The Icy Satellites

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Uranus' Satellites

 The Icy Satellites Uranus' SatellitesAriel
Ariel :
The discovery of Ariel, just like that of Umbriel, was credited to William Lassel, in 1851. This satellite has a similar composition to that of the other big satellites around Uranus, that is 40 à 50 % ice water and the rest being rock. The colour of the surface, grey, is also similar to other satellites but Ariel is the satellite with the greatest albedo effect: 0.25 a 0.30 on average. Some regions are particularly reflecting and reach an albedo of 0.45; these regions are actually made up of deposits from the interior of Ariel that were ejected in meteorite impacts, like some areas of our Moon.
Image from Voyager 2
Distance to Uranus190 930 km
Period of revolution2.52 earth days
Diameter1158 km
Mass (Earth=1)0.0002260
Density (water =1)1.66
Compositionice water (40-50%), rocks
and traces of NH3 and CO2
Temperature on the surface-187 °C
The surface contains a mix of cratered areas and valleys that are interlinked and spam many hundreds of km in length and over 10 km deep. Some craters have lost their original from due to viscous relaxation phenomena. The surface of Ariel is relatively young but nevertheless older than Enceladus'. It can therefore be said that Ariel has known past geological activity due to a hot interior. This heat that could have been created by tidal dissipation was made possible by resonance with other satellites orbiting Uranus.

The valleys are in fact a consequence of a phenomenon of expansion of the crust that could have occurred as the satellite cooled. The greater fractures were later filled with younger materials (a similar process to the grabens). These more recent deposits are very plane relative to the other areas of the surface and have fractures of their own but on a smaller scale. We don't know whether these fractures are tectonic in origin or if they are a consequence of material falling into the valleys. There is no certainty as to the structure of Ariel. The little information at the disposal of research scientists are nothing but a few images by Voyager 2 which was at 130,000 km from Ariel at closest approach. The model that agrees most with these limited observations imply a differentiated structure (model 4 in the following figure).

Ariel would have a rocky core covered in a malleable mantel that would be malleable for the most part comprised of ammoniac di-hydrate (NH3-2H20), and lastly a rigid crust of pure ice. The presence of ammoniac could explain the deposits in the valleys. Indeed, the water could not simply rise up and trickle down the valley. Due to the ambient temperature on the surface of Ariel water cannot exist in liquid state. Ammoniac makes the material in the mantel malleable (over large time scales).

Internal structure and composition
But other models, implying a less differentiated structure, have been equally put forward.
  • Model 1: a mixture of undifferentiated (homogeneous) ice and rocks with traces of ammoniac, followed by a fine ice water crust.
  • Model 2: a mix of undifferentiated (homogeneous) ice and rock followed by a crust of hydrated ammoniac.
  • Model 3: a rocky core followed by a mantel of ice and a crust of hydrated ammoniac.
Ariel's spectrum in certain bands of wavelengths was obtained from Earth (Mauna Kea). This allowed for other components that constitute the ice surface, like CO2 to be identified. CO2 is only present in one of Ariel's hemispheres.

Figures 1 to 4 (credit: JPL/NASA)

2 and 3 Details of the surface:
the red: crater degraded by viscous relaxation
in blue: valley filled with more recent deposits
in green: small sinuous fracture in the deposits
4) different models of the internal structure for Ariel (Prieto and J. Kargel)

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