ESA Science & Technology04-Jul-2005 14:23:25

Saturn's Moons


IV - Discovered by Cassini (1684)

Physical Parameters

Orbital Parameters

Radius (km)


Radius (km)

377 400

Mean Density (kgm-3)



2d 17h 41m

Mass (kg)

1.05 x 1021

Velocity (kms-1)


Magnitude (V0)


Rotational Period

2d 17h 41m

Escape Velocty (kms-1)




Mean Surface Albedo


Inclination ( °)


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 2004

Cassini Instrument: Imaging Science Subsystem

Date: 02 July 2004

Distance: 1.4 million km

Scale: 8 km per pixel

Phase Angle: 119

Image Notes: Narrow angle, magnified x2, two processed versions of the same image.

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 2004

Cassini Instrument: Imaging Science Subsystem

Dates: 19 July, 15 September and 28 September 2004

Distances: 6.2, 8.8 and 7.3 million km

Scales: 37, 53 and 44 km per pixel

Phase Angles: 96, 83 and 79 °

Image Notes:   Narrow angle, visible light, all images magnified x4,   and the middle image contrast enhanced.

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 2004

Cassini Instrument: Imaging Science Subsystem

Date: 27 October 2004

Distance: 1.2 million km

Scale: 3.5 km per pixel

Phase Angle:  28 °    

Image Notes: Narrow angle, visible light

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 2004

Cassini Instrument: Imaging Science Subsystem

Date: 14 December 2004

Image Notes: Image  centred on wispy terrain which is  not thought to hold  thick ice deposits, but rather the bright ice cliffs are created by tectonic fractures. The surface is also heavily cratered.

Date: 14 December 2004

Scale: 0.9 km per pixel

Phase Angle: 34 °

Image Notes: Narrow angle, five composite images, anti-Saturn side

Date: 14 December 2004

Distance: ~ 156 000 km

Scale: 1 km per pixel

Phase Angle: 34 °

Image Notes: Narrow angle, visible  light

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