The Icy Satellites

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Saturn's satellites

 Icy satellites Saturn's satellitesRhea

Distance to Saturn527 040 km
Period of revolution4.52 terrestrial days
Diameter1530 km
Mass (Earth=1)0.0004168
Density (water =1)1.33
CompositionIcy water and silicates
Temperature of surface-200 °C
Rhea is Saturn's second largest satellite and its fifth furthest away. Its surface has common with Dione.

's. Rheas surface has several characteristics that will tell us its age. The biggest crater is named Tarawa and has a diameter of 360km. Tarawa is a very old crater since it has smaller ones inside it. Another big crater is distinguishable by its shiny mark in a star shape that comes from the impact of materials that were ejected. These materials are still shiny, it is a more recent crater than the others. (view images 1)

The Cassini probe has permitted the photographing in detail of its surface, and to obtain images in artificial colours that clearly show the difference between some regions of the surface. (view page on Mimas for the explanation on the images in fake colour).

image 2 shows us a difference between the Northern and the Southern hemisphere while image 3 permits to point out the shiny trails on the surface. These trails would be the proof of the existence of tectonics in the past.
Internal structure and composition:
The low density of Rhea tells us that this satellite's mass is composed 75 % of ice water and 25 % of rocks. Iron could also be present but then in very small quantity.

Knowing this information, it very unlikely that the structure is separated in a metallic core, a mantle of rock and an external layer of ice since the quantity of iron is small. It is more logical to consider a core of rock with an icy mantle or a non-differentiated structure. Indeed the measure of the moment of inertia (0.391) indicates us that the body is only a bit differentiated or even homogeneous.

Figures 1 to 3(credit: JPL/NASA)

1) Tarawa crater (diameter of 360 km) and shiny trails due to a crater
2) Contrast North - South in the image in fake colours
3) The shiny furrows are put in evidence in the image in fake colours

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