Astrobiology is a relatively new term that embraces the multidisciplinary study of the living Universe. It is the investigation of the origin, evolution, distribution, and destiny of life in the Universe. Astrobiology addresses some of the most profound questions of humankind: How did life begin? Are there other planets like Earth? What is our future as terrestrial life expands beyond the home planet? These questions are age-old. In the twenty-first century, however, advances in biological sciences, informatics, and space technology may make it possible for us to provide some answers.
Although the term had been used occasionally during previous decades as a synonym for “exobiology,” astrobiology in its present incarnation was proposed by NASA Associate Administrator for Space Science Wesley Huntress in 1995. NASA encouraged this new discipline by organizing workshops and technical meetings, establishing a NASA Astrobiology Institute, providing research funds to individual investigators, ensuring that astrobiology goals are incorporated in NASA flight missions, and initiating a program of public outreach and education. NASA’s role is derived from its history of studying the origin of life and searching for evidence of life on Mars or elsewhere in our solar system. Under the umbrella of astrobiology, these efforts are expanded to include the search for life within other planetary systems, as well as investigating the response of terrestrial life to global changes on Earth and to exposure to conditions in space and in other worlds. Astrobiology addresses our origins and also our aspirations to become a space-faring civilization.
Science Goals
The NASA Astrobiology road map (1) provides an initial description of the technical content of astrobiology. This road map was formulated through a series of workshops and discussions involving more than 400 people, primarily academic scientists who are interested in this new discipline. The road map represents a snapshot of a developing science and defines its content as perceived by scientists in 1998.
Astrobiology addresses three basic questions, which have been asked in some form for generations.
• How does life begin and evolve? (Where did we come from?)
• Does life exist elsewhere in the Universe? (Are we alone?)
• What is life’s future on Earth and beyond? (Where are we going in space?)
These are very general questions, and no one expects that definitive answers will be found easily. More specific is the analysis of astrobiology in terms of 10 long-term science goals.
1. Understand How Life Arose On Earth. Terrestrial life is the only form of life that we know, and it appears to have arisen from a common ancestor. How and where did this remarkable event occur? The question can be approached using historical, observational, and experimental investigations to understand the origin of life on our planet. We can describe the conditions on Earth when life began, use phylogenetic information to study our earliest ancestors, and also assess the possibility that life formed elsewhere and subsequently migrated to Earth.
2. Determine the General Principles Governing the Organization of Matter into Living Systems. To understand the full potential of life in the Universe, we must establish the general physical and chemical principles of life. We ask if terrestrial biochemistry and molecular biology are the only such phenomena that can support life. Having only one example, we do not know which properties of life are general and necessary and which are the result of specific circumstances or historical accident. We seek these answers by pursuing laboratory experimental approaches and computational theoretical approaches.
3. Explore How Life Evolves on the Molecular, Organic, and Ecosystemic Levels. Life is a dynamic process of changes in energy and composition that occurs at all levels of assemblage from individual molecules to ecosystemic interactions. Modern genetic analysis, using novel laboratory and computational methods, allows new insights into the diversity of life and evolution at all levels. Complementary to such studies are investigations of the evolution of ecosystems consisting of many interdependent species, especially microbial communities.
4. Determine How the Terrestrial Biosphere Has Coevolved with Earth. Just as life evolves in response to changing environments, changing ecosystems alter Earth’s environment. Astrobiologists seek to understand the diversity and distribution of our ancient ancestors by developing technology to read the record of life as captured in biomolecules and in rocks (fossils), to identify specific chemical interactions between the living components of Earth (its biosphere) and other planetary subsystems, and to trace the history of Earth’s changing environment in response to external driving forces and to biological modifications.
5. Establish Limits For Life in Environments That Provide Analogs for Conditions in Other Worlds. Life is found on Earth anywhere liquid water is present, including such extreme environments as the interior of nuclear reactors, ice-covered Antarctic lakes, suboceanic hydrothermal vents, and deep subsurface rocks. To understand the possible environments for life in other worlds, we must investigate the full range of habitable environments on our own planet, for what they can tell us about the adaptability of life and also as analogs for conditions on other bodies in our solar system, such as Mars or Europa.
6. Determine What Makes a Planet Habitable and How Common These Worlds are in the Universe. Where should we look for extraterrestrial life? Based on our only example (life on Earth), liquid water is a requirement. Therefore, we must determine which sorts of planets are likely to have liquid water and how common they might be. Studying the processes of planet formation and surveying a representative sample of planetary systems will determine which planets are present and how they are distributed, essential knowledge for judging the frequency of habitable planets.
7. Determine How to Recognize the Signature of Life in Other Worlds.
Astrobiologists need to learn to recognize extraterrestrial biospheres and to detect the signatures of extraterrestrial life. Within our own solar system, we must learn to recognize structural fossils or chemical traces of extinct life that may be found in extraterrestrial rocks or other samples (such as Martian meteorite ALH84001). To understand remotely sensed information from planets circling other stars, we should develop a catalog of possible spectral signatures of life. (See article on Extraterrestrial Life, Searching for in this topic.)
8. Determine Whether There Is (or Once Was) Life Elsewhere in Our Solar System, Particularly on Mars and Europa. Exciting data have presented us with the possibility that at least two other worlds in our solar system have (or have had) liquid water present. On Mars, there is evidence for stable flowing water early in that planet’s history. Both in situ investigations and the analysis of returned samples will be necessary to understand Mars’ historical climates and its potential for life. Because their surfaces are inhospitable, exploration of the subsurface probably offers the only credible opportunity to find extant life on either Mars or Europa.
9. Determine How Ecosystems Respond to Environmental Change on Timescales Relevant to Human Life on Earth. Research at the level of the whole biosphere is needed to examine the habitability of our planet over time in the face of both natural and human-induced environmental changes. To help ensure that continuing health of this planet and to understand the potential long-term habitability of other planets, we need to assess the role of rapid changes in the environment and develop our knowledge base to enable predictive models of environment-ecosystem interaction.
10. Understand the Response of Terrestrial Life to Conditions in Space or on Other Planets. All terrestrial life has developed in a one-gravity field, protected by Earth’s atmosphere and magnetic field. What happens when terrestrial life is moved off its home planet and into space or to the Moon or Mars, where the environment is very different from that of Earth? Can organisms and ecosystems adapt to a completely novel environment and live successfully for multiple generations? Are alternative strategies practical, such as bioengineer-ing organisms for specific environments? The results from attempting to answer such questions will determine whether Earth’s life can expand its evolutionary trajectory beyond its place of origin.
Programmatic and Institutional Foundations
Astrobiology began as an effort within NASA to organize its space research programs around the theme of life in the Universe. It was given impetus by missions to Mars and Europa, by plans for telescopes in space to detect other planetary systems and measure the spectra of distant planets, and by the launch of the International Space Station and its planned suite of experimental facilities for life science. Within the NASA hierarchy, astrobiology has elements in the Space Science, Earth Science, and Human Exploration and Development of Space Enterprises. The lead management and coordination role was assigned to the Office of Space Science, and the lead NASA Center role was assigned to the Ames Research Center in California.
One of the early commitments to the development of astrobiology was the creation of a NASA Astrobiology Institute (NAI). This organization has the multiple objectives of encouraging commitments to astrobiology in the academic community, stimulating multidisciplinary research, and providing advice and technical input to NASA flight missions. Its member institutions are built around multidisciplinary research teams selected competitively. The central offices of the Astrobiology Institute are located at Ames Research Center, but the participating scientists (nearly 400 of them in 2001) remain employed in their own home institutions. Thus the NAI is a ”virtual institute” or ”collab-oratory” in structure, using communications technology, together with an annual science meeting, postdoctoral fellows, and a number of cross-institutional ”focus groups” to bind its geographically dispersed teams together. The first Director of the Astrobiology Institute is Nobel laureate Baruch Blumberg. Eleven member institutions were selected in 1998: Harvard University, Marine Biological Laboratory at Woods Hole, Carnegie Institution of Washington, Pennsylvania State University, Arizona State University, Scripps Research Institute, University of California at Los Angeles, University of Colorado, NASA Ames Research Center, NASA Jet Propulsion Laboratory, and NASA Johnson Space Center. To these, the following were added in 2001: University of Rhode Island, Michigan State University, and University of Washington. Also associated with the NAI are the Center for Astrobiology in Madrid Spain and astrobiology teams in the United Kingdom and Australia. There is also a great opportunity for public access, as indicated by such popular web-sites as <astrobiology.arc.nasa.gov>, <nai.arc.nasa.gov>, and /www.astro-biology.com>.
The NASA astrobiology science goals bear a relationship to the search for extraterrestrial intelligence (SETI). Indeed, the detection of signals from an intelligent civilization on a distant planet would provide one of the most unambiguous signatures of extraterrestrial life (Goal 7 above). Historically, however, the SETI efforts have been separated from NASA since congressional action in 1993 terminated all NASA support for SETI programs. Intellectually, however, the two efforts represent complementary ways of addressing some of the same objectives.
Astrobiology is a science that has wide public appeal, as well as potential public concern. The search for life beyond Earth and the eventual expansion of terrestrial life to Mars or other planets in our solar system carry responsibility for protecting planetary ecosystems. Astrobiologists must ensure that these programs are carried out according to generally understood ethical and scientific principles. We will not endanger terrestrial life by introducing alien life-forms, and we will consider the broad ethical and cultural implications before we undertake to change the climate and surface conditions to make another world more hospitable to terrestrial life. Astrobiologists realize that their research has implications that are felt beyond the confines of the laboratory. As our understanding of living systems and the physical universe increases, we will confront the implications of this knowledge in more than just the scientific and technical realms. To understand the consequences will require multidisciplinary consideration of areas such as economics, environment, health, theology, ethics, quality of life, the sociopolitical realm, and education.
Summary
Astrobiology deals with a broad spectrum of disciplines, working together to use space technology to answer fundamental questions about life. Recent developments suggest that astrobiology is here to stay. But its success as a field will depend primarily on the quality of research carried out and on its contributions to space missions. If (or when) life is discovered on Mars or Europa, or the signature of life is detected in the light from an Earth-like planet circling another star, or commitments are made to human visits to other planets, then we can anticipate that astrobiology will hold center stage within space science.
Glossary
Astrobiology. The multidisciplinary study of the living universe, investigating the origin, evolution, distribution, and destiny of life in the Universe (term coined by Wesley Huntress in 1995). The term is somewhat broader than exobiology and includes studies of terrestrial life as it migrates into space.
Exobiology. The study of the origin, evolution, and distribution of life in the universe (term coined by Joshua Lederberg in 1960). In the United States, the term does not include studies of terrestrial life in space, but in Europe, “exobiology” sometimes takes on this broader meaning (which is essentially the same as “astrobiology”).
