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Saturn's satellites

 The Icy Satellites Saturn's satellites Enceladus

Enceladus was discovered in 1789 by William Herschel. Because it is so small, we can't see it with the naked eye or binoculars. It is the fourteenth furthest from Saturn et the sixth largest. It is in synchronized rotation with Saturn, which means that the same side of Enceladus in always facing Saturn, just like the Moon always shows the same face to Earth.

Image of Cassini
Distance from Saturn237 378 km
Period of revolution32 h 48 m
Diameter513x503x497 km
Mass (Earth=1)0.0000144
Density (water =1)1.33
Compositionicy water and silicates
Temperature on the surface-201 °C
This small satellite was visited by tree exploration probes:
Voyager 1 and 2, and Cassini.

Its shape is practically spherical. It is in fact an ellipsoid flattened at its poles, just like Earth. Its precise dimensions have been determined by the Cassini probe:
  • 513 km in diameter in the line of sight facing Saturn
  • 503 km in diameter along the direction of Enceladus' trajectory
  • 497 km between the poles.

Enceladus surface is particularly reflective, its albedo is of 0.99, which is the biggest value so far observed in the solar system (the albedo has a maximum value of 1). Its surface is composed of icy water and its low density (1.61 while water has one of 1) suggests that its internal composition is mainly made up of icy water and only a small proportion of silicates (in volume).

Enceladus is part of the Saturnian system which is a quite a complex system. The number of satellites is impressive and some interact with each other and with the rings:

  • It interacts with another satellite Dione, in a phenomenon called resonance of ratio 2:1. Indeed while Encaladus indergoes two revolutions around Saturn, Dione does exactly one!
  • Enceladus revolves in the E ring, which is an external ring, non-visible on the usual pictures. We think that this is not a coincidence. Enceladus would be providing material that constitutes the E ring by cryovolc mechanisms.
Enceladus' surface shows signs of recent activitie and a geyser of ice particles has been photographed by Cassini. Enceladus is therefore one of the rare bodies still active in the solar system (there is also Earth, Jupiter's satellite, Io and Neptune's satellite Triton).
The discoveries of the Probes Voyager 1 and 2:
In December 1980, Voyager 1 took the first images of Enceladus but the resolution was low. We can see a smooth surface with apparently no bumps or craters. With similar resolutions, craters could also be seen on the other satellites. The age of the surface depends on the way in which it has been cratered, we have already supposed that Enceladus' surface was quite new. A spectral analysis of the surface showed that it was composed of pure ice water.

In August 1981, Voyager 2 presented an opportunity to photograph Enceladus with a resolution of $1\, km/pixel$ for the Northern hemisphere and, with a weaker resolution, up the the mid latitudes of the Southern hemisphere. The Southern polar region was badly covered and the image resolution was quite low. These images showed that Enceladus wasn't just a completely smooth ball of ice but presented varied regions, formed geologically, proving to all that Enceladus has had a long and eventful history.
  • The oldest areas are strongly cratered and the craters have been deformed by different mechanisms (including the viscous relaxation).
  • Some fields show linear and restricted furrows embossed on a km scale. Some fractures even cross others!
  • The most recent regions are smoother.
The discoveries of the Cassini probe:
Dates of the flybys
Minimal distance
17/02/20051259 km
09/03/2005497 km
14/07/2005168 km
12/03/2008995 km
The Cassini probe has studied Saturn, its satellites and its rings since 2004. Right now, tree flybys of Enceladus have occurred, with different distances of closest approaches. Much information has been gathered during these expeditions. A fourth flyby will take place there in 2008.

During the first flyby, the probe recorded a deflection, around Enceladus, of Saturn's magnetic field. This deflection of the field is due to the presence of a thin atmosphere (containing ions) around the satellite. Enceladus' mass is too small to keep an atmosphere, this is because the gravity is of 0.006 times the Earth's. Therefore the atmosphere has to be renewed as it escapes.
The mechanism for the renewing of the surface is the cryovolcanic activity. The atmosphere that escapes will then fuel the E ring. This is why the location of the E ring is denser at Enceladus' orbit. Without Enceladus, the E ring would disappear in a few thousand years. The second flyby has permitted the location of the source of the iron disturbing Saturn's magnetosphere: the Southern polar regions.

The regions that appear to be smooth on the photographs of the probe Voyager 2 appear with a better resolution as thinly fractured. The Southern polar region has been closely photographed during the third flyby. The surface there is heavily striatal and jammed by ice cubes of 10 to 100m large and almost without any impact craters.

On a global scale, the Southern pole is marked by 4 big parallel fractures of more or less 130 km long, 1 to 2 km large, 500 m deep and separated by more or less 35 km. We have named them the The tiger stripes. An analysis of the intersections between the different faults of the regions showed that the fractures are the most recent geologic formations of the Southern pole region.

Around these stripes, at a latitude of more or less 55°S, extends a circular chain of sinuous fractures that originally comes from a global modification the shape of Enceladus, which means a reduction of the diameter between the two poles.

The atmosphere is composed of water vapour and of micro crystal of frost. This atmosphere is not at all like to the Earth's atmosphere. It is thin and only located above the Southern polar regions. It is permanently renewed by the emission of material from the tiger stripes or by areas. Indeed, while the Enceladus' surface is of an average temperature of -201°C (72K), these fractures are warmer, -133°C (140 K). Plus we can see that crystalline ice near these faults (and so recently formed). At these fractures, the cryovolcanic mechanism happens and water vapour is ejected to form geysers. A big part of the vapour falls on the ground and crystallises, because of the low temperatures, while the rest (more or less 1%) will fuel the E ring.

A geyser was observed in November 2005, while Cassini distanced itself from Enceladus. The Sun light, located behind Enceladus with regard to the probe, was diffused by the plume. The plume was constituted of several jets reaching up to 500 km of altitude (which is as high as the diameter of Enceladus). This phenomenon was observed during several months, it is therefore not a one off event.

A hypothesis concerning the exact mechanism of the geysers is states that water vapour comes from reservoirs of liquid water located under the surface. The hypothesis involving the sublimation of the ice has been rejected following observations made of the quantity of ice and vapour and the analysis of the size of the particles of ice.

To have a reservoir of liquid water under the surface, the temperature must be of at least 0°C (273 K). 7 m under the surface, the temperature and pressure would allow for the existence of liquid water in a stable state. If the pressure suddenly decreases because of a fracture in the crust of the ice, a part of the water will be become vapour and the other part ice. During this process the volume of the vapour/ice mix has risen by 24000 times the one occupied by the same quantity of water but at a liquid state. The mix will therefore come out of the faults because of a lack of space. The geysers will be very thin jets that go up very high.

The volcanism of the Southern pole is due to a kind of volcanic hot spot, since they are concentrated in a few particular regions. This hot spot could have come form anywhere and then orientate itself in the direction of the Southern pole. This is because the region of the hot spot is characterised by a smaller density than the neighbouring region. A rotating body is even more stable since most of its mass is near its equator. When a hot sport appears it can create a reorientation of the axis of ration until a more stable configuration is established. The ring going around the tigers scratch would then be a consequence of this reorientation. This ring is by the way similar to the rings that we can observe on Miranda, Uranus' satellite that is just a bit smaller than Enceladus.
Enceladus' internal structure:
The size, the shape and the mass of Enceladus have been specifically determined by Cassini. We can directly find its density but not the way the mass is distributed inside, in other words not its internal structure. Knowing that the density of ice is 0.93, different models have been considered:
  • A homogeneous distribution of the masses, this means a mix of non differentiated of the ice and the silicates, whatever the depth. But this not allow for a surface entirely made of ice water.
  • A thin crust of ice (of about 10 km thick) in surface with a homogeneous mix of ice and of silicates inside. This model agrees a bit better with the entirely iced appearing surface of Enceladus than the first model.
  • An internal structure in layers, with an ice crust 70 km thick and a core of silicates of 2.7 density (the density of the silicates on Earth are of 3.3).
We cannot decide, because of our limited knowledge at the moment, which internal structure to chose. The whole cryovolcanism phenomenon needs the liquid to have a high internal temperatures, this would mean that the structure would be different so that the existence of a well heated core by radioactive disintegration and the heating of the tides is possible. The core would then provide the needed heat for the melting of the ice. To explain the cryovolcanism, the existence of simple liquid water bags would be sufficient, but we can also consider the existence of an internal global ocean, similar to Europa.


Figures 1 to 4(credit: JPL/NASA)

1)Images of Voyager 1 ( accentuated colours)
2)and of Voyager 2 (mosaic in false colour)
3)Fractures on Enceladus (Cassini)
4)Images at high resoltuion of a zone located in the Southern polar region (Cassini: 37 m/pixel et 4m/pixel))
5) The tigers scratch (in dark blue) and the sinuous chain located at more or less 55° of South lattitude (in red)
6) Geyser of steam and ice water emited by polar and South regions and same image in colour to point out the concentration of material.

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