Life requires energy sources, the nutrients necessary for building structures and synthesizing catalysts, and access to geological environments in which biosynthesis and maintenance of biostructures are possible (e.g., Wald, 1964). A habitable planetary environment, regardless if past, present, or future, would have to be one that features all of the above requirementsfor life. No life on other planets than Earth has been detected so far, and therefore it is not possible to determine what its environmental requirements are. Nevertheless the one environmental factor that is almost universally accepted as necessary for life is at least episodic access to liquid water. The availability of the above parameters through time is a function of the geologic evolution of a planet or moon. Therefore, the geological investigation of planetary systems, analogous to the Earth, is the key to our understanding of the concept of habitability. This characterization includes geochemical and mineralogical knowledge of surface or near-surface rocks, the nature of the atmosphere and climate, and the nature as well as the intensity and rates of geologic processes – all these factors are intimately related to planetary geology. The most logical place to look for life elsewhere in the solar system is Mars: It is the most Earth-like of all the other planetary bodies in terms of its geological surface record and the availability of liquid water at or near the surface throughout its evolution. Moreover, solar energy, geothermal and chemical energy, as well as nutrients, are thought to be available on Mars (e.g., Jakosky, 1998). Hence, the analysis of the surface and near-surface geology is more than just the direct observation and interpretation of landforms. It can also indirectly validate or invalidate models of atmospheric evolution, of interior geophysical processes, and of impact probabilities. Another important process is the cycling of rocks. Minerals and rocks are stable only under the conditions at which they form, though kinetics and activation energies permit sustained disequilibrium. Changing these conditions will initiate metamorphism of the rock and its minerals. Therein is disequilibrium, and resources for life. On Earth, plate tectonics regularly remodels the face of the planet. This is central to the sustained supply of ions and disequilibrium for biogeochemistry (Nisbet et al., 2007).
The geological record preserved at or near the surface provides insights into all the aspects of habitability such as: (1) the nature of the surface and the subsurface environments; (2) the temporal and geographical distribution of liquid water including the history of volatiles and climate (cf. research topic 2.1 above); (3) the interior evolution and availability of other resources (e.g., energy) that are necessary to support life (cf. research topic 2.2); and an understanding of the processes that controlled each of these factors. It also provides constraints and “ground truth” for any modelling of planetary processes. For example, volcanoes of different ages have been unambiguously observed on Mars and other planets. Any model of the interior evolution and heat flow (with the implications for outgassing, the release of volatiles and the climatic consequences) has to account for the existence and duration of volcanism (e.g. Neukum et al., 2004). The same holds for the existence of liquid water on the surface (e.g. Jaumann et al., 2005).
The key prerequisites of life, liquid water and organic material, are both also present on moons in the outer solar system (e.g. Titan, Europa and Enceladus), where subsurface conditions controlled by tidal heating forces may generate oceans of liquid water covered byice crusts. Titan has an environment very rich in organics, it is often considered as one of the best targets to look for prebiotic chemistry at a full planetary scale (e.g. Fortes, 2000). The recent discovery on Enceladus of water ice geysers with methane and the possible presence of a large internal reservoir of liquid water containing active organic chemistry make this small Saturnian satellite besides the Jovian satellite Europa a new important planetary target for astrobiology as well (Porco et al., 2006, Jaumann et al., 2007a). Besides intense ice tectonics on these moons, the surface expression of Titan’s atmospheric processes are extensive erosion by methane precipitation as well as fluvial and aeolian transportation and sedimentation of organics (Jaumann et al., 2007b).
Finally, the Moon, Mercury, Venus and a couple of asteroids are almost water-free bodies. However, with respect to the basic processes such as differentiation, volcanism, tectonism and impact cratering they are geologically similar to the other planets and moons. Thus, they can be used as water–free endmembers of the planetary evolution system that help to constrain the correlation of geological processes and habitability.