Enceladus

Icy satellites like Enceladus are among the most interesting objects in the solar system concerning the possibility of harboring life. Although they are not situated in the habitable zone around the Sun (Kasting et al., 1993), they could host life in their potential subsurface oceans. Therefore, a main task is to deepen our knowledge of possible compositions of such water/ice reservoirs and on microbes that might host this environment.
Enceladus is a rather small moon with a diameter of just about 500 kilometers (Thomas, 2010). Nevertheless, it is one of the most interesting objects in the Solar Systems because of its plumes (similar to Earth’s geysers) detected by the NASA probe Cassini. These jets of mostly pure water (90% water, 5% CO2, traces of CO, H2, N2, and other organic material; Waite et al. (2009)) are located in the extreme warm south polar terrain. This area is dominated by the four so-called tiger stripes, which are linear rifts of about 130 kilometers in length and about 2 kilometer in breadth (Porco et al. (2006)).
The first part of the thesis is the modeling of various scenarios for the inner structure of Enceladus depending on the density of its rocky core. In the first scenario (S1) we use the assumption that the mean density of Titan's rocky core should be similar to Enceladus' core because these moons were formed in the same region of the Solar System. The second scenario (S2) deals with a core density similar to the density of the uppermost part of the Earth's mantle primarily made up of olivine-rich rocks such as peridotite. The next scenario (S3) handles the idea of Schubert et al. (2007) that Enceladus' core density is similar to Io's bulk density. According to Schubert et al. (2007) this number indicates the highest possible value for a rock-metal core because Enceladus core is likely to be less dense as Io's bulk density due to inclusions of e.g. low-density hydrated silicates. The last scenario (S4) hypothesises a density similar to the grain density of typical hydrated carbonaceous CI chondrites, which might be the building blocks of icy moons like Enceladus.
The degree of differentiation of Enceladus might be determined by new measurements of the gravitational field by the Cassini spacecraft and by observations of the period of each satellite (Hussmann et al., 2010).
This first step is very important, because it gives us an idea on the boundary layers of the ocean and hence we will estimate the effects of interactions (weathering, erosion, melting, etc.) on the ocean’s composition between the water reservoir and the encasing layers (both rocky and icy). So, we will be able to estimate which compounds might be solved in such a liquid layer. Here, Zolotov et al. (2007) may serve as a basis, in which the authors describe a possible oceanic composition. Besides, we have to model the different external circumstances like pressure, density, temperature, etc. at different locations inside the moon, as assumed for Europa in Marion et al. (2003).
The follow up studies will be the analysis of the habitability of the ocean concerning microbes. At first, we will select such microbes, which would (theoretically) be able to survive/reproduce in such environments. We plan to cultivate these microorganisms in a medium corresponding to the ocean’s composition. At the moment, we prefer methanogens and/or sulfur-reducing bacteria for our studies corresponding with McKay et al. (2008).
This approach to that subject would be revolutionary because this research will combine assumptions about the structure of an icy satellite and the composition of its possible subsurface ocean with a laboratory study in which we could test the theoretical results.



Plate tectonics on Europa

Jupiter’s moon Europa is distinguished by a multiplicity of geological peculiarities, like ridges, bands and faults, that cover its surface. Most of this features also can be found on earth, where they are generated by plate tectonics. Thus it is reasonable to assume that the surface of Europa too is formed by plate tectonics. But in this case, so the current opinion, the plates are thick sheets of ice, which swim on a global, subsurface, ocean of seawater mixed with ammonia.
The numerous diverse structures of the surface and their vast currency suggest that Europa experienced considerable tectonic activity in its past. Very important for this process are the icesheets which function as tectonic plates. Especially their dimension is very important to determine the scale of the tectonic activity. Also of great interest is the differentiation of the particular surface structures, the ridges and bands, and their different peculiarities as well as the characteristics and occurrences of the shaping forces. A lot of structures indicate that the main forces are the tidal forces of Jupiter and the rotation of the frozen surface. Additional, weaker, forces are orbital evolution and the increasing thickness of the frozen surface. In fact it seems that the time of highest tectonic activity lies in the past of the moon. This decline of tectonic activity seems to be accompanied by an increase of cryovolcanic activity.
Considering that the time of highest activity lies in the past the question occurs if there is present tectonic activity to be observed. There is no proof for it at this moment because it is difficult to tell if the ice surface is too thick for plate tectonics. The problem with that is that the thickening of the ice layer is caused by an abatement of Jupiter’s tidal forces. There is, however, the chance that the active tidal forces, in combination with the dilation of the ice, is strong enough to allow tectonic activities.