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.

Dates of the flybys Minimal distance 17/02/2005 1259 km 09/03/2005 497 km 14/07/2005 168 km 12/03/2008 995 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.

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.

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