Space Sci Rev (2007) 129: 1–5 DOI 10.1007/s11214-007-9208-0
Introduction: A Multidisciplinary Approach to Habitability Kathryn E. Fishbaugh · David J. Des Marais · Oleg Korablev · Philippe Lognonne · François Raulin
Received: 26 January 2007 / Accepted: 10 May 2007 / Published online: 6 June 2007 © Springer Science+Business Media B.V. 2007
Developments during the early years of space exploration provided particular impetus toward articulating the concept of habitable planets beyond Earth. One of these developments was the collective results of the life-detection experiments conducted by the Viking missions, which demonstrated that an improved understanding of environmental context was crucial to the search for evidence of life on Mars. Another development was the expansion of the search for extraterrestrial intelligence and Frank Drake’s articulation in 1961 of his famous Drake Equation (Drake and Sobel 1992), which identifies the parameters that collectively estimate the probability of detecting intelligent life elsewhere. A meeting entitled “Life in the Universe” was held in 1979 at NASA Ames Research Center (Billingham 1981). In addition to articulating topics in astronomy and biology, the participants disseminated key factors that sustain habitable planetary environments. They addressed, among other factors, stellar and orbital influences, planetary accretion and early development, atmospheres, climate, and the origins and evolution of continents and oceans. An additional strong impetus K.E. Fishbaugh (!) International Space Science Institute (ISSI), Hallerstrasse 6, 3012 Bern, Switzerland e-mail:
[email protected] D.J. Des Marais NASA Ames Research Center, Mail Stop 239-4, Moffett Field, CA 94035, USA O. Korablev Russian Space Research Institute (IKI), Profsoyuznaya 84/32, 117997 Moscow, Russia P. Lognonne Equipe Etudies Spatiales et Planétologie (IPGP), Université de Paris VII, 4 Avenue Neptune, 94107 Saint Maur des Fossés, France F. Raulin Laboratoire de Interuniversitaire des Systèmes Atmosphériques (LISA, UMR CNRS), Université Paris VII et Paris XII, Val de Marne, 61, Avenue du Général de Gaulle, 94010, Créteil Cedex, France
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to develop models of habitable planets orbiting other stars has been provided by ESA’s Darwin and NASA’s Terrestrial Planet Finder missions (e.g., Kaltenegger and Fridlund 2005; Des Marais et al. 2002). Several additional meetings and technical papers have now established that the search for life in the universe necessarily includes efforts to locate and characterize potentially habitable planetary environments. This book has arisen from a unique workshop, entitled “Geology and Habitability of Terrestrial Planets”, held at the International Space Science Institute (ISSI) in Bern, Switzerland, on 5–9 September 2005. The goal of this workshop, as articulated by the conveners, was “to define the influence of planetary geologic evolution on habitability and to assess the conditions necessary for life, using the Earth, Mars, and Venus as examples.” The intent was to focus on the relationship between a planet’s origin and evolution and its capacity to sustain habitable environments. Many previous books and conferences have dealt extensively with specific topics in astrobiology, such as the search for life on Mars (e.g., Tokano 2005) and for intelligent life in the universe (e.g., Ulmschneider 2006). A planet’s geologic evolution and its relationship with its star are the main drivers for maintaining long-term habitability and for carrying life from apparition to eventual evolution. By focusing on the co-evolution of geology and habitability, this workshop has added a new dimension to the current focus of astrobiological studies and has combined insights from multiple relevant research disciplines into a broad-scale treatment of habitability. The “Geology and Habitability of Terrestrial Planets” workshop clearly maintained the interdisciplinary character of ISSI workshops; 53 scientists from 12 countries collaborated to weave together geologic, atmospheric, geophysical, magnetospheric, and biological planetary studies. What makes this book truly unique is the cooperation of scientists from multiple disciplines on each chapter. Thus, these chapters tackle the topic of habitability from the viewpoints of multiple branches of planetary science in an interdisciplinary way, rather than representing a compendium of individual research papers presented at the conference. In some sense, however, this book can be considered a “conference proceedings” in that the authors have taken care to include many aspects of what was presented in formal talks and discussed by the participants as a group. Defining the term “habitability” is not trivial. In this book, you will find “habitability” defined and addressed in myriad ways; no one definition can cover all aspects of the term. One could propose that a habitable planet lies within the so-called Habitable Zone around a star, a zone defined by temperature wherein water at the planet’s surface would be expected to be liquid (Kasting et al. 1993). Yet such a definition would completely exclude the actual geologic and atmospheric conditions of the planet. An alternative definition could be that a habitable planet is one on (or within) which life can exist. Even this definition is lacking, as it does not account for the fact that life may only exist on that planet for a very short period of time. If life does not thrive for long enough to spread wide enough or to multiply sufficiently for later detection of even fossils or biomarkers, can the planet still be considered to have been habitable? Our concepts of the attributes of life-sustaining planetary environments must be based, at least initially, upon the requirements of our own biosphere (Marais et al. 2003). Microorganisms tolerate greater environmental extremes than do plants and animals; therefore, biospheres having only microorganisms are expected to be more widespread than biospheres similar to our own. Microbial life requires chemical building blocks, utilizable sources of energy, and relatively stable conditions that can sustain liquid water at least intermittently. To search for evidence of these requirements on other planets and to understand our own origins, we must gain a fundamental understanding of planetary processes and history. The
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attributes of life arise not only from physical and chemical principles but also from the nature and evolution of the planetary environments in which life developed. Differences in the interplay between cosmic events and planetary processes might have shaped other life forms in ways that are far more diverse than occurred in our own biosphere. This book sets forth the necessary concepts and approaches to begin to comprehend that potentially enormous diversity. Factors that determine the habitability of a planet are multifaceted and include but are certainly not limited to the presence or absence of plate tectonics, the presence or absence of a magnetosphere, the characteristics and evolution of the planet’s atmosphere, the presence of liquid water, the presence of energy sources, variability in insolation, and even the means to transport nutrients to and wastes away from organisms. Some researchers are beginning to reassess the definition of the Habitable Zone around a star by taking some of these factors into account along with the liquid-water temperature requirement (Franck et al. 2000). It is evident that even the few factors listed here cover the entire spectrum of a planet’s properties and are interrelated. Without plate tectonics on Earth, mid-ocean ridges (a possible location for the origin of life (e.g., Russell and Hall 1997) and continental habitats would not exist. More importantly, CO2 trapped in carbonate would not be recycled, but instead it could generate global warming incompatible with life and even with liquid water (as was the case for Venus). The Earth’s magnetosphere shields our atmosphere from erosion by the solar wind, and the dynamo that generates this shield is maintained by plate tectonics; mantle viscosity is reduced by injection of water at subduction zones and by the efficiency of plate tectonics at cooling the mantle and hence the core (e.g., van Thienen et al. 2005). Both plate tectonics and the generation of a magnetosphere depend upon the nature of the internal dynamics of the planet. Clearly, particular forms of life will utilize particular atmospheric chemistries. For example, the earliest forms of terrestrial life did not benefit from any oxygen (e.g., Canfield et al. 2000). The characteristics of a planet’s atmosphere will depend on its position relative to its star, the number of volatile-bearing impacts it receives, the amount of volatile outgassing from volcanism (which is in turn, again, linked to internal dynamics), and other factors. Of course, many organisms (endoliths, chemotrophs, etc.) do not directly require an atmosphere at all and so may not even be directly affected by the atmosphere or lack thereof. Liquid water and a source of energy are two important criteria for habitability according to life as we know it on Earth. Whether water exists on the planet depends upon many of the same factors that determine the atmospheric characteristics, such as temperature and distance from its star. Sources of energy can be varied: geothermal heat, insolation, volcanic activity, or even tidal heating (as in the case of Europa (e.g., Ross and Schubert 1987)). Timing is also crucially important. Even if a planet is likely to have liquid water on its surface (like Mars at various times in its past (e.g., Carr 1996)) and a sufficient energy source, this water and energy are of little consequence to habitability unless they existed long enough for any potential life to use them and thrive off of them. Interestingly, it was suggested during workshop discussion that a flux of nutrients to and wastes away from a potential habitat is also crucially important to life. For example, if water pools in an area where it cannot leak into the groundwater system or escape via a channel and somehow be replenished, this closed system may not provide the necessary flux. External factors also play a significant role in a planet’s habitability. For example, if the obliquity of a planet changes with time, then the resultant changes in insolation will affect changes in climate. This is apparent in the imprint of the Earth’s Milankovitch cycles on climate records gleaned from such sources as ice and deep-sea cores and in the modeled
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effects of obliquity changes on Mars’ climate (e.g., Forget et al. 2006; Mischna et al. 2003). These effects are more pronounced on Mars than on Earth since the Moon stabilizes Earth’s rotation axis (Laskar et al. 1993). This discussion illustrates that a planet’s habitability depends upon a wide range of factors that cover the entire scope of a planet’s characteristics. For this reason, a multidisciplinary book on the subject is a crucial step in our general understanding of habitability. The book chapters follow the themes of the workshop. Chapter 1 (Southam et al. 2007) introduces habitability from a fundamental, biological perspective. This chapter delves into the questions of what requirements are necessary for life to exist and how geology creates those environments. The authors also tackle the difficult question of what is needed for life to originate in the first place, including speculation about the potential for a second genesis on Mars or Europa. Chapter 2 (Zahnle et al. 2007) focuses on the earliest stages of Earth history and the development of habitable conditions. The authors pay particular attention to the effects of high impact rates. Chapter 3 (Nisbet et al. 2007) then covers a broad scope of topics, describing how the geologic evolution of a planet, both on the surface and in the near subsurface, creates habitable environments that evolve along with the geology. Chapter 4 (Bertaux et al. 2007) narrows in on the importance to habitability of volatiles in the atmosphere, on the planet’s surface (e.g., liquid water), and in rocks. This chapter borrows from the writing style of Galileo, framing the multidisciplinary parts as a discussion among famous founding fathers of science. Chapter 5 (van Thienen et al. 2007) reviews the exchanges among a planet’s interior dynamics, the atmosphere, and the magnetosphere and the ways in which these exchanges modify habitability. Chapter 6 explores these topics in a different light by addressing the stability of planetary atmospheres and their protection by the magnetosphere. This chapter has been divided into three parts, with an introduction by Lammer et al. (2007). Part 1 focuses on long-term solar variability and solar forcing of planets with magnetospheres (Lundin 2007). Part 2 (Kulikov et al. 2007) provides a comparative study of the early atmospheres of Earth, Mars, and Venus and the reasons for their consequent dissimilar evolutions. Part 3 (Dehant et al. 2007) then details the generation of magnetic dynamos and the protection of the atmosphere by the magnetosphere. In the epilogue, Lognonne et al. (2007) discuss two main aspects of investigations of life in the universe—genesis and maintenance of habitability—providing directions for future studies and missions. References J.-L. Bertaux, M. Carr, D. Des Marais, E. Gaidos (2007), this volume J. Billingham (Ed.), Life in the Universe. NASA Conference Publication Series, vol. 2156 (1981) D. Canfield, K. Habicht, B. Thamdrup, Science 288(5466), 658–661 (2000) M. Carr, Water on Mars (Oxford University Press, New York, 1996) V. Dehant, H. Lammer, T. Kulikov, J.-M. Griessmeier, D. Breuer, O. Verhoeven, Ö. Karatekin, T. van Hoolst, O. Korablev, P. Lognonne (2007), this volume D. Des Marais, M. Harwitt, K. Jucks, K. Kasting, L. Lunine, D. Lin, S. Seager, J. Schneider, W. Tarub, N. Woolf, Astrobiology 2(2), 153–181 (2002) F. Drake, D. Sobel, Is Anyone Out There? The Scientific Search for Extraterrestrial Intelligence (Delacorte Press, New York, 1992) F. Forget, R. Haberle, F. Montmessin, B. Levrard, J. Head, Science 311(5759), 368–371 (2006) S. Franck, A. Block, W.V. Bloh C. Bounama, H. Schellnhuber, Y. Svirezhev, Planet. Space Sci. 48(11), 1099– 1105 (2000) L. Kaltenegger, M. Fridlund, Adv. Space Res. 36, 1114–1122 (2005)
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J.F. Kasting, D. Whitmire, R. Reynolds, Icarus 101(1), 108–128 (1993) Y. Kulikov, H. Lammer, H. Lichtenegger, T. Penz, D. Breuer, T. Spohn, R. Lundin, H. Biernat (2007), this volume H. Lammer, V. Dehant, O. Korablev, R. Lundin (2007), this volume J. Laskar, J. Joutel, P. Robutel, Nature 361, 615–617 (1993) P. Lognonne, D.D. Marais, F. Raulin, K. Fishbaugh (2007), this volume R. Lundin (2007), this volume D.D. Marais, L. Allamandola, S. Brenner, A. Boss, D. Deamer, P. Falkowski, J. Farmer, B. Hedges, B. Jakosky, A. Knoll, D. Liskowsky, V. Meadows, M. Meyer, C. Pilcher, K. Nealson, A. Spormann, J. Trent, W. Turner, N. Woolf, H. Yorke, Astrobiology 3(2), 219–235 (2003) M. Mischna, M. Richardson, R. Wilson, D. McCleese, J. Geophys. Res. 108E(06) (2003). doi: 10.1029/ 2003JE002051 E. Nisbet, K. Zahnle, J. Helbert, R. Jaumann, B. Hoffmann, K. Benzerara, F. Westall (2007). this volume M. Ross, G. Schubert, Nature 325, 133–134 (1987) M. Russell, A. Hall, J. Geol. Soc. London 154(3), 377–402 (1997) G. Southam, L. Rothschild, F. Westall (2007), this volume T. Tokano (Ed.), Water on Mars and Life, vol. XVI. Advances in Astrobiology and Biogeophysics Series, A. Brack, G. Horneck, M. Mayor, C. McKay (Eds.), (Springer, Dordrecht, 2005), 332 pp P. Ulmschneider, Intelligent Life in the Universe (Springer, Dordrecht, 2006) P. van Thienen, K. Benzerara, D. Breuer, C. Gillman, S. Labrosse, P. Lognonne, T. Spohn (2007), this volume P. van Thienen, N. Vlaar, A.v.d. Berg Phys. Earth Planet. Interiors 150, 287–315 (2005) K. Zahnle, N. Arndt, C. Cockell, A. Halliday, E. Nisbet, F. Selsis, N. Sleep (2007), this volume
