force, the outer plasmadisc of Jupiter is clearly a pressure-dominated structure.

    Comments: 23 pages, 16 figures, 2 tables; submitted for publication to MNRAS

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

    Cite as: arXiv:0909.1514v1 [astro-ph.EP]

    Submission history

    From: Patrick Guio [view email]

    [v1] Tue, 8 Sep 2009 18:26:49 GMT (1907kb)