Saturn - astronomy.

« measurements of the magnetic field made by the Voyager space probes in the 1980s. Additional Cassini findings reported in March 2007 suggested that particles originating from geysers on the moon Enceladus may provide a partial explanation for thechange.

The neutral gas particles become electrically charged and are captured by Saturn’s magnetic field, forming a disk of hot, ionized gas around the planet’sequator.

The charged particles interact with the magnetic field and slow down the rotation of the ionized gas, causing the radio period associated with the magnetic fieldto be longer than Saturn’s true rotation period.

The period of radio signals from the magnetosphere apparently varies over time, possibly reflecting the activity levels ofthe geysers on Enceladus that create the ionized gas disk.

The solar wind—charged particles from the Sun—may have an even stronger effect on the planet’smagnetosphere.

The exact rotation period for the body of Saturn is not known for sure, but it may be about 10.5 hours according to one recent estimate. Surrounding the Saturnian satellite Titan and Titan's orbit, and extending to the orbit of Saturn's moon Rhea, is an enormous doughnut-shaped cloud of neutralhydrogen atoms.

A disk of plasma, composed of hydrogen and possibly oxygen ions, extends from outside the orbit of the moon Tethys almost to the orbit of Titan.

Theplasma rotates in nearly perfect synchrony with Saturn's magnetic field. VI THE RING SYSTEM When the Italian scientist Galileo saw Saturn’s ring system through a small telescope in 1610, he did not understand that the rings were separate from the body of theplanet.

He described the rings as handles ( ansae ).

The Dutch astronomer Christiaan Huygens was the first person to describe the rings correctly.

In 1655, desiring further time to verify his explanation without losing his claim to priority, Huygens wrote a series of letters in code, which when properly arranged formed a Latinsentence that read in translation, “It is girdled by a thin flat ring, nowhere touching, inclined to the ecliptic.” The rings are named in order of their discovery, and from the planet outward they are known as the D, C, B, A, F, G, and E rings.

The Cassini probe discovered anadditional faint ring between the G ring and the F ring in 2006.

The main rings are now known to comprise more than 100,000 individual ringlets, each of which circlesthe planet.

The visible rings stretch out to a distance of 136,200 km (84,650 mi) from Saturn's center, but in many regions they may be only 5 m (16.4 ft) thick.

Theyare thought to consist of aggregates of rock, frozen gases, and water ice ranging in size from less than 0.0005 cm (0.0002 in) in diameter to about 10 m (33 ft) indiameter—from dust to boulder size. The apparent separation between the A and B rings is called Cassini's division, after its discoverer, the French astronomer Giovanni Cassini.

Voyager's television showedfive new faint rings within Cassini's division.

The wide B and C rings appear to consist of hundreds of ringlets, some slightly elliptical, that have ripples of varying density.The gravitational interaction between rings and satellites, which causes these density waves, is still not completely understood.

The B ring appears bright when viewedfrom the side illuminated by the Sun, but dark on the other side because it is dense enough to block most of the sunlight.

Voyager images have also revealed radial,rotating spokelike patterns in the B ring.

These spokelike patterns appear to be seasonal and were not visible when Cassini began orbiting Saturn in 2004.

The patternsmay be caused by electrostatic effects that elevate tiny particles above the ring plane.

The spokes may reappear when the angle of the rings to the Sun changes. Scientists continue to debate the age of the rings.

The planet Saturn itself formed about 4.5 billion years ago.

The relatively fresh appearance of the rings as seen byVoyager seemed to indicate that the rings were created as recently as 100 million years ago, a time when dinosaurs roamed the Earth.

Results from the Cassini probe,however, suggest to some researchers that parts of the ring system may date back billions of years.

The material in the ring is constantly being recycled, forming smallmoonlets that later break up, accounting for the fresh and bright appearance of many of the particles. One theory about the origin of the rings is that a comet or an asteroid smashed a small moon that orbited the planet.

The debris from the collision spread out to formthe main body of the rings (rings D through A).

Images from Cassini confirm that the rings contain chunks and particles of rock and ice in a full range of sizes asexpected from a collision.

Similar collisions with small moons may have happened at different times, supplying material to the rings.

The tenuous G ring is associatedwith an arc of icy material that may supply its particles.

The faint E ring that stretches from the orbit of the moon Mimas past the orbit of the moon Rhea is mainly madeof tiny particles released by active geysers on the moon Enceladus and is constantly being renewed. The gravitational pull from some of Saturn’s moons helps shape parts of the rings.

The Cassini division is thought to be caused by the moon Mimas, which orbits Saturnonce for every two orbits made by ring particles near the gap.

Some rings are shaped by small so-called shepherd moons that orbit inside gaps in the rings or along theedges of rings.

The gravitational pull of some of these small moons tends to keep the ring debris in place.

In other cases, small moons keep some rings narrow or causethe rings to have braided or scalloped shapes. VII MOONS Saturn has at least 60 moons.

They range up to 2,575 km (1,600 mi) in radius.

They consist mostly of the lighter, icy substances that prevailed in the outer parts of thegas and dust nebula from which the solar system was formed and where radiation from the distant Sun could not evaporate the frozen gases.

The discovery of 12 ofSaturn’s moons was reported as recently as May 2005 and 9 more were announced in June 2006.

An additional 4 moons were reported in 2007.

These moons areirregular in shape and small, ranging in diameter from 3 km (2 mi) to 7 km (4 mi).

A number of the moons orbit Saturn in a direction opposite that of Saturn’s largermoons, suggesting that some of these recently discovered satellites were originally asteroids that were captured by Saturn’s gravitational field. The five larger inner satellites—Mimas, Enceladus, Tethys, Dione, and Rhea—are roughly spherical in shape and composed mostly of water ice.

Rocky material mayconstitute up to 40 percent of Dione's mass.

The surfaces of the five moons are heavily cratered by meteorite impacts. Enceladus has a smoother surface than Saturn’s other moons, the least cratered area on its surface being less than a few hundred million years old.

The detection ofgeysers on Enceladus suggests that liquid water below the surface is being heated by some source.

Among the possibilities are tidal forces—the gravitational pull ofSaturn and other moons.

These tidal forces could cause friction that heats rocks within the interior of the moon.

Heat released by the radioactive decay of rocks deep inthe moon could also melt some of the ice.

The Cassini spacecraft detected oxygen atoms in the geyser plumes that jet out from the moon’s southern polar region.

Theplumes reach about 418 km (about 260 mi) into space.

Scientists concluded that the geysers were spewing out water molecules that then broke down into oxygen andhydrogen atoms.

Astronomers think that Enceladus supplies most of the particles in the E ring, which neighbors Enceladus’s orbit. Mimas, far from being smooth, displays an impact crater the diameter of which is one-third of the diameter of the satellite itself.

Tethys also bears a large crater and avalley 100 km (62 mi) in width that stretches more than 2,000 km (1,200 mi) across the surface.

Both Dione and Rhea have bright, wispy streaks on their alreadyhighly reflective surfaces.

Some scientists conjecture these were caused either by ice ejected from craters by meteorites, or by fresh ice that has migrated from theinterior. Several small moons have been discovered immediately outside the A ring and close to the F and G rings.

Possibly four so-called Trojan or coorbital satellites of Tethysand one of Dione have also been discovered.

Trojan satellites occur in regions of gravitational stability that lead or follow a body in its orbit around a massive centralbody, in this case, Saturn. The outer satellites Hyperion and Iapetus also consist mainly of water ice.

Iapetus has a very dark region in contrast to most of its surface, which is bright.

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