Enceladus orbits within Saturn's tenuous, outermost ring, the E-ring. The surface coating of pristine ice makes it the most reflective object known in the solar system. It is also the most geologically evolved member of Saturn's satellites, displaying a young surface with at least five distinct types of terrain. Even the most heavily cratered areas of Enceladus are much more lightly pock-marked than the other icy satellites, while some regions are crater-free down to a resolution of 2 km (1.2 miles), indicating that the surface has undergone major change within the last 1 billion years, and has been geologically active within the last 100 million years. On either edge of the ridged plains, a series of truncated craters indicate that melting of the surface occurred after the end of the intense bombardment phase. Linear markings in the southern hemisphere are rectilinear fault lines associated with movement of the crust. Other, curved lines appear to be a complex system of ridges, similar to the grooved terrain on Ganymede.
All of this suggests that Enceladus must have an interior that is kept in a liquid state. But how? One possibility is tidal heating due to the gravitational effects of Saturn and the large, neighboring moons Dione (with which Enceladus is locked in a 1:2 resonance) and Tethys. Many researchers, however, fail to see how tidal heating could be a major factor because the orbit of Enceladus is not sufficiently elongated (eccentric). A question mark hangs over other possible heating mechanisms, too. Enceladus doesn't have enough interior rocks for radioactive heating, nor does it have enough ammonia to lower its melting temperature.
Whatever is heating Enceladus may also drive cryovolcanism (ice volcanism), which has been proposed as the source of material in the E-ring and also of the moon's thin atmosphere. The latter was detected by the Cassini spacecraft in 2005. It consists primarily of water vapor with much lower amounts of nitrogen, carbon dioxide and other simple carbon-based molecules (organics). These gases are concentrated at the south pole of the moon, which is also a "hotspot", hovering at a relatively balmy minus -183°C compared to a temperature for the rest of the moon of about -201°C (-330°F).
The south pole of Enceladus, ice plumes, and astrobiology
The south pole of Enceladus is evidently the scene of significant, ongoing geological and possibly hydrological action. The south polar region is cut by parallel cracks roughly 130 km (81 miles) long and 40 km (25 miles) apart. These cracks, dubbed "tiger stripes," vent vapor and fine ice water particles that have crystallized on Enceladus' surface as recently as 1,000 years to 10 years ago. The fine ice material is probably the major source of particles that replenish Saturn's E ring.
Various ideas have been put forward to explain the activity taking place at the south pole. One suggestion is that it's being driven by liquid water perhaps as little as 10 meters below the surface. (This contrasts with the several kilometers depth suspected for liquid water oceans on Europa and several other larger moons in the solar system.) Although Cassini hasn't seen ice geysers or ice volcanoes, the lack of ammonia and the sheer volume of water vapor escaping suggests that pure-water volcanism exists on Enceladus.
Related entry Saturn, moons
Related category PLANETS AND MOONS
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