Abstract
Abstract To be habitable, a planet must be suitable at all scales [1]. The setting in relation to the star must be right, so that surface temperatures can sustain liquid water. The planetary inventory must be suitable, providing surface water, rocks, and accessible thermodynamic disequilibrium. There must be physical habitat, especially mud and hydrothermal systems around volcanoes. Planets are not static: they evolve. Habitability must evolve with the planet. On accretion, the processes of impact and formation of volatile inventory must be suitable. Tectonics and volcanism must supply redox contrasts and biochemical substrates capable not only of starting life but of sustaining it. Mud or soft sediment may be essential: it is unlikely that early life can sustain itself in open water or air. This requirement for mud has tectonic implications. Once life starts, it immediately alters its own environment, by consuming nutrient. Until photosynthesis evolves, inorganic sources must supply sustained redox contrast to the local environment. But life changes its setting, both by risky alterations to the atmospheric greenhouse (drawing down CO2, emitting CH4), and by partitioning reductants (e.g. as dead bodies) and oxidants (waste). Somehow the planet must avoid both freezing and boiling. Early in the history of the solar system, a passing galactic tourist might have rated Venus as the likeliest habitat for life, Mars next, and Earth last of the three. Venus was warm and hospitable, Mars clement, and Earth had been though an impact episode powerful enough to make a silicate atmosphere. By comparison with Earth there are many potential environmental settings on Mars in which life may once have occurred, or may even continue to exist. Perhaps Mars seeded earth? Yet today the reverse order of habitability is the case. Earth today is safeguarded by a reworked atmosphere that is 99% of biological construction, maintained in active disequilibrium with the surface. Mars, in contrast, is chilly oxidised permafrost where kinetics alone would make life difficult. Venus has achieved sustainable equilibrium, where equilibrium, being the opposite of life, equals death. Geologically, how and why did this happen? Earth was not necessarily the Goldilocks planet, neither too warm not too cold. But it did have a degree of locally-accessible disequilibrium, to sustain early life. Life itself, especially the enzyme rubisco, that mediates carbon transfer from air to organism, then actively managed to rework the surface of Earth. From the moment life began, it remade its house to maintain habitability. Life, if ever present on Mars, may have simply died from lack of resources. Any life on Venus would have died of dehydration or by being cooked. However, terraforming may now be worth attempting on both planets: as on Earth it may be self-sustaining, even after the terraforming species becomes extinct. [1] Nisbet, E.G. Zahnle, K., Gersimov, M., Helbert, J., Jaumann, R., Hofmann, B., Benzerara, K, Westall, F., Spc. Sci. Rev. (2007), 129, 79-121.
