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Iapetus
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| Iapetus from Cassini, 2004. This mosaic image was obtained with the spacecraft's narrow angle camera on Dec. 31, 2004, at a distance of about 172,400 km (107,124 miles) from the moon and at a Sun-Iapetus-spacecraft, or phase, angle of 50 degrees. The image scale is 1 km (0.6 miles) per pixel. The image has been contrast enhanced to aid visibility of surface features.
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The great ridge of Iapetus
In December 2004, the Cassini spacecraft discovered a further extraordinary feature of Iapetus: a topographic ridge that coincides almost exactly with the geographic equator. The ridge is conspicuous in the top image on this page as an approximately 20-km wide (12 miles) band that extends from the western (left) side of the disk almost to the day/night boundary on the right. On the left horizon, the peak of the ridge reaches at least 13 km (8 miles) above the surrounding terrain. Along the roughly 1,300 km (800 mile) length over which it can be traced in this image, 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.
Light and dark regions
The scene in the top image is dominated by the dark, heavily-cratered region, called Cassini Regio, that covers nearly an entire hemisphere of Iapetus. The view is centered on the moon’s equator and on roughly 90° west longitude – a location that always faces the direction of Iapetus's orbital motion around Saturn. Within Cassini Regio, and especially near the equator, dark deposits with a visual reflectivity of only about 4 percent coat nearly everything with remarkable uniformity. However, at latitudes of about 40°, the surface transitions to a much brighter, icy terrain near the pole where the brightest icy materials have reflectivity over 60 percent. However, this region is not uniform: Close inspection reveals that the surface is stained by crudely north-south trending wispy streaks of darker material, typically a few kilometers wide and sometimes tens of kilometers long. An ancient, 400-km wide (250 miles) impact basin appears just above the center of the disk. 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 (i.e. toward the pole). Often, the opposite south-facing scarps are stained with the lower-brightness material.
The origin of Cassini Regio is a long-standing debate among scientists. According to one theory, the dark material may have erupted onto Iapetus's icy surface from the interior. Another theory holds that the dark coating consists of debris that was ejected by impact events on dark, outer satellites of Saturn, such as Phoebe, and then swept up by Iapetus as it moved in its orbit. 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.
Far-out speculation
The extreme albedo range displayed by Iapetus has prompted the suggestion that the brightness variations might be artificial. For example, Donald Goldsmith and Tobias Owen in The Search for Life in the Universe1 (1980) wrote:
This unusual moon is the only object in the Solar System which we might seriously regard as an alien signpost - a natural object deliberately modified by an advanced civilization to attract our attention ...By coincidence, Iapetus is the site of the "Star Gate" in Arthur C. Clarke's 2001: A Space Odyssey.2
Cassini close flyby of 2007
On September 10, 2007, Cassini flew past Iapetus at a distance from the surface of only 1,640 kilometers (1,000 miles) – almost 100 times closer than the 2004 flyby. It sent back startling images, some of which are shown below, that promise to revolutionize our knowledge of this strange moon.
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| Mountainous terrain that reaches about 10 km (6 miles) high along the unique equatorial ridge of Iapetus. Above the middle of the image can be seen a place where an impact has exposed the bright ice beneath the dark overlying material. The image was taken on Sept. 10, 2007, with Cassini's narrow-angle camera at a distance of approximately 3,870 km (2,400 miles) from Iapetus. Image scale is 23 meters (75 feet) per pixel.
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| Cassini close-up of the patchy, bright and dark mountains originally identified in images from Voyager taken more than 25 years earlier. The terrain seen here is located on the equator of Iapetus at approximately 199° west longitude, in the transition region between the moon's bright and dark hemispheres. North is up. The image was taken on Sept. 10, 2007, with Cassini's narrow-angle camera at a distance of approximately 9,240 km (5,740 miles) from Iapetus. Image scale is 55 m (180 ft) per pixel.
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| Dark material splatters the walls and floors of craters in the surreal, frozen wastelands of Iapetus. This image shows terrain in the transition region between the moon's dark leading hemisphere and its bright trailing hemisphere. The image was taken on Sept. 10, 2007, with Cassini's narrow-angle camera at a distance of approximately 6,030 km (3,750 miles) from Iapetus. Image scale is 36 me (118 ft) per pixel.
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| Cassini surveys a bright landscape coated by dark material on Iapetus. This image shows terrain in the transition region between the moon's dark leading hemisphere and its bright trailing hemisphere. The image was taken on Sept. 10, 2007, with Cassini's narrow-angle camera at a distance of approximately 5,260 km (3,270 miles) from Iapetus. Image scale is 32 m (105 ft) per pixel.
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References
- Goldsmith, Donald, and Owen, Tobias. The Search for Life in the Universe. Menlo Park, Calif.: Benjamin/Cummings (1980).
- Clarke, Arthur C. 2001: A Space Odyssey. New York: New American Library (1968).
Related entry
• Saturn, moons
Related category
• PLANETS AND MOONS
Archived news
Have we cracked Saturn's walnut? (May 14, 2005)
Source: NASA/JPL
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