|ESA Science & Technology||04-Jul-2005 14:23:25|
Knowledge before Cassini-Huygens
Dione has the highest known density of the inner icy moons, suggesting that it contains a higher proportion of rocky materials in its core. Because of this Dione has a higher level of internal radioactive heat compared with the other moons. Dione also has a companion moon, Helene, located at the leading Lagrange point 60 ° further along the orbital path.
The surface of Dione displays a range of features: faults, valleys and depression - along with the usual cratering. This seemingly more active surface might be as a result of the increased heat from the core. The surface fractures are thought to be due to outgassing of water and possibly methane.
Dione, like many moons of Saturn, has an orbtial period that matches the rotational period so that the same face of the moon always faces the planet. This has given rise to some interesting variations in the surface. The Saturn facing side is more extensively cratered, including the large crater Dido (118 km in diameter) and contains several long running faults: including Tibur Chasmata (156 km long) and Palatine Chasma (394 km long). The away facing side is covered in dark material and bright streaks including Padua Linea measuring 780 km in length.
*Note: in the following section all images courtesy NASA/JPL/Space Science Institute unless stated otherwise.
19 July 2004Cassini Instrument: Imaging Science Subsystem
The view at left, with only mild enhancement, shows a crescent with large craters visible. The version at right has been greatly contrast enhanced to show the side of Dione lit faintly by reflected light from Saturn. A similar phenomenon can be seen from Earth, when the Moon's dark side is visible due to earthshine. The crater at the top of the image appears to have a sunlit central peak in the enhanced view - a common characteristic of craters on Dione as seen in Voyager images. Slight variations in brightness on the moon's dark side hint at the bright curvilinear streaks seen by Voyager. These streaks are thought to be deposits of water ice.
04 November 2004Cassini Instrument: Imaging Science Subsystem
Some of the bright wispy streaks that cover much of Dione's trailing hemisphere can be seen in these images. The streaks are thought to be deposits of icy material that has been extruded onto the moon's surface from the interior.
25 November 2004Cassini Instrument: Imaging Science Subsystem
Apart from the wispy features subtle variations in brightness across the surface of Dione are visible. The image shows primarily the trailing hemisphere of Dione, which is the side opposite the moon's direction of motion in its orbit. The image has been rotated so that north is up.
16 December 2004Cassini Instrument: Imaging Science Subsystem
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