The Geology and Habitability of Terrestrial Planets: Fundamental Requirements for Life

Abstract

The current approach to the study of the origin of life and to the search for life elsewhere is based on two assumptions. First, life is a purely physical phenomenon closely linked to specific environmental conditions. From this, we hypothesise that when these environmental conditions are met, life will arise and evolve. If these assumptions are valid, the search for life elsewhere should be a matter of mapping what we know about the range of environments in which life can exist, and then simply trying to find these environments elsewhere. Second, life can be clearly distinguished from the non-living world. While a single feature of a living organism left in the rock record is not always sufficient to determine unequivocally whether life was present, life often leaves multiple structural, mineralogical and chemical biomarkers that, in sum, support a conclusion that life was present. Our understanding of the habitats that can sustain or have sustained life has grown tremendously with the characterisation of extremophiles. In this chapter, we highlight the range of environments that are known to harbour life on Earth, describe the environments that existed during the period of time when life originated on Earth, and compare these habitats to the suitable environments that are found elsewhere in our solar system, where life could have arisen and evolved.

Theoretical Spectra of Terrestrial Exoplanet Surfaces

“…As shown in Figure 4, a bright surface can be feldspathic, granitoid, and clay; all are silica-rich (termed "felsic"). A felsic surface on a planet of size larger than Mars is probably produced by slow intrusion of molten lava, which indicates plate-tectonics and potential geological setting for the origins of life (Best & Christiansen 2001;Southam et al 2007). A caveat here is that high VNIR reflectivity can also be attributed to atmospheric effects, such as bright clouds and hazes.…”

Volcaniclastic habitats for early life on Earth and Mars: A case study from ∼3.5Ga-old rocks from the Pilbara, Australia

“…Chemical energy is provided in the form of thermodynamic disequilibria between chemical species in close proximity, such as reduction-oxidation (redox) contrasts. For example, reduction of ferric oxides in hot lava in contact with seawater produces hydrogen, a common electron donor for primitive microbial life (Nisbet et al, 2007;Southam et al, 2007b). Solar photons are a more effective source of energy but the biological mechanisms needed to capture this energy (photosynthesis) are relatively sophisticated and most likely developed at a later stage (at least on Earth) after the initial appearance of cellular life.…”

“…in water depths that could be penetrated by sunlight) that could have hosted photosynthetic microorganisms obtaining their energy from photons and forming microbial mats e.g. stromatolites (Southam et al, 2007b;Nisbet et al, 2007). Hydrothermal vents are also loci of much microbial diversity (Schrenk et al, 2003) and there is evidence of significant hydrothermal activity on the early Earth (Hofmann and Bohlar, 2007).…”

Astrobiology: An astronomer's perspective

“…3, the geological evidence points toward the emergence of life on our planet as the frequency of energetic impacts decayed. Southam et al [50] argues that the Earth was essentially habitable throughout the Hadean, while Sleep [46] makes an argument for photosynthesis as early as 3.8 Byr. The important point here is not the details, which are complex, but that simple life did not take billions of years to take root.…”