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The planet Saturn, the largest moon Titan and Saturn’s rings are described on other pages of this web site.
Click on any of the small images below to see a larger picture.
A family portrait of Saturn’s moons (five out of its 61 or 62 known major satellites) taken by NASA’s Cassini spacecraft. Janus (179 km across) is on the far left; Pandora (81 km) orbits between the inner A ring and thin outer F ring near the middle of the image; bright Enceladus (504 km) is above the centre of the image; half of Saturn’s second largest moon, Rhea (1,528 km) is seen on the right; the smaller moon Mimas (396 km) can be seen beyond Rhea.
| Index | Name | Diameter (km) | Mass (×1015 kg) | Semi-major axis of orbit (km) |
Orbital period (days) |
Position | Discovery |
|---|---|---|---|---|---|---|---|
| 0 | S/2009 S 1 (innermost known satellite) |
≈0.3 | <0.0001 | ≈117,000 | ≈0.47 | Moonlet in outer B Ring |
2009 Cassini-Huygens spacecraft images |
| 1 | Pan | 28.2±2.6 (34×31×20) | 4.95±0.75 | 133,584 | +0.57505 | In Encke Division | 1990 Mark R. Showalter |
| 2 | Daphnis | 7.6±1.6 (9×8×6) | 0.084±0.012 | 136,505 | +0.59408 | In Keeler Gap | 2005 Cassini-Huygens spacecraft images |
| 9 | Mimas | 396.4±0.8 (416×393×381) | 37,493±31 | 185,404 | +0.942422 | 1789 William Herschel | |
| 13 | Enceladus | 504.2±0.4 (513×503×497) | 108,022 ±101 | 237,950 | +1.370218 | Generates E Ring | 1789 William Herschel |
| 14 | Tethys | 1062±1.2 (1077 ×1057×1053) |
617,449 ±132 | 294,619 | +1.887802 | 1684 Giovanni Cassini | |
| 17 | Dione | 1122.8±0.8 (1128 ×1123×1119) |
1,095,452 ±168 | 377,396 | +2.736915 | 1684 Giovanni Cassini | |
| 20 | Rhea | 1527.0±1.2 (1530 ×1526×1525) |
2,306,518 ±353 | 527,108 | +4.518212 | 1672 Giovanni Cassini | |
| 21 | Titan | 5151 | 134,520,000 ±20,000 |
1,221,930 | +15.94542 | 1655 Christiaan Huygens | |
| 23 | Iapetus | 1468.6±5.6 (1491×1491×1424) |
1,805,635 ±375 | 3,560,820 | +79.3215 | 1671 Giovanni Cassini | |
| 26 | Phoebe | 213.0±1.4 (219×217×204) | 8292±10 | 12,869,700 | −545.09 | Norse group | 1899 William Pickering |
| 61 | Fornjot (outermost confirmed satellite) |
≈6 | ≈0.15 | 24,504,879 | −1432.16 | Norse group | 2004 Scott S. Sheppard, David C. Jewitt, Jan Kleyna |
| Notes: A negative orbital period indicates a retrograde orbit | ‘±’ shows the uncertainty in the figure | ||||||
Enceladus is the sixth-largest of the moons of Saturn. It was discovered in 1789 by William Herschel. Enceladus seems to have liquid water under its icy surface. Its surface gravity is 0.113 m/s2, its density is 1.61 g/cm3 and it orbits Saturn in 33 hours. With its icy surface, Enceladus reflects nearly 100% of the sunlight that strikes it (the exact percentage depends on wavelength), giving it the highest albedo of any known object in the solar system.
The Cassini spacecraft took this image [right] of jets of water ice and vapour, mixed with organic compounds, over the south polar region of Saturn’s moon Enceladus.
Enceladus shoots plumes hundreds of kilometres into space [below left]; they include water-ice particles, water vapour, carbon dioxide, nitrogen, methane, ammonia and other gases. In other words, they contain the ingredients for life.
Cassini scientists use false-colour images such as this [left] to identify individual plumes with source regions on “tiger stripes” (sulci) on the moon’s surface. The plumes shoot hundreds of kilometres into space and include water-ice particles, water vapour, carbon dioxide, nitrogen, methane, ammonia and other gases. In other words, they contain the ingredients for life.
[Above right] Sulci up close – In August [left] and October [right] 2008, Cassini swooped under Enceladus’s south pole at close range, giving scientists high-resolution views of the sulci, which might behave much like spreading ridges on Earth’s ocean floors, for example, the mid-Atlantic Ridge.
Gravitational measurements from Cassini now show that there may be an underground ocean of salty water inside Saturn’s moon Enceladus; it is kept from freezing by tidal heating.
[See cut-away on the left]
Enceladus and Iapetus are both considered medium-sized satellites. They are much smaller than Earth’s Moon, Jupiter’s Galilean satellites, and Titan, but are big enough for their self-gravity to pull them into a nearly spherical shape.
Cassini to meet Enceladus
In October 2015 Cassini is scheduled to undertake a close flyby of Saturn’s moon Enceladus. The flyby will allow the spacecraft to get close enough to fly through the geysers of water that have been discovered emanating from this very interesting icy moon and hopefully reveal secrets of a possible subsurface ocean.
Saturn’s moon Iapetus has huge intriguing landslides. Iapetus frequently plays host to a huge type of landslide or avalanche that is rare elsewhere in the Solar System, scientists report. Sturzstroms or “long-runout landslides” move faster and farther than geological models predict they should. They have been seen on Earth and Mars, but there is debate about their causes.
Now, images from the Cassini space mission, reported in Nature Geoscience, suggest that heating of icy surfaces helps the landslides keep going. On Earth, landslides typically travel a horizontal distance that is less than twice the distance that the material has fallen. Long-runout landslides, by contrast, can travel as much as 30 times the vertical falling distance.
A great many mechanisms have been proposed to explain this phenomenon, ranging from simple sliding on ice to the sound waves from the slide making rock and debris behave more like a fluid. But there is little consensus on which of these theories, if any, is correct.
Now, Kelsi Singer of Washington University in St Louis, US, and colleagues report that the geography of Iapetus is a unique setting to test these theories. “The landslides on Iapetus are a planet-scale experiment that we cannot do in a laboratory or observe on Earth,” Ms Singer said. “They give us examples of giant landslides in ice, instead of rock, with a different gravity, and no atmosphere. So any theory of long-runout landslides on Earth must also work for avalanches on Iapetus.”
Iapetus is a geologically interesting place to investigate; it is a squashed sphere, fatter at its equator than its poles, and is mostly encircled by a ridge that reaches peaks some 20km high. It also has a number of giant impact craters reaching depths of 25km. The icy satellite has more giant landslides than any Solar System body other than Mars. The reason, says Prof William McKinnon, also from Washington University, is Iapetus’ spectacular topography.
“Not only is the moon out-of-round, but the giant impact basins are very deep, and there’s this great mountain ridge that’s 20km (12 miles) high, far higher than Mount Everest,” he explained. “So there’s a lot of topography and it’s just sitting around, and then, from time to time, it gives way.”
Ms Singer was looking for stress fractures in the moon’s ice, but instead found evidence of 30 massive landslides – 17 along crater walls and 13 along the giant equatorial ridge. Analysis of the images from these events suggests that the “coefficient of friction” – a measure of how much the slip-sliding of material in a landslide tends to slow it down – on Iapetus is far lower than expected for ice. It appears that this faster-moving ice seen on Iapetus has a lower friction coefficient than that of slow-moving ice measured in Earth-bound laboratories. The team suggests that the tiny contact points between bits of ice debris in such a landslide may heat up considerably, melting it and forming a more fluid – and thus less friction-limited – mass of material. They suggest that physicists here on Earth test the idea in the laboratory, giving insight not only into what is happening on Iapetus, but closer to home as well.
Soon after Giovanni Domenico Cassini discovered Iapetus in 1671 on the western side of Saturn at magnitude 10, he noticed that the moon disappeared on the eastern side. With an improved telescope, he finally found it on the eastern side in 1705, at magnitude 12. He brilliantly and correctly deduced that Iapetus has a dark and light hemisphere, and that it’s tidally locked to Saturn.
Iapetus’s leading hemisphere is as dark as coal, while the trailing hemisphere is considerably lighter in tone. The image on the far left shows the equatorial ridge, which stretches halfway around the moon. If the ridge formed off the equator, tidal interactions with Saturn would have forced it to straddle the equator.
These high-resolution Cassini images, taken during its flyby of Iapetus on 10th September 2007, show small sections of the equatorial ridge. In places, the mountains rise nearly 20 km above the average elevation, meaning they would dwarf the highest mountains on Earth. Only Mars’s giant volcanoes outrank these peaks in our solar system.
During its 10th September 2007 flyby, Cassini obtained the first high-resolution images of Iapetus. The images partially solved the mystery of Iapetus’s two-toned surface. The image taken in a bright region [left] shows that equator-facing crater rims remain dark. This pattern, along with areas where light material is found in dark regions [right], suggests runaway thermal segregation. Ice more quickly vaporizes on the warmer, dark surfaces, so they remain dark, and water vapour freezes on bright, colder terrain.
Tethys is a small moon 1,066 km in diameter that orbits 294,660 km from Saturn. This cold, airless and heavily scarred body is very similar to sister moons Dione and Rhea except that Tethys is not as heavily cratered as the other two. This may be because its proximity to Saturn causes more tidal warming, and that warming kept Tethys partially molten longer, erasing or dulling more of the early terrain.
Tethys’s density is 0.97 times that of liquid water, which suggests that Tethys is composed almost entirely of water ice plus a small amount of rock.
As with all but two of the major Saturnian moons, Tethys is tidally locked in phase with its parent planet – one side always faces toward Saturn. Likewise, Tethys has gravitationally locked two smaller moons into its own subsystem – Telesto and Calypso. Telesto is an irregular body 34 × 28 × 36 km that orbits 60° ahead of Tethys. Calypso is an irregular body 34 × 22 × 22 km that orbits 60° behind Tethys. These smaller moons are held in Lagrangian points (L4 and L5, respectively), where objects are stable with the larger controlling body. The fact that Tethys and other Saturnian moons have such objects implies that Lagrangian points might be stable for millions of years. This, in turn, implies that Lagrangian points around the Earth and the Moon might be more stable than previously thought to be in the past.
Tethys has a high visual albedo of 1.229, again suggesting a composition largely of water ice, which would behave like rock in the Tethyan average temperature of −187°C. Many of the crater floors on Tethys are bright, which also suggest an abundance of water ice. Also contributing to the high reflectivity is that Tethys is bombarded by Saturn E-ring water-ice particles generated by geysers on Enceladus.
The Tethyan northern hemisphere is lighter coloured and heavily reworked from ages of bombardment. For instance, near the prominent peaked crater Telemachus (Odysseus’ son in “The Odyssey”) are the remnants of Teiresias crater (named after a famous soothsayer of ancient Greece). The ancient Teiresias impact site is so badly overprinted and eroded by impact weathering and degradation that only a circular pattern of hummocks remains to indicate the old crater rim.
Closer to the equator, the terrain is darker and has fewer craters. This less-dense cratering suggests past internal activity and resurfacing of the terrain.
Meanwhile, a mysterious dark band on Tethys first observed in Voyager images has failed to yield to explanation. The band is obvious in Cassini images, darkening a patch of territory on the leading side, centred almost perfectly on the equator, and apparently disregarding local topography. Its relatively dark colour might mean its surface ice contains different-size ice crystals than other areas of Tethys. The equatorial location might suggest that the E ring has something to do with its formation, but no one has yet proposed a mechanism to relate the two.
Tethys has two overpowering features, a giant impact crater and a great valley. Odysseus crater dominates the Tethyan western hemisphere. Odysseus crater is 400 km in diameter. That diameter is nearly two-fifths of Tethys itself. Such an impact could have shattered a solid body, which suggests that the internal composition of Tethys was still partially molten. The crater’s rim and central peak have largely collapsed, leaving a shallow crater, and this also suggests a terrain that was elastic enough to change shape. The subdued features of Odysseus crater are in contrast to the many steep cliffs found elsewhere on the moon, which again suggests that the ancient terrain was still elastic enough to change shape.
The second major feature, a valley called Ithaca Chasma (named after the country ruled by Odysseus), runs roughly from the Tethyan north pole to its south pole. It is 100 km wide, 3 to 5 km deep and extends for 2000 km. Ithaca Chasma may have been caused by expansion of internal liquid water as it froze into ice after the surface had already frozen. An alternate theory is that the impact that created the Odysseus crater also generated forces that created Ithaca Chasma, especially since the chasm is on the opposite side of Tethys from the Odysseus crater. The chasm and surrounding area are heavily cratered, indicating that it was formed long ago.
Tethys appeared as a tiny dot to astronomers until the Voyager (1 and 2) encounters in 1980 and 1981. The Voyager images showed the major impact crater and the great chasm. The Cassini spacecraft has added details including a great variety of colours at small scales suggesting a variety of materials not seen elsewhere.
Giovanni Cassini discovered Tethys on 21 March 1684.
Cassini referred to Tethys as one of the four Sidera Lodoicea (Stars of Louis) after King Louis XIV (the other three were Iapetus, Dione and Rhea). Other astronomers called the Saturn moons by number in terms of their distance from Saturn. Thus, Tethys was Saturn III. John Herschel suggested that the moons of Saturn be associated with the Greek mythical brothers and sisters of Kronus. (Kronus is the equivalent of the Roman god Saturn in Greek mythology.)
The name comes from the Greek goddess (or Titan) Tethys, who was the daughter of Uranus and Gaea, a sister to Kronus and the wife of Oceanus. She was said to be the mother of the chief rivers, the mother of three thousand daughters called the Oceanids and the embodiment of the waters of the world.
Geological features on Tethys generally get their names from “The Odyssey” by Homer. The International Astronomical Union now controls naming of astronomical bodies.
Telesto, a Tethys Trojan originally designated S/1981 S1, is about 24 km across and appears to have a smooth, icy surface. It does not show the signs of instense cratering seen on many of Saturn’s other moons. It was discovered in 1980 using ground-based observations by Brad Smith, Harold Reitsema, Stephen Larson, and John Fountain. Telesto is a daughter of the Titans, Oceanus and Tethys in Greek mythology.
Calypso is a Trojan of Tethys, trailing Tethys in its orbit by 60°. Calypso is 22 km across. Like many other small Saturnian moons and small asteroids, Calypso is irregularly shaped by overlapping large craters. This moon appears to also have loose surface material capable of smoothing the appearance of craters. It was discovered by D. Pascu, P.K. Seidelmann, W. Baum, and D. Currie in March 1980 using a ground-based telescope.
Calypso was a nymph whose name means “I hide”. A daughter of the Titans, Oceanus and Tethys, she lived alone on her island until she fell in love with the explorer Odysseus (called Ulysses by the Romans; his name means “one who suffers”). Calypso helped Odysseus find his way home after his long voyage and dangerous adventures.
Rhea, Saturn’s second largest moon, diameter about 1530 km, and (above it) Saturn’s rings almost edge on.
On a 2005 flyby of Rhea – Saturn’s second-largest moon – Cassini’s Magnetospheric Imaging Instrument (MIMI) detected what may be a system of three or more rings surrounding the satellite. MIMI found that something unseen blocks the flow of electrons through the magnetosphere in Rhea’s neighbourhood. MIMI detected three sharp drops in its electron counts on each side of Rhea. If the rings exist, they’re made of particles too large to be seen in the forward-scattering geometry that made Enceladus’s plumes visible. And they’re too sparse to be noticed by regular reflected light. So it might not be possible for Cassini to perform follow-up observations that could confirm a Rhea ring system.
Rhea has a low density, indicating it’s mostly water ice with a dash of rock thrown in (probably a 3-to-1 ratio).
As you can see, it’s heavily cratered, and you can also see fractures running along the surface; both are typical for icy, airless worlds.
Cassini took some incredibly detailed photographs of Rhea in February 2015. [The thumbnail on the left doesn’t do it justice!]
The irregular crater near the top is called Wakonda; here’s a better view
Typical of a low-gravity, icy body, the crater walls are steep and the floor flat. After impact, the material flows back in from the sides of the crater, and the melted material can form a pond in the centre. The odd shape of the wall may be due to the cracks and fractures you can see running by and even through the crater itself. The surrounding terrain can guide the shape of a crater wall. The big bite out of the bottom right looks at first like it may have been from another impact, but the shape and height suggest it’s part of the original crater.
These images used green, ultraviolet, and infrared filters (as well as some with no filter at all), so they are not the same colour you’d see if you were there. Call it enhanced colour if you like.
Whatever you call it, expect more fantastic images like this in the coming months as more moons are visited by Cassini. It’s spent an amazing 11 years now orbiting Saturn, and it’s nice to see it can still deliver.
Hyperion, taken from the Cassini orbiter spacecraft in 2005. This sponge-like object is one of the most bizarre of Saturn’s family of more than sixty moons. Measuring 410 by 260 km, Hyperion, is one of the largest bodies in the solar system known to have such an irregular form; it is made largely of ice and has the consistency of a loose pile of rubble. Its surface is covered with irregular sharp-edged craters dusted with a mysterious dark material that may have originated on Phoebe, another of Saturn’s moons. During its flyby, Cassini was blasted by a burst of charged particles from Hyperion, in effect experiencing a 200-volt electric shock. The zap was due to the small moon’s surface becoming electrostatically charged within Saturn’s magnetic field.
Hyperion’s “spongy” appearance is due to its very low density; the moon is porous, with well-preserved craters of all shapes and sizes packed together across its surface.
Scientists believe Hyperion is mostly made from water ice, with small amounts of rock. It has a naturally reddish colour which was toned down in this image to enhance the visibility of its surface features.
Cassini was about 62,000 km from Hyperion when the picture was taken.
NASA’s Cassini spacecraft captured this image of three Saturn moons (Tethys at centre; Hyperion at upper left; and Prometheus at lower left near the boundary of the F ring) on 14th July 2014. [NASA/JPL-Caltech/Space Science Institute]
Less than 400 km in diameter, crater-covered Mimas is the smallest and innermost of Saturn’s major moons. Its most distinguishing feature is a giant impact crater – named Herschel after its discoverer – which stretches a third of the way across the face of the moon, making it look like the Death Star from “Star Wars”. Herschel is 130 km across, with outer walls about 5 km high and a central peak 6 km high. The impact that blasted this crater out of Mimas probably came close to breaking the moon apart.
At a mean distance of less than 200,000 km from the massive planet, Mimas takes only about 23 hours to complete an orbit. It keeps the same face toward Saturn as it flies around the planet, just as our Moon does with Earth. Its low density suggests that it consists almost entirely of water ice, which is the only substance ever detected on Mimas.
The Cassini magnetometer has seen hints of a source of plasma at Dione, and the Visual and Infrared Mapping Spectrometer (VIMS) team has also reported the possibility of a tenuous atmosphere of methane and water ice surrounding this moon. The ISS team has searched for plumes from Dione like the ones seen at Enceladus, but has so far come up empty-handed.
That Mimas appears to be frozen solid is puzzling because Mimas is closer to Saturn and has a much more eccentric (elongated) orbit than Enceladus, which should mean that Mimas has more tidal heating than Enceladus. Yet Enceladus displays geysers of water, which implies internal heat, while Mimas has one of the most heavily cratered surfaces in the solar system, which suggests a frozen surface that has persisted for enough time to preserve all those craters. This paradox has prompted the “Mimas Test” by which any theory that claims to explain the partially thawed water of Enceladus must also explain the entirely frozen water of Mimas.
Mimas orbits Saturn exactly twice as often as the more distant moon, Tethys, a phenomenon known as “orbital resonance”. Similar orbital resonances between Mimas and parts of Saturn’s rings are thought to be responsible for the Huygens gap, which marks the boundary between the B Ring and the Cassini Division, and for several density waves within the A Ring. In addition, Mimas’s slight inclination (1.574 ° with respect to the ring plane) gives rise to several vertical bending waves within the A Ring.
Mimas is also in resonance with Dione and Enceladus, and perturbs the orbits of Methone, Pallene and Anthe.
See this map of Mimas.
Phoebe, discovered in August 1898 by American astronomer William Pickering, is one of Saturn’s most intriguing satellites, orbiting at 12,952,000 km from the planet, almost four times the distance from Saturn than its nearest neighbour, Iapetus. Phoebe and Iapetus are the only major moons in the Saturnian system that do not orbit closely to the plane of Saturn’s equator.
Phoebe is roughly spherical and has a diameter of about 220 km, about one-fifteenth the diameter of Earth’s Moon. Phoebe rotates on its axis every nine hours, and it completes a full orbit around Saturn in about 18 months. Its irregular, elliptical orbit is inclined about 30° to Saturn’s equator. Phoebe’s orbit is also retrograde (it goes around Saturn in the opposite direction to most other moons, as well as most objects in the solar system).
Unlike most major moons orbiting Saturn, Phoebe is very dark and reflects only 6% of the sunlight it receives. Its darkness and irregular, retrograde orbit suggest Phoebe is most likely a captured object (one trapped by the gravitational pull of a much bigger body). Phoebe’s darkness, in particular, suggests that the small moon comes from the outer solar system, an area where there is plenty of dark material. Some scientists think Phoebe could be a captured Centaur.
Moons of Saturn were originally named after Greco-Roman Titans and descendants of the Titans. But as many new moons were discovered scientists began selecting names from other mythologies, including Gallic, Inuit and Norse. Phoebe is another name for the Greek Artemis and the Romans Diana. She was the youthful goddess of Earth’s Moon, forests, wild animals, and hunting. Sworn to chastity and independence, she never married and was closely identified with her brother Apollo.
Shepherd satellites are small moons that orbit within, or just beyond, a planet’s ring system. They have the effect of sculpting the rings: giving them sharp edges, and creating gaps between them. Saturn’s shepherd moons are Pan (Encke gap), Daphnis (Keeler gap), Atlas (A Ring), Prometheus (F Ring) and Pandora (F Ring). These moons together with co-orbitals (see right) probably formed as a result of accretion of the friable ring material on preexisting denser cores. The cores with sizes from one-third to one-half the present day moons may be themselves collisional shards formed when a parental satellite of the rings disintegrated.
More about the moons like Pan that “shepherd” the planet’s rings.
Janus and Epimetheus are co-orbital moons: they are of roughly equal size, with Janus being slightly larger than Epimetheus; they have orbits with only a few kilometres difference in semi-major axis, close enough that they would collide if they attempted to pass each other. Instead of colliding, however, their gravitational interaction causes them to swap orbits every four years.
Pan is seen in this colour view as it sweeps through the Encke Gap with its attendant ringlets. 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 opening in the 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 kilometres 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.
Pan, like Atlas, has a prominent equatorial ridge that gives it a distinctive flying saucer shape. It was discovered by M.R. Showalter in 1990 using images taken by the Voyager 2 spacecraft nine years earlier.
Pan, a satyr (a creature resembling a man with the hind legs and hooves of a goat), is a Greek god of nature and the forest.
From left to right: a view of Atlas’s trailing hemisphere, with north up, at a spatial scale of about 1 km per pixel; Atlas seen at about 250 m per pixel from mid-southern latitudes, with the sub-Saturn hemisphere at the top and leading hemisphere to the left.
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 across. Atlas was discovered in 1980 by R. Terrile and the Voyager 1 team.
Originally designated S/1980 S28, this moon is named after Atlas, a Titan, and a son of Iapetus. Atlas was ordered by Zeus to uphold the vault of the sky after the defeat of the Titans. Atlas was so strong that he supported the weight of the Universe on his shoulders.
Saturn’s moon Prometheus continues its dance with the planet’s F ring, creating channels in the ring and streamers of extracted ring material as a result. Prometheus acts as a shepherding satellite, constraining the extent of the inner edge of Saturn’s F Ring. Prometheus is extremely irregular and has visible craters – some up to 20 km in diameter. However, it is much less cratered than its nearby neighbours Pandora, Janus and Epimetheus. The density of Prometheus has been estimated to be low; it is probably a porous, icy body.
The potato-shaped moon is about 86 km across, and was originally designated S/1980 S27. The Voyager 1 science team discovered Prometheus in October 1980.
Prometheus was the son of the Titan Iapetus and brother of Atlas and Epimetheus. He is best known in Greek mythology for stealing fire from the gods and giving it to humanity.
Saturn already has 62 moons but this new addition could be its last. New images from the Cassini–Huygens space probe suggest that Saturn may be in the process of forming a new moon, which has already been affectionately named by scientists as ‘Peggy’. The tiny, icy satellite has not been spotted directly, but a bulge in Saturn’s A Ring – the brightest and outermost ring – suggests that the new arrival could soon join the other 62 moons.
“We have not seen anything like this before,” said astronomer Carl Murray, lead author of a study in Icarus which outlined the findings and the discoverer of the moon. “We may be looking at the act of birth, where this object is leaving the rings and heading off to be a moon in its own right.” Saturn’s rings are comprised almost entirely of ice with a trace amount of rocky material and stretch from 7,000 km to 80,000 km above the surface of Saturn’s equator with a thickness varying from 10 metres to 1 kilometre.
The bright smear at the bottom edge of the ring is estimated to be about 750 miles long and shows where ‘Peggy’s’ gravity is thought to be effecting ring particles. [Image credit: Nasa/JPL]
The rings are not solid but are made up of countless individual fragments ranging in size from particles as small as a grain of sand to boulder-like lumps metres across. It’s thought that the rings act as a galactic nursery for the planet’s many moons, with material gradually clumping together in orbit until it gathers enough momentum to separate. “The theory holds that Saturn long ago had a much more massive ring system capable of giving birth to larger moons,” Murray said. “As the moons formed near the edge, they depleted the rings and evolved, so the ones that formed earliest are the largest and the farthest out.” The largest of Saturn’s moons is Titan with a diameter of more than 5,000 km and a mass nearly double that of our Moon. If ‘Peggy’ does eventually grow up enough to leave home it would be tiny in comparison, perhaps only 0.5 miles in diameter. Although many of Saturn’s satellites eventually take on orbits many thousands of miles away from the planet, others stay closer to home, sculpting the rings by either collecting stray matter to sharpen their edges or carving out thin gaps like someone a giant snowball rolled through a snowy field.
However, ‘Peggy’s’ diminutive size suggests that Saturn’s satellite-bearing days may soon be over, with each successive moon appearing smaller than the last as the supply of potentially moon-forming material is depleted. “The object is not expected to grow any larger, and may even be falling apart,” said NASA’s Jet Propulsion Laboratory. “But the process of its formation and outward movement aids in our understanding of how Saturn’s icy moons, including the cloud-wrapped Titan and ocean-holding Enceladus, may have formed in more massive rings long ago.”