Editorial_ Special issue “Planetary evolution and life”

Given the enormous number of stars in the universe and the number of confirmed and postulated planets in our galaxy, it is generally agreed that our home planet Earth is not likely to be unique (e.g., Sagan, 1980; Bignami et al., 2005; Hawking and Mlodinow, 2010). But is it? Although the number of known extrasolar planets grows almost by the day, observational bias caused by the technological challenges of finding Earth-size, rocky extrasolar planets and determining their masses and sizes have thus far prohibited the detection of a second Earth. But even if a second Earth were to be found—located in what is termed the habitable zone (e.g., Kasting et al., 1993)—can we expect that life would have originated there and have evolved beyond the most primitive forms? Is the universe “bio-friendly” as Paul Davies said (cited after Sullivan and Baross, 2007) using the Anthropic Principle (Barrow and Tipler, 1986) or is the origin of life so complex and our home planet so peculiar (Ward and Brownlee, 2000) that we are the unlikely product of a chain of unlikely events (Gould, 1989)? And if life existed on a second Earth or on many other planets, would we be able to detect it? Would life have shaped these planets such as life has shaped the Earth? Looking at our neighborhood—the solar system—it is clearly recognized that the Earth stands out. Not only does it have an atmosphere at the triple point of water allowing the three phases of the substance to coexist (a fundamental requirement of surface habitability) and abundant life, it is also the only planet of which we know that it has plate tectonics operating. Plate tectonics is considered an important element of habitability (e.g., Lammer et al., 2010). First, it is a vital element of the carbon–silicate cycle that stabilizes the Earth‘s climate (e.g., Kasting and Catling, 2003). It is also instrumental in cooling the core and keeping the geodynamo process alive that produces the magnetic field (e.g., Breuer et al., 2010). The latter helps to protect the atmosphere against erosion and life on Earth against harmful radiation. Plate tectonics also provides diversity and thermodynamic disequilibrium by e.g., generating continental crust and shelf areas and by powering mid-oceanic ridge volcanism. Mid-oceanic ridge volcanoes and continental shelves have been identified as the most likely regions on Earth where life could have originated. There may still be other places in the solar system that may be habitable to at least primitive life forms. The most prominent example, of course, being Mars but Europa, Enceladus and even Titan being candidates. The Martian surface shows evidence for water having shaped the surface in its early history (the Noachian, more specifically) with lacustrine and fluvial land forms. Europa and Enceladus are speculated to have oceans in contact with rock (e.g., Iess et al., 2014 for a most recent study) providing water and nutrients. (The energy source for life still posing problems, though.) Titan, finally, has been repeatedly speculated to be a habitat for life that may use other solvents than water (e.g., Bains, 2004; Benner et al., 2004) On Earth we know from extremophiles that life adapts to varying environments on a time scale of a few generations. But is the complimentary true? If life adapts to a planet, will a planet adapt to life? Can life stabilize the environment and even the tectonic mechanism? Will it influence the plate tectonics engine and perhaps even stabilize this tectonic mechanism? In 2008, the Helmholtz Association began funding a research Alliance that studies the habitability of planets such as Mars but also of satellites and of extrasolar planets. Moreover, it considers the interactions between the evolution of a planet and its habitability as well as the interactions between a planet and life. About one hundred scientists were involved in the Alliance up to the end of 2013 and 630 peer reviewed papers have been published between 2008 and now. This special issue collects 21 papers, some of which review the progress of the Alliance (e.g., Jaumann et al., 2014; Fritz et al., 2014) while others are research papers in their own right. The subjects of the papers range from mantle dynamics and plate tectonics to atmosphere studies and surface geology. Other studies consider the physics, chemistry and biology of primitive life on present Mars and the formation of habitable planetary systems, that is of planetary systems that have rocky planets (or satellites) at an orbital distance from the central star where liquid water on the surface is possible and where the available insolation could power a biosphere. The subject of the first in this collection of papers (Höning et al., 2014) is the interaction between life and plate tectonics. The authors argue that life through its effect on erosion and sedimentation and water cycling between the interior and the surface reservoirs will buffer the mantle water content and the surface area of the continents. An abiotic Earth after reaching an equilibrium state of balance between continental crust production and destruction and mantle water degassing and regassing would differ significantly from the present biotic Earth in the surface area of continents and mantle water content. The authors speculate that their abiotic Earth with a much smaller continental coverage and much drier mantle would probably not be able to sustain plate tectonics. If plate tectonics is an important element of habitability, then it will be important to see how likely plate tectonics would be on rocky planets bigger or smaller than Earth. We would conclude from the few examples of the solar system that there is no compelling evidence for plate tectonics on planets smaller than Earth. The problem of plate tectonics on super-Earths (up to 10 Earth radii) has been widely discussed in the literature