On August 1^st, 2014 Doris Breuer, her daughter Nizara and I had the pleasure of visiting ELSI and I had the privilege of giving a talk on the work of the HGF-Research Alliance "Planetary Evolution and Life" (http://www.dlr.de/pf/desktopdefault.aspx/tabid-4843/). HGF - the Helmholtz Gemeinschaft Deutscher Forschungszentren - had generously funded this Alliance between 2008 and the end of 2013 and about 150 scientists from Germany but also from other European countries and the US had participated. The DLR Institute of Planetary Research in Berlin, Germany, coordinated the Alliance. The work reached from models to explain the formation of the terrestrial planets in the solar system to the experimental verification of the viability of terrestrial extremophiles under Martian conditions. It included atmospheric investigations of potentially habitable super-Earths and calculations of theoretical atmospheric spectra for a range of planetary scenarios of Earth-like exoplanets and early Earth. Models of the interior structure and dynamics of Earth-size and super-Earth-size planets have been calculated that suggest that plate tectonics and magnetic field generation may be more likely for Earth-size planets and that the propensity for these features may decrease with increasing planetary mass beyond Earth's mass. Geological mappings of Mars reveal the (paleo)-habitable environments of this planet and mappings of Titan show lakes and rivers of carbon-hydrates. The research is summarized in a special issue "Planetary Evolution and Life" of Planetary and Space Sciences, Vol., 98, August 2014. Matthieu Laneuville - a post-doc now at ELSI - was one of the graduate students supported by the Alliance.

The Alliance - an affiliate member of the NASA Astrobiology Institute - is presently refocusing its activities for the upcoming years and is looking to partner with other institutions and networks in the field. It has already excellent ties to the Belgian network "Planet Topers" and to the Swiss "Center for Space and Habitability".

Below are a few thoughts of mine on the subject of "Life and Planetary Evolution":

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. 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 - 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 using the Anthropic Principle or is the origin of life so complex and our home planet so peculiar that we are the unlikely product of a chain of unlikely events? 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 widely considered an important element of habitability. First, it is a vital element of the long-term carbon-silicate cycle that stabilizes the Earth's climate (and of other chemical cycles). It is also instrumental in cooling the core and keeping the geodynamo process alive that produces the magnetic field. Models suggest that single-plate tectonics with stagnant lid convection would rather cool the outer layers of the planet by thickening its lithosphere. A magnetic field helps 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. But is a complimentary statement true? Can plate tectonics be considered a biosignature? And would life help stabilize this tectonic style?

We have recently published a paper (Höning et al., 2014) - in the aforementioned special issue in Planetary and Space Science - on possible interactions between life and plate tectonics. We argue that life through its effect on erosion and sedimentation and water cycling between the interior and the surface reservoirs will help to 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 abiotic Earth with a much smaller continental coverage and much drier mantle would probably not be able to sustain plate tectonics. Furthering the model, we are currently calculating evolution models that include time dependence.

Let me conclude this blog by emphasizing that finding extraterrestrial life is a task of fundamental importance to mankind. Its fulfillment would be philosophically profound as it would complete the Copernican and Darwinian revolutions! Evaluating the interactions between planetary evolution and life will help to put the evolution of our home planet (even anthropogenic effects) into a wider perspective.

Thank you ELSI for having us visit!

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Tilman Spohn
DLR Institute of Planetary Research
Berlin