The Icy Satellites

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Saturn's satellites

 The icy satellites Saturn's satellites Dione

Large shiny fractures
Dione is the fourth furthest satellite from Saturn and the fourth largest. It is in resonance of ratio 1:2 with Enceladus, which means that it does one revolution around Saturn while Enceladus does two. Its principal characteristic is the combination of the shiny lines that go across its surface. (image 1). This comes from several generations of fractured tectonics, that are younger than the cratered regions that constitute the rest of the surface, since we can see that they pass through some craters. There are also some fractures that don't shine, because they are older (image 2). Craters have shiny slopes and dark deposits at their bottom. (images 3).

There is a big crater that is called Amata, its diameter is of about 250 km; it is located at the point convergence of the several shiny fractures and so is deformed and so is hardly detectable The shiny fractures are proof of a recent geological activity, but since they are not around Amata, it is possible that this impact made the surface more fragile so that the tectonic movements could have happened. There also has been a theory saying that this crater of impact is actually an illusion caused by the entanglement of the faults that would have an almost circular shape. But the presence of fields with only a few craters at the antipodes favours the theory about real impacts. Indeed the major impacts have consequences on the aspect of the surface of the affected body (view Tethys and the odyssey crater for example).

Distance to Saturn377 400 km
Period of revolution2.74 terrestrial days
Diameter1120 km
Mass (Terre=1)0.0001758
Density (water =1)1.43
Composition2/3 of ice and
1/3 of rock/silicates
Temperature on the surface-186 °C
Other big craters are easier to see (image 2). In short Dione has two very different faces:
  • A cratered face in which the albedo is quite high. The density of the craters is variable.
  • A face in average darker but with shiny faults that go across it.

If we look at the surface on a smaller scale, for example on image 3a, we can observe the superposition of different generations of fault lines. On image 3b, in addition to its shiny fractures, the surface is noticeably striped by very thin parallel lines (their direction is shown in red), a bit like if the surface had been 'raked'. They could be the consequence of an impact or the one of a geologically active history for Dione. In a certain way they remind us of the parallel line that we find at the surface of Phobo's, Mars' biggest satellite.

Intern structure and composition :
Dione is Saturn's most dense satellite (apart from Titan). This density suggests an internal structure giving of a central of rock representing a third of the total mass. The rest of the mass is mainly composed of icy water.

It is also easier to justify an internal activity if we consider that the structure is differentiated, since the heat needed for this activity can then be supplied by the radioactive disintegration inside the rocky core. In Dione's case, taking its orbital resonance with Enceladus into account, the internal heat is also supplied by tidal dissipation.

Figures 1 to 4(credit: JPL/NASA)

2) Cratered surface and non-shiny fractures
3) Phobos', Mars' satellite, and its parallel strips
4a) resolution of 230 m/pixel 4b) resolution of 23 m/pixel

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