Visions of the Cosmos

Planetary Science

The Solar System

The Moons of Saturn

The Moons of Saturn

 On 15 October the Cassini-Huygens Spacecraft was launched on its long journey to investigate Saturn and its many moons and on 30June/1 July 2004 it was inserted into an orbit round Saturn.  Details of this spectacular international enterprise are given in this web-site under the page on Titan.

Of all the planets, Saturn still has the highest number of known satellites, currently counted at 31. Titan is by far the largest moon.  It is between Mercury and Mars in diameter.  It discovered by Huygens in 1655. Four more were first observed by Cassini and, thanks to better telescopes, nine more moons were known by the beginning of the 20th Century.

Pan appears to be the nearest to Saturn and Phoebe the furthest.  They are listed in the table below together with the eight largest.  Titan is comprehensively dealt under its own page of the web-site.  Except for tiny Pan, the other moons listed are each separately discussed..   The statistics in the table below are credited to the European Space Agency Statistics.

 Vital Statistics of the Moons of Saturn

Name

Distance in kms

Diameter in kms

‘Day’

Pan

134,000

20

0.58

Mimas

186,000

383/418

0.94

Enceladus

238,000

489/513

1.37

Tethys

295,000

1,051/1071

1.89

Dione

377,000

1120

2,74

Rhea

527,000

1528

4.52

Titan

1,222,000

5,150

15.95

Hyperion

1,481,000

225/360 very irregular

21.28

Iapetus

3,561,000

1,436

79.33

Phoebe

12,592,000

220

550.48

 

Distances are the mean distances of the moon  from the centre of Saturn to the nearest kilometre.   Note that Saturn is approximately 120,000 kilometres in diameter.

The ‘day’ is the time in Earth days for the moon to orbit Saturn.  Looked at another way – from the point of view from Saturn it is a ‘month’.  Notice that they orbit far faster than Earth’s Moon orbits Earth.  This is because of the huge mass of Saturn compared to the Earth. 

MIMAS

 Mimas is one of the innermost moons of Saturn.   It is only 186,000 kilometres from Saturn. William Herschel discovered the moon in 1789. It is only 398 kilometres in diameter.  The surface is icy and heavily cratered. Mimas has a low density, meaning it probably consists mostly of ice. Because Mimas has such a low temperature of about -200° C the impact features may date back to the time of the moon's creation.

Soon after orbital insertion, Cassini returned its best look yet at the heavily cratered moon Mimas (398 kilometres in diameter). The enormous crater at the top of this image, named Herschel, is about 130 wide and 10 kilometres deep.

The image was taken in visible light with the Cassini spacecraft narrow angle camera on July 3, 2004, from a distance of 1.7 million kilometres from Mimas and at a Sun-Mimas-spacecraft, or phase angle of about 102 degrees. The image scale is 10 kilometres per pixel. The image has been magnified by a factor of two to aid visibility.

 

 Acknowledgement NASA/JPL/Space Science Institute

 

ENCELADUS

William Herschel discovered Enceladus in 1789.  It’s mean distance from Saturn is 238,020 kilometres. It has a diameter of 499 kilometres. 

It is rather similar in size to another small moon Mimas but has a smoother, brighter surface. Unlike Mimas, Enceladus displays at least five different types of terrain. Some parts of Enceladus show craters no larger than 35 kilometres in diameter. Other areas show regions with no craters at all indicating major resurfacing events in the geologically recent past. There may well have been incidences of cryovolcanic activity (for explanation see Titan).   There are fissures, plains, corrugated terrain and other crustal deformations. All of this indicates that the interior of the moon may be liquid today, even though it should have frozen aeons ago. It is postulated that Enceladus is heated by tidal mechanisms similar to Io – one of Jupiter’s moons. It is perturbed in its orbit by Saturn's gravitational field and by the larger moons.

Intriguing Enceladus
The Cassini-Huygens image shows the trailing hemisphere of Enceladus, which is the side opposite the moon's direction of motion in its orbit.

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on 27 October 2004, at a distance of about 766,000 kilometres from Enceladus. The image scale is 4.6 kilometres per pixel.

It is characterised by its extraordinary brilliance – it reflects nearly 100% of the light reaching it.  Because of this it has a very low surface temperature of –201 degrees Celsius.  This compares with Titan, which has a surface temperature of around –180 degrees Celsius because of its thick cloud cover and the presence of considerable quantities of methane a powerful greenhouse gas.

Credit NASA/JPL/Space Science Institute

March 9, 2006: NASA's Cassini spacecraft may have found evidence of liquid water reservoirs that erupt in Yellowstone-like geysers on  Enceladus. (From NASA' News 03/09/06)

March 9, 2006: NASA's Cassini spacecraft may have found evidence of liquid water reservoirs that erupt in Yellowstone-like geysers on  Enceladus. "We realize that this is a radical conclusion -- that we may have evidence for liquid water within a body so small and so cold," said Carolyn Porco, Cassini imaging team leader at the Space Science Institute, Boulder, Colorado. "However, if we are right, we have significantly broadened the diversity of solar system environments where we might possibly have conditions suitable for living organisms."

High-resolution Cassini images show icy jets and towering plumes ejecting huge quantities of particles at high speed. Scientists examined several models to explain the process.  The jets might be erupting from near-surface pockets of liquid water above 0 degrees Celsius, like cold versions of the Old Faithful geyser in Yellowstone:"

Other moons in the solar system probably have liquid-water oceans covered by kilometers of icy crust," said Andrew Ingersoll, imaging team member and atmospheric scientist at the California Institute of Technology, Pasadena,  "What's different here is that pockets of liquid water may be no more than tens of meters below the surface."

The picture on the top right shows the tiger stripes on Enceladus in the south polar regions. The blue-green "tiger stripes" are thought to be the source of Enceladus's water jets. Cassini's infrared spectrometer took the temperature of a tiger stripe.  The fissure is at least 15 degrees K warmer than its surroundings--a sign of geothermal activity.  The diagram on the right is marked in both Celsius and Fahrenheit degrees

The exciting mystery of an active south polar region on Saturn’s icy moon Enceladus continues to unfold as scientists make the correlation between geologically youthful surface fractures and unusually warm temperatures.

This view shows excess heat radiation from cracks near the moon’s south pole. These warm fissures are the source of plumes of dust and gas seen by multiple instruments on the Cassini spacecraft during its flyby of Enceladus on July 14, 2005, as described in a series of papers in the March 10, 2006, issue of the journal Science.

As Cassini approached Saturn, we discovered that the Saturn system is filled with oxygen atoms. At the time we had no idea where the oxygen was coming from," said Candy Hansen, Cassini scientist at NASA's Jet Propulsion Laboratory (JPL) in Pasadena. "Now we know Enceladus is spewing out water molecules, which break down into oxygen and hydrogen."

Scientists still have many questions. Why is Enceladus so active? Might this activity have been continuous enough over the moon's history for life to have had a chance to take hold in the moon's interior? In the spring of 2008, scientists will get another chance to look at the geysers--and another crack at answering these questions--when Cassini flies within 350 kilometers of Enceladus.

"There's no question, along with the moon Titan, Enceladus should be a very high priority for us," said Jonathan Lunine, Cassini interdisciplinary scientist, University of Arizona at Tuscon. "Saturn has given us two exciting worlds to explore."

TETHYS

Cassini discovered Tethys in 1684.  It orbits Saturn  synchronously at a mean distance of  294,660 kilometres.  The diameter of Tethys is 1060 kilometres.    It’s low density indicates that it is almost completely composed of water ice.  It appears to be similar to Rhea and Dione.

The western hemisphere is dominated by a huge impact crater, called Odysseus, whose 400 km diameter is nearly 2/5 of that of Tethys itself (right). That such an impact didn't shatter Tethys completely indicates that it may have been liquid or at least not very solid at the time. The crater is now quite flat (or more precisely, it conforms to Tethys' spherical shape). In this it is like the craters on Jupiter’s moon Callisto and unlike the high ring mountains and central peaks commonly seen on the our Moon and on Mercury

The second major feature seen on Tethys is a huge valley (called Ithaca Chasma) 100 km wide and 3 to 5 km deep which runs 2000 km or 3/4 of the way around Tethys' circumference (above).

Clearly then, Tethys has not always been frozen solid. At some point in its past it was probably liquid. The impact craters from that era have been smoothed out. As it froze and expanded, the surface must have cracked to accommodate the extra volume producing Ithaca Chasma. The smaller impact craters we see today are more recent.

 

 

Voyager 2 Mission

Tethys and the Odysseus Crater from Voyager 2 Mission take at a distance of 825, 955 kilometres 25 August 1981. 

The image shows the giant impact crater, Odysseus. Tethys is 1050 km in diameter and the crater is nearly 500 km across. The north pole is on the edge of the disk at 1:30 and Odysseus is centered at 30 N, 130W. This image was taken on 26 August 1981 from a distance of 825,955 km and has a resolution of 8 km/pixel.  (Voyager 2, NASA Image ID Number FDS 43980.27

 Acknowledgement to NASA/JPL/Space Science Institute

 

Cassini-Huygens Mission

The battered surface of Tethys (1060 kilometres across) seen here has a neutral hue, and is a mosaic of two Cassini-Huygens images. Three images taken in the red, green and blue filters were taken to form a natural colour composite.

The view shows primarily the trailing hemisphere of Tethys, which is the side opposite the moon's direction of motion in its orbit. The image has been rotated so that north on Tethys is up.

The red, blue and green images comprising this colour view were taken with the Cassini-Huygens spacecraft narrow-angle camera on 28 October 2004, at a distance of about 256 000 kilometres from Tethys. The image scale is 1.5 kilometres per pixel.

Acknowledgement NASA/JPL/Space Science Institute.

 

 DIONE

Giovanni Cassini Dione discovered Dione in 1684.   It is an icy body similar to the other small satellites Tethys and Rhea. Its density is 1.43 gm/cm3, makes it the densest moon of Saturn after Titan.   Dione is probably composed of a rocky core making up one-third of the moon's mass, with the rest composed of water ice. Dione is only 1,120 miles in diameter and has probably been tidally locked in its current position for the past several billion years.

Because Dione is relatively small an impact causing a 35kilometre crater could have spun the satellite. Since there are many craters larger than this Dione could have been repeatedly spun.

The origin of the bright wispy material found on Dione is somewhat obscure. Apparently, it is thin enough not to have obscured the surface feature underneath. It might have formed from eruptions along cracks in Dione's surface that fell back to the surface as snow or ash.  This is an example of cryovolcanism (see Titan).

On 13 December 2004 the Cassini Spacecraft made its first fly-by of Dione and took the remarkable photograph shown below

On 13 December 2004 the Cassini Spacecraft made a close fly-by of Dione.  The Cassini cameras caught a set of images that was combined into a mosaic to give the stunning global view seen on the right.

Cassini imaging scientists are intrigued by the complex braided structure of surface fractures on Dione. To the surprise of scientists, the wispy terrain features do not consist of thick ice deposits, but bright ice cliffs created by tectonic features. This is one of the most surprising results so far. It just wasn't what we expected," said Dr. Carolyn Porco, Cassini imaging team leader, Space Science Institute, Boulder, Colorado

Its mean distance of Dione from Saturn is 277,400 kilometres and its orbital period (‘day’) relative to the Sun.  Its mean surface temperature is –186.56 degrees Celsius.

Its surface gravity is about 0.223 of the Earth

Credit NASA/JPL/Space Science Institute

 

 

RHEA

 Cassini discovered Rhea in 1672.  It is the second largest moon of Saturn.   It has a diameter of 1528 kilometres   It orbits at a mean distance of 527,040 kilometres from Saturn.  Rhea rotates round Saturn synchronously.   Though somewhat larger it is similar to Dione.  Both satellites have varied terrain and have dissimilar leading and trailing hemispheres.  Rhea is composed primarily of water ice with rock making up less than 1/3 of its mass.

Rhea's ancient surface
The sunlight angle in this sharp view of Saturn's second-largest moon, Rhea, highlights the moon's crater-strewn surface. Rhea's diameter is 1528 kilometres.

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on 1 November 2004, at a distance of 1.6 million kilometres from Rhea. North is up. The image scale is about 10 kilometres per pixel. The image has been slightly contrast enhanced to aid visibility of surface features.

Acknowledgements NASA/JPL/Space Science Institute

TITAN

Discovered by Huygens in 1655, Titan is by far the biggest of the moons of Saturn.  It so important that it has a whole section of the web-site to itself

HYPERION

 Hyperion was discovered by Lassel and Bond in 1848. It is one of the smaller moons of Saturn. It has a cratered body and is the largest irregularly shaped satellite ever observed.   The dimensions are 410 x 260 x 220 kilometres   Hyperion might have had a major collision that blew part of the moon away.  Its eccentric orbit makes it subject to gravitational forces from Saturn that have set it tumbling out of control. The moon's rotational period is not constant and varies from one orbit to the next.

The pictures taken by Voyager and subsequent ground based photometry indicate that Hyperion's rotation is chaotic. i.e. its axis of rotation wobbles so much that its orientation in space is completely unpredictable. Hyperion is unique in that it is very irregularly shaped, has a highly eccentric orbit, and is near Titan.  These factors combine to restrict the set of conditions under which stable rotation is possible. It’s mean distance from Saturn is 1,481,100 kilometres.

 Like most of Saturn's moons, Hyperion's low density indicates that it is composed largely of water ice with only a small amount of rock. But, unlike most of Saturn's moons, Hyperion has a low reflexion (albedo) indicating that it is covered by at least a thin layer of dark material. This may be material blown over from Phoebe.  This may be the cause the dark colour of one of the hemispheres of Iapetus although some authorities attribute the colour on Iapetus due to internal causes.  Hyperion is redder than the little moon Phoebe and closely matches the colour of the dark material found on Iapetus. Hyperion's odd rotation probably accounts for the fact that Hyperion's surface is more or less uniform, in contrast to many of Saturn's other moons which have distinctly different leading and trailing hemispheres. The largest crater on its surface is approximately 120 kilometres in diameter and 10 kilometres deep. The irregular shape of Hyperion and evidence of bombardment by meteors makes it appear to be the oldest surface in the Saturn System.

IAPETUS

  

With a diameter of 1,436 kilometres, Iapetus is Saturn’s third largest moon.  It is famous for the dramatic contrasts in the brightness of its surface.  When Giovanni Cassini first discovered Iapetus using a telescope in 1671, he could only see the bright side of the moon. The leading hemisphere is as dark as asphalt and the trailing hemisphere is as bright as snow.  It is sometimes called the two-faced moon.

One hemisphere of Iapetus is very dark, while the other is very bright. Whether the moon is being coated by foreign material or being resurfaced by material from within is not yet known.

Its diameter is about one third that of our own Moon at 1436 kilometres. The latest image was taken in visible light with the Cassini-Huygens spacecraft narrow-angle camera on 3 July 2004, from a distance of 3 million kilometres.

Credits NASA/JPL/Space Science Institute.

Waning Iapetus

These spectacular Cassini images of Iapetus are the sharpest views yet obtained.   The views show parts of the anti-Saturn side  - that is to say the side that faces away from Saturn.  These will not be photographed again until September 2007.

 The enhanced colour views on the left and centre were obtained by combining ultraviolet  (338 nanometres), green (568 nanometres) and infrared (930 nanometres). The image on the left was obtained using visible white light.  The images on the bottom row are identical to those on the top row with the addition of an overlaying co-ordinate grid.

 Many impact craters can be seen in the bright terrain and a few in parts of the dark terrain.  A line of mountains can also be discerned as a string of bright dots in the left hand and the middle views.  They may compete in height with mountains on Earth or even Mars.

 The images were taken with the Cassini spacecraft’s narrow angle camera between 15-20 October 2004 at distances from Iapetus of approximately 1,200,000, 1,100,000 and 1,300,000 kilometres respectively.

Credits NASA/JPL/Space Science Institute

PHOEBE

Phoebe was discovered by Pickering in 1898.   It is by far the most distant moon of Saturn and orbits at a mean distance of 12,952,000 kilometres.  It is a small moon with a diameter of only 220 kilometres

On June 11 2004 the Cassini spacecraft passed close to Phoebe on its way into the Saturn system, giving us our first real look at it since Voyager and at much higher resolution.

Apart from Phoebe and Iapetus all of Saturn’s moons orbit very nearly in the plane of Saturn's equator. Phoebe's orbit is inclined almost 175° and its north pole is in the opposite direction to Saturn's.    Phoebe does not rotate synchronously round Saturn and rotates in a retrograde direction.

Phoebe may be a captured Kuiper Belt object or comet.  Cassini's 2004 flyby reinforces this idea with the detection of CO2 ice. The detection of CO2 rules out the earlier idea that it might have been a captured asteroid.   It appears that Phoebe is chemically similar to both Triton (a moon of Neptune) and Pluto. It is probably very similar to in chemical composition of the outer solar system at the very beginning.

Most of Saturn's moons are bright but Phoebe is as dark as lampblack. Material knocked off of Phoebe's surface by microscopic meteor impacts may be responsible for it’s dark surfaces as well as those of Hyperion and of the leading hemisphere of Iapetus.

Cassini-Huygens sees probable evidence on Phoebe of an ice-rich body overlain with a thin layer of dark material. The sharply defined crater in the right hand picture at above centre exhibits two or more layers of alternating bright and dark material.

Imaging scientists on the Cassini-Huygens mission have hypothesised that the layering might occur during the crater formation, when ejecta thrown out from the crater buries the pre-existing surface that was itself covered by a relatively thin, dark deposit over an icy mantle.

The lower thin dark layer on the crater wall appears to define the base of the ejecta blanket. The ejecta blanket itself appears to be mantled by a more recent dark surface 'lag'.

This image was obtained on 11 June 2004 from a distance of 13, 377 kilometres. The image scale is approximately 80 metres per pixel. No enhancement was performed on this image.

Acknowledgement NASA/JPL/Space Science Institute

 Two of many images photographed from Cassini are shown above. “What spectacular images,” said Dr. Carolyn Porco, Cassini Imaging Team leader at the Space Science Institute in Boulder Colorado. “So sharp and clear and showing a great many geological features, large and small. It's obvious a lot of new insights into the origin of this strange body will come as a result of all this.”

Dr. Gerhard Neukum, an imaging team member from Freie University in Berlin, said, “It is very interesting and quite clear that a lot of craters smaller than a kilometre are visible. This means, besides the big-ones, lots of projectiles smaller than 100 meters (328 feet) have hit Phoebe.” Whether these projectiles came from outside or within the Saturn system is debatable. Cassini's flyby of Phoebe shows a moon with a battered past

 Scientists plan to use the data obtained in the Phoebe fly-by to create global maps of the cratered moon, and to determine Phoebe’s composition, mass and density and verify its suspected origin as a Kuiper Belt Object.

 

 

Solar System