ESA Science & Technology04-Jul-2005 14:22:59

Saturn's Moons


III - Discovered by Cassini (1684)

Physical Parameters

Orbital Parameters

Radius (km)


Radius (km)

294 660

Mean Density (kgm-3)



1d 21h 19m

Mass (kg)

6.27 x 1020

Velocity (kms-1)


Magnitude (V0)


Rotational Period

1d 21h 19m

Escape Velocty (kms-1)




Mean Surface Albedo


Inclination ( °)


Knowledge before Cassini-Huygens

Thethys is a heavily cratered moon - including the 400 km diameter Odysseus crater. This crater is of some interest as the floor appears to be relatively shallow, suggesting some kind of flow process (possibly ice) has occurred to partially fill the crater since its formation.

The other major feature on the moon's surface is Ithaca Chasma, a 2000 km long valley that spans upto three-quarters of the surface of the planet. It is typically 100 km wide and a few kilometres deep. There are currently two competing ideas for its formation: it formed as a surface fracture as a result of the impact which formed Odysseus; or it formed when liquid water expanded in the core as it froze.

Tethys also has two companions in its orbit - Telesto and Calypso. These are small moons, roughly 20 km in diameter, that orbit at Lagrange points 60 ° ahead and behind the position of Tethys. [Note: these orbital positions are highly stable and are the same orbital points that the Trojan asteroids occupy around Jupiter's orbit]

*Note: in the following section all images courtesy NASA/JPL/Space Science Institute unless stated otherwise.

21 July 2004

Cassini Instrument: Imaging Science Subsystem

Date: 03 July 2004

Distance: 1.7 million km

Phase Angle: 97 °

Image Notes: Narrow angle camera, visible light, neither Odysseus or Ithaca Chasma can be seen

3 August 2004

Cassini Instrument: Imaging Science Subsystem

Date: 13 July 2004

Distance: 4.8 million km

Scale: 29 km per pixel

Phase Angle: 97 °

Images Notes:  Narrow angle, visible light, magnified x2, large crater with central peak visible lower right

11 November 2004

Cassini Instrument: Imaging Science Subsystem

20 July, 09 September and 23 September 2004

Distances: 6.1 million km, 8.8 million km and 7.9 million km

Scales: 37, 53 and 48 km per pixel

Phase Angles: 92 °, 81 °, 80 °    

Image Notes: Narrow angle,  magnified x4, images 2 and contrast-enhanced.    A large crater is visible in first and thrid image.

23 November 2004

Cassini Instrument: Imaging Science Subsystem

Date: 28 October 2004

Distance: 256 000 km

Scale: 1.5 km per pixel

Phase Angle: 50 °

Image Notes: Narrow angle, filtered image in the red, green and blue .

Having passed closer to Tethys than the Voyager 2 spacecraft, Cassini has returned the best-ever natural colour view of this icy moon. As seen here, the battered surface of Tethys has a neutral hue. The image is a mosaic of two footprints.

The result reveals a world  awash with craters - many small craters lie on top of older, larger ones, suggesting an ancient surface. At the top and along the boundary between day and night, the moon's terrain has a grooved appearance.

17 December 2004

Cassini Instrument: Imaging Science Subsystem

Date: 15 December 2004

Distance: 560 000 km

Scale: 3 km per pixel

Phase Angle: 91 °

Image Notes: Narrow angle, visible light, Ithaca Chasma visibile lower left

This view of Tethys shows principally the Saturn-facing hemisphere of the moon. In the centre of the image part of the tremendous rift called Ithaca Chasma can be seen. It is 100 kilometres wide in places, and runs nearly three-quarters of the way around the icy moon. Adjacent to the great Chasma is a large multi-ring impact basin with a diameter of about 300 kilometres. The inner ring of the basin is about 130 kilometres in diameter.

For further information please contact: