Astrobiology - astronomy. I INTRODUCTION Astrobiology or Exobiology, study of the origin, evolution, distribution, and future of life in the universe, including life on Earth. The term exobiology may be used interchangeably with the term astrobiology. Some scientists, however, restrict exobiology to mean only the study of how life might exist beyond Earth (extraterrestrial life). Exobiology is accepted as a study area within astrobiology in both approaches. Astrobiologists investigate how the formation of stars and solar systems led to the existence of planets suitable for life and how life originated on Earth and perhaps elsewhere. Astrobiologists also explore which factors have influenced biological evolution in the past and in the present, or may influence evolution in the future. The understanding of these events shapes the study of how life arises and evolves in the universe. Astrobiology brings together a wide range of scientific fields within space sciences, planetary sciences, Earth sciences, chemistry, and life sciences, including astronomy, microbiology, molecular biology, ecology, and paleontology. The term astrobiology comes from the Greek word astron ("star") as in astronomy, combined with biology, the scientific study of life; exobiology comes from the Greek prefix exo- ("outside"), referring to a perspective on life that includes the possibility of life beyond Earth. II THE PROBABILITY OF LIFE IN THE GALAXY Earth is the only planet that we know harbors life. Nonetheless, the basic chemicals and processes needed for life appear to be widespread in our Milky Way Galaxy and in the universe beyond. A significant number of scientists think that some form of life beyond Earth is possible, even probable. Astrobiologists can use their knowledge about life on Earth to guide their search for extraterrestrial life. A Conditions for Life Life elsewhere in the universe might also form near a star like our sun. The Sun is an average star, bright and hot enough to warm the inner planets but calm and cool enough that Earth is relatively safe from some forms of destructive radiation. Most importantly, our sun has been stable for billions of years. Life would also benefit from a planet like Earth, large enough to provide the gravitational force to hold an atmosphere. The atmosphere protects the surface against radiation and rapid temperature changes and holds elements that may be important to sustaining life. The atmosphere also allows water to exist in liquid form on the surface. The combination of a suitable star and planet might be vital to the formation of life. Scientists use the term "habitable zone" to describe regions around a star where suitable planets might enjoy Earthlike conditions of temperature or radiation exposure. For a star that is cooler and smaller than the Sun, the habitable zone would be closer in than in our solar system while a star hotter than the Sun might have a habitable zone farther out than in our solar system. The recent discovery of ocean waters deep beneath the frozen surface of Europa, a moon of Jupiter, suggests that other habitable zones are possible, even when a star is not at a convenient distance. B Drake Equation Detecting life on planets outside our solar system presents many challenges. If a planet with Earthlike temperatures was detected at a distance of many light-years, chemicals in its atmosphere such as oxygen, methane, or water vapor could be possible indicators of biological activity or at least of conditions where life might exist. In some ways, however, an intelligent, communicating civilization might be much easier to detect than primitive life. Intelligent life might have the technology to produce signals such as radio waves that could be much more powerful than even natural light from a star. To calculate the likelihood that intelligent life could be detected elsewhere in the galaxy, American astronomer Frank Drake developed an equation for the number of communicating civilizations that might exist. This equation is called the Drake equation and is represented by N = R* fp n e fl fi fc L. N is the number of communicating civilizations in the Milky Way galaxy. R* is the rate of formation of suitable stars, fp is the fraction of those stars that have planets, n e is the average number of suitable planets around a star, fl is the fraction of those planets that develop life, fi is the fraction of those planets with intelligent life, fc is the fraction of such planets with a technological civilization that communicates, and L is the average lifetime of such a civilization. The only term for which astrobiologists currently have a good estimate is R*, although recent success in detecting planets around other stars suggests that the value of fp is greater than one-half. Astrobiologists need to learn about the galaxy and life on the Earth (and perhaps elsewhere in the solar system) to come up with appropriate estimates for the other terms in the Drake equation. One particular Drake-equation factor, fl (the fraction of suitable planets that develop life), depends on how life originates. III LIFE ON EARTH Current astrobiological research focuses on understanding how life arose on Earth and discovering potential life-supporting environments other than Earth. Scientists now believe that life on Earth dates back to at least 3.85 billion years before present, so living organisms have populated Earth for more than 80 percent of its history. A Carbon and Organic Chemistry All known life on Earth is based on the element carbon. Carbon, hydrogen, oxygen, nitrogen, and phosphorus are elements that exist in all organisms on Earth. Astrobiologists can conceive of organisms that would not rely on these elements, but these elements are among the most abundant elements in the universe and would probably be available elsewhere as a basis for living systems. Carbon is particularly important to life because it forms three-dimensional molecules of large size and complexity in organic (carbon-containing) compounds (see Organic Chemistry). Large organic molecules include amino acids, enzymes, sugars, and other chemicals vital to life on Earth. Organic molecules can become complex enough to store genetic information, as in deoxyribonucleic acid (DNA). Carbon molecules are also capable of an amazing variety of chemical reactions in liquid water. The presence of water vastly increases the number of possible organic molecules, increasing the likelihood that the right combination of molecules for life can form. Based on the available evidence, there is no reason to believe that carbonbased life should be limited to Earth alone. Laboratory results have shown that nucleic acids associated with primitive life forms can change through natural selection--scientists are employing such selection techniques to develop new drugs to fight a variety of diseases. During the 1920s Russian biologist Aleksandr Oparin and British biologist J. B. S. Haldane proposed that life could have arisen as a consequence of the physical and chemical formation of Earth. The early Earth had an environment very different from the conditions on Earth today. The young Earth had more volcanic activity than today's Earth, warming the atmosphere and filling it with chemicals that trapped the Sun's heat. Debris from the young solar system impacting Earth, lightning, and radiation from the Sun provided energy necessary to break apart molecules, allowing new compounds to form. Earth had oceans even in its early existence, providing water to help reactions along. American chemists Stanley Miller and Harold Urey tested part of Oparin and Haldane's hypothesis in the early 1950s by simulating conditions of the early Earth. In what has become known as the Miller-Urey experiment, the two scientists connected two flasks with a loop of glass tubing that allowed the gases to pass between the flasks. They filled the upper flask with methane, ammonia, and hydrogen--components thought to have been in the early atmosphere. They filled the lower flask with water. The scientists then applied electric sparks--the equivalent of lightning on the early Earth--to the gas mixture. After less than a day, the water in the lower flask contained a variety of amino acids and other organic molecules--the building blocks of life. The Miller-Urey experiment showed that it was possible to form organic materials from inorganic components on the early Earth. Forming organic materials in this way is only one possibility for the origin of the first building blocks of life. Other scientists have shown how organic compounds could have come to Earth from space in cosmic dust particles, asteroids, comets, and meteorites. The chemistry of deep sea hydrothermal vents is another possible source of life's building blocks. Many potential sources of organic material exist on Earth and possibly on other planets. B Adaptability and Survival of Life Biologists are focusing new attention on the ability of life on Earth to live in extreme environments--from the cold, dry deserts of Antarctica to superheated hydrothermal vents in the dark depths of the ocean--and life is proving to be remarkably robust. So-called extremophiles have also adapted to exist without sunlight or oxygen; to thrive in highly acidic, alkaline, or salty environments; to feed on minerals in rocks or on substances otherwise considered toxic; to live under massive pressure; or to survive deadly levels of radiation. Astrobiology includes the study of how life has responded to changing conditions on Earth through evolution, documented by fossils. Of particular interest in the history of life are mass extinctions that drastically reduce the number of species of plants and animals, and the role of catastrophic geological or astronomical events such as gigantic volcanic eruptions, impacts from space, or supernova explosions. Astrobiologists are also concerned with the future of life on Earth. Scientists are only beginning to understand the basic mechanisms that keep the Earth habitable in the present-day. In the future, global warming might vastly change Earth's environment or ice ages might return. In millions of years our sun will grow hotter, changing conditions on the surface of our planet. Oceans will evaporate and the chemistry of the atmosphere will change. In billions of years, the Milky Way galaxy will collide with the Andromeda galaxy, changing the orbit of our sun. Scientists want to know if any life forms on Earth could survive such future conditions, or conditions now existing elsewhere in the universe. Taking a broader perspective, astrobiologists are also interested in the destiny of life anywhere in the universe. The recent discovery that the expansion of the universe is accelerating because of dark energy raises questions of how life would respond or survive as the cosmos evolves. Other areas of research include how humans and other Earth organisms can respond and adapt to life in space. Humans traveling in space to live on distant planets or other bodies face conditions such as reduced or increased gravity, exposure to cosmic rays and other radiation, and possible encounters with extraterrestrial organisms. Using space conditions in biological experiments may also enable new insights into the basic mechanisms of life on a cellular level. Some research in this area has been done on space stations and laboratories orbiting Earth. In a more speculative realm is the question of what would happen if an extraterrestrial life form were introduced into Earth's biosphere. Astrobiologists are interested in how such an organism might affect or respond to the present-day environment on Earth, or if such an organism might pose a danger to terrestrial life forms. Of equal interest is if extraterrestrial organisms reached Earth in the past. During the early history of Earth, large impact events may have showered our planet with rocks from other solar-system bodies, including Mars. If life forms had evolved independently on Mars or another body, such organisms might have affected or even helped initiate the development of life on Earth. IV LOOKING FOR LIFE BEYOND EARTH Exploring space with space probes is one method of searching for extraterrestrial life. For example, orbital probes and robot landers may be able to detect chemical indicators of life called biomarkers. Humans have so far sent spacecraft only to other planets and their moons within our solar system. The planet that has received the most attention is Mars, but the moons of the outer planets such as Jupiter and Saturn are coming under increasing scrutiny as places that might be able to support life. Planetary bodies such as Mars and Europa show evidence of environments no worse than those in some parts of Earth. A Mars The planet Mars appears to have been similar to Earth throughout much of its early history, and some of the missions to that planet have included experiments designed to look for signs of life. In 1976 the American Viking missions placed two landers on the surface of Mars and conducted tests to detect Martian organisms. The Viking landers carried cameras to take pictures of the surrounding landscape and possibly reveal visual clues to life on Mars. They also carried instruments that could analyze soil samples to determine their composition and look for organic compounds. The Viking missions had miniature laboratories onboard specifically designed to detect evidence of life in samples of the Martian soil and atmosphere. Scientists hoped that any life on Mars could be cultured, or grown, in these laboratories. Instruments connected to the experiments could then determine whether something was growing in the cultures. None of the Viking experiments returned definite evidence of life. Biologists now know that about 99 percent of Earth microbes do not grow in cultures, so the Viking experiments may have failed to detect life even if there were microbes on Mars. Viking did provide scientists with information that allowed them to identify meteorites on Earth that originally came from Mars. Geologists compared the gases in Mars's atmosphere with gases trapped in meteorites found on Earth and discovered that at least 12 meteorites had reached Earth from Mars. A team of scientists from the National Aeronautics and Space Administration (NASA) and several universities analyzed one of these meteorites, designated ALH84001, and found structures that they believed could be fossils of ancient microorganisms, as well as organic compounds. Other researchers disputed the identification of the structures as evidence of extraterrestrial life, concluding instead that they resulted from geological or chemical action. Nonetheless, the composition of ALH84001 has shown that the Martian surface today is much different than its early subsurface, of which the meteorite was a part, and that the processes that led to the meteorite containing organic materials and structures could be widespread in the universe. Given recent discoveries on Earth such as oases of life in the deep sea and widespread evidence of microbes living deep underground, the hostile environment of Mars does not rule out the possibility that life once existed on the planet. Many more Martian missions are under way or are planned, culminating in a mission that will bring samples of Martian soil back to Earth. The Mars Exploration Rover mission, which began in 2003, has continued long past its original 90-day schedule on the surface of Mars. This mission, along with data from advanced orbiting spacecraft, confirmed that Mars once had liquid water on its surface. Liquid water is essential to life. However, the chemistry of ancient Mars may have been different from that on early Earth. Sulfur-based compounds rather then carbon-based compounds appear to have been dominant for most of the time that Mars may have had a denser atmosphere and liquid water. Liquid water on Mars may have been very acidic, although some extremophile microbes on Earth have adapted to even harsher conditions. The exploration of Mars, including ancient Mars, has only just begun. Scientists hope that future missions intended to bring back samples from Mars will land in areas where water existed. Such samples may provide vital clues for determining if life did exist or still exists on Mars. Possible evidence could be chemicals associated with life (biomarkers), fossils of extinct life, or even surviving organisms. If Martian samples are successfully returned one day, they will be treated very carefully, both for the scientific results they may contain, and to ensure that any possible Martian life is detected before exposing a sample to Earth's biosphere. Scientists are also making efforts to avoid contaminating Mars with microorganisms from Earth that might be carried on space probes. B Outer Planets and Their Moons Astrobiologists are increasingly turning their attention to other places in the solar system. The Pioneer and Voyager missions of the 1970s and 1980s returned data showing that Saturn's moon Titan had an atmosphere made up of gases similar to those in the Miller-Urey experiment. Jupiter's moon Europa is even more intriguing, with a smooth, icy surface and puzzlelike cracks that suggest, along with other data, that a liquid ocean exists underneath. The Galileo mission orbited Jupiter from 1995 to 2003, studying Jupiter's moons, with an extended mission focusing on Europa. Results indicate that streaks on the surface of Europa may include salts and even organic compounds that erupted from under the ice. Gravitational interactions with Jupiter and its other moons create internal heat on Europa, which, along with the high-intensity radiation fields around Jupiter, can provide energy for possible life in an ocean beneath its surface. Scientists targeted Galileo to burn up in Jupiter's atmosphere to prevent the craft from accidentally crashing into Europa and contaminating the moon with microbes from Earth. The Cassini spacecraft, which was launched in 1997, went into orbit around Saturn in 2004 and dropped a probe through the atmosphere and onto the surface of Titan in 2005. Organic compounds are abundant on Titan, including lakes of liquid methane and dunes made of complex organic molecules. However, extreme cold likely inhibits the development of possible life on Titan's surface. Titan may also have subsurface bodies of water, similar to Europa, but future missions will have to determine their nature and extent. Cassini's most dramatic discovery in regard to life outside of Earth came from the detection of icy geysers from Saturn's moon Enceladus. In 2006 planetary scientists reported that Enceladus had three of the ingredients essential to life: water, carbon molecules, and heat. The observations of Enceladus from Cassini continue. V LIFE BEYOND OUR SOLAR SYSTEM Since the mid-1990s astronomers have used special techniques to search for planets around other stars and have found that planets are much more common than previously thought. More than 200 planets have been detected orbiting other stars in the Milky Way galaxy. The discovery of these so-called extrasolar planets greatly increases the possibility of life outside our solar system. However, due to the low sensitivity of the techniques used to discover them, the extrasolar planets identified so far are larger than Earth and often orbit very close to their home stars, creating conditions that appear hostile to life as we know it. The chemistry needed for life may be common in our galaxy, and scientists are actively searching for smaller and more diverse planets around other stars. In 2005 scientists using NASA's Spitzer Space Telescope discovered gaseous chemicals that are precursors to protein and DNA around a star 375 light-years from Earth. The star, known as IRS 46, is circled by a flat disk of gas and dust and has a region known as a terrestrial planet zone. This is a region where rocky planets such as Earth are believed to form. The telescope's infrared spectrometer detected acetylene and hydrogen cyanide. It was the first time these chemicals had been detected in a terrestrial planet zone outside our solar system. In the laboratory, scientists have combined acetylene and hydrogen cyanide with water, and the ensuing combination has yielded organic compounds, such as amino acids and a DNA base called adenine. In 2007 astronomers announced the discovery of two large rocky planets or "super-Earths" orbiting the star Gliese 581--the first identified exoplanets where Earthlike life might be possible. As a red dwarf star, Gliese 581 is much cooler than our sun and its habitable zone is much closer to the star. Analysis suggests the closer planet has about five times the mass of Earth and the planet farther out has about eight times. Conditions on both worlds would be harsh, but liquid water could possibly exist on the surface of one or the other of the planets. As technology improves and special planet-detecting space telescopes such as COROT (Convection, Rotation, and planetary Transits) and Kepler become operational, scientists may be able to find smaller, possibly Earthlike planets around other stars. The detection of free oxygen in a distant planet's atmosphere or liquid water on its surface would be important clues to an Earthlike environment where life similar to what we know could have developed. Such life, however, may not be as advanced as here on Earth, but only time and further exploration will provide that knowledge. The Milky Way may be full of possible locations for life. The future poses many opportunities to discover new aspects and capabilities of life on Earth and to study exciting places in space that may also have life. VI SEARCH FOR INTELLIGENT LIFE Solar system exploration may detect extraterrestrial life in the solar system that is not advanced enough to communicate with Earth. However, astrobiologists have employed other strategies of searching for intelligent, technological life--strategies aimed at communicating with or detecting communication from other worlds. The Pioneer and Voyager missions carried messages from Earth for their eventual journeys through interstellar space. Pioneer 10 and 11 were the first objects planned to leave the solar system and carried small metal plaques depicting male and female humans with a coded message identifying the time and place of spacecraft origin. A more ambitious message was placed aboard the Voyager 1 and 2 spacecraft as a kind of time capsule. Each carried a gold-plated copper disk recording of sounds and images portraying the diversity of life and culture on Earth--including a variety of natural sounds, musical selections, and spoken greetings in 55 languages. These messages are on their way to the stars as the spacecraft enter regions beyond our solar system. Spacecraft are not the fastest or most efficient way to send messages out of the solar system, or for cultures on other planets to send messages to Earth. Radio waves travel at the speed of light and can be sent out in many different directions. Astrobiologists began searching the skies for radio signals from extraterrestrial life in 1960, in the first Search for Extraterrestrial Intelligence (SETI) experiment. Frank Drake used the National Radio Astronomy Observatory in Green Bank, West Virginia, to search for radio signals for four months in 1960. This attempt was named Project Ozma after the queen in American writer L. Frank Baum's novels about the imaginary land of Oz. Project Ozma focused on the stars Tau Ceti in the constellation Cetus and Epsilon Eridani in the constellation Eridanus, both about 11 light-years (about 106 trillion km, about 66 trillion miles) from Earth. Drake's search lasted six hours a day from April to July 1960, using a 26 m (85 ft) radio telescope tuned to the wavelength of radiation that cold hydrogen gas in interstellar space emits (a frequency of 1420 megahertz). With the exception of an early false alarm caused by a secret military experiment, no signals were detected. See also Radio Astronomy. Project Ozma used just one single-channel receiver, but NASA eventually developed the capability to monitor millions of channels simultaneously. On October 12, 1992, NASA's two-part SETI effort initiated observations with the All-Sky Survey, a survey of space using a 34 m (112 ft) diameter radio telescope in Goldstone, California, and the Targeted Search, which examined solar-type stars using the National Science Foundation's 305 m (1,000 ft) telescope in Arecibo, Puerto Rico. The government- funded project was not appreciated by some in the United States Congress, however, and was canceled in October 1993. Despite that setback, the private SETI Institute took over the search for extraterrestrial signals. The institute conducted Project Phoenix during a nine-year period from 1995 to 2004, using some of the equipment developed for the canceled NASA search. Phoenix made a targeted search of more than 800 relatively nearby stars similar to our sun with radio telescopes located in New South Wales, Australia, as well as at Arecibo and at Green Bank. No signals identifiable as originating from intelligent life were detected. Another approach to searching for signals is to "piggyback" a special receiver onto radio astronomy work being done for conventional scientific research. The SETI receiver does not interfere with the data being collected by the main radio telescope. Beginning with a project dubbed SERENDIP, a number of versions of this untargeted type of search have taken place. SERENDIP IV conducted with the Arecibo dish collected a vast amount of data covering about 30 percent of the sky. The data need to be carefully analyzed to extract possible signals coming from an artificial source. To do this, a project called SETI@home was started by the Space Sciences Laboratory at the University of California at Berkeley. SETI@home has signed up millions of volunteers on the Internet to process parts of the data on their personal computers at home, using special software. Under development is the Allen Telescope Array (ATA) radio telescope, planned for completion in 2010 in the form of 350 antennas located northeast of San Francisco. The ATA radio telescope complex, sponsored by Microsoft cofounder Paul Allen, will carry out regular radio astronomy as well as searches for extraterrestrial signals for SETI. Another technique to detect other possible signals from extraterrestrial life is called Optical SETI, which searches for laser pulses instead of radio waves. Versions of Optical SETI are being carried out by Harvard University and the Columbus Observatory in Ohio, and have been conducted at California's Lick Observatory and other locations, including Australia. The detection of intelligent life elsewhere in the universe would be one of the most momentous events in human history, with profound scientific, philosophical, and even religious implications. Contributed By: John D. Rummel Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.
