RESEARCH STARTER

Extraterrestrial life in the Solar System

Extraterrestrial life in the Solar System refers to the ongoing scientific inquiry into the existence of life beyond Earth, particularly within our planetary neighborhood. Astrobiology, the study of potential life on other celestial bodies, has focused on planets like Mars and Venus, as well as moons of Jupiter and Saturn, which may harbor conditions suitable for life. Historically, speculations about extraterrestrial civilizations were often imaginative, but advancements in technology and understanding of life's origins have shifted focus toward simpler life forms, such as microorganisms.

The discovery of liquid water, organic compounds, and geological activity on Mars, as well as the icy oceans beneath the surfaces of Europa and Enceladus, has reinvigorated interest in the potential for life in these environments. Researchers have also considered extreme life forms on Earth that exist in harsh conditions, suggesting that life could potentially thrive in ways previously unimagined.

Modern missions aim to directly measure conditions on these celestial bodies, with the hope of finding signs of biological activity or evidence of past life. The study of extraterrestrial life remains an evolving field, combining discoveries from various planetary bodies, leading to a broader understanding of life's potential diversity in our Solar System.

Full Article

Astrobiology or exobiology is the search for and study of life on celestial bodies other than Earth. Much work in the field has focused on objects within the solar system, in part due to fact that they are the closest to Earth and therefore most conducive to study. Many scientists consider the planets Mars and Venus, as well as several satellites of the planets Jupiter and Saturn, to have the best conditions for potentially developing life.

Overview

The possibility that life might have developed elsewhere in the solar system has been the subject of speculation for hundreds of years. Early theories tended to focus on hypothetical human-like civilizations on other planets or moons. In 1820, Carl Gauss, a German mathematician, suggested cutting geometrical patterns into the Siberian forest large enough to be seen by an observer using a telescope from the moon or Mars. The idea was to motivate any inhabitants of the moon or Mars to engineer similar geometrical patterns, initiating crude communication with Earth. Other suggestions for communication with extraterrestrial intelligent life included setting huge fires in the Sahara desert and constructing large mirrors to reflect sunlight into space. These early ideas of communicating with intelligent life elsewhere in the solar system did not focus on particular sites where the conditions were expected to be appropriate for the development of life.

The growth of radio astronomy after World War II brought new avenues in research into potential extraterrestrial life. Researchers soon discovered that the natural universe was far from radio-quiet. Some scientists, beginning with astronomer Frank Drake, wondered about and then tested the idea that intelligence beyond Earth might transmit recognizable radio signals. This eventually gave rise to the coordinated Search for Extraterrestrial Intelligence (SETI) program. However, no decipherable communications were immediately found. This, along with deepening scientific understanding of the environments of known planets and moons, convinced most researchers against the possibility of advanced civilizations existing in the solar system.

Meanwhile, however, only in the second half of the twentieth century did biologists begin to develop a modern scientific understanding of how life likely originated on Earth. This knowledge provided clues as to the conditions needed for similar forms of life to develop elsewhere in the solar system. The study of terrestrial life indicates that it originated as simple, single-celled microorganisms—and that these simple microorganisms might be able to develop on other celestial bodies as well, given a few basic conditions such as the presence of water. With no signs of intelligent life apparent, the focus of solar system exobiology thus largely shifted to the search for simple microorganisms.

The dawn of the space age inaugurated an era in which spacecraft could be used to directly search for environments favorable to the development of life, perform experiments designed to detect living organisms on the surface of other planets and/or their satellites, and return samples to Earth so that scientists could examine them for evidence of biological activity or fossil evidence of past life. However, early efforts reinforced the idea that the other bodies in the solar system were barren. After the 1976 Mars Viking lander failed to find any biological signs in Martian soil, hopes of finding extraterrestrial life dwindled.

Despite those setbacks, there was a resurgence of interest in exobiology by the end of the twentieth century as technology improved and new discoveries were made. Evidence of river channels on Mars, hints of water ice on the moon and Mercury, observations of cryovolcanism on multiple natural satellites, and other findings suggested that the solar system might not be as inhospitable to the development of life as once believed. Research continued to accelerate in the twenty-first century, with growing amounts of data and theoretical refinements. One important trend was the recognition that extraterrestrial life might take forms wholly unfamiliar to terrestrial biology.

Context

Traditionally, research into potential extraterrestrial life in the solar system has been rooted in understanding of life on Earth. Terrestrial life is based on complex organic molecules consisting of chains of carbon, hydrogen, nitrogen, and oxygen. However, these molecules can be produced by simple chemical reactions as well as by biological activity. Thus, to determine if a process is truly biological rather than simply a chemical reaction, it is necessary to define the criteria for life. The ability of an organism to reproduce itself is considered an essential feature of life. Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are the organic molecules that control heredity in terrestrial life-forms, and so are considered essential for the reproduction of life on Earth. Therefore, a major focus of exobiology has been to understand how DNA, RNA, and the proteins and amino acids essential in their production originated.

Beginning in the 1950s, a series of experiments suggested that three sufficient conditions must be met to produce amino acids: a supply of carbon-rich material must be present, liquid water must be available, and some energy source (electrical discharge, high-energy particles, or possibly heat and sunlight) is required. While some scientists continued to doubt that the formation of organic molecules can necessarily lead to the development of life, these basic ideas guided much subsequent research into potential extraterrestrial life. Scientists examined the planets and satellites of the solar system, searching for locations where all three conditions are met. Water may be the most critical restriction since it remains a liquid only over a very narrow range of temperatures; the concept of a "habitable zone" developed to describe the orbital range around a star in which liquid water is possible. As space science advanced, it was understood that the surfaces of Venus and Mercury are too hot for liquid water to be present, while Jupiter, Saturn, Uranus, and Neptune (as well as Pluto, eventually reclassified as a dwarf planet) are too cold. This led to strong focus on Mars as the most suitable candidate for life in the solar system.

Scientists have gone back and forth over whether Mars may support life or did so in the past, with consensus shifting as new evidence is made available. One method of study involves looking for secondary evidence of life, a technique used to observe distant exoplanets as well as Mars. Where life is abundant, it can produce changes in the atmosphere of a planet, allowing astronomers to search for unusual signatures of biological activity. The present composition of Earth’s atmosphere, dominated by nitrogen and oxygen, is regulated by the life cycle processes of respiration and photosynthesis of Earthly organisms. The atmosphere of Mars, on the other hand, is dominated by carbon dioxide, and it contains only a trace amount of oxygen. Thus, by the 1960s, astronomers had observed that, at least in the present era, living organisms were not present in sufficient abundance to perturb the atmospheric chemistry of Mars.

The beginning of the space age made it possible to employ robotic spacecraft to perform direct measurements on the surface of some planets in the expanding search for evidence of life. The first such search was performed on Mars by two Viking spacecraft, developed by the National Aeronautics and Space Administration (NASA), which landed safely in 1976. Each Viking spacecraft was equipped with instruments designed to examine the soils of Mars for evidence of Earth-like life. No direct evidence was found, which for a time contributed to the prevailing belief that the solar system was wholly inhospitable beyond Earth. However, later missions and discoveries renewed exobiologists' hope that the conditions for life could still exist on Mars or at least have existed in the past. By 2015, scientists had confirmed the presence of liquid water on the planet, further suggesting that some kind of microscopic life could exist there. Into the 2020s, evidence continued to accumulate that Mars once had large amounts of surface water, and still retained substantial amounts underground. Although scientists noted that the presence of water did not necessarily mean the presence of life, such findings brought fresh attention to Mars as a candidate for extraterrestrial life.

New concepts in the search for extraterrestrial water also revitalized the search for extraterrestrial life elsewhere in the solar system. By the 2010s, scientists had confirmed or hypothesized the presence of liquid or frozen water on several moons, including Jupiter’s Europa and Saturn’s Titan and Enceladus. These satellites became chief subjects of exobiology research, with a growing body of evidence suggesting the presence of large, relatively warm oceans beneath shells of ice—considered prime locations for potential life. Meanwhile, technological improvements also allowed the detection of previously unknown celestial bodies. While much of this research focused on extrasolar planets (those beyond the solar system), scientists also increased their understanding of trans-Neptunian objects within the solar system.

While scientists improved their chances of detecting Earth-like, carbon-based life-forms reliant on water, they also broadened their understanding of how life could be radically different. During the 1980s and 1990s, developments in terrestrial biology changed how exobiologists looked at the essential conditions to develop life. Single-celled organisms called archaebacteria, which may have developed very early in Earth’s history, were discovered. These organisms live in oxygen-deprived places, such as the hot springs or even tar pits. Archaebacteria take in carbon dioxide and give off methane, and they actually cannot thrive in the presence of oxygen. They have genetic material different from that of other terrestrial life-forms, suggesting that they possibly evolved independently from the more common life-forms very early in Earth’s history at a time before the oxygen-rich atmosphere arose. Other terrestrial microorganisms were discovered that live on sulfur from geothermal sources rather than relying on the Sun to supply energy. The discovery of these unusual terrestrial life-forms suggests that conditions required for the development of the common life-forms on Earth may not be required for the development of all life. Thus, some planets and satellites previously believed to be unsuitable for the development of life may be habitable by organisms very different from the common life-forms on Earth.

Such discoveries complicate the search for extraterrestrial life in multiple ways. While exobiologists can broaden their search and pay attention to previously overlooked environments such as thermal vents, the possibility has been raised that life encountered on another planet or moon might be so foreign to known science that it may not be recognized as life. For example, some researchers have proposed that there is no reason life would have to follow the same conditions as on Earth if it developed under drastically different conditions. A non-carbon-based life-form or one that evolved in the absence of water, for example, in the liquid methane seas theorized to exist on Titan, might be so alien as to be unimaginable.

Methods of Study

One focus of the search for extraterrestrial life is to identify the carbon-rich compounds available for life’s development. Impacts of meteorites, asteroids, and comets are believed to have contributed a carbon-rich layer to the Earth’s early surface and other planets and their satellites. One particularly carbon-rich meteorite, called Murchison, fell in Australia in 1969. Detailed studies of Murchison established that it contains numerous organic compounds, including amino acids.

In 1986, five spacecraft, two launched by the Soviet Union, two by Japan, and one by the European Space Agency, flew past Halley’s comet. Dust analyzers on some of these spacecraft determined the chemical composition of individual dust particles emitted by the comet. These instruments detected a large number of carbon-rich particles, many of which also contained hydrogen, suggesting the presence of organic molecules in the dust. However, detailed analysis of organic molecules requires sophisticated scientific instruments too large and complicated to be flown on those spacecraft. NASA launched a spacecraft called Stardust to fly to Comet Wild 2 to collect dust emitted by that comet. It successfully returned samples to Earth in 2006. Laboratory study of the dust established the abundances and types of organic compounds present in Wild 2.

The second focus of the search for life is to perform direct tests for the presence of biological activity on other planets or satellites. Apollo astronauts collected the first samples from the moon in 1969. When they returned to Earth, the astronauts, their spacecraft, and their prized lunar rocks were subjected to a twenty-one-day quarantine during which scientists searched for living microorganisms that might be hazardous to life on Earth. Fragments from lunar rocks were crushed and placed in a standard culture medium, a nutrient-rich soup that promotes the growth of microorganisms. Microscopic examination of these samples showed no evidence of living microorganisms. More detailed studies of the lunar rocks have shown no fossil evidence of life-forms that might once have developed on the moon but are now extinct. Examination of lunar samples revealed them to be exceptionally dry, with none showing any evidence of liquid water. The absence of liquid water was taken to indicate that the moon was always a lifeless body.

Initial experiments in the search for life on another planet were conducted in 1976 by the two Viking spacecraft that landed on Mars. Each Viking carried four instruments to examine the soil samples for evidence of such basic life-cycle processes as respiration or photosynthesis. The Gas Exchange Experiment deposited samples of Martian soil in a chamber containing a culture medium. This apparatus monitored the composition of gas within its chamber, looking for changes in the abundance of carbon dioxide, oxygen, or hydrogen that would signal metabolic activity by microorganisms in the soil.

In a second experiment, the Labeled Release Experiment, radioactive carbon atoms were incorporated into the culture medium. A detector looked for the appearance of radioactive carbon in released gas, signaling that the addition of Martian soil to the nutrient had resulted in a reaction of biological origin. Both experiments produced positive results, but the effects were much more dramatic than the scientists had expected. These positive results were eventually explained as chemical reactions initiated because of the highly reactive nature of the surface materials on Mars resulting from their exposure to ultraviolet light from the sun, a superoxide chemical reaction.

The Pyrolytic Release Experiment provided an opportunity to test that explanation. It was also a labeled release experiment, but this apparatus had the additional capability of heating soil samples between experiments. Scientists heated soil to 548 kelvins, well above the temperature expected to kill any microorganisms present in the soil. Even then, the Pyrolytic Release Experiment yielded positive results, suggesting that the release was produced by a chemical reaction involving superoxides rather than a biological process.

A fourth experiment, the Gas Chromatograph Mass Spectrometry Experiment, produced the most convincing evidence that the soils at the Viking landing sites contained no microorganisms. This instrument found no organic molecules within the soil down to a limit of a few parts per million. Even the organic molecules that would be expected in the soils from the accumulation of meteorites like Murchison were absent. Subsequent studies indicated that the high chemical reactivity of the soils and intense ultraviolet radiation striking the surface would rapidly destroy most organic molecules. Thus, if there is life on Mars, the two Viking spacecraft, which could only sample the near-surface soils, were probably looking in the wrong places.

Although instruments on both Viking landers found no evidence of biological activity in their soil samples, the two Viking orbiters obtained high-resolution photographs of Mars’s surface, producing results that excited exobiologists. Several regions on Mars revealed features similar to extensive water flow channels on Earth, leading many geologists to conclude that water had flowed freely on the surface of Mars at some earlier period in its history. Later missions found ice deposits and eventually even liquid water on Mars. Because of the assumed importance of liquid water in the development of life, some exobiologists suggested that life might have developed on Mars in that earlier era and that life might exist in subsurface layers protected from ultraviolet radiation. Or perhaps such life had gone extinct but left fossil evidence behind.

In 1996, scientists from NASA’s Johnson Space Center reported that a meteorite called ALH 84001, one ejected from the surface of Mars and deposited in the Antarctic about thirteen thousand years ago, contained microscopic features that might indicate ancient Martian biological activity. This resulted in renewed interest in the search for life on Mars. These suspected fossils resembled wormlike creatures, but their size was extraordinarily small. Many scientists pointed out, however, that similar nanometer-sized structures could be produced geochemically and had nothing to do with life.

The same techniques used to search for existing or fossil life on Mars can be applied to other planets or satellites that are identified as suitable candidates for the development of life. The Galileo spacecraft, placed in orbit around Jupiter in late 1995, obtained close-up photographs of Jupiter’s four largest satellites. One of these, Europa, emerged as another potential site for the development of life. One of Galileo’s orbits around Jupiter took it within 363 miles of Europa’s surface, allowing its cameras to photograph objects as small as seventy-five feet across. These images showed evidence of ice flows that had broken from a solid sheet and been displaced, suggesting that they had floated or slipped across a liquid ocean or on a layer of slush below. Calculations indicated that Jupiter’s extreme gravitational pull could introduce tidal distortions that produce sufficient heat to allow liquid water to exist beneath Europa’s icy surface. Other photographs showed dark deposits, possibly carbon-rich material contributed by meteorites.

An observation by the Hubble Space Telescope in 2013 identified a spout of water vapor from the surface of Europa, generating further interest in the possibility of a liquid ocean beneath the satellite's ice shell. Exobiologists were excited to see the possible existence of the three conditions believed necessary for the development of life: carbon-rich material, water, and energy from the Jovian tides. These findings led NASA to plan a mission to further study Europa as a prime candidate for extraterrestrial life. A spacecraft placed into orbit around Europa could use radar to see through several miles of ice, detecting any water below and providing a clear test of the ocean model. More ambitious proposals included a spacecraft that would fling a nine-kilogram projectile into the surface of Europa, catch some of the debris lofted by the collision, and return it to terrestrial laboratories for examination. Another common proposal would see a submersible vehicle melt its way through Europa’s icy crust to reach a potential subsurface layer of liquid water and image the local environment directly. In 2024, NASA launched the spacecraft Europa Clipper, beginning a mission aimed at gathering more information about Europa's ice shell, composition, and geology through a series of flybys.

Titan, the largest satellite of Saturn, has a methane-rich atmosphere believed to be similar in composition to that of the early Earth. High-energy electrons and protons, trapped in the magnetic field of Saturn, continually bombard the upper region of Titan’s atmosphere. This bombardment is believed to produce complex organic molecules that rain down onto Titan’s surface. Titan is too cold to have liquid water. Titan remained the primary target of study for the Cassini spacecraft, which was launched in October 1997 and arrived in the Saturn system in early July 2004. Cassini dropped its Huygens probe, loaded with instruments to measure the types and abundances of the organic molecules, into Titan’s atmosphere. The Huygens probe showed its surface may be covered with lakes of methane or ethane, which some scientists speculate might be sufficient to allow primitive life to develop. Also, Titan’s crust appears to move significantly as if floating on a subsurface ocean, adding another intriguing aspect to the possibility of organic chemistry and/or primitive life on Titan.

Even Saturn's smaller satellite, Enceladus, displays unexpected geyser activity at its south polar regions. This suggested the possibility of liquid water underneath the surface and, therefore, the potential for primitive life. The presence of an ocean of salt water beneath the moon's ice crust was confirmed in 2014, and data about the moon captured by Cassini continued to be studied even after the mission ended in 2017, with a team of researchers arguing in 2018 that such data included indications of the existence of bigger, more complex organic molecules. Neptune’s Triton also exhibits cryovolcanism at an even lower temperature. More research was needed to determine the nature of this mechanism, and that investigation would likely have to await a Neptune orbiter. Meanwhile, there was also hope that the James Webb Space Telescope launched in 2021 could make identifications helpful to understanding and finding the conditions for life, including possible relevant chemical detections.


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Full Article

Astrobiology or exobiology is the search for and study of life on celestial bodies other than Earth. Much work in the field has focused on objects within the solar system, in part due to fact that they are the closest to Earth and therefore most conducive to study. Many scientists consider the planets Mars and Venus, as well as several satellites of the planets Jupiter and Saturn, to have the best conditions for potentially developing life.

Overview

The possibility that life might have developed elsewhere in the solar system has been the subject of speculation for hundreds of years. Early theories tended to focus on hypothetical human-like civilizations on other planets or moons. In 1820, Carl Gauss, a German mathematician, suggested cutting geometrical patterns into the Siberian forest large enough to be seen by an observer using a telescope from the moon or Mars. The idea was to motivate any inhabitants of the moon or Mars to engineer similar geometrical patterns, initiating crude communication with Earth. Other suggestions for communication with extraterrestrial intelligent life included setting huge fires in the Sahara desert and constructing large mirrors to reflect sunlight into space. These early ideas of communicating with intelligent life elsewhere in the solar system did not focus on particular sites where the conditions were expected to be appropriate for the development of life.

The growth of radio astronomy after World War II brought new avenues in research into potential extraterrestrial life. Researchers soon discovered that the natural universe was far from radio-quiet. Some scientists, beginning with astronomer Frank Drake, wondered about and then tested the idea that intelligence beyond Earth might transmit recognizable radio signals. This eventually gave rise to the coordinated Search for Extraterrestrial Intelligence (SETI) program. However, no decipherable communications were immediately found. This, along with deepening scientific understanding of the environments of known planets and moons, convinced most researchers against the possibility of advanced civilizations existing in the solar system.

Meanwhile, however, only in the second half of the twentieth century did biologists begin to develop a modern scientific understanding of how life likely originated on Earth. This knowledge provided clues as to the conditions needed for similar forms of life to develop elsewhere in the solar system. The study of terrestrial life indicates that it originated as simple, single-celled microorganisms—and that these simple microorganisms might be able to develop on other celestial bodies as well, given a few basic conditions such as the presence of water. With no signs of intelligent life apparent, the focus of solar system exobiology thus largely shifted to the search for simple microorganisms.

The dawn of the space age inaugurated an era in which spacecraft could be used to directly search for environments favorable to the development of life, perform experiments designed to detect living organisms on the surface of other planets and/or their satellites, and return samples to Earth so that scientists could examine them for evidence of biological activity or fossil evidence of past life. However, early efforts reinforced the idea that the other bodies in the solar system were barren. After the 1976 Mars Viking lander failed to find any biological signs in Martian soil, hopes of finding extraterrestrial life dwindled.

Despite those setbacks, there was a resurgence of interest in exobiology by the end of the twentieth century as technology improved and new discoveries were made. Evidence of river channels on Mars, hints of water ice on the moon and Mercury, observations of cryovolcanism on multiple natural satellites, and other findings suggested that the solar system might not be as inhospitable to the development of life as once believed. Research continued to accelerate in the twenty-first century, with growing amounts of data and theoretical refinements. One important trend was the recognition that extraterrestrial life might take forms wholly unfamiliar to terrestrial biology.

Context

Traditionally, research into potential extraterrestrial life in the solar system has been rooted in understanding of life on Earth. Terrestrial life is based on complex organic molecules consisting of chains of carbon, hydrogen, nitrogen, and oxygen. However, these molecules can be produced by simple chemical reactions as well as by biological activity. Thus, to determine if a process is truly biological rather than simply a chemical reaction, it is necessary to define the criteria for life. The ability of an organism to reproduce itself is considered an essential feature of life. Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are the organic molecules that control heredity in terrestrial life-forms, and so are considered essential for the reproduction of life on Earth. Therefore, a major focus of exobiology has been to understand how DNA, RNA, and the proteins and amino acids essential in their production originated.

Beginning in the 1950s, a series of experiments suggested that three sufficient conditions must be met to produce amino acids: a supply of carbon-rich material must be present, liquid water must be available, and some energy source (electrical discharge, high-energy particles, or possibly heat and sunlight) is required. While some scientists continued to doubt that the formation of organic molecules can necessarily lead to the development of life, these basic ideas guided much subsequent research into potential extraterrestrial life. Scientists examined the planets and satellites of the solar system, searching for locations where all three conditions are met. Water may be the most critical restriction since it remains a liquid only over a very narrow range of temperatures; the concept of a "habitable zone" developed to describe the orbital range around a star in which liquid water is possible. As space science advanced, it was understood that the surfaces of Venus and Mercury are too hot for liquid water to be present, while Jupiter, Saturn, Uranus, and Neptune (as well as Pluto, eventually reclassified as a dwarf planet) are too cold. This led to strong focus on Mars as the most suitable candidate for life in the solar system.

Scientists have gone back and forth over whether Mars may support life or did so in the past, with consensus shifting as new evidence is made available. One method of study involves looking for secondary evidence of life, a technique used to observe distant exoplanets as well as Mars. Where life is abundant, it can produce changes in the atmosphere of a planet, allowing astronomers to search for unusual signatures of biological activity. The present composition of Earth’s atmosphere, dominated by nitrogen and oxygen, is regulated by the life cycle processes of respiration and photosynthesis of Earthly organisms. The atmosphere of Mars, on the other hand, is dominated by carbon dioxide, and it contains only a trace amount of oxygen. Thus, by the 1960s, astronomers had observed that, at least in the present era, living organisms were not present in sufficient abundance to perturb the atmospheric chemistry of Mars.

The beginning of the space age made it possible to employ robotic spacecraft to perform direct measurements on the surface of some planets in the expanding search for evidence of life. The first such search was performed on Mars by two Viking spacecraft, developed by the National Aeronautics and Space Administration (NASA), which landed safely in 1976. Each Viking spacecraft was equipped with instruments designed to examine the soils of Mars for evidence of Earth-like life. No direct evidence was found, which for a time contributed to the prevailing belief that the solar system was wholly inhospitable beyond Earth. However, later missions and discoveries renewed exobiologists' hope that the conditions for life could still exist on Mars or at least have existed in the past. By 2015, scientists had confirmed the presence of liquid water on the planet, further suggesting that some kind of microscopic life could exist there. Into the 2020s, evidence continued to accumulate that Mars once had large amounts of surface water, and still retained substantial amounts underground. Although scientists noted that the presence of water did not necessarily mean the presence of life, such findings brought fresh attention to Mars as a candidate for extraterrestrial life.

New concepts in the search for extraterrestrial water also revitalized the search for extraterrestrial life elsewhere in the solar system. By the 2010s, scientists had confirmed or hypothesized the presence of liquid or frozen water on several moons, including Jupiter’s Europa and Saturn’s Titan and Enceladus. These satellites became chief subjects of exobiology research, with a growing body of evidence suggesting the presence of large, relatively warm oceans beneath shells of ice—considered prime locations for potential life. Meanwhile, technological improvements also allowed the detection of previously unknown celestial bodies. While much of this research focused on extrasolar planets (those beyond the solar system), scientists also increased their understanding of trans-Neptunian objects within the solar system.

While scientists improved their chances of detecting Earth-like, carbon-based life-forms reliant on water, they also broadened their understanding of how life could be radically different. During the 1980s and 1990s, developments in terrestrial biology changed how exobiologists looked at the essential conditions to develop life. Single-celled organisms called archaebacteria, which may have developed very early in Earth’s history, were discovered. These organisms live in oxygen-deprived places, such as the hot springs or even tar pits. Archaebacteria take in carbon dioxide and give off methane, and they actually cannot thrive in the presence of oxygen. They have genetic material different from that of other terrestrial life-forms, suggesting that they possibly evolved independently from the more common life-forms very early in Earth’s history at a time before the oxygen-rich atmosphere arose. Other terrestrial microorganisms were discovered that live on sulfur from geothermal sources rather than relying on the Sun to supply energy. The discovery of these unusual terrestrial life-forms suggests that conditions required for the development of the common life-forms on Earth may not be required for the development of all life. Thus, some planets and satellites previously believed to be unsuitable for the development of life may be habitable by organisms very different from the common life-forms on Earth.

Such discoveries complicate the search for extraterrestrial life in multiple ways. While exobiologists can broaden their search and pay attention to previously overlooked environments such as thermal vents, the possibility has been raised that life encountered on another planet or moon might be so foreign to known science that it may not be recognized as life. For example, some researchers have proposed that there is no reason life would have to follow the same conditions as on Earth if it developed under drastically different conditions. A non-carbon-based life-form or one that evolved in the absence of water, for example, in the liquid methane seas theorized to exist on Titan, might be so alien as to be unimaginable.

Methods of Study

One focus of the search for extraterrestrial life is to identify the carbon-rich compounds available for life’s development. Impacts of meteorites, asteroids, and comets are believed to have contributed a carbon-rich layer to the Earth’s early surface and other planets and their satellites. One particularly carbon-rich meteorite, called Murchison, fell in Australia in 1969. Detailed studies of Murchison established that it contains numerous organic compounds, including amino acids.

In 1986, five spacecraft, two launched by the Soviet Union, two by Japan, and one by the European Space Agency, flew past Halley’s comet. Dust analyzers on some of these spacecraft determined the chemical composition of individual dust particles emitted by the comet. These instruments detected a large number of carbon-rich particles, many of which also contained hydrogen, suggesting the presence of organic molecules in the dust. However, detailed analysis of organic molecules requires sophisticated scientific instruments too large and complicated to be flown on those spacecraft. NASA launched a spacecraft called Stardust to fly to Comet Wild 2 to collect dust emitted by that comet. It successfully returned samples to Earth in 2006. Laboratory study of the dust established the abundances and types of organic compounds present in Wild 2.

The second focus of the search for life is to perform direct tests for the presence of biological activity on other planets or satellites. Apollo astronauts collected the first samples from the moon in 1969. When they returned to Earth, the astronauts, their spacecraft, and their prized lunar rocks were subjected to a twenty-one-day quarantine during which scientists searched for living microorganisms that might be hazardous to life on Earth. Fragments from lunar rocks were crushed and placed in a standard culture medium, a nutrient-rich soup that promotes the growth of microorganisms. Microscopic examination of these samples showed no evidence of living microorganisms. More detailed studies of the lunar rocks have shown no fossil evidence of life-forms that might once have developed on the moon but are now extinct. Examination of lunar samples revealed them to be exceptionally dry, with none showing any evidence of liquid water. The absence of liquid water was taken to indicate that the moon was always a lifeless body.

Initial experiments in the search for life on another planet were conducted in 1976 by the two Viking spacecraft that landed on Mars. Each Viking carried four instruments to examine the soil samples for evidence of such basic life-cycle processes as respiration or photosynthesis. The Gas Exchange Experiment deposited samples of Martian soil in a chamber containing a culture medium. This apparatus monitored the composition of gas within its chamber, looking for changes in the abundance of carbon dioxide, oxygen, or hydrogen that would signal metabolic activity by microorganisms in the soil.

In a second experiment, the Labeled Release Experiment, radioactive carbon atoms were incorporated into the culture medium. A detector looked for the appearance of radioactive carbon in released gas, signaling that the addition of Martian soil to the nutrient had resulted in a reaction of biological origin. Both experiments produced positive results, but the effects were much more dramatic than the scientists had expected. These positive results were eventually explained as chemical reactions initiated because of the highly reactive nature of the surface materials on Mars resulting from their exposure to ultraviolet light from the sun, a superoxide chemical reaction.

The Pyrolytic Release Experiment provided an opportunity to test that explanation. It was also a labeled release experiment, but this apparatus had the additional capability of heating soil samples between experiments. Scientists heated soil to 548 kelvins, well above the temperature expected to kill any microorganisms present in the soil. Even then, the Pyrolytic Release Experiment yielded positive results, suggesting that the release was produced by a chemical reaction involving superoxides rather than a biological process.

A fourth experiment, the Gas Chromatograph Mass Spectrometry Experiment, produced the most convincing evidence that the soils at the Viking landing sites contained no microorganisms. This instrument found no organic molecules within the soil down to a limit of a few parts per million. Even the organic molecules that would be expected in the soils from the accumulation of meteorites like Murchison were absent. Subsequent studies indicated that the high chemical reactivity of the soils and intense ultraviolet radiation striking the surface would rapidly destroy most organic molecules. Thus, if there is life on Mars, the two Viking spacecraft, which could only sample the near-surface soils, were probably looking in the wrong places.

Although instruments on both Viking landers found no evidence of biological activity in their soil samples, the two Viking orbiters obtained high-resolution photographs of Mars’s surface, producing results that excited exobiologists. Several regions on Mars revealed features similar to extensive water flow channels on Earth, leading many geologists to conclude that water had flowed freely on the surface of Mars at some earlier period in its history. Later missions found ice deposits and eventually even liquid water on Mars. Because of the assumed importance of liquid water in the development of life, some exobiologists suggested that life might have developed on Mars in that earlier era and that life might exist in subsurface layers protected from ultraviolet radiation. Or perhaps such life had gone extinct but left fossil evidence behind.

In 1996, scientists from NASA’s Johnson Space Center reported that a meteorite called ALH 84001, one ejected from the surface of Mars and deposited in the Antarctic about thirteen thousand years ago, contained microscopic features that might indicate ancient Martian biological activity. This resulted in renewed interest in the search for life on Mars. These suspected fossils resembled wormlike creatures, but their size was extraordinarily small. Many scientists pointed out, however, that similar nanometer-sized structures could be produced geochemically and had nothing to do with life.

The same techniques used to search for existing or fossil life on Mars can be applied to other planets or satellites that are identified as suitable candidates for the development of life. The Galileo spacecraft, placed in orbit around Jupiter in late 1995, obtained close-up photographs of Jupiter’s four largest satellites. One of these, Europa, emerged as another potential site for the development of life. One of Galileo’s orbits around Jupiter took it within 363 miles of Europa’s surface, allowing its cameras to photograph objects as small as seventy-five feet across. These images showed evidence of ice flows that had broken from a solid sheet and been displaced, suggesting that they had floated or slipped across a liquid ocean or on a layer of slush below. Calculations indicated that Jupiter’s extreme gravitational pull could introduce tidal distortions that produce sufficient heat to allow liquid water to exist beneath Europa’s icy surface. Other photographs showed dark deposits, possibly carbon-rich material contributed by meteorites.

An observation by the Hubble Space Telescope in 2013 identified a spout of water vapor from the surface of Europa, generating further interest in the possibility of a liquid ocean beneath the satellite's ice shell. Exobiologists were excited to see the possible existence of the three conditions believed necessary for the development of life: carbon-rich material, water, and energy from the Jovian tides. These findings led NASA to plan a mission to further study Europa as a prime candidate for extraterrestrial life. A spacecraft placed into orbit around Europa could use radar to see through several miles of ice, detecting any water below and providing a clear test of the ocean model. More ambitious proposals included a spacecraft that would fling a nine-kilogram projectile into the surface of Europa, catch some of the debris lofted by the collision, and return it to terrestrial laboratories for examination. Another common proposal would see a submersible vehicle melt its way through Europa’s icy crust to reach a potential subsurface layer of liquid water and image the local environment directly. In 2024, NASA launched the spacecraft Europa Clipper, beginning a mission aimed at gathering more information about Europa's ice shell, composition, and geology through a series of flybys.

Titan, the largest satellite of Saturn, has a methane-rich atmosphere believed to be similar in composition to that of the early Earth. High-energy electrons and protons, trapped in the magnetic field of Saturn, continually bombard the upper region of Titan’s atmosphere. This bombardment is believed to produce complex organic molecules that rain down onto Titan’s surface. Titan is too cold to have liquid water. Titan remained the primary target of study for the Cassini spacecraft, which was launched in October 1997 and arrived in the Saturn system in early July 2004. Cassini dropped its Huygens probe, loaded with instruments to measure the types and abundances of the organic molecules, into Titan’s atmosphere. The Huygens probe showed its surface may be covered with lakes of methane or ethane, which some scientists speculate might be sufficient to allow primitive life to develop. Also, Titan’s crust appears to move significantly as if floating on a subsurface ocean, adding another intriguing aspect to the possibility of organic chemistry and/or primitive life on Titan.

Even Saturn's smaller satellite, Enceladus, displays unexpected geyser activity at its south polar regions. This suggested the possibility of liquid water underneath the surface and, therefore, the potential for primitive life. The presence of an ocean of salt water beneath the moon's ice crust was confirmed in 2014, and data about the moon captured by Cassini continued to be studied even after the mission ended in 2017, with a team of researchers arguing in 2018 that such data included indications of the existence of bigger, more complex organic molecules. Neptune’s Triton also exhibits cryovolcanism at an even lower temperature. More research was needed to determine the nature of this mechanism, and that investigation would likely have to await a Neptune orbiter. Meanwhile, there was also hope that the James Webb Space Telescope launched in 2021 could make identifications helpful to understanding and finding the conditions for life, including possible relevant chemical detections.


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