Theoretically Possible Habitable Worlds: But Will We Get Soon Answers by Observations?

Abstract

Based on the chapter presented within this volume, one of the conclusions might be, that there is no doubt, that habitable worlds exist. Even in our neighborhood, in our own solar system, it is not improbable that planets like Mars, with some limitations Venus and the satellites or the so-called moons in the Jovian and Saturnian systems (e.g., Europa and Titan resp.), are habitable. The question, which is hidden behind this argument, is if implications are suggested that “there is no life – no life as we know it in the universe” (see chapter “Factors of Planetary Habitability” by Hengeveld) – could it be possible that our terrestrial inhabitants, the big varieties of present microorganisms on our home planet Earth, could probably live on these extraterrestrial worlds? Could they be able to find habitats, where they would be able to survive? In that case, it seems mainly a question of presence of liquid water and an energy source. However, we also have to be aware that for the classification of a planet or moon as a habitable object, we have to take into account which reference groups of organisms might survive in such conditions. Some of the potential species are described in this book (chapter “The Role of Terrestrial Analogs in the Exploration of the Habitability of Martian Evaporitic Environments” by Barbieri, chapter “Microbial Scale Habitability on Mars” by Westall, chapters “Organic Molecules in Lunar Ice: A Window to the Early Evolution of Life on Earth” and “Extremophiles on Alien Worlds: What Types of Organismic Adaptations Are Feasible on Other Planetary Bodies” by Schulze-Makuch and Seckbach et al., 2013). Examples which have been discussed are mainly focusing on Earth-like life as a reference system. Some problems might be obvious if we try to think about alternative life forms, which appear to be much different from our Earth-centric view. An approach was given in particular in Part 5 with the title “Alternatives to Earth-like Life.” An option to gain energy besides photosynthesis and chemolithotrophic processes, which are known as conservative main power source due to the effective use of redox potentials, has also been given by two innovative examples. The proposed idea of “thermosynthesis” by imaginable thermotrophic life forms is dealing with the idea of thermal phase differences and transitions in the natural environment from which alternative life forms might gain the needed energy comparable to the charge differences at membranes which are necessary for the synthesis of ATP in the previously mentioned synthetic pathways of photosynthesis and chemoautotrophic synthesis (Schulze-Makuch). The “osmosynthesis” is another postulated alternative (see chapters “Organic Molecules in Lunar Ice: A Window to the Early Evolution of Life on Earth” and “Extremophiles on Alien Worlds: What Types of Organismic Adaptations Are Feasible on Other Planetary Bodies” by Schulze-Makuch), where a cell can be hypo-osmotic and a surrounding saltier ocean can act as a counterpart. This osmotic pressure gradient is again a potential engine to generate energy for life’s requirements of self-preservation, maintenance of reproduction capacities, and life cycle properties.