• Author
  • Ph.D. Jack D. Farmer
    School of Earth and Space Exploration, Arizona State University, Tempe, USA
  • Editors
  • Dr. Joachim Reitner
    Department of Geobiology, Center for Geosciences at the University of Göttingen, Göttingen, Germany
  • Dr. Volker Thiel
    Geoscience Center, Geobiology Group, Georg-August University of Göttingen, Göttingen, Germany
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Astrobiology

Synonyms

Bioastronomy; Exobiology

Definition

Astrobiology is the interdisciplinary science that studies the origin, evolution, distribution, and future of life in the Universe. It is a highly interdisciplinary field that engages many scientific disciplines, including biology, biogeochemistry, paleontology, earth and atmospheric sciences, planetology, and astrophysics among others.

Historical perspective

Astrobiology as a term was first coined in the 1940s to encompass early scientific ideas about how to explore for extraterrestrial life (Dick and Strick, 2004). During the 1960s, astrobiological research was focused more narrowly than today, with an emphasis on the origin of life, and programs funding these activities developed under the umbrella term "exobiology." This early period of activity culminated in the Viking mission to Mars, which carried life detection experiments designed to explore for life in Martian surface materials (Klein et al., 1976). With the failure of the Viking experiments to detect unambiguous evidence for life on Mars, interest in the search for life elsewhere in the solar system waned. Then in 1995, Dr. Wesley Huntress, who was the National Aeronautics and Space Administration's (NASA) Associate Administrator for Space Science at the time, reintroduced astrobiology to refer to a broad range of scientific activities that had begun to coalesce around questions of the potential for habitable environments and life beyond Earth. The following year, growth of the field of astrobiology was accelerated by the announcement by a team of scientists at NASA Johnson Space Center of possible signs of life in Martian meteorite, ALH84001 (McKay et al., 1996) (Figure 1). With this came a renewed interest in the search for Martian life. Over the next few years, NASA expanded research to develop more reliable approaches to extraterrestrial life detection, which helped to revitalize the field of astrobiology.
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Astrobiology. Figure 1 The ALH8400 meteorite was discovered in 1984, in the Alan Hills of Antarctica, where it had emerged from glacial ice. In the early 1990s, the meteorite was subjected to a battery of studies by a team at NASA Johnson Spaceflight Center (McKay et al., 1996). The group suggested that they had discovered evidence in the meteorite that was consistent with the presence of fossilized microorganisms. Most of the evidence for a Martian biology was associated with orange-colored globules of iron carbonate shown in this image (the image is about 5.5 cm across). Subsequent studies by the broader science community have shown that the suggested lines of evidence for life in the meteorite are more likely to be a result of inorganic processes that mimic the signatures of life.

In 1998, NASA helped promote the growth of astrobiology by creating the NASA Astrobiology Institute (NAI), a virtual research organization comprised of interdisciplinary teams of scientists from NASA Research Centers and universities around the United States. An important goal of the NAI has been to promote the growth of astrobiology through the sponsorship of collaborative, transdisciplinary research that could provide a scientific and technology development framework for future space missions. Following the establishment of the NAI, the growing community of scientists converged on a strategic plan that would help guide astrobiological research, including a scientific roadmap, with well-defined goals and objectives, and technology developments that would be needed to enable missions (Des Marais et al., 2008). Since its inception, the astrobiology roadmap has been revised and updated every few years to reexamine scientific priorities in the light of new scientific discoveries. The NAI has also embraced partnerships with other astrobiology institutions in Spain, Australia, and Great Britain. This includes international cosponsorship of yearly workshops designed to promote collaborative research and train the next generation of astrobiologists. Maturing of the field is also apparent in the development of two international journals for Astrobiology, international conferences each year and the establishment of an Astrobiology Society.

In 2008, the National Research Council reviewed the growth of the science of astrobiology (NRC, 2007a), summarizing the important collaborative efforts and scientific discoveries that had occurred since the inception of the NAI in 1998. Studies of the deep geological record of Earth have helped refine our knowledge of the early evolution of life on Earth, based on the fossil record, while providing new constraints on the nature and evolution of early environments. Studies of prebiotic organic chemical systems have revealed important new insights into the steps that may have preceded the emergence of terrestrial life, as well as the environments where life may have emerged (Russell et al., 1993; Deamer et al., 2002; Ricardo et al., 2004; Ferris, 2005).

Environmental limits for life

Biological studies have also significantly expanded our knowledge of the environmental limits of life on Earth (see Rothschild and Mancinelli, 2001), while the application of new tools of environmental molecular microbiology have revealed the presence of an extraordinarily diverse microbial biosphere, with an equally impressive array of metabolic strategies for extracting energy from Earth's environments (e.g., Staley and Reysenbach, 2001). Life has been shown to occupy a broad range of temperature (−15°C up to 122°C; see Kashefi and Lovely, 2003; also, Figure 2 from Takai et al., 2008) and nearly the full range of pH from acidic to alkaline (pH ∼1.0 to 13.0), over a range of salinities from freshwater to hypersaline brines and water activities >0.6. We have also learned of the importance of the deep subsurface biosphere, which may account for possibly half or more of the Earth's biomass (Gold, 1998). The discovery of lithoautotrophs that can use the simple chemical compounds, such as hydrogen and methane (CH4), released during the low-temperature inorganic weathering of rocks or sulfide (H2S) formed by hydrothermal processes has opened up opportunities for the survival of microbial life in the deep subsurface, independent of photosynthesis (Stevens, 1997). These discoveries and others in the fields of biology and paleontology have helped broaden the search space for habitable environments and life elsewhere in our Solar System and beyond.
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Astrobiology. Figure 2 Life is known to exist over a broad range of physical and chemical conditions and uses an equally impressive array of metabolic processes for extracting energy from the environment. This image was obtained a using a fluorescence microscope. The form shown is known as Methanopyrus kandleri and is a hyperthermophile (growth optimum >80°C) that uses carbon dioxide and hydrogen for its metabolism, producing methane as a by-product. The species was discovered at a submarine hydrothermal vent in the Gulf of California, living at temperatures between 84°C and 110°C. Strain 116 of this species has been shown to grow up to 122°C and survive up to ∼130°C. This is currently the highest temperature organisms known (Kashefi and Lovely, 2003; Takai et al., 2008). Each cell in this image is only a few microns in length.

Defining life

Earth has the distinction of being the only place in the Universe where life is known to exist. Thus, terrestrial life provides a logical starting point for evaluating the prospects for the existence of life elsewhere and identifying strategies for astrobiological exploration. For exploration, we need to identify basic attributes of living systems that can be used to inform exploration strategies to search for life beyond the Earth. Ideally, we would like to follow universal criteria that are not tied to the particular circumstances found on Earth. But, universal definitions of life have proven elusive and challenging to apply (Cleland and Chyba, 2002). As a practical approach to exploration, the search for life begins with what is known of terrestrial life, following a terracentric definition that combines features like a water-based carbon chemistry, and self-replication, with cellular systems that evolve through processes of Darwinian natural selection to adapt to environmental changes. Although such attributes are regarded as essential properties of life, many inorganic systems display similar features, such that a single property, while necessary, is not regarded as sufficient to define life. Methods of life detection for astrobiological exploration of other planetary environments are still in their infancy but presently combine several instrumental techniques that can independently measure life's attributes, as a basis for detection. An example is the Pasteur instrument, which combines the detection of amino acids in soils and ices, with measurements of the chirality (right- versus left-handedness) in amino acid building blocks of proteins (Skelley et al., 2005). Terrestrial life selects left-handed amino acids when making proteins, and this selectivity has been suggested to be a biosignature that could be used to explore for life elsewhere (Figure 3).
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Astrobiology. Figure 3 In nature, amino acids come in right- and left-handed forms called isomers that are identical in composition and differ only in structure. On Earth, amino acid isomers occur in nature in equal abundances (i.e., racemic mixtures). However, when making proteins, organisms select only left-handed amino acids. While the origins of chirality remain unclear, some have suggested that this unique property of terrestrial life may provide a useful signature for identifying life on other worlds.

Exploring for life in the Solar System

In exploring for life in the Solar System, NASA has adopted a simple approach that emphasizes the search for those environmental factors that are known requirements of terrestrial life. At the top of the list is liquid water, which is required by all known life-forms. In their exploration of the Solar System and beyond, NASA has pursued an exploration strategy designed to "Follow the Water." Liquid water plays an essential role in living systems as a medium of biochemical transport and exchange during metabolic reactions. The unique solvent properties of liquid water are primarily derived from the dipolar charge distribution of water molecules and the ease with which hydrogen bonds may be formed and broken in solution. These properties also determine many of the unique physical properties of water, including the broad temperature range, over which it remains liquid and the decrease in density that occurs upon freezing. This has the interesting consequence that the crystalline structure of water expands and density decreases during freezing, so that the ice floats at the surface instead of sinking to the bottom. Thus, oceans freeze from the top down, thereby enhancing long-term habitability. Terrestrial examples that illustrate the importance of this for habitability include perennially ice-covered lakes of the "dry valleys" of Antarctica. High salinity (which depresses the freezing point), combined with the self-insulating properties of the frozen surface of the lake, promotes the persistence of habitable zones of liquid water environments, even in the winter when air temperatures have fallen far below the freezing point (Andersen et al., 1995). On Mars, one consequence of this is that ice-covered lakes could have persisted on Mars long after the planet began to lose its atmosphere, thus rendering liquid water at the surface unstable (Wharton et al., 1995; McKay and Davis, 1991). Recent climatic variations on Mars mediated by changes in the obliquity and recorded in polar deposits, suggests the possibility that transient habitable conditions could have developed at the Martian surface very recently in the planet's history (Figure 4).
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Astrobiology. Figure 4 The High Resolution Stereo Camera (HRSC), onboard the European Space Agency's Mars Express Orbiter, acquired this image of the Chasma Boreale region of the northern polar cap of Mars. Erosive winds carved the large canyon, Chasma Boreale, cutting deeply into polar deposits and exposing a record of past climate changes. The image shows finely layered, ice-rich sedimentary sequences that appear to reflect cyclical obliquity changes in the climate of Mars, occurring over the past few hundred thousand years. This caused the polar caps to wax and wane, affecting the habitability of the planet's surface.
It is also widely appreciated that living systems require more than liquid water. Habitable environments are also sources of the biological elements (C, H, N, O, P, S), inclusive of about two-dozen transition metals that are critical for enzyme function (Crabb and Moore, 2009), and sources of energy for sustaining cellular metabolism, growth, and replication (Hoehler et al., 2007). Thus, the exploration for potentially habitable zones in the Solar System and beyond has been focused on the search for environments where liquid water coexists with sources of biogenic elements and energy (Figure 5).
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Astrobiology. Figure 5 In January 2004, the Mars Exploration Rover (MER) mission landed twin rovers on the surface of Mars to explore for evidence of ancient habitable environments. The Opportunity Rover, which landed at Meridiani Planum, discovered unequivocal evidence of past water in the form of sulfate minerals, which only form in the presence of water (Knoll et al., 2005). This image, taken by the MER Opportunity Panoramic Camera (Pancam), shows light-toned slabs of sulfate-rich sedimentary rocks that have been partially buried by migrating sand dunes. (Image Credit: NASA.)

Recent progress in astrobiology

In parallel with the studies of life on Earth, missions to other bodies in our Solar System have revealed many examples of potentially habitable environments, with evidence for the past, or present existence of liquid water, biologically important elements, and potential energy sources. The exploration of Mars has identified widespread geological evidence for climatic changes that could have affected ancient surface water systems, while also strengthening the case for liquid water in the subsurface today (NRC, 2007b). NASA missions have also "followed the water" to the outer Solar System, with the discovery of brines beneath the surfaces of three of the icy Galilean satellites of Jupiter (Europa, Ganymede, and Callisto; Kivelson et al., 2000; Greenberg, 2008), as well as Enceladus, one of the moons of Saturn (Kieffer et al., 2006). In addition, Saturn's moon, Titan, has also been shown to harbor water ice as well as lakes of liquid methane (Mitri et al., 2007) (Figure 6).
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Astrobiology. Figure 6 During a close flyby, the Galileo spacecraft imaged the surface of Europa, one of the moons of Jupiter, showing a pervasively fractured surface composed of water ice that has apparently formed by the upwelling of warm ice and/or brines, from deep within the moon's interior. The internal energy believed to drive melting has been attributed to tidal flexing of the interior due to gravitational forces from Jupiter and the other moons that orbit the planet. Whether there is a global subsurface ocean on Europa, or warm, convecting ice is still uncertain, but magnetometer experiments onboard the Galileo Orbiter suggest the presence of subsurface brines (Kivelson et al., 2000). (Image Credit: NASA.)
The discovery of lakes of methane on Titan has led to a debate over the potential for life to originate within highly reducing, hydrocarbon-rich environments, where liquid water may be replaced by alternative solvents, such as liquid methane or ammonia, as alternative solvents for life processes (e.g., Benner et al., 2004) (Figures 7 and 8).
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Astrobiology. Figure 7 The Cassini-Huygens mission to Saturn has revealed the presence of hydrocarbon lakes on the surface of Saturn's moon, Titan. Beyond Earth, these are the only other lakes in the Solar System. Yet, they are composed of liquid methane and other hydrocarbons - not water (Mitri et al., 2006). This discovery has led some scientists to consider the possibility that liquid hydrocarbons such as methane or ammonia might provide alternative solvents to sustain life-forms quite different from those found on Earth. (Image Credit: NASA.)
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Astrobiology. Figure 8 Enceladus, a moon of Saturn, imaged during flybys of the Cassini orbiter, has been shown to actively erupt plumes of water ice, vapor, and organic compounds from the moon's interior (Kiefer et al., 2006). These eruptions occur along fracture systems located near the south pole. (Image credit: NASA.)
Finally, discoveries in observational astronomy have shown that planets and planetary systems to be common features of other stars in the nearby Milky Way Galaxy. Since the first confirmed discovery in 1992, more than 450 extrasolar planets have been identified, and the number continues to grow each year (Schneider, 2010). Orbiting planets have been detected by a variety of methods, but primarily by measuring small perturbations in the position of the parent star due to the gravitational tug and pull of an orbiting planet(s), or as a result of slight decreases in the stars brightness as an orbiting planetary body passes in front of the star. More rarely, extrasolar planets have also been discovered using direct imaging methods (Figure 9).
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Astrobiology. Figure 9 Formalhaut b is a multiple Jupiter-mass planet that was directly imaged in its orbit around the star Formalhaut, located about 25 light years away from Earth in the constellation Piscus Austrinus (Kalas et al., 2008).

Summary

The emerging field of astrobiology is taking a highly interdisciplinary approach to the basic question: "Are we alone in the Universe?" An answer to this question is being pursued on a large number of fronts both within and beyond our Solar System. For example, studies of biology on Earth has continued to expand the environmental limits for life while revealing a dizzying array of potential energy sources. This has significantly expanded the potential number of habitable zones in the solar system, opening up new opportunities for astrobiological exploration in the solar system. Ongoing exploration of the planet Mars, long regarded as most Earth-like in its potential habitability, has provided abundant evidence for past water on the planet's surface, along with potential building blocks for life and sources of energy to support living systems. Although the present surface of Mars appears to be uninhabitable due to an absence of liquid water, life could still exist in the subsurface (Warner and Farmer, 2010), or its remains be preserved in ancient aqueous sediments exposed at the surface (Farmer, 2000; Farmer and Des Marais, 1999). Future exploration will continue to pursue the search for fossil or extant biosignatures of microbial life on Mars, in the hope of making a transformative and revolutionary discovery of life. But exploration is also pursuing the question of microbial life in potentially habitable icy moons of the outer Solar System, specifically three of the Galilean moons of Jupiter (Europa, Ganymede, and Callisto), as well as Saturn's moon, Enceladus, where internal zones of liquid water are maintained not by solar radiation but by the internal tidal heating resulting from gravitational flexing of the interior of these moons. In addition, Jupiter's moon, Titan, has provided access to potentially habitable environments that are fundamentally different from Earth. On Titan, we have discovered alternative environments potentially capable of supporting non-terran forms of life, sustained by alternative solvents (oceans of liquid methane and ammonia) and energy sources. Finally, the discovery of numerous planets and planetary systems orbiting other stars in the nearby galaxy has widened the search for habitable environments beyond the confines of our Solar System. These and other important discoveries have shaped the way astrobiologists evaluate potential for the origin, evolution, and persistence of life elsewhere, contributing to the development of a robust conceptual framework for planning missions and developing the technologies needed to explore for extraterrestrial life (e.g., Mix et al., 2006; Plaxco and Gross, 2006; Sullivan and Baross, 2007).

Cross-references

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