The exploration of the Saturnian system by the Cassini spacecraft has yielded a collection of new data about the planet and its rings and moons, including the icy satellite Enceladus (Fig. 1). Enceladus has been found to be one of the most geologically active objects in the Solar System and has raised major questions in understanding the evolution and dynamics of small and mid-sized icy satellites. It is thus among the priority targets for Cassini''s extended mission.
As already revealed by Voyager data (Smith et al. 1981), its surface is very heterogeneous, displaying young, tectonically modified terrain, as well as old, heavily cratered surface areas. The youngest features, including the "tiger stripes", roughly parallel lineaments about 500m deep, 2 km wide, ~ 130 km in length and flanked by about 100-m-high ridges, are found in the south-pole region (Porco et al., 2006). An outstanding discovery of the Cassini mission was the detection of venting plumes composed of gas and dust, emanating from the south pole region of Enceladus (Fig.2), the sources of which are probably correlated with the location of the tiger stripes. Furthermore, the vigorous activity near the south-pole is associated with strong thermal activity (Fig.3), which is completely unexpected for such a small satellite. Temperatures of about 90 K at Baghdad Sulcus1 - almost exactly located at the south-pole- are well exceeding the expected equilibrium temperature of 60 K due to solar insolation at the poles (Spencer et al., 2006). The intrinsic power derived from these temperature anomalies using Cassini''s CIRS-instrument (Composite Infrared Spectrometer) was estimated to be 5.8 ± 1.9 × 109 W (Spencer et al., 2006), an extremely large value for such a small icy satellite. It is significantly exceeding the output of heat expected from radioactive decay of long-lived isotopes in Enceladus'' silicate component, which would be ~ 0.3 × 109 W, only.
Tidal heating in Enceladus'' ice and silicates is one suggestion in the search for an efficient heat source, already motivated by Voyager images revealing Enceladus'' strongly modified surface. Due to its proximity to Saturn and its elliptic orbit, Enceladus is subject to periodic tidal deformation which will lead to internal friction and thus heating. Enceladus'' orbital eccentricity is forced by Dione, which is locked in a 2:1 mean motion resonance with Enceladus, thus keeping the eccentricity finite, presently at 0.0045.
Although tidal heating can serve as an explanation for Enceladus'' activity, yielding the right range of thermal output, there are two major problems still longing for a satisfactory explanation:
Suggestions to solve the first problem include the possibilities a) that Enceladus may be heterogeneous in its interior, e.g. due to incomplete differentiation or b) that the temperature distribution and consequently the viscosity - one of the main parameters determining the amount of tidal heating- is asymmetric in Enceladus, e.g. due to degree-one convection or other locally acting processes. To solve the second problem, the fact that Enceladus contains more rock (about 50% rock and 50% ice) than Mimas (~ 20% rock and 80% ice) may be a starting point. If the higher radiogenic heating rate in Enceladus heats up the interior sufficiently for tidal heating to kick in because of the resulting lower viscosity of the ice or rock, is still an open question.
The very sparse constraints on Enceladus'' interior structure are mainly the radius and shape of the body and its mean density. To precisely determine the gravity field from close flybys would be a key to infer the degree of differentiation or to detect density anomalies. However, additional clues for the satellite''s evolution and present state are the surface chemical composition and texture as well as the chemical composition of the plume which was measured by Cassini and which will be determined in much more detail in flybys through the plumes in the extended mission.
The gaseous plume component is dominated by water vapor (~90%) with additional constituents, such as CO2, CO or N2, and CH4 (Waite jr. et al., 2006). This raises the question if liquid water is enclosed in the near-surface ice, or if there might even exist a global subsurface ocean. Such a scenario, implying low-viscosity ice close to the melting point, would also enhance tidal heating in the ice shell.
If the activity of Enceladus is recent or persisting is connected to the orbital state of Enceladus, which is mainly controlled by the 2:1 mean motion resonance lock with Dione. The resonance forces
the orbital eccentricity of Enceladus, maintaining tidal heating on geological timescales.
To understand the mechanism that drives such an intense activity including the linkage between interior and surface and the connection to Enceladus'' thermal-orbital evolution is still a challenge. It involves investigations of Enceladus'' interior structure, surface geology, composition and texture, in-situ measurements of plume composition and dynamics, and characterization of the satellite''s orbital and rotational state.
1The most prominent "tiger stripes" are called Alexandria, Cairo, Baghdad, and Damascus, following the convention that features on Enceladus are named for people and places from Richard Burton''s 1900 version of the Arabian Nights. A sulcus (plural: sulci) is a set of sub-parallel furrows and ridges.
References:
Porco et al., Science 311 (2006)
Smith et al., Science 212 (1981)
Spencer et al., Science 311 (2006)
Waite jr. et al., Science 311, (2006).
Last update: 27/05/2010 14:48
