The Mountains of Saturn's Mysterious Moon Iapetus
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Saturn's moon Iapetus has mystified observers for centuries because of its hemispheric albedo dichotomy: a bright side and a dark side. One side, or hemisphere, is extremely dark and reflects very little light, while the other hemisphere is quite bright and reflects light from the surface. A comparable brightness difference is seen nowhere else in the solar system. Observations from the Cassini spacecraft, in orbit around Saturn since July 2004, have found another Iapetus mystery: during the flyby of December 2004, Cassini cameras discovered a ridge or chain of mountains that traces Iapetus' equator and is over at least 1300 km (~ 800 miles) high, within the dark hemisphere known as Cassini Regio. The highest peak rises ~20 km (12 miles) above the surroundings. The ridge itself appears to be located on a broader bulge, extending a few hundred km to the north and south. Some of the highest mountains of the ridge are comparable to Olympus Mons on Mars, which is surprising, since Iapetus is about 5 times smaller than Mars. Such an equatorial ridge has not been observed anywhere else in the solar system. The fact that the mountainous ridge occurs precisely along the equator gives Iapetus a walnut-like appearance, or as if the moon has a seam as a result of the two hemispheres being sewn together. The formation mechanism of this enigmatic "bellyband" is currently under investigation. One possibility is that the ridge is a compressional feature, a mountain belt that has folded upward. Another idea is that a crack formed, as a result of extension in the surface, and subsequently material from inside Iapetus erupted onto the surface and accumulated in the immediate vicinity, forming the ridge.
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The bellyband is seen primarily in the dark terrain of Iapetus, but seems to extend into the bright terrain on the anti-Saturnian hemisphere, where bright individual mountains are apparent. These bright mountains were initially seen by the Voyager spacecraft, at lower resolution than subsequently observed by Cassini. It is interesting how clean and bright the mountains look, when they appear so close to the dark terrain.
What is the source of Iapetus' hemispheric brightness asymmetry? Is it related to the equatorial ridge? The origin of Cassini Regio is a long-standing debate among scientists. According to one theory, the dark material has accumulated as a result of debris that has been ejected by impact events on the moon Phoebe, or on the dark satellites of Saturn that orbit outside Phoebe's orbit. Some Cassini measurements may support this idea: the dark material appears to coat the surface uniformly at low latitudes and may become thinner at higher latitudes. Dark wispy streaks near the bright-dark boundary and high latitudes are consistent with predictions of patterns of exogenic emplacement (deposits from space). Many large, evidently old impact craters are present in the dark terrain, and do not seem to have been disturbed as might be expected if the dark material were endogenically derived (from within Iapetus).
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Another theory suggests that the dark material erupted onto Iapetus' icy surface from the interior. The presence of few dark-floored craters on the bright side of Iapetus, first observed by Voyager, may be consistent with this idea. Though evidence from Cassini images may seem to rule out this idea of an endogenic source for the dark material, and no obvious volcanic landforms are apparent, the newly-discovered bizarre equatorial ridge leads some scientists to wonder whether there is a connection between the ridge and the dark terrain. The dark terrain of Cassini Regio may have had its origin in eruptions of material, which may have occurred in conjunction with the creation of the equatorial ridge. Such eruptions may have resulted in the accumulation of dark particulate materials on the surface as fallout.
Upcoming Cassini observations may help to resolve the Iapetus mysteries. Cassini's best Iapetus flyby will occur on September 10, 2007. The flyby distance is currently planned for ~1500 km (~ 900 miles), that's close! The December 2004 flyby, was at 123,000 km (~ 76,000 miles). The observations near closest-approach will cover regions not previously seen by Cassini at high resolution - excellent view of the mountains in the bright-dark boundary on the anti-Saturnian hemisphere will be available. The extent of the mountainous ridge into the bright hemisphere will be evaluated for the first time. Flyby activities include surface measurements by cameras and spectrometers, plus a stellar occultation to assess the possibility of a thin atmosphere. RADAR measurements of both hemispheres will take place and will be valuable to understanding surface composition. The increased resolution of the September 2007 observations may shed light on the whether or not the equatorial mountainous ridge on Iapetus has been volcanically active in the past.
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