Exobiology, study of the origin, evolution and distribution of life in the universe. Exobiologists investigate how the formation of stars and solar systems led to the existence of planets suitable for life, how life originated on Earth and perhaps elsewhere, and which factors influenced biological evolution. The understanding of these events shape the study of how life arises and evolves in the universe.
Earth is the only planet that we know harbors life. Exobiologists can use their knowledge about life on Earth to begin their search for life elsewhere. 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. Exobiologists can conceive of organisms that would not rely on those elements, but those 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 carbon-based life should be limited to Earth alone.
II THE PROBABILITY OF LIFE IN THE GALAXY
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 combination of a suitable star and planet might be vital to the formation of life.
An intelligent, communicating civilization might be much easier to detect than primitive life, because it might 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 ne fl fi ft 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, ne 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, ft is the fraction of such planets with a civilization that communicates, and L is the average lifetime of such a civilization. The only term for which exobiologists currently have a good estimate is R*. Exobiologists 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. During the 1920s Russian biologist Alexander 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. Many potential sources of organic material exist on Earth and possibly on other planets.
Exploring space with space probes is one method of searching for extraterrestrial life. 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.
The planet Mars appears to have been similar to Earth throughout much of its 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 90 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. 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. Since then, other scientists have found evidence that the structures were more likely caused by geological or chemical action, or by the preparation of the rock for electron microscopy. Organic compounds found in the meteorite have been shown to be contamination from Earth. 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. 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 planned. NASA plans to launch missions to Mars every 18 months in the same series as the mapping orbiter spacecraft Mars Global Surveyor and the lander spacecraft Mars Pathfinder, culminating in a mission that will bring samples of Martian soil back to Earth in 2008. Orbiters will provide pictures to be analyzed for signs of water and will measure the composition of the Martian surface. Landers will analyze rocks and soil and collect samples for return to Earth. Mars samples 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.
Exobiologists are increasingly turning their attention to other places in the solar system. The Pioneer and Voyager missions of the 1970s and 1980s returned data that showed 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 puzzle-like cracks suggesting that a liquid ocean may exist underneath. The Galileo orbiter began an orbit around Jupiter in 1995, studying Jupiter's moons, with an extended mission focusing on Europa. The Cassini probe was launched in 1997 toward Saturn and Saturn's moon Titan.
Solar system exploration may detect extraterrestrial life in the solar system that is not advanced enough to communicate with Earth. However, exobiologists have employed other strategies of searching for 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, with Pioneer 10 and Voyager 1 both more than 11 billion km (more than 6.5 billion mi) from Earth.
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. Exobiologists 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 an 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 (1000 ft) telescope in Arecibo, Puerto Rico. The project was not appreciated by some in the United States Congress, however, and was canceled in October 1993. The equipment developed for the Targeted Search was transferred to the private SETI Institute, which deployed it to Australia in 1995. SETI continues to operate the search with other equipment at a radio telescope in Green Bank—Project Ozma's former home.
Current exobiology 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. Laboratory results have shown that nucleic acids associated with primitive life forms can change through natural selection. 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 springs in the dark depths of the ocean—and life is proving to be remarkably robust. In comparison, planetary bodies such as Mars and Europa show evidence of environments no worse than those in some parts of Earth.
Astronomers have made many recent discoveries of interest to exobiologists. In the mid-1990s astronomers began using special techniques to search for planets around other stars, and found that planets are much more common than previously thought. Scientists are developing instruments that will be able to look more closely at extrasolar planets such as 16 Cygni B. In 1996 the European Space Agency's (ESA) Infrared Space Observatory (ISO) reported the first galactic water vapor found in a deep space object—in a cloud around the dying star NGC 7027. 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.
Contributed By:
John D. Rummel
Martian Meteorite ALH84001
This meteorite was probably blasted off of the surface of the planet Mars about 16 million years ago by an impact with an asteroid and travelled through space to the earth, where it landed on Antarctica about 13,000 years ago. Some scientists believe that the rod-shaped structures across the top and center of this image may be tiny fossilized bacteria. Many other scientists believe that the structures were formed by processes other than life.
Photo Researchers, Inc./NASA/Science Source