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While observing the heavens with his telescope one night in 1671, the Italian-born French astronomer Giovanni Cassini detected a faint star-like object to the west of Saturn. Suspecting it might be a new moon of the ringed planet, Cassini kept the object under observation - or at least tried to. For as the object moved to the east of the planet, it disappeared, only to reappear again as it came round to the other side.
Cassini concluded that the object was indeed a moon of Saturn, which he named Iapetus, after one of the titans of Greek mythology. As for its strange disappearing act, he suggested: "One part of [Iapetus's] surface is not so capable of reflecting to us the light of the sun which maketh it visible as the other part is."

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Title: Probing the origin of the dark material on Iapetus
Authors: F. Tosi, D. Turrini, A. Coradini, the VIMS Team

Among the icy satellites of Saturn, Iapetus shows a striking dichotomy between its leading and trailing hemispheres, the former being significantly darker than the latter. Thanks to the VIMS imaging spectrometer onboard Cassini, it is now possible to investigate the spectral features of the satellites in Saturn system within a wider spectral range and with an enhanced accuracy than with the previously available data. Statistical techniques such as the clustering methods are powerful tools for exploring the spectral data of planetary and satellite surfaces since they allow to identify spectral units between the data of a single celestial body in correlation with the surface geology and to emphasize correlations among the spectra of different bodies. In this work, we present an application of the G-mode method to the VIMS visible and infrared spectra of Phoebe, Iapetus and Hyperion, in order to search for compositional correlations. We also present the results of a dynamical study on the efficiency of Iapetus in capturing putative dust grains travelling inward in Saturn system with the aim of evaluating the viability of Poynting-Robertson drag as the physical mechanism transferring the dark material to the satellite. The results of spectoscopic classification are used jointly with the ones of the dynamical study to describe a plausible physical scenario for the origin of Iapetus' dichotomy.

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PIA11116-br500.jpg
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Credit NASA/JPL/Space Science Institute

This global map of Saturn's moon Iapetus was created using images taken during Cassini spacecraft flybys, with Voyager images filling in the gaps in Cassini's coverage.

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PIA11115-br500.jpg
Expand (107kb, 920 x 700)
Credit NASA/JPL/Space Science Institute

Presented here is a complete set of cartographic map sheets from a high-resolution Iapetus atlas, a project of the Cassini Imaging Team.
The map sheets form a three-quadrangle series covering the entire surface of Iapetus. As noted on the map, while both Saragossa Terra and Roncevaux Terra are bright regions on the moon's surface, they are distinct from each other in that the former has a slightly reddish colour and the latter does not.

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The IAU has approved names for 39 craters, 9 montes, and 1 terra on Iapetus.

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This global map of Iapetus was created using images taken during Cassini spacecraft flybys, with Voyager images filling in the poles.

Iapetus
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Expand (2.64mb, 7748 x 4250 )
Credit: NASA/JPL/Space Science Institute

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Cassini sails low over the surface of Iapetus on approach to its close encounter with the enigmatic moon on Sept. 10, 2007.


Its flight takes it over the rugged, mountainous ridge along the moon's equator, where ancient, impact battered peaks -- some topping 10 kilometres in height -- are seen rising over the horizon and slipping beneath the spacecraft as it flies. Frames used in this movie were acquired with the Cassini wide-angle camera on Sept. 10, 2007, as the intrepid robot soared past Iapetus, within a few thousand kilometres of the surface. Additional simulated images were inserted between the Cassini images in this movie in order to smooth the appearance of the movement, a scheme called interpolation.

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This mosaic view looks out onto an area close to the northern bright/dark boundary, within the dark region called Cassini Regio on Iapetus.
Near upper left is a large crater with terraced walls, a mostly flat floor and a prominent group of peaks in its centre. The sharp features make this likely one of the youngest craters in this area of Iapetus.

IAP_e1
Credit: NASA/JPL/Space Science Institute

The mosaic consists of three clear spectral filter images centred near 43.3 degrees north latitude, 138 degrees west longitude.
The images were obtained with the Cassini spacecraft narrow-angle camera on Sept. 10, 2007, when it was approximately 13,500 kilometres from Iapetus and at a sun-Iapetus-spacecraft, or phase, angle of 139 degrees.

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Speckled Surface
Speckled Surface
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Spotty Iapetus
Spotty Iapetus
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A Complex Transition
A Complex Transition
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The Transition Zone
The Transition Zone
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Towering Peaks of Iapetus
Towering Peaks of Iapetus
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Closest View of Iapetus
Closest View of Iapetus
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A Scene of Craters
A Scene of Craters
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Approaching Iapetus
Approaching Iapetus

Credit NASA/JPL/GSFC/SwRI/SSI

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