Saturn's Moons (Satellites)
Some of the small images can be cliked for a larger one.
In addition to its rings, Saturn has to date, 53 satellites (moons) that have been officially named. 33 of these satellites measure at least 6 miles (10 kilometers) in diameter, and several smaller satellites. The largest of Saturn's satellites, Titan, has a diameter of about 3,200 miles (5,150 kilometers) -- larger than the planets Mercury and Pluto. Titan is one of the few satellites in the solar system known to have an atmosphere. Its atmosphere consists largely of nitrogen.
Many of Saturn's satellites have large craters. For example, Mimas has a crater that covers about one-third the diameter of the satellite. Another satellite, Iapetus, has a bright side and a dark side. The bright side of this satellite reflects about 10 times as much sunlight as the dark side. The satellite Hyperion is shaped somewhat like a squat cylinder rather than like a sphere. Unlike Saturn's other satellites, Hyperion's axis does not point toward the planet.
The innermost satellite, Atlas, orbits near the outer edge of the A-ring and is about 40 by 20 kilometers (25 by 15 miles) in size. It was discovered in Voyager 1 images by R. Terrile in 1980.
Shown here are the highest resolution images of the moons Pan and Atlas, showing their distinctive "flying saucer" shapes, owing to equatorial ridges not seen on the other moons of Saturn.
From left to right: a view of Atlas' trailing hemisphere, with north up, at a spatial scale of about 1 kilometer (0.6 mile) per pixel; Atlas seen at about 250 meters (820 feet) per pixel from mid-southern latitudes, with the sub-Saturn hemisphere at the top and leading hemisphere to the left; Pan's trailing hemisphere seen at about 3 kilometers (2 miles) per pixel from low southern latitudes; an equatorial view, with Saturn in the background, of Pan's anti-Saturn hemisphere at about 1 kilometer (0.6 mile) per pixel.
Atlas orbits around the outer edge of Saturn's A Ring and acts as a shepherding satellite, constraining the extent of the outer edge of this ring.
Like Pan, Atlas has a distinctive flying saucer shape created by a prominent equatorial ridge not seen on the other small moons of Saturn. Cassini images revealed in 2004 that a faint ring of material coincides with the orbit of Atlas.
The small, pointy moon is about 32 km (20 miles) across.
April 13, 2017
Cassini Sees 'Flying-Saucer' Moon Atlas Up Close
These raw, unprocessed images of Saturn's moon, Atlas, were taken on April 12, 2017, by NASA's Cassini spacecraft. The flyby had a close-approach distance of about 7,000 miles (11,000 kilometers).
These images are the closest ever taken of Atlas and will help to characterize its shape and geology. Atlas (19 miles, or 30 kilometers across) orbits Saturn just outside the A ring -- the outermost of the planet's bright, main rings.
Credits: NASA/JPL-Caltech/Space Science Institute
Pan, the innermost of Saturn's known moons, is located within the Encke Gap of Saturn's A-ring. It acts as a shepherd satellite and is responsible for keeping the Encke Gap open. The gap is a 325 km (200 mile) opening in Saturn's A ring.
Pan creates stripes, called "wakes," in the ring material on either side of it. Since ring particles closer to Saturn than Pan move faster in their orbits, these particles pass the moon and receive a gravitational "kick" from Pan as they do. This kick causes waves to develop in the gap and also throughout the ring, extending hundreds of kilometers into the rings. These waves intersect downstream to create the wakes, places where ring material has bunched up in an orderly manner thanks to Pan's gravitational kick.
On Atlas, the ridge extends 20 to 30 degrees in latitude on either side of the equator; on Pan, its latitudinal extent is 15 to 20 degrees. Atlas shows more asymmetry than Pan in having a more rounded ridge in the leading and sub-Saturn quadrants.
The heights of the ridges can be crudely estimated by assuming (ellipsoidal) shapes that lack ridges and vary smoothly cross the equator. Heights of Atlas' ridge range from about 3 kilometers (2 miles) at 270 degrees west longitude to 5 kilometers (3 miles) at 180 and 0 degrees. Pan's ridge reaches about 4 kilometers (2.5 miles) at 0 degrees west longitude, and is about 1.5 kilometers (0.9 mile) high over most of the rest of the equator.
The ridges represent about 27 percent of Atlas' volume and 10 percent of Pan's volume.
The images were acquired with the Cassini spacecraft. Image Credit: NASA/JPL/Space Science Institute
March 16, 2017
These two images from NASA's Cassini spacecraft show how the spacecraft's perspective changed as it passed within 15,300 miles (24,600 kilometers) of Saturn's moon Pan on March 7, 2017. This was Cassini's closest-ever encounter with Pan, improving the level of detail seen on the little moon by a factor of eight over previous observations.
The views show the northern and southern hemispheres of Pan, at left and right, respectively. Both views look toward Pan's trailing side, which is the side opposite the moon's direction of motion as it orbits Saturn.
Cassini imaging scientists think that Pan formed within Saturn's rings, with ring material accreting onto it and forming the rounded shape of its central mass, when the outer part of the ring system was quite young and the ring system was vertically thicker. Thus, Pan probably has a core of icy material that is denser than the softer mantle around it.
The distinctive, thin ridge around Pan's equator is thought to have come after the moon formed and had cleared the gap in the rings in which it resides today. At that point the ring was as thin as it is today, yet there was still ring material accreting onto Pan. However, at the tail end of the process, that material was raining down on the moon solely in (or close to) its equatorial region. Thus, the infalling material formed a tall, narrow ridge of material. On a larger, more massive body, this ridge would not be so tall (relative to the body) because gravity would cause it to flatten out. But Pan's gravity is so feeble that the ring material simply settles onto Pan and builds up. Other dynamical forces keep the ridge from growing indefinitely.
Credits: NASA/JPL-Caltech/Space Science Institute
The next satellite outward, Prometheus (left), shepherds the inner edge of the F-ring and is about 140 by 100 by 80 kilometers (90 by 60 by 50 miles).
Prometheus has a number of ridges and valleys and several craters about 20 km in diameter but appears to be less cratered than the neighboring moons Pandora, Janus and Epimetheus.
Saturn's moons Janus and Prometheus (right) look close enough to touch in this stunningly detailed view.
From just beneath the ringplane, Cassini stares at Janus (181 kilometers, or 113 miles across) on the near side of the rings and Prometheus (102 kilometers, or 63 miles across) on the far side. The image shows that Prometheus is more elongated than Janus.
The view takes in the Cassini Division (4,800 kilometers, or 2,980 miles wide), from its outer edge to about halfway across its width.
The image was taken in visible light with the Cassini spacecraft narrow-angle camera on April 29, 2006
Next is Pandora (left), outer shepherd of the F-ring, 110 by 90 by 80 kilometers (70 by 55 by 50 miles).
Craters formed on this object by impacts appear to be covered by debris, a process that probably happens rapidly in a geologic sense. The grooves and small ridges on Pandora suggest that fractures affect the overlying smooth material.
Both shepherds were found by Voyager 1. Discovered by S. Collins and others in 1980.
Next are Epimetheus and Janus (right), which share about the same orbit "co-orbital"-- 91,000 kilometers (56,600 miles) above the clouds. The orbital radii of Janus and Epimetheus differ by only 50 km, less than the diameter of either. In their orbital ballet, Janus and Epimetheus (left) swap positions every four years -- one moon moving closer to Saturn, the other moving farther away. The two recently changed positions (the swap occurring on Jan. 21, 2006), and Janus will remain the innermost of the pair until 2010, when they will switch positions again.
Although the moons appear to be close in the image, they are not. Janus (181 kilometers, or 113 miles across at right) is about 40,000 kilometers (25,000 miles) farther away from Cassini than Epimetheus (116 kilometers, or 72 miles across, at left) in this view. In fact, even when they are at their closest, tugging at each other and swapping orbital positions, they are never closer than about 15,000 kilometers (9,000 miles).
The image was taken in visible light with the Cassini spacecraft narrow-angle camera on March 20, 2006
Epimetheus and Janus may have formed by the break-up of one moon. If so, it would have happened early in the life of the Saturn system because both moons have ancient cratered surfaces, many with soft edges because of dust. They also have some grooves (similar to grooves on the Martian moon Phobos) suggesting some glancing blows from other bodies. Together, the moons trail enough particles to generate a feint ring. However, except for very powerful telescopes, the region of their common orbit (and the faint ring) appears as a gap between Saturn's more prominent F and G rings.
There are several craters larger than 30 km in diameter as well as both large and small ridges and grooves. The extensive cratering indicates that Epimetheus (left) must be quite old.
The orbital data given here is as of the time of the Voyager encounters. Epimetheus was first observed by Walker in 1966. But the situation was confused since Janus is in a very similar orbit. So Walker officially shares the discovery of Epimetheus with Fountain and Larson who showed in 1977 that there were two satellites involved.
Janus is extensively cratered with several craters larger than 30 km but few linear features. Its surface appears to be older than Prometheus' but younger than Pandora's.
The French astronomer Audouin Dollfus is credited with the discovery of Janus but it's not really certain whether the object he saw was Janus or Epimetheus and his observations led to a spurious orbit. (Walker discovered it independently but his telegram arrived a few hours after Dollfus'.) Larson and Fountain determined in 1978 that there are in fact two moons at about 151000 km from Saturn. This was confirmed in 1980.
This shadowy scene is one of the Cassini spacecraft's closest views of Saturn's moon Janus (right).
The slopes of some craters here display hints of the darker material better seen on Epimetheus in Epimetheus Revealed. A bright linear feature runs up the wall of the large crater at bottom center.
The view looks toward southern latitudes on Janus (179 kilometers, or 111 miles across). North is toward the top of the image and rotated 58 degrees to the right.
One new satellite, Helene (left and right), shares the orbit of Dione, about 60 degrees ahead of its larger companion, and is called the Dione Trojan. It is about 36 by 32 by 30 kilometers (22 by 20 by 19 miles).
Helene is in Dione's leading Lagrange point and hence was sometimes referred to as "Dione B". It was discovered in ground-based photographs Laques and Lecacheux in 1980.
Dione is the densest of Saturn's moons (aside from Titan, whose density is increased by gravitational compression). It is composed primarily of water ice but must have a considerable fraction of denser material like silicate rock.
Though somewhat smaller, Dione is otherwise very similar to Rhea. They both have similar compositions, albedo features and varied terrain. Both rotate synchronously and have dissimilar leading and trailing hemispheres.
Dione's features include heavily cratered terrain with craters as large as 100 kilometers (62 miles) across, moderately cratered plains, lightly cratered plains, and fractured areas. The heavily cratered areas are most common on the trailing hemisphere. Logically, a moon's leading hemisphere should be the more heavily cratered, so it has been theorized that a more recent impact spun Dione around. It has been calculated that bodies as small as those that made 35-kilometer (22-mile) craters could have spun Dione around. However, the fact that Dione seems to have spun exactly 180 degrees is a mystery.
Fractured areas, seen in Voyager images as bright thin wispy lines, have lengths of tens to hundreds of kilometers, often cutting through plains and craters. Cassini flybys starting in 2005 showed "the wisps" as bright canyon ice walls (some of them several hundred meters high), probably caused by subsidence cracking. The walls are bright because darker material falls off them, exposing bright water ice. These fracture cliffs suggest Dione experienced tectonic activity in its past. They could be a mature phase of the so-called tiger stripes on Enceladus.
Very fine ice powder (equivalent to cigarette smoke) from Saturn's E-ring constantly bombards Dione. The dust in the E-ring ultimately comes from Enceladus, which has prominent geyser activity.
Dione's density is 1.48 times that of liquid water, suggesting that about a third of Dione is a dense core (probably silicate rock) and the rest is ice. At Dione's extremely cold average temperature, ice is very hard and behaves like rock.
As with Earth's Moon, Dione is phase locked with its parent; one side always faces toward Saturn. Likewise, Dione has gravitationally locked two much smaller moons: Helene orbits Saturn 60 degrees ahead of Dione, and Polydeuces orbits Saturn 60 degrees behind Dione. (These tidally locked moon locations are also referred to as Lagrange points L4 for the leading spot and L5 for the trailing spot, based on calculations of the French astronomer Joseph-Louis Lagrange in 1772.)
Dione is in resonance with two nearby moons, Mimas and Enceladus. That is, these moons speed up slightly as they approach each other and slow down as they draw away, causing their orbits to vary slightly in a long series of complex changes, which helps keep them locked in their positions. Dione keeps Enceladus locked at a period exactly one half of the Dione orbit.
Helene orbits in Dione's leading Lagrange point. The tiny moon Polydeuces (S/2004 S5), discovered by Cassini in 2004, occupies the trailing Lagrange point.
Discovered by Cassini in 1684.
Click on each image below for a larger one.
Two more satellites are called the Tethys Trojans because they circle Saturn in the same orbit as Tethys, about 60 degrees ahead of and behind that body. They areTelesto on the left (the leading Trojan) and Calypso on the right (the trailing Trojan). Both were found in 1981 among ground-based observations made in 1980.
Calypso and Telesto are among the smallest moons in the solar system.
Telesto is 34 by 28 by 26 kilometers (21 by 17 by 16 miles) and Calypso is 34 by 22 by 22 kilometers (21 by 14 by 14 miles). Telesto is in Tethys' leading Lagrange point. Discovered by Smith, Reitsema, Larson and Fountain in 1980.
Calypso is in Tethys' trailing Lagrange point.
Discovered by Pascu, Seidelmann, Baum and Currie in 1980 from ground-based observations with prototype cameras destined for the HST.
There are three unconfirmed satellites. One circles Saturn in the orbit of Dione, a second is located between the orbits of Tethys and Dione, and the third, between Dione and Rhea. All three were found in Voyager photographs, but were not confirmed by more than one sighting.