Saturn's Spoke: Lost and Found
Yunfeng Sui
Abstract: In the begining of this report, some basic facts related to the saturn's spoke are introduced. Then, some theory about the spoke are discussed. At last, the article of "saturn's spoke: lost and found" is discussed in details.
Some basic facts of Saturn are: 1. it is the sixth planet from the Sun; the second-largest planet in the solar system after Jupiter; 2. it is a gas giant, less dense than water; 3. it has oblate spheroid: it is flattened at the poles and bulges at the equator;
The saturn can be viewed as three part for easy understanding: Interior, Atmosphere, Rings. Interior part has a Rocky core, a liquid metallic hydrogen layer above that, and a molecular hydrogen layer above that. And it is very hot, reaching 12,000 kelvins (11,700 °C). Saturn's atmosphere exhibits a banded pattern, and Saturn's northern hemisphere is changing colors. Saturn's rings consists mostly of ice particles with a smaller amount of rocky debris and dust and rings of Saturn possess their own atmosphere.
Spokes in the B ring, imaged by Voyager 2 in 1981.Until 1980, the structure of the rings of Saturn was explained exclusively as the action of gravitational forces. The Voyager spacecraft found radial features in the B ring, called spokes, which could not be explained in this manner, as their persistence and rotation around the rings were not consistent with orbital mechanics. The spokes appear dark against the lit side of the rings, and light when seen against the unlit side. It is assumed that they are connected to electromagnetic interactions, as they rotate almost synchronously with the magnetosphere of Saturn. However, the precise mechanism behind the spokes is still unknown.
Twenty-five years later, Cassini observed the spokes again. They appear to be a seasonal phenomenon, disappearing in the Saturnian midwinter/midsummer and reappearing as Saturn comes closer to equinox. The spokes were not visible when Cassini arrived at Saturn in early 2004. Some scientists speculated that the spokes would not be visible again until 2007, based on models attempting to describe spoke formation. Nevertheless, the Cassini imaging team kept looking for spokes in images of the rings, and the spokes reappeared in images taken September 5, 2005.
Spokes of this nature were observed to persist at times for two or three rotations of the ring about the planet. Freshly-formed spokes seemed to revolve around the planet at the same rate as the rotation of the magnetic field and the interior of Saturn, independent of their distance from the center of Saturn. It is therefore suspected that the tiny dust grains which form the spokes are electrically charged. Older spokes, which presumably have lost their electrical charge, revolve with the underlying larger ring particles.
Author shows that the notable effect of the rings being more open to the Sun is capable of stopping spoke formation entirely and restricting the size of the particles in the spokes.
The spokes were observed to fade as the ring opening angle increased. Light-scattering geometry explans this phenomenon as: light must pass through the spokes at a shallow angle for them to be observed. On the basis of these arguments, Cassini was expected to see spokes during periods when it was near the ring plane. Contrary to these expectations, Cassini has not yet seen spokes on the illuminated side of the rings, despite many dedicated observations .
So the author extends an earlier suggestion that the value of B' may affect spoke activity and shows that for large values of B', spokes cannot form at all. The early suggestion is first bring out by G. E. Morfill* and C. K. Goertz in "Plasma clouds in Saturn's rings" in July 1983. In this article, the author assume the early suggestion is correst and build a mathmatic modle upon that, and find out the the open angle and the size of the dust are the main factors to the formation of Saturn's spoke.
I n this mathe modle, the particles are initially in the transient dense (nT = 100 cm–3) plasma for 3 s, after which the plasma is changed to the ambient plasma sheath with nmax = 0.11 cm–3. For opening angles smaller than B' 20°, particles in the ambient plasma are charged positively because the photoelectric effect produces a larger current than that produced by electron collection. Because the electric field points away from the ring, the grains are accelerated upward and form a spoke. For larger values of B', the plasma density increases enough to charge the grains negatively as the electron collection current becomes dominant. In this case, the dust grains are quickly pulled back down onto the ring, preventing the formation of spokes. The abrupt change in the lifetime of the particles with radii 0.1 < a < 1 um, indicates that spoke activity is expected to turn on and off rapidly as a function of B'. The exact value at which this transition happens is a function of the ambient plasma density nmax. The cut-off for large particles is a function of the amount of time the grains spend in the transient plasma.
Reference:
1. All Images except the last two are from NASA.
2. Horanyi, Havnes and Porco, Saturn's Spoke: Lost and Found, Mitchell, Science 17 March 2006, Vol. 311. no. 5767, pp. 1587 - 1589.
3. G. E. Morfill* and C. K. Goertz, Plasma clouds in Saturn's rings, Icarus, Volume 55, Issue 1 , July 1983, Pages 111-123.
4. Web resourse, wikipedia saturn, http://en.wikipedia.org/wiki/Saturn