The Icy Satellites

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Saturns satellites

 The icy satellites  Saturns satellitesTethys

Key fig.1
Tethys' cratered surface, Ithaca Chasma flaw and different albedo
Tethys' surface can be devided into 4 parts:
  • The heavily cratered areas: this is the oldest part of the surface. Most of the craters have lost their original shape. Some parts of these cratered fields are damaged by the flaws. (view image 1)

  • The Odyssey crater : It is an enormous crater with a diameter of 400 km, with a central peak (image 2). It is so high that on a few images, we can notice a slight flattening of the satellites shape (image 4). This flattening has been reduced since the birth of the crater by the movement of the mass inside de satellite. The crater contains steep hooked slopes. A very small fraction of the material ejected during this impact is placed in an orbit around Saturn close to Tethys' before being dragged back to the satellite, creating secondary impacts in this way.

Distance to Saturn294 660 km
Period of revolution1.89 terrestrial days
Diameter1060 km
Mass (Earth=1)0.0001041
Density (water =1)1
Compositionicy water predominantly and rock
Temperature on the surface-188 °C
  • The falts : The Ithaca Chasma structure takes about 5 to 10 % of Tethys' surface area. It's a system of Canyons of about 100 km wide by 4 km deep at some points and that travels more than 1000 km from the North to the South. These canyons are nearly concentric around the Odyssey crater. The edges of these canyons have been heavily damaged by more recent impacts than the ones that have created the cratered areas. The canyons are therefore very old but younger than the heavily cratered zones. A big crater, Telemachus, even covers this same canyon. This crater is therefore relatively recent. (view images 1,5 and 7)

Other smaller faults are also more visible on more or less the whole surface.

To explain the presence of this flaw, we have two possibilities: either they are the consequence of the Odyssey impact, or they have appeared during an expansion of the whole satellite due to the cooling that has followed its creation.

  • The plains : They actually aren't really plains but regions, less cratered, located at the antipodes of Odyssey (view image 5). These regions should show the trail of the seismic activity following to the impact in the form of a mountain range and fractures (like the region located at the antipodes of the Caloris basin on Mercury). But it is possible that this region has been through a renewal of its surface that would have cleared the trails. A flow of material in direction of the surface would have been possible by a weakening of the surface layer because of the impact.
Other important craters are also noticeable on Cassini's images : The Penelope and Melanthius craters (view image 6). This one does not have a central peak but a small mountain range (view image 5).

Changing the images like we have done with Mimas has permitted us to obtain the images in artificial colours that show that the materials at the surface are slightly different in composition and texture. This difference on the images 3 and 4 in artificial colours coincide with the variations in albedo that we can observe on image 1.

Internal structure and composition:
Tethys' shape has been determined with the help of photographs from the probes Voyager 1 and 2. The shape of the satellite is represented at best by a tri-axial ellipsoid. The average diameter is then of 1060 km and the difference between the biggest and the smallest axis is of about 10 km. This difference allows us to establish whether a body is differentiated or not, in comparison with theoretical values. In Tethys' case, this would indicate a concentration of mass in the centre, in the core of rock. But since Tethys' density is close by to the one of icy water, this core must be small (10 % of the total mass if we consider that Tethys doesn't present any porosity). The core is covered by a mantle of icy water.

Figures 1 to 7(credit: JPL/NASA)

2)Odyssey crater (at the top) Melanthius crater (at the bottom)
3) Odyssey crater
4) black and white images and in artificial colours of the satellites face turned toward Saturn Penelope crater on the left, Melanthius crater at the top and flattening of the limb at the top right
5) black and white and in artificial colours images: Ithaca Chasma and Telemachus crater at the top, Penelope crater and plains at the bottom
6)Melanthius crater
7) Ithaca Chasma: details

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