Moons de Saturn

Satellites of Saturn

Saturn has 63 known natural satellites in 2008, including Titan. Titan is larger than Mercury or Pluto, it contains elements of the primitive Earth, but there's very cold. The numerous passages in 2005 of the Cassini probe above Titan suggest that there would be little chance of discovering life forms. In fact, the total number of satellites of Saturn is unknown, because there are lot of objects in orbit around the planet.
More than a dozen moons have been discovered since the late 2000s on unusual orbits, probably fragments of larger bodies captured by Saturn, most of them are only a few kilometers in diameter. Some have even been recently discovered through the rings of Saturn by the Cassini probe.
Ripples in the rings, photographed by the probe, has intrigued scientists. They saw small points that have proved to be tiny moons of a few kilometers in diameter.

The orbits of three pairs Mimas-Tethys, Enceladus-Dione and Titan-Hyperion are in echo: mimed and Tethys is in echo 1:2 (the period of revolution of Mimas is exactly half of that of Tethys); Enceladus and Dione is also in echo 1:2; Titan and Hyperion are in echo 3:4.

Image: Comparative sizes and masses of the largest moons of Saturn. Titan (5151 km diameter) is the largest moon of Saturn, is the second largest satellite in the Solar System after Ganymede (moon of Jupiter), larger than Mercury is the only satellite to have an atmosphere.

Titan

The infrared spectrometer of Cassini observed a gigantic system of clouds recovering the North Pole of Titan.

Image: Sight of Titan by the mission Cassini, on October 26th, 2004. This mosaic of 9 images shows variations of brightness of Titan's surface, and brilliant clouds near the south pole.
The most brilliant region of the right-hand side and the equatorial region carry the name of Xanadu.
The surface seems young and there is no visible crater.

Rhea

Rhea is the second moon of Saturn by the size (after Titan). It was discovered in 1672 by Giovanni Domenico Cassini.
Rhea was glanced through for the first time to Travel 1 in November, 1980. The Cassini probe is crossed in November 26th, 2005, in 500 km above its surface. Rhea is an ice-cold body of weak density (1240 kg / m3), indicating that the moon consists of a rocky nucleus counting only for a third of the mass of Rhea, the rest being mainly some ice-cold water.
The temperature on the surface of Rhea is of-174°C in the sun, and of-200°C in-220°C in the shade.
In orbit synchronous around Saturn, Rhea always presents the same hemisphere to Saturn.
The space probe Cassini presents us here the "back" face of Rhea.

Image: Image taken with the telephoto lens of the probe Cassini on January 17th, 2007 at a distance about 597000 kilometers of Rhea. The resolution is 4 kilometers by pixel.

Iapetus

Iapetus is the third moon of Saturn by the size). During its flying by the probe Cassini, the passed on images revealed the existence of an equatorial crest stretching on approximately 1 300 km long, reaching by places the exceptional height about 20 000 m.
This crest is not without arousing the interest of the planetologists who advance several hypotheses as for its formation: she could notably result from the accretion of former rings or still the progressive collapse, by effect of tide, from an equatorial sausage due to a massive centrifugation during the formation of the satellite.

Dione

Dione was discovered in 1672 by Giovanni Domenico Cassini.
Dione mainly consists of water in the form of ice; but, in view of its density higher than that of the other moons of Saturn (except Titan, whose density is increased by the gravitational compression), it contains probably a rather important quantity of denser matter, such as rocks of silicates.
The sights taken in 500 kilometers of distance, allow to confirm that the observed white volutes are gigantic cliffs of ice which cover Dione.

Tethys

Tethys is an ice-cold body, similar to Dione and Rhea. Its density indicates that it mainly consists of ice.
The surface of Tethys is covered with craters and counts numerous cracks in the ice.
There are two types of ground on Tethys: the one consists of regions strongly craterised; other one consists of a belt of dark color and little craterised which crosses the moon throughout.
In orbit synchronous around Saturn, Tethys the fifth the bigger moon of Saturn, always present the same hemisphere to its planet.

Image: Image taken with the telephoto lens of the probe Cassini on December 31st, 2006 at a distance about 414 000 kilometers of Tethys. The resolution is 2 kilometers by pixel.

Enceladus

Enceladus is one of the rare objects of the solar system (with Io, satellite of Jupiter, Triton, that of the Neptune and naturally Earth) on which the eruptions or the ejection of matter were able to be directly observed.
The distance averages between Enceladus and Saturn is about 180 000 kilometers, that is three times the beam of this planet, the tour of which it makes in approximately 53 minutes past 32 am.
As many natural satellites, Enceladus is in synchronous rotation around Saturn, the duration of its orbit being equal to that of its rotation on himself, it presents so always the same face towards the planet, as makes it the Moon with the Earth.

Mimas

The first thing which we notice on Mimas is the enormous crater of impact 130 km in diameter which carries the name of the discoverer of Mimas, Herschel. The sides have approximately 5 km high with abysses 10 km deep and central peak which rises of 6 km over the floor of the crater.
These characteristics make a quasi-perfect example of crater of impact.
An equivalent crater on Earth would be not less than 4000 km in diameter, is bigger than Canada.
The shock which produced this crater nearly destroyed of few totally Mimed because the object is broken up to the face opposite to the impact point, probably because of the shock wave which had to cross completely the body.

Hyperion

Hyperion the biggest celestial body of the solar system the shape of which is strongly irregular. It is likely that Hyperion is a fragment of a bigger object having undergone an impact in past.
The biggest crater measures approximately 120 km in diameter, for a 10 km depth.
As most of the moons of Saturn, the weak density of Hyperion indicate that it mainly consists of ice with a small quantity of rock.
The images of Voyager 2 as well as the measures of ground photometry indicate that the rotation of Hyperion is chaotic, its axis of rotation varies so strongly as its orientation in the space is impossible to predict.
Hyperion is unique by its very irregular shape, its very eccentric orbit, and its nearness with Titan.

Phoebe

The first space probe to have supplied with Phoebe's images other one than a tiny white point is to Travel 2, on September 4th, 1981.
However, these images, taken at a distance of 2,2 million kilometers, had a very weak resolution which did not allow to study in detail the surface of the satellite.
The following stage of the exploration of Phoebe is the flying by the probe Cassini on June 11th, 2004, heading for Saturn.
Phoebe was moreover the first satellite which the Cassini probe visited during its arrival in the planetary system of Saturn.
This flying, the only one during all the duration of the mission Cassini, took place at a distance of 2068 kilometers and has allowed to obtain the quasi-totality of the current data on Phoebe (shape, dimensions, physical characteristics of the surface).

Janus

The orbits of Janus and Epimetheus are distant only about 50 km and they could not thus cross the one next to the other one without colliding. Epimetheus and Janus could result from the same object which would have split into two.
Approximately every four years, a surprising ballet occurs: these two objects get closer one of the other one and switch around then their orbits.
The most internal become the most outside, and conversely.
The internal object catching up the outside object, slows down this one by gravitational attraction and eventually falls then on the internal orbit, whereas the internal moon is accelerated by the attraction of the outside moon and thus rise on the outside orbit.

Image: above the ballet amazing at Janus and Epimetheus photographed by the probe Cassini September 8th, 2005 - To the left we perceive the ring F, Janus and Epimetheus are situated between the ring F (140 180 km) and the ring G (170 000 km).

Epimetheus

Epimetheus situated on the same orbit as Janus, measures only 138x110x110 km.
The shape of this satellite is very irregular and its surface presents big valleys.
We can perceive several craters there furthermore of 30 kilometers wide. Janus and Epimetheus participate in a gravitational ballet which makes them exchange their positions respective orbital, approximately every four years the most external moon becomes the most internal moon both and vice versa.

Image: [see above the ballet amazing with Janus].
In the Roman mythology, Janus is a god in a head but two set faces, a guard of the passages and the crossings, the divinity of the change, the transition.

Prometheus

Prometheus is an irregular satellite of Saturn of a size of 148×100×68 km. It presents numerous valleys and cliffs as well as several craters of about twenty kilometers in diameter. It seems less craterised than the nearby moons, Pandora, Janus and Epimetheus. Its weak density and its high albedo seem to indicate that Prometheus is a very porous celestial body of ice.
The observations of 1995 and 1996 put in evidence that Prometheus was late of 20° with regard to its position of 1981. This distance cannot explain by errors of observations. It is thus possible that the orbit of Prometheus changed following a collision or that it possesses the moon partner who shares her orbit.

Image: Prometheus in the foreground above the F ring and under the F ring, Pandora.
Credit: Cassini imaging team NASA.

Pandora

Pandora is a small irregular satellite.
He presents more craters than his neighbor Prometheus, at least two of them measure around thirty kilometers. Its weak density and its high albedo seem to indicate that Pandora is a very porous ice-cold celestial body.
Pandora acts as satellite external shepherd of the ring F of Saturn. A satellite shepherd A satellite shepherd is an object of modest size which orbits near the edge of a ring (unstable structure consisted of a multitude of very small objects) and playing guard's role of the structure. The gravity engendered by the satellite confines the ring and bounds its a precise edge. Indeed, the materials which go away from it are either returned in the ring, or ejected by this one either are captured by the satellite. This effect explains the existence of narrow rings confined by two satellites "shepherds", as it is the case for the ring F of Saturn and certain rings of Uranus. is, in flight mechanics, a natural satellite of a planet which limits the area of a ring of this planet.

Helene

Helene is the sixth moon from Saturn, it is located at a distance of 377 400 km. On June 18, 2011, the NASA spacecraft Cassini has successfully completed its second close encounter with Helene, the small moon of Saturn's ice. Closest, Cassini flew 4 330 miles to Helena is 6 968 km. The image was taken against during the second closest flyby of the mission. Helene (Helene S XII) is a small moon of Saturn, discovered February 29, 1980, by French astronomer, Pierre Lacquers, Raymond and Jean Despiau Lecacheux (Meudon Observatory), with the 1 meter telescope of the Pic du Midi observatory. She received the temporary designation S/1980 S 6. Lecacheux lacquers and made part of the team who confirmed a few months later, the discovery, thanks to photographs taken by Voyager 2.
The small moon officially received the name of Helen in 1988. Helene orbits at the Lagrange point L4 - Dione Saturn system, it is therefore the same orbit as Dione, ahead of 1/6th revolution.

Image: image of the surface of Helen lit by low sunlight.
Credit: NASA/JPL-Caltech/Space Science Institute