Summary How does life begin and evolve, does life exist elsewhere in the universe, and what is the future of life on Earth and beyond? These basic questions of astrobiology will be addressed in the project PLACES. The project focuses on the co-evolution of biosphere and geosphere on very long time and spatial scales and pursues two main directions. The first aim is to model and analyse the limits of self-regulation under extreme forcing and internal variability in the past and future of planet Earth by refining our coupled biosphere-geosphere model. The second aim is to investigate extrasolar Earth-like planets by simplifying our model and varying planetary properties.

Zusammenfassung Wie beginnt und entwickelt sich das Leben? Existiert es irgendwo anders im Universum? Was wird aus dem Leben auf der Erde und jenseits von ihr in der Zukunft? Diese grundlegenden Fragen der Astrobiologie sollen im Projekt PLACES angesprochen werden. Das Projekt konzentriert sich auf die Koevolution von Geo- und Biosphäre auf sehr langen zeitlichen und räumlichen Skalen und verfolgt zwei Hauptrichtungen. Das erste Ziel ist die Modellierung und Analyse der Grenzen der Selbstregulation unter extremen Einflüssen ("forcing") und bei innerer Variabilität für die Vergangenheit und die Zukunft des Planeten Erde mittels Verfeinerung unseres gekoppelten Modells fr die Geo- und Biosphäre. Das zweite Ziel ist die Untersuchung extrasolarer, erdähnlicher Planeten mittels Vereinfachung unseres Modells und der Variation der planetaren Eigenschaften.

State of the art

The origin and evolution of life on Earth is closely related to the state of the Earth system in the Archaean. According to the presently most fashionable model life began in a hydrothermal environment at high temperatures between 85C and 110C (Nisbet, 2001). Such a hyperthermophile "Noah" could even survive frequent major impacts capable of heating the ocean to over 100C. On the other hand, frequent impacts create a lot of mechanically damaged material that could easily be attacked chemically by weathering reactions. Depending on the kinetics of weathering (the main sink of atmospheric CO2 at long time scales) there could have been even a strong decrease of the Archaean surface temperatures creating an Archaean icehouse. But even an ice-covered Earth has itself been recommended as an environment for the origin of life (Sleep and Zahnle, 2001). The Earth underwent several global glaciations during the late Proterozoic era. During each Snowball Earth event the entire Earth froze over completely for tens of million of years. Such Snowball Earth events were terminated by extreme CO2 levels built up by volcanic outgassing (Hoffman and Schrag, 2002). The switch of Snowball Earth states to ultra-greenhouse conditions may have imposed an intense environmental filter on the evolution of life.

Figure 1: The habitable zone of Kasting et al. (1993)

The search for extrasolar Earth-like planets is one of the main goals of present research. Over 100 extrasolar giant planets are known to orbit nearby Sun-like stars including several multiple-planet systems. These giant planets, with hydrogen and helium as the main constituents, have atmospheres too turbulent to permit the emergence of life and have no underlying solid surfaces or oceans that could support a biosphere. The distribution of masses of all known exoplanets lets scientists suppose that there must be a multitude of planets with lower masses (Marcy et al., 2003). Even if it is today beyond the technical feasibility to detect Earth-mass planets we can apply computer models to investigate known exoplanetary systems to determine whether they could host Earth-like planets with surface conditions sufficient for the emergence and maintenance of life on a stable orbit. Such a configuration is described as dynamically habitable. Jones et al. (2001) have investigated the dynamical habitability of known exoplanetary systems. They used the boundaries of the habitable zone (HZ) originating from Kasting et al. (1993) . The inner boundary is defined as the maximum distance from the star where a runaway greenhouse effect would lead to the evaporation of all the surface water, and the outer boundary as the maximum distance at which a cloud-free carbon dioxide atmosphere could maintain a surface temperature above 0C. To test the intersection of stable orbits and the habitable zone, putative Earth-mass planets were launched into various orbits in the HZ and a symplectic integrator was used to calculate the celestial evolution of the extrasolar planetary system.

Own groundwork

In the former TRIPEDES project (2001-2003) we developed a coupled biosphere-geosphere model (CBGM) in order to describe the evolution of the Earth system (Franck et al., 2002b). This model describes the global carbon cycle between the reservoirs mantle, ocean floor, continental crust, continental biosphere, and the kerogen, as well as the aggregated reservoir ocean and atmosphere (Fig.1). We found a pronounced global minimum of the mean surface temperature at the present state and an extinction of the biosphere in about 1.2 billion years. This general model was later extended by introducing three different types of biosphere (prokaryotes, eucaryotes, and complex multicellular life). They differ in their environmental tolerances and additionally in their ability to increase life's promotion of weathering over geologic time (von Bloh et al., 2003c). Based on this model, a new hypothesis for the Cambrian explosion was presented. Due to a positive feedback mechanism a gradual cooling of the climate at the beginning of the Cambrian triggered a boost in biodiversity.

Figure 2: A coupled biosphere-geosphere model (CBGM). The fluxes from and to the different pools are indicated by arrows.

A simplified CBGM was used to investigate the habitability of selected extrasolar planetary systems. According to Franck et al. (2000b) habitability does not just depend on the parameters of the central star, but also on the properties of the planetary climate model. In particular, habitability is linked to the photosynthetic activity of the planet, which in turn depends on the planetary atmospheric carbon dioxide concentration, and is thus strongly influenced by the planetary geodynamics. The first investigated system, 47 UMa, consists of a solar-like star and two Jupiter-size planets beyond the outer edge of the stellar habitable zone, and thus resembles our own solar system most closely. It was found that Earth-type habitable planets around 47 UMa are in principle possible (Cuntz et al., 2003). This study was extended by considering Earth-like planets with different land/ocean coverages (Franck et al., 2003). We found that the likelihood for habitable Earth-like planets is significantly increased for planets with a high percentage of ocean surface (water worlds). Furthermore, we discussed the possibility of Earth-type planets in the planetary system 55 Cancri, which might host three giant planets (von Bloh et al., 2003d). In this case the probability for dynamically habitable Earth-like planets is even higher than for 47 UMa.

References

Hoffman, P.F., and Schrag, D.P. 2002. The snowball Earth hypothesis: testing the limits of global change. Terra Nova 14 , 129-155. (abstract)

Jones, B.W., Sleep, P.N., and Chambers, J.E. 2001. The stability of the orbits of terrestrial planets in the habitable zones of known exoplanetary systems. Astron. Astrophys. 366 , 254-262. (abstract)

Kasting, J.F., Whitmire, D.P., and Reynolds, R.T. 1993. Habitable zones around main sequence stars. Icarus 101 , 108-128. (abstract)

Marcy, G.W., Butler, R.P., Fischer, D.A., and Vogt, S.S. 2003. Properties of Extrasolar Planets. In: D. Deming and S. Saeger (eds.) Scientific Frontiers in Research on Extrasolar Planets. ASP Conf. Ser. 294, 1-16. (abstract)

Nisbet, E.G. 2001. Where Did Early Life Live, and What Was It Like? In: S. Guerzoni, S. Harding, T. Lenton, F.R. Lucchi (eds.) Earth System Science. ISEPS, Siena, 43-52.

Sleep, N.H., and Zahnle, K. 2001. Carbon dioxide cycling and implications for climate on ancient Earth. J. Geophys. Res. 106 (E1), 1373-1400. (link)