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 Icy Satellites Vocabulary


The albedo of a planet is the amount of light energy received by the body that is reflected off its surface.

In our solar system, only the Sun radiates light that it has produced. All other bodies only reflect some of the light they receive from the Sun. The amount of reflected light, and so the albedo, depends on the composition of the surface of the object in question. For example, ice water is reflects light better than rocks.

The Kuiper belt

Beyond the orbit of Neptune, the Kuiper belt spans the space 30 AU to 150 AU from the Sun. (1 AU= one astronomical unit= the distance between the Earth and the Sun) Like the Asteroid belt between Mars and Jupiter, it is made up of a multitude of little objects. These bodies contain a large proportion of ice.

These objects in the belt are also called transneptunian. Pluto is one of them along with the new dwarf planets of the solar system (Eris for example). Some short lived comets could be originally from the belt.

The Kuiper belt is thought to be what is left of the Sun' accretion disc. The inner radii of the disc gave birth to the 8 planets, whereas the outer part of the disc condensed to form an assembly of lots of smaller objects.


A particular form of volcanic activity that is found on bodies with no initial internal heat from the accretion phase, like the ice satellites! Cryovolcanism is therefore not a volcanic activity that involves lava flows like on Earth or Io, Jupiter's first Galilean satellite. The materials are ejected in plumes and are mainly water, ammoniac, nitrogen and methane, all volatile (low fusion and sublimation temperatures). These materials are liquid and or gaseous and then condense to form a solid that covers the surface, modifying its physical aspect.

The necessary heat for cryovolcanic phenomena to occur comes from tidal dissipation due to interaction between the satellites and its planet. Cryovolcanism was observed on Enceladus, Triton and Titan. The tidal dissipation can also be at the root of the existence of internal liquid oceans. There could therefore be internal cryovolcanoes in the oceans. This hypothesis still needs to be confirmed. The candidates for this type of cryovolcanism are Europa and Ganymede.


The change as a body moves from a gaseous phase to a liquid or solid phase. Moving to the liquid phase is only possible under certain conditions of pressure and temperature. If these conditions are not met, the vapour goes straight to its solid phase.

For example, in the atmosphere of our planet Earth, water can exist as a liquid an so condenses to a liquid. But on the surface of ice satellites in our solar system, water cannot exist as a liquid and so more commonly condenses to a solid.

Tidal dissipation (Tidal friction)

Tidal phenomena on bodies is the response of that body to the gravitational attraction of another body. For example, the side of the Earth that is facing the Moon is more attracted to it than the Earth's centre of mass. The side facing away from the Moon however is less attracted. Given that the centre of mass is a point in the Earth that is at a constant distance from the Moon in the rotational system, both sides of the Earth are attracted in opposite directions, making what is called the tidal bulge.

(Video on tides)

The position of the tidal bulge evolves as the body rotates and is directed towards the body responsible for the gravitational attraction. (we also have two high tides a day on Earth). But due to the materials' properties (the friction between the different constituents), the bulge resists the change in position. The friction produces heat, this is what is called tidal dissipation or friction. On Earth, there are two types of tides : oceanic tides and terrestrial tides. Both cause a dissipation of heat energy.

All ice satellites are in synchronous rotation with their planets so the tidal bulge, to a small approximation, is always located in the same place. There are therefore no dissipative force creating heat energy. But the orbits of the ice satellites are not perfectly circular, they are slightly elliptical (they are said to have a non zero eccentricity); the planet is therefore at one of the foci of the ellipse. Due to this eccentricity, the distance between the planet and the satellites varies which varies the amplitude of the tide which also creates friction. A second effect is added to this: the eccentricity is responsible for the change in speed of the satellite around its orbit. Satellites move "faster" when they are close to the planet. The speed of rotation of the satellite about it's own axis however is constant. So the rotation stay constant on average, but small changes in orbital revolution speed do exist, which creates a displacement in position of the bulge on the satellite's surface and is a second source of friction.

The heat produced by the body is hence available to melt the ice and maintain water in the liquid phase over large time scales.

We can show the production of this heat energy by the satellites has a direct consequence on its orbit: the eccentricity decreases! So, after a long while the tidal friction will cease... except in there is an other phenomenon in play maintaining the eccentricity at a constant value. This other phenomenon is resonance which is the an effect of the satellites on each other (like laplacienne resonance). It has been shown that these particular situations allow for the value of the eccentricity to be maintained and for tidal friction to persist.


Tectonic sink holes located in normal fissures, that is two pieces of the surface layer that are separating.

We refer to an effect as resonance in two cases:
  • When the ratio of numbers between the periods of orbits of two or more planets is a series of integers as given for the 2:1 ratio for the orbits of Enceladus and Dione, or even for the laplacienne resonance of ratios 4:2:1 that links Io, Europa and Ganymede. Enceladus make two revolution around Saturn while Dione only does one. Io makes 4 around Jupiter while Europa makes 2 and Ganymede makes one. Laplacienne resonance is the only resonance that involves three bodies in our solar system. The two body resonance systems are more common.

  • When the ratio between the rotation and revolution periods of a body is an integer number (this is Spin-Orbit coupling) : for example, the rotation of the Moon has a 1:1 period ratio with its revolution around the Earth (this is a synchronous orbit), or even Mercury's orbit which has a 3:2 ratio with its rotation about its own axis. Mercury does three rotation on itself while it orbits the Sun twice.
Synchronous rotation
A moon revolution around its parent-planet also rotates around itself with a proper rotation. When the rotation period and the orbital period are the same, it is called synchronous rotation.

Materials formed from Silicon and Oxygen. For example, olivine and quartz are silicates. Silicates enter into the composition of rocks that make up the planets and Earth-like satellites.


The boundary between the solid and gaseous phases without passing through the liquid phase is the sublimation line.

For example, the CO2 icecaps on Mars sublimate in the seasonal cycle; they do not melt like polar icecaps of Earth.

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