ESA Science & Technology04-Jul-2005 10:44:10

Saturn's Moons


VIII - Discovered by Cassini (1671)

Physical Parameters

Orbital Parameters

Radius (km)


Radius (km)
  • Pericentre
  • Apocentre

  • 3 461 100
    3 660 500

    Mean Density (kgm-3)



    79d 07h 55m

    Mass (kg)

    1.6 x 1021

    Velocity (kms-1)
  • Pericentre
  • Apocentre

  • 3.36

    Magnitude (V0)

    10.2 - 11.9

    Rotational Period

    79d 07h 55m

    Escape Velocty (kms-1)




    Mean Surface Albedo


    Inclination ( °)


    Knowledge before Cassini-Huygens

    Iapetus has a highly eccentric and inclined orbit over 3.5 million km away from Saturn. The moon is similar in size to Rhea and is one of the denser moons. Its surface may contain methance and amonia ices. These ices exist here, and possibly only here in the Saturnian system, because the inner moons would have been too warm for the volatile gases to remain during the early history of the solar system.

    The main feature of Iapetus is its split hemispheres - one seemingly bright the other around 6 to 10 times darker. Cassini noticed that the moon was only visible on one side of its orbit. His hypothesis, later confirmed by the Voyager probes, was that the same face of the moon must always face Saturn and that the leading hemisphere is darker than the trailing hemisphere.

    At the moment, the mechanism that has produced this difference is not known.

    *Note: in the following section all images courtesy NASA/JPL/Space Science Institute unless stated otherwise.

    15 July 2004

    Cassini Instrument: Imaging Science Subsystem

    Date: 03 July 2004

    Distance: 3 million km

    Scale: 18 km per pixel

    Phase Angle: 10 °

    Image Notes: Narrow angle, visible light, magnified x2, brightness variations are real

    31 August 2004

    Cassini Instrument: Imaging Science Subsystem

    Date: 19 July 2004

    Distance: 2.9 million km

    Scale: 17 km per pixel

    Phase Angle: 89 °  

    Image Notes: Narrow angle, visible light, magnified x4. Each image proceesed differently: left image cleaned from cosmic rays - right also contrast enhanced.

    08 December 2004

    Cassini Instrument: Imaging Science Subsystem

    Dates: 15 to 20 October 2004

    Distances: 1.2, 1.1 and 1.3 million km

    Scales: ~7 kilometres per pixel

    Phase Angle: Ranges from 88 to 144 °

    Image Notes: Images obtained using ultraviolet (centred at 338 nanometres), green (568 nanometres) and infrared (930 nanometres) filters were combined to produce the enhanced colour views at left and centre; the image at the right was obtained in visible white light. The images on the bottom row are identical to those on top, with the addition of an overlying coordinate grid.

    These views show parts of the moon's anti-Saturn side. In the central view, part of the moon's eastern edge was not imaged and appears to be cut off.   The large circular feature rotating into view in the southern hemisphere is probably an impact structure with a diameter of more than 400 kilometres.

    Date: 22 October 2004

    Distance: 1.6 million km

    Scale: 9.4 km per pixel

    Phase Angle: 161 °

    Image Notes: Narrow angle camera, visible light, view centred on 0.4 ° N 317 ° W

    New details on Iapetus are illuminated by reflected light from Saturn in this image. New features on the Saturn-facing part of Iapetus' bright trailing hemisphere are visible for the first time, including many dark spots, and a sharper view of a dark, circular structure that was first seen at very low resolution by NASA's Voyager 1 in 1980.

    The image shows mainly the night side of Iapetus; part of the far brighter sunlit side appears at the right and is overexposed due to the long integration time of 180 seconds. Despite this long exposure time, almost no blurring due to the spacecraft's motion is apparent.

    07 January 2005

    Cassini Instrument: Imaging Science Subsystem

    Date: 31 December 2004

    Distance: 172 400 km

    Scale: 1 km per pixel

    Phase Angle: 50 °  

    Image Notes: Narrow angle, visible light, 4 composite images, contrast enhanced, view centred on 0.0 ° N 90 ° W, north is uppermost

    The scene is dominated by a dark, heavily-cratered region, called Cassini Regio, within  which  dark deposits (visual reflectivity  ~ 4%) coat nearly everything with remarkable uniformity. Above latitudes of about 40 ° N, the surface  changes to a much brighter, icy terrain where the brightest materials have a reflectivity over 60%. This region is not, however,  uniform: Close inspection reveals that the surface is stained by crudely north-south trending wispy streaks of darker material, typically a few kilometres wide and sometimes tens of kilometres long.

    An ancient, 400-kilometer wide impact basin appears just above the centre of the disc. The basin is heavily overprinted by more recent, smaller impact craters. The basin rim is delineated by steep scarps that descend to the basin floor. Many of these scarps, as well as walls of nearby craters, appear bright, probably due to exposed outcrops of relatively clean ice. Particularly at mid-latitudes, the brightest scarp exposures appear to face away from the equator (toward the pole). Often, the opposite south-facing scarps are stained with the lower-brightness material.

    The most unique, and perhaps most remarkable feature discovered on Iapetus is a topographic ridge that coincides almost exactly with the geographic equator. The ridge is conspicuous in the picture as an approximately 20-kilometre wide band that extends from the western (left) side of the disc almost to the day/night boundary on the right. On the left horizon, the peak of the ridge reaches at least 13 kilometres above the surrounding terrain. Along the roughly 1300-kilometre length over which it can be traced in this picture, it remains almost exactly parallel to the equator within a couple of degrees. The physical origin of the ridge has yet to be explained. It is not yet clear whether the ridge is a mountain belt that has folded upward, or an extensional crack in the surface through which material from inside Iapetus erupted onto the surface and accumulated locally, forming the ridge.

    The origin of Cassini Regio is a long-standing debate among scientists. One theory proposes that its dark material may have erupted onto Iapetus's icy surface from the interior. Another theory holds that the dark material represented accumulated debris ejected by impact events on dark, outer satellites of Saturn. Details of this Cassini image mosaic do not definitively rule out either of the theories. However, they do provide important new insights and constraints.

    The uniform appearance of the dark materials at the equator, the apparent thinning and spottiness of the dark materials at progressively higher latitudes and dark wispy streaks near the distal margin of Cassini Regio strongly suggest that dark material was emplaced as a coating. One of the important new results is that no clear evidence can be found that erupted fluids have resurfaced Cassini Regio. The high density of impact craters argues that the terrain underlying the dark coating is relatively ancient and has not been eradicated by its emplacement. Thus, Cassini Regio may have had its origin in plume-style eruptions in which dark particulate materials accumulated on the surface as fallout, perhaps in conjunction with the creation of the equatorial ridge. On the other hand, the dark deposits in Cassini Regio may be a surface coating consistent with, and perhaps more simply explained by, the fall of dark materials from outside.

    Date: 31 December 2004

    Distance: 172 900 km

    Scale:   1 km per pixel

    Image Notes: Narrow angle   taken with infrared (centred at 930 nanometres), green (568 nanometres), and ultraviolet light (338 nanometres) filters

    This near-true colour view from Cassini reveals the colourful and intriguing surface of Iapetus. The images taken with different spectral filters and used for this composite were taken at the same time as the clear frames used in the above composite view of Iapetus' surface. The use of colour on Iapetus is particularly helpful for discriminating between shadows (which appear black) and the intrinsically dark terrain (which appears brownish).

    This image shows the northern part of the dark Cassini Regio and the transition zone to a brighter surface at high northern latitudes. Within the transition zone, the surface is stained by roughly north-south trending wispy streaks of dark material. The absence of an atmosphere on Iapetus means that the material was deposited by some means other than precipitation, such as ballistic placement from impacts occurring elsewhere on Iapetus, or was captured from elsewhere in the Saturn system. Iapetus's north pole is not visible here, nor is any part of the bright trailing hemisphere.

    Date: 31 December 2004

    Distance: 123 370 km

    Scale: Resolution achieved in the original image was 732 meters per pixel.

    Phase Angle: 93 °

    Image Notes: Narrow angle, visible light, mosaic of two images



    This oblique view of Iapetus from high latitude shows how the dark, heavily cratered terrain of Cassini Regio transitions to a bright, icy terrain at high latitudes.

    At the equator terrains are uniformly covered with a dark mantle of material that has a reflectivity of about 4 percent. At latitudes toward the pole of about 40 degrees, the dark deposits become patchy and diffuse as the surface transitions to a much brighter, icy terrain near the pole. The brightest icy materials exhibit visual reflectivity over 60 percent.

    Superimposed on the bright terrain is a subtle pattern of crudely parallel, north-south trending wispy streaks. The streaks, which were discovered during this flyby of Iapetus, are typically a few kilometres wide and sometimes tens of kilometres long. Their appearance and orientation may be connected with the emplacement of dark materials that cover Cassini Regio. The dark materials might represent the gradual accumulation of dark debris falling from space, or alternatively, may represent fallout from plume-style eruptions that may have accompanied the formation of Iapetus's enigmatic equatorial ridge.

    Also seen in this mosaic are conspicuous, north-facing bright crater walls. An example can be seen in the botom left where the bright, 4-kilometer-high walls of a 70 kilometre central-peak crater lies.

    The bright crater walls are often higher in brightness than the corresponding south-facing walls of the same crater. They are vaguely reminiscent of bright north-facing crater walls that were discovered by NASA's Voyager and Galileo spacecraft in craters near the poles of the Jovian satellites Callisto and Ganymede. In the case of the Jovian satellites, cold-trapping of frosts on north-facing slopes and sublimation of ices from south-facing slopes are thought to produce the north-south asymmetries in crater wall brightness. However, the occurrence of some young-appearing craters on Iapetus that have bright north-facing and dark south-facing slopes, and the pattern of streaks near the north pole of Iapetus suggests that another mechanism may be responsible for the crater wall brightness asymmetries on Iapetus.

    One possibility is that the south-facing slopes may be stained by the same process that emplaced the low brightness coating throughout the region. In this case, the north-pointing scarps might be bright because they face away and are shielded from the putative falling spray of dark materials. Bright south-facing slopes would exist primarily on young craters that have not been exposed to the darkening agent long enough to be stained.


    Date: 31 December 2004

    Distance: 123 400 km

    Scale: Resolution achieved in the original image was 740 metres per pixel.

    Phase Angle: 78 °

    Notes: Narrow angle, visible light, magnified x2, contrast enhanced, north is at the bottom.

    A spectacular landslide within the low-brightness region of Iapetus's surface known as Cassini Regio is visible in the image. The landslide material appears to have collapsed from a scarp 15 kilometres high that forms the rim of an ancient 600-kilometre impact basin. Unconsolidated rubble from the landslide extends halfway across a conspicuous, 120-kilometer diameter flat-floored impact crater that lies just inside the basin scarp.

    Landslides are common geological phenomena on many planetary bodies, including Earth and Mars. The appearance of this landslide on an icy satellite with low-brightness cratered terrain is reminiscent of landslide features that were observed during NASA's Galileo mission on the Jovian satellite Callisto. The fact that the Iapetus landslide travelled many kilometres from the basin scarp could indicate that the surface material is very fine-grained, and perhaps was fluffed by mechanical forces that allowed the landslide debris to flow extended distances.

    10 January 2005

    Cassini Instrument: Visual and Infrared Mapping Spectrometer

    Date: 31 December 2004

    Image Notes: The image at left shows the reflectance at 4-microns, which is dominated by the minerals on Iapetus' surface. Two large craters are seen in this image. The polar water ice is relatively dark at this wavelength, so the ice cap is not seen.

    The second frame shows carbon dioxide on the surface. The carbon dioxide peaks at mid latitudes and shows less strength at the pole and along the equator (the dark band curving near the left edge of the image).

    The third frame shows the strength of water absorption on Iapetus. The brightest regions are due to water ice near the pole. The greyer areas indicate water bound to minerals on the surface.

    The colour composite shows water as blue, carbon dioxide as green, and non-ice minerals as red.

    Cassini Instrument: Composite Infrared Spectrometer

    Date: 31 december 2004

    Image Notes: he orange asterisk marks the point on Iapetus where the Sun is directly overhead.

    Image credit: NASA/JPL/GSFC

    Temperatures reach nearly 130  K at noon on the equator on the dark material that covers most of this side of Iapetus, making noon on Iapetus's dark side probably the warmest place in the Saturn system. This is much warmer than temperatures on the moon Phoebe measured by the composite infrared spectrometer in June 2004, which peaked near 112 K. That is because, although Phoebe is almost as dark as Iapetus's dark material and absorbs nearly as much sunlight, Phoebe rotates much more quickly (once every 9 hours, compared to 79 days for Iapetus). That means the surface has less time to heat up during the day.

    Temperatures on Iapetus' bright material are much colder, peaking near 100 K, both because the bright material absorbs less sunlight and because it is further from the equator on this side of Iapetus.

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