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Planetary News: Cassini-Huygens (2007)

Cassini Zeroes in on Saturn's Yin-Yang Moon Iapetus

Click to enlarge > Iapetus in Cassini's forward view Cassini captured the images for this view of Iapetus as it approached for its closest flyby of the moon on September 3, 2007 from a distance of about 1,500,000 kilometers. The image is a color composite of images taken through infrared, green, and ultraviolet images, superimposed on another image composed of five separate exposures stacked together (to remove artifacts). The full-size image has been enlarged by a factor of three. Credit: NASA / JPL / SSI / Color composite by Emily Lakdawalla

Cassini is, at last, homing in on Saturn's two-faced moon Iapetus. The flyby, which will take place on September 10 at 14:15 UTC (07:15 PDT) at an altitude of a mere 1,644 kilometers (1,022 miles), is the only close encounter with the moon planned for the spacecraft's primary or extended missions. During the flyby, Cassini's dozen science instruments will fill its data recorders with clues to the mystery that has puzzled astronomers for more than 300 years: why is Iapetus dark on one side and bright on the other?

When Giovanni Cassini discovered Iapetus in 1671, he was puzzled by the fact that the moon, which circles Saturn in an inclined orbit that is three times farther from Saturn than Titan, could only be seen when it was on the east side of Saturn. He correctly deduced that Iapetus, like our own Moon, rotates once each orbit, so that it keeps one face continuously pointed toward Saturn; and, furthermore, that the face that points backward in its orbit (the "trailing hemisphere") is brightly reflective, while the face that points forward along its orbit (the "leading hemisphere") is dark and thus invisible to Cassini's telescope. The dark terrain is now called Cassini Regio after the moon's discoverer.

Scientists explained the bright-dark dichotomy with the hypothesis that dark material from some other place in the Saturn system -- perhaps the moons Phoebe or Hyperion -- was being swept up by Iapetus as it circled Saturn; such material would preferentially land on the leading hemisphere, just as snow is swept up on the windshield of a moving car. However, there is a problem with this hypothesis: reflecting only four percent of the sunlight that strikes it, Iapetus' dark terrain is darker than any other surface in the Saturn system. The only comparably dark body is Phoebe, with an albedo of five or six percent; but Phoebe's dark material is gray, where Iapetus' is noticeably red.

The images that Voyager returned of Iapetus deepened the mystery. If the sweeping-up-dark-stuff hypothesis were true, then Iapetus should be dark all over the leading hemisphere, and bright all over the trailing hemisphere. But although the dark stuff mostly occupies the leading hemisphere, both poles were bright. At the same time, dark material could be seen to wrap around from the leading hemisphere to the trailing hemisphere along the equator.

Cassini's namesake spacecraft arrived, of course, in 2004, and has had several distant encounters with the moon. Because Iapetus is relatively large (it is Saturn's third-largest satellite at 1,460 kilometers or 907 miles in diameter), even distant flybys can reveal volumes. The best of these occurred very early in the mission, even before Huygens landed, on December 31, 2004, when Cassini approached to within 130,000 kilometers. That flyby only added to Iapetus' mystery. Cassini's sharp images revealed that Iapetus' dark terrain was just as heavily cratered as its bright terrain, meaning that both are of extreme age; the dark material appears to be a thin coating covering a surface that is pretty much the same from the leading to the trailing hemispheres.

Click to enlarge > Saturn's moon Iapetus Iapetus is Saturn's third-largest moon, famous for its dark leading hemisphere and bright trailing hemisphere. During Cassini's December 31, 2004 flyby, Iapetus was discovered to have a "belly band" of mountains exactly on its equator. Credit: NASA / JPL / SSI

The craters are likely extremely old, but it appears that the bright-dark dichotomy is youthful or even ongoing. Not a single crater has been observed that punches through the dark material to what must be brighter ice below. Iapetus should have some craters that formed geologically recently, within the last few million or even few thousand years; the fact that not a single crater punches through the dark stuff means that any relatively recent crater that did punch through must have been repainted by the mysterious dark-stuff-emplacing process.

The bright-dark dichotomy is weird enough, but Cassini's close-up images revealed something even stranger: the dark terrain is bisected by an incredibly straight, tall, steep mountain ridge that perfectly girds the moon's equator. The ridge reaches heights of 13 kilometers (8 miles) and is only about 20 kilometers (12 miles) wide. Parts of this "belly band" had been spotted in Voyager images as a line of bright moutain peaks -- now referred to as the Voyager mountains -- jutting above the dark terrain along the equator. No other solid-surfaced body in the solar system has a self-defined equator.

Cassini's sharp images also allowed scientists to measure Iapetus' shape very exactly, and they found that the moon is bloated at the equator and squashed at the poles, exactly the shape you would expect if it were rotating once every 16 hours. But it's not; it rotates only once every 79 days. So somehow Iapetus' shape got "locked in" very early in its history, when it was still rotating rapidly, and the moon hasn't changed much since.

To date, there have been three other "Voyager-class" encounters including imaging from within a million kilometers, and the data from each encounter have produced even more mysteries. For instance, there is a color difference between Iapetus' leading and trailing hemispheres, such that the terrain on the leading hemisphere (both the bright stuff and the dark stuff) is redder than the terrain on the trailing hemisphere.

Observation Plans for Cassini's Iapetus Flyby

The Cassini spacecraft gets only one shot at a close visit to Iapetus because of the dynamical difficulty of reaching the moon. Unlike the rest of Saturn's mid-sized satellites, which orbit quickly in close circles within Saturn's ring plane, Iapetus occupies a leisurely, distant, 15-degree-inclined orbit. Many reasonably close flybys of the inner moons have come about by chance as Cassini travels on its looping path; to get close to Iapetus, the designers of Cassini's tour had to plan very far ahead, and had to use a significant portion of Cassini's precious and limited maneuvering fuel. Because of the cost (in fuel and time) of planning a flyby of Iapetus, this will be the only one throughout Cassini's primary or extended missions. So Iapetus will be hit by all the instruments in a science plan that is absolutely packed from minute to minute. In order to make the most of this flyby, Cassini will turn to Earth for a few quick transmission sessions as the flyby is taking place, freeing space on the solid-state recorders for more data.

Cassini is approaching Iapetus from its night side, so the moon appears as a crescent, with only a narrow sliver of terrain lit by the Sun. Since Cassini is approaching almost directly from Saturn, the night side of Iapetus is illuminated by Saturnshine, so it may be visible to Cassini's cameras, and Saturnshine observations will be attempted. Previous Saturnshine observations at Iapetus have worked, but they primarily focused on Iapetus' bright terrain; the night side of Iapetus visible in Cassini's forward view is mostly covered by dark terrain, so may not be visible in the dim reflected light from Saturn.

At closest approach, Cassini will zoom over Iapetus' equator and the "belly band" of mountains. Some of the most exciting images of the flyby will be captured as Cassini watches the Voyager mountains rise over the horizon.

Click to enlarge > Impact crater Ksa on Titan This 30-kilometer-diameter impact crater was spotted on Titan by the RADAR instrument during the September 7, 2006 "T17" flyby. Cassini will capture similar SAR images on Iapetus during the September 10, 2007 flyby; comparing Iapetus' cratered icy surface to Titan's may help scientists interpret the Titan SAR images. Credit: NASA / JPL

One unusual opportunity that the flyby affords is the ability to do Synthetic Aperture Radar (SAR) imaging on a body other than Titan. For most encounters with the icy moons, which orbit close to Saturn, Cassini moves too quickly for the SAR technique to work. As a result, the only icy body in the entire solar system for which SAR images have been acquired is Titan. But because Iapetus is far from Saturn, both the moon and the spacecraft will be moving relatively slowly during the encounter, permitting the use of the SAR technique. Underneath its clouds, Titan is an icy satellite, so comparison of Titan SAR images with those that Cassini will get at Iapetus -- in combination with Cassini's camera images of Iapetus -- may help the RADAR team understand better the properties of the surface of Titan, and may even help the Cassini imaging team interpret their Titan images.

There is another advantage to SAR imaging: the technique requires no sunlight. Sadly, because of Iapetus' slow 79-day rotation, Cassini will see only about half of Iapetus's surface in sunlight. There are plans to attempt images on Iapetus' night side taken by the reflected light of Saturn, but it is not certain that they will work. The first data from the approach phase of the encounter will be downlinked on September 9. The majority of the data will be downlinked in the early hours of the morning UTC on September 11, while the final data should reach Earth midday, UTC, on September 12.

The following table gives detailed information on the timing of observations planned throughout Cassini's one and only close flyby of Iapetus. For even more information, visit a special page on the flyby composed by imaging team associate Tilmann Denk of the Freie Universität Berlin.

Click to enlarge > The view from Iapetus On September 9, 2007 at 02:10 UTC, Cassini will capture a huge portrait of the Saturn system: a 5-frame wide-angle view of the planet, plus narrow-angle color views of Hyperion, Titan, Tethys, Mimas, Rhea, Enceladus, and Dione; and finally a huge, 30-panel, 3-color, narrow-angle mosaic on the rings. Credit: NASA / JPL / Freie Universität Berlin

Animated map of Cassini's observations 1: far approach images Only a tiny sliver of Iapetus' surface is both Sunlit and visible to Cassini during the approach phase of the encounter. Throughout September 9, most images will show a thin crescent, with a few longer exposures taken in Saturnshine on Iapetus' night side. How to read these plots: The map shows the entire globe of Iapetus in a simple cylindrical projection. The blue line represents the terminator or night/day boundary. Everything north of the blue line is Sunlit, while everything south of the blue line is in shadow. The yellow line represents the limb of Iapetus, Cassini's horizon. Terrain on the map that is shown in high contrast is both Sunlit and on the side of Iapetus that is visible to Cassini. Terrain that is shown at low contrast is in the dark, but still may be visible to Cassini by the dim light of Saturnshine. Green boxes represent the fields of view of the narrow-angle and wide-angle cameras as they photograph Iapetus' surface. The green boxes will only appear on the side of Iapetus that is visible to Cassini. The map titles indicate the coded name of each planned observation, which are also given in the tables above. Credit: NASA / JPL / Freie Universität Berlin / Steve Albers

Animated map of Cassini's observations 2: just before closest approach Near closest approach, Cassini snaps photo after photo over the dark terrain on the leading hemisphere and the "belly band" of mountains as it begins to pass from the moon's night side to its day side. The horizon (yellow line) closes in as Cassini zooms toward Iapetus' surface. How to read these plots: The map shows the entire globe of Iapetus in a simple cylindrical projection. The blue line represents the terminator or night/day boundary. Everything north of the blue line is Sunlit, while everything south of the blue line is in shadow. The yellow line represents the limb of Iapetus, Cassini's horizon. Terrain on the map that is shown in high contrast is both Sunlit and on the side of Iapetus that is visible to Cassini. Terrain that is shown at low contrast is in the dark, but still may be visible to Cassini by the dim light of Saturnshine. Green boxes represent the fields of view of the narrow-angle and wide-angle cameras as they photograph Iapetus' surface. The green boxes will only appear on the side of Iapetus that is visible to Cassini. The map titles indicate the coded name of each planned observation, which are also given in the tables above. Credit: NASA / JPL / Freie Universität Berlin / Steve Albers

Cassini ISS Iapetus observations, 3: just after closest approach Just after closest approach, Cassini is moving rapidly across Iapetus' surface to the sunlit side, grabbing high-resolution images across the "belly band" of mountains and complicated boundary between the dark terrain on the leading hemisphere and the bright terrain on the trailing hemisphere. How to read these plots: The map shows the entire globe of Iapetus in a simple cylindrical projection. The blue line represents the terminator or night/day boundary. Everything north of the blue line is Sunlit, while everything south of the blue line is in shadow. The yellow line represents the limb of Iapetus, Cassini's horizon. Terrain on the map that is shown in high contrast is both Sunlit and on the side of Iapetus that is visible to Cassini. Terrain that is shown at low contrast is in the dark, but still may be visible to Cassini by the dim light of Saturnshine. Green boxes represent the fields of view of the narrow-angle and wide-angle cameras as they photograph Iapetus' surface. The green boxes will only appear on the side of Iapetus that is visible to Cassini. The map titles indicate the coded name of each planned observation, which are also given in the tables above. Credit: NASA / JPL / Freie Universität Berlin / Steve Albers

Cassini ISS Iapetus observations, 4: outbound As Cassini recedes from Iapetus, it captures a huge multicolor mosaic on the nearly-fully-lit moon, and continues snapping images as the moon diminishes in the distance. How to read these plots: The map shows the entire globe of Iapetus in a simple cylindrical projection. The blue line represents the terminator or night/day boundary. Everything north of the blue line is Sunlit, while everything south of the blue line is in shadow. The yellow line represents the limb of Iapetus, Cassini's horizon. Terrain on the map that is shown in high contrast is both Sunlit and on the side of Iapetus that is visible to Cassini. Terrain that is shown at low contrast is in the dark, but still may be visible to Cassini by the dim light of Saturnshine. Green boxes represent the fields of view of the narrow-angle and wide-angle cameras as they photograph Iapetus' surface. The green boxes will only appear on the side of Iapetus that is visible to Cassini. The map titles indicate the coded name of each planned observation. Credit: NASA / JPL / Freie Universität Berlin / Steve Albers