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.
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
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.
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
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.
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
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.)
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.)
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.)
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).
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).
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