Siegfried Franck

The habitability of super-Earths in Gliese 581

Aims. The planetary system around the M star Gliese 581 consists of a hot Neptune (Gl 581b) and two super-Earths (Gl 581c and Gl 581d). The habitability of this system with respect to the super-Earths is investigated following a concept that studies the long-term possibility of photosynthetic biomass production on a dynamically active planet. Methods. A thermal evolution model for a super-Earth is used to calculate the sources and sinks of atmospheric carbon dioxide. The habitable zone is determined by the limits of photosynthetic life on the planetary surface. Models with different ratios of land / ocean coverage are investigated. Results. The super-Earth Gl 581c is clearly outside the habitable zone, since it is too close to the star. In contrast, Gl 581d is a tidally locked habitable super-Earth near the outer edge of the habitable zone. Despite the adverse conditions on this planet, at least some primitive forms of life may be able to exist on its surface. Therefore, Gl 581d is an interesting target for the planned TPF/Darwin missions to search for biomarkers in planetary atmospheres.

Determination of habitable zones in extrasolar planetary systems: Where are Gaia's sisters?

A general modeling scheme for assessing the suitability for life of extrasolar planets is presented. The scheme focuses on the identification of the “habitable zone” in main sequence star planetary systems accommodating Earth-like components. Our definition of habitability is based on the long-term possibility of photosynthetic biomass production under geodynamic conditions. Therefore all the pertinent astrophysical, climatological, biogeochemical, and geodynamic processes involved in the generation of photosynthesis-driven life conditions are taken into account. Implicitly, a cogenetic origin of the central star and the orbiting planet is assumed. A geostatic model version is developed and investigated in parallel for demonstration purposes. The numerical solution of the advanced geodynamic model yields realistic lookup diagrams for convenient habitability determination. As an illustration, the MACHO-98-BLG-35 event is scrutinized. It is shown that this event is definitely not tantamount to the discovery of one of Gaias sisters.

Modelling the global carbon cycle for the past and future evolution of the earth system

The Earth may be described as a global system consisting of the components solid Earth, hydrosphere, atmosphere, and biosphere. This system evolves under the external influence of increasing solar luminosity. In spite of this changing external forcing, the Earths climate has been stabilized by negative feedbacks against global freezing in the past (faint young Sun paradox). The future long-term trend of further increasing solar luminosity will cause a further atmospheric CO2 decrease. Atmospheric CO2 will fall below the critical level for photosynthesis and the plant based biosphere will die out. In the present paper we propose a modelling study of the evolution of the carbon cycle from the Archaean to the planetary future. Our model is based on a paper published previously by Caldeira and Kasting [Caldeira, K., Kasting, J.F., 1992. The life span of the biosphere revisited. Nature 360, 721–723]. The difference of the current study with respect to this work resides in the forcing function used for the silicate weathering rate. While Caldeira and Kasting used a constant weathering rate over time, we calculate the time evolution of this rate by assuming a balance between the weathering flux and the CO2 release flux by volcanism and metamorphism. We use the geodynamics theory to couple the two internal forcing functions continental area (for weathering) and spreading (for CO2 release flux) which were generally considered as independent in previous models. This coupling introduces an additional feedback in the system. We find a warmer climate in the past and a shortening of the life span of the biosphere up to some hundred million years.

Reduction of biosphere life span as a consequence of geodynamics

The long-term co-evolution of the geosphere’biospere complex from the Proterozoic up to 1.5 billion years into the planet’s future is investigated using a conceptual earth system model including the basic geodynamic processes. The model focusses on the global carbon cycle as mediated by life and driven by increasing solar luminosity and plate tectonics. The main CO2 sink, the weathering of silicates, is calculated as a function of biologic activity, global run-off and continental growth. The main CO2 source, tectonic processes dominated by sea-floor spreading, is determined using a novel semi-empirical scheme. Thus, a geodynamic extension of previous geostatic approaches can be achieved. As a major result of extensive numerical investigations, the “terrestrial life corridor”, i.e., the biogeophysical domain supporting a photosynthesis-based ecosphere in the planetary past and in the future, can be identified. Our findings imply, in particular, that the remaining life-span of the biosphere is considerably shorter (by a few hundred million years) than the value computed with geostatic models by other groups. The “habitablezone concept” is also revisited, revealing the band of orbital distances from the sun warranting earth-like conditions. It turns out that this habitable zone collapses completely in some 1.4 billion years from now as a consequence of geodynamics.

Habitable zone for Earth-like planets in the solar system

Abstract We present a new conceptual Earth system model to investigate the long-term co-evolution of geosphere and biosphere from the geological past upto 1.5 billion years into the planets future. The model is based on the global carbon cycle as mediated by life and driven by increasing solar luminosity and plate tectonics. As a major result of our investigations we calculate the “terrestrial life corridor”, i.e. the biogeophysical domain supporting a photosynthesis-based ecosphere during planetary history and future. Furthermore, we calculate the behavior of our virtual Earth system at various distances from the Sun, using different insolations. In this way, we can find the habitable zone as the band of orbital distances from the Sun within which an Earth-like planet might enjoy moderate surface temperatures and CO2-partial pressures needed for advanced life forms. We calculate an optimum position at 1.08 astronomical units for an Earth-like planet at which the biosphere would realize the maximum life span. According to our results, an Earth-like planet at Martian distance would have been habitable upto about 500 Ma ago while the position of Venus was always outside the habitable zone.

Continental growth and volatile exchange during Earth's evolution

Abstract We investigate the thermal and degassing history of the Earth with the help of a parameterized mantle convection model including the volatile exchange between mantle and surface reservoirs. The weakening of mantle silicates by dissolved volatiles is described by a functional relationship between creep rate and water fugacity. We use flow law parameters of diffusion creep in olivine under dry and wet conditions. The mantle degassing rate is considered as directly proportional to the seafloor spreading rate, which is also dependent on the mantle heat flow and the continental area. To calculate the spreading rate, we assume three different continental growth models: constant growth, delayed growth, and the one proposed by Reymer and Schubert (1984, Tectonics, 3: 63–77). The rate of regassing also depends on the seafloor spreading rate, as well as on other factors. Both mechanisms (degassing and regassing) are coupled self-consistently with the help of a parameterized convection model under implementation of a temperature and volatile-content dependent mantle viscosity. We calculate time series for the Earths evolution over 4.6 Gyr for the average mantle temperature, the mantle heat flow, the mantle viscosity, the Rayleigh number, the Urey ratio, the volatile loss, and the seafloor spreading rate. In those numerical simulations with continental growth from the beginning and a high initial average mantle temperature water is outgassed rapidly. In the delayed continental growth model there is a very early outgassing event and the delayed continental growth has no remarkable influence on the thermal and outgassing history. A similar situation is found for the linear continental growth model but not for the Reymer and Schubert (1984) model.

Effects of water-dependent creep rate on the volatile exchange between mantle and surface reservoirs

Abstract A parameterized model of mantle convection, including the volatile exchange between mantle and surface reservoirs, is used to study the thermal history of the Earth. The influence of dissolved volatiles on mantle rheology is reformulated. The weakening of mantle materials is described by a functional relationship between creep rate and water fugacity. We use flow law parameters of diffusion creep in olivine under dry and wet conditions. The mantle degassing rate is considered as directly proportional to the seafloor spreading rate, which, in turn, is dependent on the mantle heat flow. To calculate the spreading rate, we assume that the heat flow under the mid-ocean ridges is double the average mantle heat flow. The rate of regassing also depends on the seafloor spreading rate, as well as on other factors such as the efficiency of volatile recycling through island arc volcanism. Both mechanisms (degassing and regassing) are coupled self-consistently with the help of a parameterized convection model under implementation of a different formulation of mantle viscosity. The model is run for 4.6 Gyr. Time series of average mantle temperature, volatile loss, mantle viscosity, mantle heat flow, Rayleigh number and Urey ratio are calculated. The effects of changing initial parameters have been tested and found negligible for the final results. Mantle water is outgassed rapidly within a time scale of less than 200 Myr for all numerical simulations. The present-day values of calculated parameters are in the generally accepted range.

On the possibility of Earth-type habitable planets around 47 UMa

Abstract We investigate whether Earth-type habitable planets can in principle exist in the planetary system of 47 UMa. The system of 47 UMa consists of two Jupiter-size planets beyond the outer edge of the stellar habitable zone, and thus resembles our own Solar System most closely compared to all exosolar planetary systems discovered so far. Our study of habitability deliberately follows an Earth-based view according to the concept of Franck and colleagues, which assumes the long-term possibility of photosynthetic biomass production under geodynamic conditions. Consequently, a broad variety of climatological, biogeochemical, and geodynamical processes involved in the generation of photosynthesis-driven life conditions is taken into account. The stellar luminosity and the age of the star/planet system are of fundamental importance for planetary habitability. Our study considers different types of planetary continental growth models and takes into account a careful assessment of the stellar parameters. In the event of successful formation and orbital stability, two subjects of intense research, we find that Earth-type habitable planets around 47 UMa are in principle possible! The likelihood of those planets is increased if assumed that 47 UMa is relatively young (≲6 Gyr) and has a relatively small stellar luminosity as permitted by the observational range of those parameters.

EVOLUTION OF THE GLOBAL MEAN HEAT FLOW OVER 4.6 GYR

Abstract The mean global heat flow of a planet depends mainly on the initial mantle temperature, the distribution of the radiogenic heat sources, and the mechanism of the heat transport within the mantle. It is well known that subsolidus convection is the dominant mechanism of the heat transport within the mantle. With the help of parametrized convection models (Franck and Bounama, 1995; and references given therein) temporal variations of the average mantle temperature and the surface heat flow in terms of the Rayleigh number can be calculated under certain initial conditions. In our numerical experiments we calculate the thermal and the degassing history for two different initial average mantle temperatures: 2400 K and 3000 K. Looking at the two different starting temperatures, we find qualitatively different behaviour in the first 1–1.5 Gyr of thermal evolution. After this stage the two curves converge rapidly showing the so-called readjustment-effect. After readjustment the quantities decrease monotonically. Furthermore, we investigate the influence of continental growth and the thermal and degassing history by using simple analytical continental growth models. We compare our results with that of laboratory melting experiments of komatiites providing estimates for the secular mantle cooling and discuss relations between global mean heat flow and the areal spreading rate.

Cambrian explosion triggered by geosphere-biosphere feedbacks

[1] A new hypothesis for the cause of the Cambrian explosion is presented. For that the evolution of the planet Earth is described by the co-evolution of the geospherebiosphere system. Here we specify our previously published Earth system model for the long-term carbon cycle by introducing three different types of biosphere: procaryotes, eucaryotes, and complex multicellular life. They are characterized by different global temperature tolerance windows. The biotic enhancement of silicate weathering by complex multicellular life adds an additional feedback to the system and triggers the Cambrian explosion. The Cambrian explosion is characterized by a sudden increase of biomass and a rapid cooling, which amplified the spread of complex multicellular life. Cooling events in the Neoproterozoic, however, could force a premature appearance of complex multicellular life. INDEX TERMS: 0330 Atmospheric Composition and Structure: Geochemical cycles; 8125 Tectonophysics: Evolution of the Earth; 9699 Information Related to Geologic Time: General or miscellaneous; 3220 Mathematical Geophysics: Nonlinear dynamics; 0315 Atmospheric Composition and Structure: Biosphere/atmosphere interactions. Citation: von Bloh, W., C. Bounama, and S. Franck, Cambrian explosion triggered by geosphere-biosphere feedbacks, Geophys. Res. Lett., 30(18), 1963, doi:10.1029/2003GL017928, 2003.