The Icy Satellites

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Uranus' Satellites

 The Icy Satellites Uranus' Satellites Miranda

Distance to Uranus129 850 km
Period of revolution1.41 earth days
Diameter472 km
Mass (Earth=1)0.0000105
Density (water =1)1.14
CompositionIce water and silicates
Temperature on the surface-187°C
Miranda was discovered in 1948 by G. Kuiper and presents a differentiated geology among satellites of the solar system. The only images at our disposal are those of the probe Voyager 2. Miranda has three big crowns that are more recent than the more cratered terrain which contributes to the vast majority of the surface. The crowns have a oval or trapezium shape. Their outer belt is made up of parallel crests and can reach a depth of 20 km. The internal area is made up of smooth material and/or crests and dips of varying orientations.
  • Arden Corona: this is the largest, it has an oval shape and a diameter of about 300 km!

  • Inverness Corona: it has a trapezium shape and is located on the south pole of Miranda.

  • Elsinore Corona: this one has an oval shape.

The origins of these peculiar geological formations on Miranda are still a mystery today. Two hypotheses have been emitted:
  • The first solution proposed was that Miranda was once broken in the course of an impact with an other object soon after formation, to then be reassembled later. During the last stage of the reformation, the pieces, rich in silicates slid under the mantel, rich in ice and less dense. The crowns were then formed in response to the compression of the lithosphère located above it.

    This reconstruction of the satellites could even have happened more than once. One argument in favour of this hypothesis is the proximity of Miranda to Uranus, which makes this satellites more exposed than the others to impacts that the bigger satellites. Indeed, bombardment from meteorites is more intense close to the planet due to gravitational attraction. Nevertheless, it is possible that Ariel, Umbriel and Titania also experienced periods of reconstruction.

  • The second solution is less spectacular and seems more in line with the morphology of the crests and crevices. The crowns could be a consequence of internal activity in the satellite. Just like on Enceladus, it is possible that, early in Miranda's history, the less dense materials rose up towards the surface (a phenomenon called diapirism, made possible by dissipation of heat due to tides), which would have caused and extension of the surface leading to the formation of crowns.
It is impossible with our present understanding to make an absolute choice between the two hypotheses. But the second one seems far more likely... Given that no mission is scheduled or in operation around Uranus it will be a long wait before this strange world comes to light.

Figures 1 to 3 (credit: JPL/NASA)

1) Image of Voyager 2: we can distinguish three crowns (Arden,Inverness et Alsinore)
The South Pole is located in the Inverness crown.

2) Details of Inverness Corona. Cresta and troughs are visible,
brihter and draker terrains are too
Image by Voyager 2

3) details of the Inverness crowns
(bottom left) and Elsinore (top right)
Image by Voyager 2

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