Rings of Saturn

The rings of Saturn together form a ring system around the planet Saturn. They are the most prominent feature of the planet and can already be seen through a telescope with about 40 - fold magnification. The rings are composed primarily of ice, but also from rocks that orbit Saturn. The particle size varies between the dust grains and several meters. The ring system has many larger and smaller gaps and thick with a diameter of almost one million kilometers between 10 and 100 meters, and thus, relatively speaking, extremely thin.

The rings cast a visible shadow on Saturn - and vice versa of Saturn on its rings. The shadow cast on Saturn's surface is more pronounced the longer the thin ring system is tilted in the course of a Saturn year with its narrow " edge " to the sun.

Discovery and designation

The ring system was discovered in 1610 by Galileo Galilei with one of the first telescopes. He did not understand the rings as isolated objects, but as she pointed Henkel ( ansae ). The Dutch astronomer Christiaan Huygens described the rings 45 years later correctly: " The Saturn is surrounded by a thin, flat ring, nowhere touching of him and which is inclined to the ecliptic ." Giovanni Domenico Cassini was the first suspected that the rings consist of individual particles, and discovered in 1675, the most striking gap in the ring system, which is named after him Cassini division.

The rings of Saturn, are named in order of their discovery, and are referred to from the inside outwards as a D, C ​​, B ​​, A, F, G, and E-ring. In astronomical survey radiographs is usually only the A and the B- ring and the Cassini division separating them, possibly even the Encke division seen in the A ring. The latter was first observed on January 7, 1888 by James Edward Keeler at the Lick Observatory. Only by space probes was realized that the rings have gaps and in turn be divided into even smaller narrowly defined sub-rings.

The Saturn orbiter Cassini has discovered another, weak dust ring on 17 September 2006. This ring is outside the bright main rings, between the weak rings F and G, in the area of the orbits of the moons Janus and Epimetheus small. It probably consists of particles that were hurled from these satellites in strikes from meteorites into space.

With the Spitzer Space Telescope is a much further outboard, independent from the main ring system ring was discovered by its infrared radiation in 2009. Visually, the ring can not be seen due to its very low matter density and the weak reflection of sunlight. The ring extends over a distance of Saturn between six and twelve million kilometers is about twenty times as thick as the planet. According to the JPL, which manages the Spitzer spacecraft for NASA, would find the Earth about a billion times square in the ring. If he were visible from Earth, it would appear twice as large as the full Moon. Against the inner ring system it is inclined by 27 degrees. Shortly after its discovery, scientists assumed that the ring comes from material of the moon Phoebe. This rotates with the newly discovered ring system, compared to the previously known rings, in the opposite direction around Saturn.

Structure

It is now known that there are more than 100,000 individual rings with different compositions and colors, which are separated by clear-cut gaps. The innermost begins about 7000 km above the surface of Saturn and has a diameter of 134,000 km, the outermost has a diameter of 960,000 km.

The ring particles orbit Saturn prograde in the equatorial plane; Thus, the ring system as well as the equatorial plane by 27 ° with respect to the web plane Saturn. All 14.8 years, ie approximately every half year Saturn, the soil passes through the plane of the ring so that the ring system is almost invisible. In September 2009, the Earth has changed from the southern to the northern hemisphere of Saturn.

Another phenomenon are radial, spoke-like structures that extend from the inside to the outside of the rings of Saturn and thereby assume enormous proportions: with a width of about 100 kilometers can be up to 20,000 kilometers long. These "spokes" were first discovered by the Voyager 2 spacecraft as it passes in 1981, and later was able to observe, inter alia, from the Hubble Space Telescope are confirmed. Mysteriously, these structures disappeared but gradually from 1998 and then could only be detected from September 2005 to shoot the Cassini spacecraft again. The cause of the banding initially a short-lived interaction has been suggested with the magnetic field of Saturn.

U.S. astronomers discovered 2006, however, another explanation for the mystery of the spokes structures: Accordingly, there are the spokes of tiny (a few microns ) charged dust particles whose trajectory is affected by UV light from the sun so that the particles from resulting electrostatic forces in be brought and lifted a state of suspension ( levitation ). Depending on the position of Saturn in its orbit, the angle between the rings of Saturn and the Sun, and thus the angle of incidence of the ultraviolet light. The dark streaks occur at periodic intervals whenever the sun is then made ​​for about eight years in the ring plane of Saturn. A streak -free phase holds against six or seven years. The reason for the electrostatic charging of the rings is controversial. One explanation is that lightning occur in the upper atmosphere of Saturn, which generate electron beams by complex processes that meet the rings.

Formation

On the origin of Saturn's rings, there are several theories. According to the theory proposed by Édouard Albert Roche in the 19th century, the rings created by a moon that has the Saturn approached so far that it is broken apart by tidal forces. The critical distance is called the Roche limit. The spatial variation of the attractive forces by the Saturn in this case exceeds the moon 's internal gravitational forces, so that the moon is held together only by its material structure. After a modification of this theory, the moon was broken by a collision with a comet or asteroid. According to another theory, the rings are formed together with the Saturn itself from the same cloud of matter. This theory was hardly represented until recently, because it was assumed that the rings represent a by astronomical standards rather short-lived phenomenon of no more than several hundred million years.

However, recent data from the Cassini spacecraft in December 2007 led to a new age determination, after Saturn's rings are formed before 4.5 billion years ago. Thus, the rings would be about the same age as the solar system.

Dynamics

The gaps between the rings based on the gravitational interaction with the many moons of Saturn, the rings themselves. It also resonance phenomena play a role, which occur when the orbital periods are in the ratio of small whole numbers. Thus, the Cassini division is caused by the moon Mimas. Some smaller moons, so called shepherds or shepherd moons circling directly in the gaps and at the edges of the ring system and stabilize its structure. New measurements and photographs of the Cassini spacecraft have revealed that the ring edges, and thus the separation of the rings are even sharper than previously thought. Thus one had suspected that you can also find some chunks of ice in the gaps, but this is not the case.

The extremely small thickness of the ring system is due to collisions of the particles. Each chunk individually revolves around the center of Saturn and not the rings as a rigid structure. Therefore commutes every boulder, which eventually located on the surface of the ring system, during a revolution once vertically through the ring system through and back again. By inelastic collisions with other chunks reduces this vertical velocity component and thus the thickness of the ring system.

More ring and disk phenomena in astronomy

Significantly weaker rings are also found in the other large gas planets of the solar system, Jupiter, Uranus and Neptune. In addition, circular discs in astronomy are a common phenomenon that occurs in very different orders of magnitude. In addition to planetary rings, these include accretion disks in X-ray binaries, and those which develop during the formation of stars, such as the asteroid belt, but also the spiral galaxies. Again, the thickness of these plates is determined by the frequency of inelastic collisions of its components.

Radii and orbital periods of Saturn's rings

Appendix

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