Astrobiology (Published in the McGraw-Hill Yearbook of Science & Technology, 2002)

Lynn J. Rothschild
Mail Stop 239-20
NASA Ames Research Center
Moffett Field, CA 94035-1000

Lrothschild@mail.arc.nasa.gov
(650) 604-6525

Astrobiology is the scientific study of the living universe, how it arrived at this point in time, and where it is heading. It starts with life on Earth, the only place where life is known to exist, and extends into the farthest reaches of the cosmos. It ranges in time from the big bang and continues on into the future.

Astrobiology covers a diverse range of topics which can be categorized under major questions: Where did life come from? What is its future? Are we alone in the universe? Like any newborn, Astrobiology is growing and changing rapidly. Thus, this article can only give an overview of Astrobiology rather than covering all the latest research findings. Several established web sites, listed at the end of the article, exist that provide current, reviewed information.

A new discipline with a 7000-year history

Astrobiology (from the Greek words astron meaning star, bios meaning life and logos or description) is arguably the most ancient of sciences. Gazing up at the sky it is easy to imagine that we have an intimate link with the heavens. Over 7000 years ago in Scotland, England and Ireland, stones were aligned to the rising or setting moon, and later the Sun. The connection between life on Earth and astronomy was recognized in many ancient cultures, including the Egyptian where it was used to calculate complex calendars for commerce and agriculture. The Babylonians used it for astrology (the idea that human events were linked to heavenly events), the Greeks for agriculture and navigation, the Muslims for its religious implications, and the Chinese in the belief that the heavens reflected the Emperor's actions. Even earlier, man noticed the wondrous diversity of living creatures, which he depicted in cave paintings at least 31,000 years ago. Using a telescope, the night sky revealed distant bodies such as Mars, with features that appeared to wax and wane with the seasons. Could some of these harbor life, he must have wondered, perhaps even like our own? Where would all this lead in the future? Would our descendants travel to these distant bodies, or would we be visited by extraterrestrial creatures?

While these questions have been asked for millennia, rapid advances in the sciences and the ability to travel out into space have set the stage for a concerted scientific examination of these questions. Then, in 1996 David McKay of NASA's Johnson Space Center and colleagues interpreted chemical and geological evidence from a meteorite found in the Antarctic that originated on Mars to argue that life was once present on the red planet. Suddenly scientists from disparate fields coalesced under the new meta-discipline. "Astrobiology", the name for the emerging discipline, is a word first used in the early 1950s as a Russian translation of exobiology, and revived in 1995 by then NASA Associate Administrator for Space Science, Wesley T. Huntress, Jr. to describe on-going activities at NASA's Ames Research Center. In many ways Astrobiology is evolutionary biology writ large, on a canvas that stretches over billions of years and across the entire universe.

NASA has taken the initiative in establishing Astrobiology as a scientific discipline, with Ames Research Center in California designated as the lead Center. In 1998 Ames hosted a workshop to develop a "roadmap" for Astrobiology, with results that were not definitive or exclusive, but rather suggested areas for research. Future roadmap workshops are likely, however, such planning is quickly being overcome by other events shaping the field.

NASA has founded a "virtual" Astrobiology Institute, which now includes 14 lead member institutions culled from universities, government labs and research centers in the United States, and a growing number of affiliates from around the world. The first Astrobiology Science Conference was held at NASA Ames April 2000, with over 600 participants from dozens of countries, and a second one is scheduled for 2002. Publications to date include journals devoted to Astrobiology, plus special issues of Nature (February 11, 2001) and Proceedings of the National Academy of Sciences, USA (January and February 2001).

Why has Astrobiology been such an immediate success? Clearly, there are practical reasons such as improved forecasting of changing global climate, implications for human health, and biotechnology spin-offs from studying life in extreme environments. But Astrobiology also has an innate intellectual fascination. Aristotle (Politics, section 1259a) taught that the quest for knowledge is important in itself, even divorced from its commercial utility. Here this quest resonates with our very being: what is our connection to the living world, to our planet and to whatever may exist beyond Earth?

The scope of Astrobiology: current research

How did life begin and evolve? This question has pre-occupied every culture. Astrobiology is a novel approach that begins within the boundaries of biological studies of origins and evolutionary biology but expands to include the physical factors shaping evolution on Earth, the origin of the chemical compounds that make up living organisms, and the history of our home, the universe.

One area of interest is the origin of biogenic elements (elements that are particularly important in living organisms such as carbon, hydrogen, oxygen, nitrogen, sulfur, phosphorus, iron and magnesium), and the origin of organic compounds. From studies in cosmo-chemistry we know that elements other than hydrogen, helium and lithium are formed in stars (thus, we are literally made of stardust). Organic compounds have been produced in various aquatic locations on Earth, in the early atmosphere, and in space such as on comets where they are subsequently delivered to the Earth. Cosmo-chemists have demonstrated that a complex mixture of organic compounds can be produced in interstellar ice from water, methanol, ammonia and carbon monoxide. These materials could have been transported to Earth and other bodies via meteorites, comets or interplanetary dust particles. The European Space Agency's Infrared Space Telescope has detected benzene, a complex organic compound that forms the basis for many biological compounds, in a protoplanetary nebula.

To a large extent, environmental factors determine whether life is possible and what evolutionary direction it can take. Thus, Astrobiology also focuses on the environment that shaped the Earth and other bodies that could conceivably harbor life or at least some form of prebiotic (organic) chemistry. As water is the sine qua non of life, it is critical to study the source of water, and environmental factors that permit liquid water to remain in a stable form.

Water on the Earth coalesced as the Earth was being formed, but it has also been delivered to Earth from comets. Early in 2001 an American and an Australian team announced simultaneously that they had evidence from zircon crystals that there was liquid water and continental crusts on the Earth as early as 4.3 - 4.4 billion years ago, a brief geological interval after the origin of the Earth 4.5 billion years ago. This discovery was exciting because the heavy bombardment of the Earth by meteorites obliterated the rock record prior to 3.9 billion years ago.

It has been recognized for decades that water ice is found on Mars, but the recent interpretation of Malin and Edgett suggests that there may have been near-surface liquid water recently, perhaps still there even today. Current excitement is focused on evidence that a moon of Jupiter - Europa - harbors a liquid water ocean under a layer of water ice. The largest moon in the solar system, Jupiter's moon Ganymede, may once have had volcanism and liquid water and may even today have a subsurface ocean today (although its low heat production and less varied chemistry makes it unlikely to have harbored life).

FIGURE: Environmental factors that shape life

Life itself has yet to be created artificially, but certain reactions simulated in a laboratory suggest that given adequate physical and chemical conditions, some steps in the origin of life are inevitable. For example, metabolic activity can be incorporated into lipid vesicles, possibly as in the first cells. RNA can act both for information storage (like DNA) and as a enzyme for self-processing, leading to the suggestion that the first organisms were based on RNA, the so-called "RNA World". Evolution of biomolecules is done in vitro which allows the observation of the evolution of new or improved functions.

Reconstructing early evolution has relied on three approaches. First, the fossil record provides chemical fossils such as the ratio of carbon 12 to carbon 13 ( 13C ratio) that indicates photosynthesis, and morphological fossils. The earliest morphological record shows cellularly preserved microfossils similar in structure to modern cyanobacteria, and laminated microbial communities called stromatolites.

Second, comparative biology of modern organisms provides clues to reconstructing evolutionary events. While in the past this approach was based on gross anatomy, comparative biology today relies most heavily on DNA sequence comparisons and secondarily on biochemical and structural analyses. These studies currently suggest that the last common ancestor of all living organisms today may have lived at high temperature. This suggestion is consistent with the fact that the fossil record begins almost immediately after the late bombardment period ended, and the oceans were heated by the bombardment.

Third, astrobiologists may use present analogs of ancient organisms under modern or simulated ancient conditions to learn how past communities functioned. Such studies are termed actualistic paleontology. Because of the prevalence of stromatolites in the fossil record from 3.5 to 0.6 billion years ago, microbial mats which are modern analogs of such communities are studied with particular intensity. This allows astrobiologists to bring modern techniques such as molecular biology and geochemistry to bear on reconstructing how life functioned in the past, from the molecular level of gene induction all the way to ecosystem function.

At the same time that organisms have evolved and diversified, the physical environment of the Earth has changed. The Sun has become more luminous over time as it burns its supply of hydrogen. The atmosphere of the Earth has changed from an anaerobic one dominated by carbon dioxide to an aerobic atmosphere with 21% oxygen. The continents have formed, drifted, and collided. The Earth has been struck by comets and meteorites, some large enough to have caused the extinction of 90% of life at the end of the Permian 225 million years ago and of the dinosaurs 65 million years ago.

Organic chemistry occurs in space. Meteorites, comets and interplanetary dust rain down on the Earth providing a supply of organic material. Mars during its first half billion years may have been more hospitable to life than Earth itself at that time, and we know that rocks from Mars have struck the Earth. Further, the European Space Agency in collaboration with European and American researchers have flown organisms in Earth orbit exposed to the space environment and found that some have survived (BioPan and LDEF experiments). All this has encouraged a re-examination of the possibility that life can travel from body to body within our Solar System - a process called panspermia. Further experiments testing the survival of organisms in space are planned for the external Expose Facility on the International Space Station.

What is life's future? Currently humans are altering the physical environment and the biodiversity of life on Earth to an unprecedented degree. The Earth is warming because of a rise in atmospheric levels of carbon dioxide. Humans are altering habitats causing the extinction and dislocation of organisms. Non-native species are being transported to new habitats where they often alter the indigenous ecosystem.

We are observing these changes from the ground and from the vantage point of space. We are using data from ground and space to develop predictive models for the future. In the 1960s and '70s, astronauts explored the moon as part of NASA's Apollo program. NASA's unmanned exploration (e.g., Voyagers 1 and 2, Pioneer 10 and 11) has taken us beyond the solar system, to the surface of Mars (Viking Landers, Mars Global Surveyor) and on the asteroid Eros (NEAR); Russian and U.S. missions have studied Venus. The Russian space station Mir (which was destroyed March 23, 2001) led the way in long duration human flight in low earth orbit, and an International Space Station is being assembled in the first years of the 21st century.

At the same time, advances in astronomy and planetary science have revealed the ultimate fate of the Earth. In 1-2 billion years (Gyr), our moon will be lost to space. Without the stabilizing effect of the moon, the obliquity of the Earth will vary chaotically resulting in horrendous shifts in climate. In the next 5 Gyr, the Sun will increase 60% in brightness, and then, its hydrogen being spent, leave the main sequence phase of stellar evolution. Thus, we see that ultimately the fate of our planet and our species will rely on a knowledge of astronomy and the ability to take dramatic steps to avert oblivion.

Are we alone in the universe? While Astrobiology is not synonymous with the search for life in the universe, it is a vital part of the enterprise. This search uses several approaches, from searching for microbes to intelligent beings. Although the idea that there may be other intelligent beings in our universe is tremendously exciting, we now know enough about our neighbors to realize that if such creatures exist, they must be beyond our solar system and thus communication with them will be difficult.

In 1961 Frank Drake, now Chairman of the Board of Trustees of the SETI Institute, provided the framework for these studies in the form of the Drake equation which lists the parameters needed to calculate the probability of intelligent life elsewhere. Currently the most sensitive and comprehensive SETI (Search for Extraterrestrial Intelligence) effort has been Project Phoenix. Begun in 1995, several radio telescopes have been used to listen for radio signals that were deliberately or inadvertently transmitted from another planet. The telescopes include the Parkes and Mopra in Australia, then the National Radio Astronomy Observatory's 140 foot telescope in West Virginia plus the 30 M Georgia Tech telescope in Woodbury, Georgia and now the Arecibo Observatory in Puerto Rico paired with the Lovell telescope in Jodrell Bank, England.

The main focus for searching for extraterrestrial life, both within and eventually outside of our solar system, is on microscopic life forms because larger organisms already would have been detected and because microbes in particular are often the champions in surviving in the extreme environments that the rest of the solar system has to offer. NASA's Viking missions were directed at detecting either visual or metabolic signals of life on Mars, but the results were negative or highly ambiguous at best. Current research focuses on the possibility of a former martian biota that may be detected in meteorites or other samples from Mars, or a possible extant biota shielded from solar radiation existing deep beneath the surface, in the polar ice caps or in salt deposits. Deep drilling on Mars is technically difficult, but may be the most promising approach. Recent suggestions that liquid water may have been present on Mars in the recent past or even in the present are forcing a re-thinking of strategy.

Europa has an advantage over Mars in that a liquid water ocean is thought to exist beneath a thick ice crust. Unfortunately Europa is quite a bit farther from the Earth and has the thick ice crust, so will also provide technical challenges.

Planetary theory suggests that the majority of stars, especially sun-like stars, should have planetary systems. Since 1995, the discovery of extrasolar planets - those orbiting other stars - has occurred at an increasing rate. By 2001 over 50 nearby stars with planets have been detected. The excitement lies in the fact that planets should provide more stable environments for life to take hold and evolve than more transitory bodies like comets. The extrasolar planets discovered to date have altered our conception of what constitutes a "normal" solar system, but have not yet revealed planets that are likely to be habitable. It is possible that habitable planets may orbit red dwarfs. While these stars are only 6 to 60% of the mass of the Sun, they may constitute 80% of the stars. Thus, if they do have solar systems with habitable planets, it would greatly expand the number of potential habitable planets in the universe.

What will the future hold?

It is easy to predict an exciting future for Astrobiology. Just as the concept of evolution provided a unifying theme for biology for the last 140 years, Astrobiology provides a novel approach for the study of life itself. The intellectual fascination is indisputable and the tools increasingly powerful.

Universities around the world are beginning to recognize this as they institute programs in Astrobiology. Academic recognition and realignment is important because an astrobiologist requires both depth in an area of specialization and the versatility to recognize and develop multi-disciplinary approaches to research questions.

In the meantime, the tools of molecular biology are advancing at a tremendous pace. This has allowed rapid DNA sequencing, determinations of gene regulation and analyses of protein function. To handle the flood of data, advances in computer systems have led to the creation of computational astrobiology. Access to space and the opportunity to perform biological experiments there have increased through unmanned missions as well as manned missions such as NASA's Space Shuttle and the Russian station Mir, and in the future the International Space Station.

Future destinations for scientific exploration will include Mars, Europa and various comets and asteroids. Back on Earth there will be further studies of environments that are at the physical and chemical limits for survival which will give us a better sense of what are the "possible" conditions for life. One thing is certain: in this rapidly advancing meta-discipline, unanticipated breakthroughs will be increasingly common and undoubtedly startling in the way that they require reevaluation of our view of life.

References:

Chyba, Christopher F. and Cynthia B. Phillips (2001) Possible ecosystems and the search for life on Europa. Proc. Natl. Acad. Sci.. USA 98: 801-804.

Croswell, Ken. THE RIGHT PLACES TO LOOK FOR ALIEN LIFE. New Scientist issue: 27 January 2001

Dworkin, Jason P., David W. Deamer, Scott A. Sandford, and Louis J. Allamandola. Self-assembling amphiphilic molecules: Synthesis in simulated interstellar /precometary ices., Proc. Natl. Acad. Sci., USA 98: 815-819.

Lunine, Jonathan I. (2001) The occurrence of Jovian planets and the habitability of planetary systems. Proc. Natl. Acad. Sci., USA 98: 809-814.

Lunine, Jonathan I. (1999) In search of planets and life around other stars. Proc. Natl. Acad. Sci., USA 96: 5353-5355.

Malin, Michael & Edgett, Kenneth. (2000) Evidence for recent groundwater seepage and surface run-off on Mars. SCIENCE 288: 2330-2335

Marcy, Geoffrey (1998) Extrasolar Planets. Back in focus. Nature 391: 127.

Marcy, Geoffrey W. and R. Paul Butler (1998) Detection of extrasolar giant planets. Ann. Rev. Astron. Astrophys. 36: 57-97.

McKay, David S., Everett K. Gibson Jr., Kathie L. Thomas-Keprta, Hojatollah Vali, Christopher S. Romanek, Simon J. Clemett, Xavier D. F. Chillier, Claude R. Maechling, Richard N. Zare (1996) Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001. Science 273: 924-930.

Pace, Norman. (2001) The universal nature of biochemistry. Proc. Natl. Acad. Sci., USA 98: 805-808.

Rothschild, L.J. & R.L. Mancinelli (2001) Life in extreme environments. Nature (London) 409: 1092-1101.

Schenk, Paul M., McKinnon, William B., Gwynn, David and Jeffrey M. Moore (2001) Flooding of Ganymede's bright terrains by low-viscosity water-ice lavas. Nature 410: 57-60.

Websites:

Spaceref.com Astrobiology Web Site: http://astrobiology.com/

NASA Astrobiology Institute: http://nai.arc.nasa.gov/

Astrobiology at NASA: http://astrobiology.arc.nasa.gov/

On-line Astrobiology Articles and links:

http://www.lyon.edu/webdata/users/dthomas/astrobiology/online_articles1.html

Space.com Astrobiology site: http://www.space.com/scienceastronomy/index.html

SETI Institute: http://www.seti.org/

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