Astrobiology, formerly known as exobiology, is an interdisciplinary scientific field concerned with the origins, early evolution, distribution, and future of life in the universe. Astrobiology considers the question of whether extraterrestrial life exists, and if it does, how humans can detect it.
Astrobiology makes use of molecular biology, biophysics, biochemistry, chemistry, astronomy, physical cosmology, exoplanetology and geology to investigate the possibility of life on other worlds and help recognize biospheres that might be different from that on Earth. The origin and early evolution of life is an inseparable part of the discipline of astrobiology. Astrobiology concerns itself with interpretation of existing scientific data, and although speculation is entertained to give context, astrobiology concerns itself primarily with hypotheses that fit firmly into existing scientific theories.
This interdisciplinary field encompasses research on the origin of planetary systems, origins of organic compounds in space, rock-water-carbon interactions, abiogenesis on Earth, planetary habitability, research on biosignatures for life detection, and studies on the potential for life to adapt to challenges on Earth and in outer space.
Biochemistry may have begun shortly after the Big Bang, 13.8 billion years ago, during a habitable epoch when the Universe was only 10–17 million years old. According to the panspermia hypothesis, microscopic life—distributed by meteoroids, asteroids and other small Solar System bodies—may exist throughout the universe. According to research published in August 2015, very large galaxies may be more favorable to the creation and development of habitable planets than such smaller galaxies as the Milky Way. Nonetheless, Earth is the only place in the universe humans know to harbor life. Estimates of habitable zones around other stars, sometimes referred to as "Goldilocks zones," along with the discovery of hundreds of extrasolar planets and new insights into extreme habitats here on Earth, suggest that there may be many more habitable places in the universe than considered possible until very recently.
Current studies on the planet Mars by the Curiosity and Opportunity rovers are searching for evidence of ancient life as well as plains related to ancient rivers or lakes that may have been habitable. The search for evidence of habitability, taphonomy (related to fossils), and organic molecules on the planet Mars is now a primary NASA and ESA objective.
Even if extraterrestrial life is never discovered, the interdisciplinary nature of astrobiology, and the cosmic and evolutionary perspectives engendered by it, may still result in a range of benefits here on Earth.
The term was first proposed by the Russian (Soviet) astronomer Gavriil Tikhov in 1953. Astrobiology is etymologically derived from the Greek ἄστρον, astron, "constellation, star"; βίος, bios, "life"; and -λογία, -logia, study. The synonyms of astrobiology are diverse; however, the synonyms were structured in relation to the most important sciences implied in its development: astronomy and biology. A close synonym is exobiology from the Greek Έξω, "external"; Βίος, bios, "life"; and λογία, -logia, study. The term exobiology was coined by molecular biologist and Nobel Prize winner Joshua Lederberg. Exobiology is considered to have a narrow scope limited to search of life external to Earth, whereas subject area of astrobiology is wider and investigates the link between life and the universe, which includes the search for extraterrestrial life, but also includes the study of life on Earth, its origin, evolution and limits.
Another term used in the past is xenobiology, ("biology of the foreigners") a word used in 1954 by science fiction writer Robert Heinlein in his work The Star Beast. The term xenobiology is now used in a more specialized sense, to mean "biology based on foreign chemistry", whether of extraterrestrial or terrestrial (possibly synthetic) origin. Since alternate chemistry analogs to some life-processes have been created in the laboratory, xenobiology is now considered as an extant subject.
While it is an emerging and developing field, the question of whether life exists elsewhere in the universe is a verifiable hypothesis and thus a valid line of scientific inquiry. Though once considered outside the mainstream of scientific inquiry, astrobiology has become a formalized field of study. Planetary scientist David Grinspoon calls astrobiology a field of natural philosophy, grounding speculation on the unknown, in known scientific theory. NASA's interest in exobiology first began with the development of the U.S. Space Program. In 1959, NASA funded its first exobiology project, and in 1960, NASA founded an Exobiology Program, which is now one of four main elements of NASA's current Astrobiology Program. In 1971, NASA funded the search for extraterrestrial intelligence (SETI) to search radio frequencies of the electromagnetic spectrum for interstellar communications transmitted by extraterrestrial life outside the Solar System. NASA's Viking missions to Mars, launched in 1976, included three biology experiments designed to look for metabolism of present life on Mars.
Advancements in the fields of astrobiology, observational astronomy and discovery of large varieties of extremophiles with extraordinary capability to thrive in the harshest environments on Earth, have led to speculation that life may possibly be thriving on many of the extraterrestrial bodies in the universe. A particular focus of current astrobiology research is the search for life on Mars due to this planet's proximity to Earth and geological history. There is a growing body of evidence to suggest that Mars has previously had a considerable amount of water on its surface, water being considered an essential precursor to the development of carbon-based life.
Missions specifically designed to search for current life on Mars were the Viking program and Beagle 2 probes. The Viking results were inconclusive, and Beagle 2 failed minutes after landing. A future mission with a strong astrobiology role would have been the Jupiter Icy Moons Orbiter, designed to study the frozen moons of Jupiter—some of which may have liquid water—had it not been cancelled. In late 2008, the Phoenix lander probed the environment for past and present planetary habitability of microbial life on Mars, and researched the history of water there.
The European Space Agency's astrobiology roadmap from 2016, identified five main research topics, and specifies several key scientific objectives for each topic. The five research topics are: 1) Origin and evolution of planetary systems; 2) Origins of organic compounds in space; 3) Rock-water-carbon interactions, organic synthesis on Earth, and steps to life; 4) Life and habitability; 5) Biosignatures as facilitating life detection.
In November 2011, NASA launched the Mars Science Laboratory mission carrying the Curiosity rover, which landed on Mars at Gale Crater in August 2012. The Curiosity rover is currently probing the environment for past and present planetary habitability of microbial life on Mars. On 9 December 2013, NASA reported that, based on evidence from Curiosity studying Aeolis Palus, Gale Crater contained an ancient freshwater lake which could have been a hospitable environment for microbial life.
The European Space Agency is currently collaborating with the Russian Federal Space Agency (Roscosmos) and developing the ExoMars astrobiology rover, which is to be launched in July 2020. Meanwhile, NASA is developing the Mars 2020 astrobiology rover and sample cacher for a later return to Earth.
When looking for life on other planets like Earth, some simplifying assumptions are useful to reduce the size of the task of the astrobiologist. One is the informed assumption that the vast majority of life forms in our galaxy are based on carbon chemistries, as are all life forms on Earth. Carbon is well known for the unusually wide variety of molecules that can be formed around it. Carbon is the fourth most abundant element in the universe and the energy required to make or break a bond is at just the appropriate level for building molecules which are not only stable, but also reactive. The fact that carbon atoms bond readily to other carbon atoms allows for the building of extremely long and complex molecules.
The presence of liquid water is an assumed requirement, as it is a common molecule and provides an excellent environment for the formation of complicated carbon-based molecules that could eventually lead to the emergence of life. Some researchers posit environments of water-ammonia mixtures as possible solvents for hypothetical types of biochemistry.
A third assumption is to focus on planets orbiting Sun-like stars for increased probabilities of planetary habitability. Very large stars have relatively short lifetimes, meaning that life might not have time to emerge on planets orbiting them. Very small stars provide so little heat and warmth that only planets in very close orbits around them would not be frozen solid, and in such close orbits these planets would be tidally "locked" to the star. The long lifetimes of red dwarfs could allow the development of habitable environments on planets with thick atmospheres. This is significant, as red dwarfs are extremely common. (See Habitability of red dwarf systems).
Since Earth is the only planet known to harbor life, there is no evident way to know if any of these simplifying assumptions are correct.
Research on communication with extraterrestrial intelligence (CETI) focuses on composing and deciphering messages that could theoretically be understood by another technological civilization. Communication attempts by humans have included broadcasting mathematical languages, pictorial systems such as the Arecibo message and computational approaches to detecting and deciphering 'natural' language communication. The SETI program, for example, uses both radio telescopes and optical telescopes to search for deliberate signals from an extraterrestrial intelligence.
While some high-profile scientists, such as Carl Sagan, have advocated the transmission of messages, scientist Stephen Hawking warned against it, suggesting that aliens might simply raid Earth for its resources and then move on.
Elements of astrobiology
Most astronomy-related astrobiology research falls into the category of extrasolar planet (exoplanet) detection, the hypothesis being that if life arose on Earth, then it could also arise on other planets with similar characteristics. To that end, a number of instruments designed to detect Earth-sized exoplanets have been considered, most notably NASA's Terrestrial Planet Finder (TPF) and ESA's Darwin programs, both of which have been cancelled. NASA launched the Kepler mission in March 2009, and the French Space Agency launched the COROT space mission in 2006. There are also several less ambitious ground-based efforts underway.
The goal of these missions is not only to detect Earth-sized planets, but also to directly detect light from the planet so that it may be studied spectroscopically. By examining planetary spectra, it would be possible to determine the basic composition of an extrasolar planet's atmosphere and/or surface. Given this knowledge, it may be possible to assess the likelihood of life being found on that planet. A NASA research group, the Virtual Planet Laboratory, is using computer modeling to generate a wide variety of virtual planets to see what they would look like if viewed by TPF or Darwin. It is hoped that once these missions come online, their spectra can be cross-checked with these virtual planetary spectra for features that might indicate the presence of life.
An estimate for the number of planets with intelligent communicative extraterrestrial life can be gleaned from the Drake equation, essentially an equation expressing the probability of intelligent life as the product of factors such as the fraction of planets that might be habitable and the fraction of planets on which life might arise:
- N = The number of communicative civilizations
- R* = The rate of formation of suitable stars (stars such as our Sun)
- fp = The fraction of those stars with planets (current evidence indicates that planetary systems may be common for stars like the Sun)
- ne = The number of Earth-sized worlds per planetary system
- fl = The fraction of those Earth-sized planets where life actually develops
- fi = The fraction of life sites where intelligence develops
- fc = The fraction of communicative planets (those on which electromagnetic communications technology develops)
- L = The "lifetime" of communicating civilizations
However, whilst the rationale behind the equation is sound, it is unlikely that the equation will be constrained to reasonable limits of error any time soon. The problem with the formula is that it is not usable to generate or support hypotheses because it contains factors that can never be verified. The first term, R*, number of stars, is generally constrained within a few orders of magnitude. The second and third terms, fp, stars with planets and fe, planets with habitable conditions, are being evaluated for the star's neighborhood. Drake originally formulated the equation merely as an agenda for discussion at the Green Bank conference, but some applications of the formula had been taken literally and related to simplistic or pseudoscientific arguments. Another associated topic is the Fermi paradox, which suggests that if intelligent life is common in the universe, then there should be obvious signs of it.
Another active research area in astrobiology is planetary system formation. It has been suggested that the peculiarities of the Solar System (for example, the presence of Jupiter as a protective shield) may have greatly increased the probability of intelligent life arising on our planet.
Biology cannot state that a process or phenomenon, by being mathematically possible, has to exist forcibly in an extraterrestrial body. Biologists specify what is speculative and what is not. The discovery of extremophiles, organisms able to survive in extreme environments, became a core research element for astrobiologists, as they are important to understand four areas in the limits of life in planetary context: the potential for panspermia, forward contamination due to human exploration ventures, planetary colonization by humans, and the exploration of extinct and extant extraterrestrial life.
Until the 1970s, life was thought to be entirely dependent on energy from the Sun. Plants on Earth's surface capture energy from sunlight to photosynthesize sugars from carbon dioxide and water, releasing oxygen in the process that is then consumed by oxygen-respiring organisms, passing their energy up the food chain. Even life in the ocean depths, where sunlight cannot reach, was thought to obtain its nourishment either from consuming organic detritus rained down from the surface waters or from eating animals that did. The world's ability to support life was thought to depend on its access to sunlight. However, in 1977, during an exploratory dive to the Galapagos Rift in the deep-sea exploration submersible Alvin, scientists discovered colonies of giant tube worms, clams, crustaceans, mussels, and other assorted creatures clustered around undersea volcanic features known as black smokers. These creatures thrive despite having no access to sunlight, and it was soon discovered that they comprise an entirely independent ecosystem. Although most of these multicellular lifeforms need dissolved oxygen (produced by oxygenic photosynthesis) for their aerobic cellular respiration and thus are not completely independent from sunlight by themselves, the basis for their food chain is a form of bacterium that derives its energy from oxidization of reactive chemicals, such as hydrogen or hydrogen sulfide, that bubble up from the Earth's interior. Other lifeforms entirely decoupled from the energy from sunlight are green sulphur bacteria which are capturing geothermal light for anoxygenic photosynthesis or bacteria running chemolithoautotrophy based on the radioactive decay of uranium. This chemosynthesis revolutionized the study of biology and astrobiology by revealing that life need not be sun-dependent; it only requires water and an energy gradient in order to exist.
Biologists have found extremophiles that thrive in ice, boiling water, acid, alkali, the water core of nuclear reactors, salt crystals, toxic waste and in a range of other extreme habitats that were previously thought to be inhospitable for life. This opened up a new avenue in astrobiology by massively expanding the number of possible extraterrestrial habitats. Characterization of these organisms, their environments and their evolutionary pathways, is considered a crucial component to understanding how life might evolve elsewhere in the universe. For example, some organisms able to withstand exposure to the vacuum and radiation of outer space include the lichen fungi Rhizocarpon geographicum and Xanthoria elegans, the bacterium Bacillus safensis, Deinococcus radiodurans, Bacillus subtilis, yeast Saccharomyces cerevisiae, seeds from Arabidopsis thaliana ('mouse-ear cress'), as well as the invertebrate animal Tardigrade. While tardigrades are not considered true extremophiles, they are considered extremotolerant microorganisms that have contributed to the field of astrobiology. Their extreme radiation tolerance and presence of DNA protection proteins may provide answers as to whether life can survive away from the protection of the Earth's atmosphere.
Jupiter's moon, Europa, and Saturn's moon, Enceladus, are now considered the most likely locations for extant extraterrestrial life in the Solar System due to their subsurface water oceans where radiogenic and tidal heating enables liquid water to exist.
The origin of life, known as abiogenesis, distinct from the evolution of life, is another ongoing field of research. Oparin and Haldane postulated that the conditions on the early Earth were conducive to the formation of organic compounds from inorganic elements and thus to the formation of many of the chemicals common to all forms of life we see today. The study of this process, known as prebiotic chemistry, has made some progress, but it is still unclear whether or not life could have formed in such a manner on Earth. The alternative hypothesis of panspermia is that the first elements of life may have formed on another planet with even more favorable conditions (or even in interstellar space, asteroids, etc.) and then have been carried over to Earth—the panspermia hypothesis.
The cosmic dust permeating the universe contains complex organic compounds ("amorphous organic solids with a mixed aromatic-aliphatic structure") that could be created naturally, and rapidly, by stars. Further, a scientist suggested that these compounds may have been related to the development of life on Earth and said that, "If this is the case, life on Earth may have had an easier time getting started as these organics can serve as basic ingredients for life."
More than 20% of the carbon in the universe may be associated with polycyclic aromatic hydrocarbons (PAHs), possible starting materials for the formation of life. PAHs seem to have been formed shortly after the Big Bang, are widespread throughout the universe, and are associated with new stars and exoplanets. PAHs are subjected to interstellar medium conditions and are transformed through hydrogenation, oxygenation and hydroxylation, to more complex organics – "a step along the path toward amino acids and nucleotides, the raw materials of proteins and DNA, respectively".
Astroecology concerns the interactions of life with space environments and resources, in planets, asteroids and comets. On a larger scale, astroecology concerns resources for life about stars in the galaxy through the cosmological future. Astroecology attempts to quantify future life in space, addressing this area of astrobiology.
Experimental astroecology investigates resources in planetary soils, using actual space materials in meteorites. The results suggest that Martian and carbonaceous chondrite materials can support bacteria, algae and plant (asparagus, potato) cultures, with high soil fertilities. The results support that life could have survived in early aqueous asteroids and on similar materials imported to Earth by dust, comets and meteorites, and that such asteroid materials can be used as soil for future space colonies.
On the largest scale, cosmoecology concerns life in the universe over cosmological times. The main sources of energy may be red giant stars and white and red dwarf stars, sustaining life for 1020 years. Astroecologists suggest that their mathematical models may quantify the potential amounts of future life in space, allowing a comparable expansion in biodiversity, potentially leading to diverse intelligent life forms.
Astrogeology is a planetary science discipline concerned with the geology of celestial bodies such as the planets and their moons, asteroids, comets, and meteorites. The information gathered by this discipline allows the measure of a planet's or a natural satellite's potential to develop and sustain life, or planetary habitability.
An additional discipline of astrogeology is geochemistry, which involves study of the chemical composition of the Earth and other planets, chemical processes and reactions that govern the composition of rocks and soils, the cycles of matter and energy and their interaction with the hydrosphere and the atmosphere of the planet. Specializations include cosmochemistry, biochemistry and organic geochemistry.
The fossil record provides the oldest known evidence for life on Earth. By examining the fossil evidence, paleontologists are able to better understand the types of organisms that arose on the early Earth. Some regions on Earth, such as the Pilbara in Western Australia and the McMurdo Dry Valleys of Antarctica, are also considered to be geological analogs to regions of Mars, and as such, might be able to provide clues on how to search for past life on Mars.
The various organic functional groups, composed of hydrogen, oxygen, nitrogen, phosphorus, sulfur, and a host of metals, such as iron, magnesium, and zinc, provide the enormous diversity of chemical reactions necessarily catalyzed by a living organism. Silicon, in contrast, interacts with only a few other atoms, and the large silicon molecules are monotonous compared with the combinatorial universe of organic macromolecules. Indeed, it seems likely that the basic building blocks of life anywhere will be similar those on Earth, in the generality if not in the detail. Although terrestrial life and life that might arise independently of Earth are expected to use many similar, if not identical, building blocks, they also are expected to have some biochemical qualities that are unique. If life has had a comparable impact elsewhere in the Solar System, the relative abundances of chemicals key for its survival – whatever they may be – could betray its presence. Whatever extraterrestrial life may be, its tendency to chemically alter its environment might just give it away.
Life in the Solar System
People have long speculated about the possibility of life in settings other than Earth, however, speculation on the nature of life elsewhere often has paid little heed to constraints imposed by the nature of biochemistry. The likelihood that life throughout the universe is probably carbon-based is suggested by the fact that carbon is one of the most abundant of the higher elements. Only two of the natural atoms, carbon and silicon, are known to serve as the backbones of molecules sufficiently large to carry biological information. As the structural basis for life, one of carbon's important features is that unlike silicon, it can readily engage in the formation of chemical bonds with many other atoms, thereby allowing for the chemical versatility required to conduct the reactions of biological metabolism and propagation.
Thought on where in the Solar System life might occur, was limited historically by the understanding that life relies ultimately on light and warmth from the Sun and, therefore, is restricted to the surfaces of planets. The four most likely candidates for life in the Solar System are the planet Mars, the Jovian moon Europa, and Saturn's moons Titan, and Enceladus.
Mars, Enceladus and Europa are considered likely candidates in the search for life primarily because they may have underground liquid water, a molecule essential for life as we know it for its use as a solvent in cells. Water on Mars is found frozen in its polar ice caps, and newly carved gullies recently observed on Mars suggest that liquid water may exist, at least transiently, on the planet's surface. At the Martian low temperatures and low pressure, liquid water is likely to be highly saline. As for Europa and Enceladus, large global oceans of liquid water exist beneath these moons' icy outer crusts. This water may be warmed to a liquid state by volcanic vents on the ocean floor, but the primary source of heat is probably tidal heating. On 11 December 2013, NASA reported the detection of "clay-like minerals" (specifically, phyllosilicates), often associated with organic materials, on the icy crust of Europa. The presence of the minerals may have been the result of a collision with an asteroid or comet according to the scientists. Additionally, on 27 June 2018, astronomers reported the detection of complex macromolecular organics on Enceladus and, according to NASA scientists in May 2011, "is emerging as the most habitable spot beyond Earth in the Solar System for life as we know it".
Another planetary body that could potentially sustain extraterrestrial life is Saturn's largest moon, Titan. Titan has been described as having conditions similar to those of early Earth. On its surface, scientists have discovered the first liquid lakes outside Earth, but these lakes seem to be composed of ethane and/or methane, not water. Some scientists think it possible that these liquid hydrocarbons might take the place of water in living cells different from those on Earth. After Cassini data were studied, it was reported on March 2008 that Titan may also have an underground ocean composed of liquid water and ammonia.
Measuring the ratio of hydrogen and methane levels on Mars may help determine the likelihood of life on Mars. According to the scientists, "...low H2/CH4 ratios (less than approximately 40) indicate that life is likely present and active." Other scientists have recently reported methods of detecting hydrogen and methane in extraterrestrial atmospheres.
Complex organic compounds of life, including uracil, cytosine and thymine, have been formed in a laboratory under outer space conditions, using starting chemicals such as pyrimidine, found in meteorites. Pyrimidine, like polycyclic aromatic hydrocarbons (PAHs), is the most carbon-rich chemical found in the universe.
Rare Earth hypothesis
The Rare Earth hypothesis postulates that multicellular life forms found on Earth may actually be more of a rarity than scientists assume. It provides a possible answer to the Fermi paradox which suggests, "If extraterrestrial aliens are common, why aren't they obvious?" It is apparently in opposition to the principle of mediocrity, assumed by famed astronomers Frank Drake, Carl Sagan, and others. The Principle of Mediocrity suggests that life on Earth is not exceptional, and it is more than likely to be found on innumerable other worlds.
The systematic search for possible life outside Earth is a valid multidisciplinary scientific endeavor. However, hypotheses and predictions as to its existence and origin vary widely, and at the present, the development of hypotheses firmly grounded on science may be considered astrobiology's most concrete practical application. It has been proposed that viruses are likely to be encountered on other life-bearing planets, and may be present even if there are no biological cells.
As of 2019[update], no evidence of extraterrestrial life has been identified. Examination of the Allan Hills 84001 meteorite, which was recovered in Antarctica in 1984 and originated from Mars, is thought by David McKay, as well as few other scientists, to contain microfossils of extraterrestrial origin; this interpretation is controversial.
Yamato 000593, the second largest meteorite from Mars, was found on Earth in 2000. At a microscopic level, spheres are found in the meteorite that are rich in carbon compared to surrounding areas that lack such spheres. The carbon-rich spheres may have been formed by biotic activity according to some NASA scientists.
On 5 March 2011, Richard B. Hoover, a scientist with the Marshall Space Flight Center, speculated on the finding of alleged microfossils similar to cyanobacteria in CI1 carbonaceous meteorites in the fringe Journal of Cosmology, a story widely reported on by mainstream media. However, NASA formally distanced itself from Hoover's claim. According to American astrophysicist Neil deGrasse Tyson: "At the moment, life on Earth is the only known life in the universe, but there are compelling arguments to suggest we are not alone."
- Extreme environments on Earth
On 17 March 2013, researchers reported that microbial life forms thrive in the Mariana Trench, the deepest spot on the Earth. Other researchers reported that microbes thrive inside rocks up to 1,900 feet (580 m) below the sea floor under 8,500 feet (2,600 m) of ocean off the coast of the northwestern United States. According to one of the researchers, "You can find microbes everywhere—they're extremely adaptable to conditions, and survive wherever they are." These finds expand the potential habitability of certain niches of other planets.
In 2004, the spectral signature of methane (CH
4) was detected in the Martian atmosphere by both Earth-based telescopes as well as by the Mars Express orbiter. Because of solar radiation and cosmic radiation, methane is predicted to disappear from the Martian atmosphere within several years, so the gas must be actively replenished in order to maintain the present concentration. On 7 June 2018, NASA announced a cyclical seasonal variation in atmospheric methane, which may be produced by geological or biological sources. The European ExoMars Trace Gas Orbiter is currently measuring and mapping the atmospheric methane.
- Planetary systems
It is possible that some exoplanets may have moons with solid surfaces or liquid oceans that are hospitable. Most of the planets so far discovered outside the Solar System are hot gas giants thought to be inhospitable to life, so it is not yet known whether the Solar System, with a warm, rocky, metal-rich inner planet such as Earth, is of an aberrant composition. Improved detection methods and increased observation time will undoubtedly discover more planetary systems, and possibly some more like ours. For example, NASA's Kepler Mission seeks to discover Earth-sized planets around other stars by measuring minute changes in the star's light curve as the planet passes between the star and the spacecraft. Progress in infrared astronomy and submillimeter astronomy has revealed the constituents of other star systems.
- Planetary habitability
Efforts to answer questions such as the abundance of potentially habitable planets in habitable zones and chemical precursors have had much success. Numerous extrasolar planets have been detected using the wobble method and transit method, showing that planets around other stars are more numerous than previously postulated. The first Earth-sized extrasolar planet to be discovered within its star's habitable zone is Gliese 581 c.
Studying extremophiles is useful for understanding the possible origin of life on Earth as well as for finding the most likely candidates for future colonization of other planets. The aim is to detect those organisms that are able to survive space travel conditions and to maintain the proliferating capacity. The best candidates are extremophiles, since they have adapted to survive in different kind of extreme conditions on earth. During the course of evolution, extremophiles have developed various strategies to survive the different stress conditions of different extreme environments. These stress responses could also allow them to survive in harsh space conditions.
Thermophilic species G. thermantarcticus is a good example of a microorganism that could survive space travel. It is a bacterium of the spore-forming genus Bacillus. The formation of spores allows for it to survive extreme environments while still being able to restart cellular growth. It is capable of effectively protecting its DNA, membrane and proteins integrity in different extreme conditions (desiccation, temperatures up to -196 °C, UVC and C-ray radiation...). It is also able to repair the damage produced by space environment.
By understanding how extremophilic organisms can survive the Earth's extreme environments, we can also understand how microorganisms could have survived space travel and how the panspermia hypothesis could be possible.
Research into the environmental limits of life and the workings of extreme ecosystems is ongoing, enabling researchers to better predict what planetary environments might be most likely to harbor life. Missions such as the Phoenix lander, Mars Science Laboratory, ExoMars, Mars 2020 rover to Mars, and the Cassini probe to Saturn's moons aim to further explore the possibilities of life on other planets in the Solar System.
- Viking program
The two Viking landers each carried four types of biological experiments to the surface of Mars in the late 1970s. These were the only Mars landers to carry out experiments looking specifically for metabolism by current microbial life on Mars. The landers used a robotic arm to collect soil samples into sealed test containers on the craft. The two landers were identical, so the same tests were carried out at two places on Mars' surface; Viking 1 near the equator and Viking 2 further north. The result was inconclusive, and is still disputed by some scientists.
- Beagle 2
Beagle 2 was an unsuccessful British Mars lander that formed part of the European Space Agency's 2003 Mars Express mission. Its primary purpose was to search for signs of life on Mars, past or present. Although it landed safely, it was unable to correctly deploy its solar panels and telecom antenna.
EXPOSE is a multi-user facility mounted in 2008 outside the International Space Station dedicated to astrobiology. EXPOSE was developed by the European Space Agency (ESA) for long-term spaceflights that allow exposure of organic chemicals and biological samples to outer space in low Earth orbit.
- Mars Science Laboratory
The Mars Science Laboratory (MSL) mission landed the Curiosity rover that is currently in operation on Mars. It was launched 26 November 2011, and landed at Gale Crater on 6 August 2012. Mission objectives are to help assess Mars' habitability and in doing so, determine whether Mars is or has ever been able to support life, collect data for a future human mission, study Martian geology, its climate, and further assess the role that water, an essential ingredient for life as we know it, played in forming minerals on Mars.
The Tanpopo mission is an orbital astrobiology experiment investigating the potential interplanetary transfer of life, organic compounds, and possible terrestrial particles in the low Earth orbit. The purpose is to assess the panspermia hypothesis and the possibility of natural interplanetary transport of microbial life as well as prebiotic organic compounds. Early mission results show evidence that some clumps of microorganism can survive for at least one year in space. This may support the idea that clumps greater than 0.5 millimeters of microorganisms could be one way for life to spread from planet to planet.
- ExoMars rover
ExoMars rover is a robotic mission to Mars to search for possible biosignatures of Martian life, past or present. This astrobiological mission is currently under development by the European Space Agency (ESA) in partnership with the Russian Federal Space Agency (Roscosmos); it is planned for a 2018 launch.
- Mars 2020
Mars 2020 rover mission is under development by NASA for a launch in 2020. It will investigate environments on Mars relevant to astrobiology, investigate its surface geological processes and history, including the assessment of its past habitability and potential for preservation of biosignatures and biomolecules within accessible geological materials. The Science Definition Team is proposing the rover collect and package at least 31 samples of rock cores and soil for a later mission to bring back for more definitive analysis in laboratories on Earth. The rover could make measurements and technology demonstrations to help designers of a human expedition understand any hazards posed by Martian dust and demonstrate how to collect carbon dioxide (CO2), which could be a resource for making molecular oxygen (O2) and rocket fuel.
- Europa Clipper
Europa Clipper is a mission planned by NASA for a 2025 launch that will conduct detailed reconnaissance of Jupiter's moon Europa and will investigate whether its internal ocean could harbor conditions suitable for life. It will also aid in the selection of future landing sites.
- Icebreaker Life
Icebreaker Life is a lander mission that proposed for NASA's Discovery Program for the 2021 launch opportunity, but it was not selected for development. It would have had a stationary lander that would be a near copy of the successful 2008 Phoenix and it would have carried an upgraded astrobiology scientific payload, including a 1-meter-long core drill to sample ice-cemented ground in the northern plains to conduct a search for organic molecules and evidence of current or past life on Mars. One of the key goals of the Icebreaker Life mission is to test the hypothesis that the ice-rich ground in the polar regions has significant concentrations of organics due to protection by the ice from oxidants and radiation.
- Journey to Enceladus and Titan
- Enceladus Life Finder
Enceladus Life Finder (ELF) is a proposed astrobiology mission concept for a space probe intended to assess the habitability of the internal aquatic ocean of Enceladus, Saturn's sixth-largest moon.
- Life Investigation For Enceladus
Life Investigation For Enceladus (LIFE) is a proposed astrobiology sample-return mission concept. The spacecraft would enter into Saturn orbit and enable multiple flybys through Enceladus' icy plumes to collect icy plume particles and volatiles and return them to Earth on a capsule. The spacecraft may sample Enceladus' plumes, the E ring of Saturn, and the upper atmosphere of Titan.
Oceanus is an orbiter proposed in 2017 for the New Frontiers mission No. 4. It would travel to the moon of Saturn, Titan, to assess its habitability. Oceanus' objectives are to reveal Titan's organic chemistry, geology, gravity, topography, collect 3D reconnaissance data, catalog the organics and determine where they may interact with liquid water.
- Explorer of Enceladus and Titan
Explorer of Enceladus and Titan (E2T) is an orbiter mission concept that would investigate the evolution and habitability of the Saturnian satellites Enceladus and Titan. The mission concept was proposed in 2017 by the European Space Agency.
- Active SETI
- Astrobiology Magazine
- Cosmic dust
- Extraterrestrial life
- Extraterrestrial sample curation
- Hypothetical types of biochemistry
- List of microorganisms tested in outer space
- Nexus for Exoplanet System Science
- Planetary habitability
- Planetary protection
- Planet Simulator
- "Launching the Alien Debates (part 1 of 7)". Astrobiology Magazine. NASA. 8 December 2006. Retrieved 5 May 2014.
- "About Astrobiology". NASA Astrobiology Institute. NASA. 21 January 2008. Archived from the original on 11 October 2008. Retrieved 20 October 2008.
- Kaufman, Marc. "A History of Astrobiology". NASA. Retrieved 14 February 2019.
- Ward, P. D.; Brownlee, D. (2004). The life and death of planet Earth. New York: Owl Books. ISBN 978-0-8050-7512-0.
- "Origins of Life and Evolution of Biospheres". Journal: Origins of Life and Evolution of Biospheres. Retrieved 6 April 2015.
- "Release of the First Roadmap for European Astrobiology". European Science Foundation. Astrobiology Web. 29 March 2016. Retrieved 2 April 2016.
- Corum, Jonathan (18 December 2015). "Mapping Saturn's Moons". The New York Times. Retrieved 18 December 2015.
- Cockell, Charles S. (4 October 2012). "How the search for aliens can help sustain life on Earth". CNN News. Retrieved 8 October 2012.
- Loeb, Abraham (October 2014). "The Habitable Epoch of the Early Universe". International Journal of Astrobiology. 13 (4): 337–339. arXiv:1312.0613. Bibcode:2014IJAsB..13..337L. CiteSeerX 10.1.1.748.4820. doi:10.1017/S1473550414000196.
- Dreifus, Claudia (2 December 2014). "Much-Discussed Views That Go Way Back – Avi Loeb Ponders the Early Universe, Nature and Life". The New York Times. Retrieved 3 December 2014.
- Rampelotto, P.H. (2010). "Panspermia: A Promising Field of Research" (PDF). Astrobiology Science Conference. Retrieved 3 December 2014.
- Reuell, Peter (8 July 2019). "Harvard study suggests asteroids might play key role in spreading life". Harvard Gazette. Retrieved 29 September 2019.
- Choi, Charles Q. (21 August 2015). "Giant Galaxies May Be Better Cradles for Habitable Planets". Space.com. Retrieved 24 August 2015.
- Graham, Robert W. (February 1990). "NASA Technical Memorandum 102363 – Extraterrestrial Life in the Universe" (PDF). NASA. Lewis Research Center, Ohio. Retrieved 7 July 2014.
- Altermann, Wladyslaw (2008). "From Fossils to Astrobiology – A Roadmap to Fata Morgana?". In Seckbach, Joseph; Walsh, Maud (eds.). From Fossils to Astrobiology: Records of Life on Earth and the Search for Extraterrestrial Biosignatures. 12. p. xvii. ISBN 978-1-4020-8836-0.
- Horneck, Gerda; Petra Rettberg (2007). Complete Course in Astrobiology. Wiley-VCH. ISBN 978-3-527-40660-9.
- Davies, Paul (18 November 2013). "Are We Alone in the Universe?". The New York Times. Retrieved 20 November 2013.
- "BBC Solar System - Earth orbits in the Goldilocks zone". Archived from the original on 28 July 2018. Retrieved 27 March 2018.
- Gary, Stuart (22 February 2016). "What is the Goldilocks Zone and why does it matter in the search for ET?". ABC News. Retrieved 27 March 2018.
- Overbye, Dennis (4 November 2013). "Far-Off Planets Like the Earth Dot the Galaxy". The New York Times. Retrieved 5 November 2013.
- Petigura, Eric A.; Howard, Andrew W.; Marcy, Geoffrey W. (31 October 2013). "Prevalence of Earth-size planets orbiting Sun-like stars". Proceedings of the National Academy of Sciences of the United States of America. 110 (48): 19273–19278. arXiv:1311.6806. Bibcode:2013PNAS..11019273P. doi:10.1073/pnas.1319909110. PMC 3845182. PMID 24191033. Retrieved 5 November 2013.
- Khan, Amina (4 November 2013). "Milky Way may host billions of Earth-size planets". Los Angeles Times. Retrieved 5 November 2013.
- Grotzinger, John P. (24 January 2014). "Introduction to Special Issue – Habitability, Taphonomy, and the Search for Organic Carbon on Mars". Science. 343 (6169): 386–387. Bibcode:2014Sci...343..386G. doi:10.1126/science.1249944. PMID 24458635.
- Various (24 January 2014). "Exploring Martian Habitability – Table of Contents". Science. 343 (6169): 345–452. Retrieved 24 January 2014.
- Various (24 January 2014). "Special Collection Curiosity – Exploring Martian Habitability". Science. Retrieved 24 January 2014.
- Grotzinger, J.P.; et al. (24 January 2014). "A Habitable Fluvio-Lacustrine Environment at Yellowknife Bay, Gale Crater, Mars". Science. 343 (6169): 1242777. Bibcode:2014Sci...343A.386G. CiteSeerX 10.1.1.455.3973. doi:10.1126/science.1242777. PMID 24324272.
- Crawford, I. A. (2018). "Widening perspectives: The intellectual and social benefits of astrobiology (regardless of whether extraterrestrial life is discovered or not)". International Journal of Astrobiology. 17 (1): 57–60. arXiv:1703.06239. Bibcode:2018IJAsB..17...57C. doi:10.1017/S1473550417000088.</
- Cockell, Charles S. (2001). "'Astrobiology' and the ethics of new science". Interdisciplinary Science Reviews. 26 (2): 90–96. doi:10.1179/0308018012772533.
- Launching a New Science: Exobiology and the Exploration of Space The National Library of Medicine.
- Gutro, Robert (4 November 2007). "NASA Predicts Non-Green Plants on Other Planets". Goddard Space Flight Center. Archived from the original on 6 October 2008. Retrieved 20 October 2008.
- Heinlein R & Harold W (21 July 1961). "Xenobiology". Science. 134 (3473): 223–225. Bibcode:1961Sci...134..223H. doi:10.1126/science.134.3473.223. JSTOR 1708323. PMID 17818726.
- Markus Schmidt (9 March 2010). "Xenobiology: A new form of life as the ultimate biosafety tool". BioEssays. 32 (4): 322–331. doi:10.1002/bies.200900147. PMC 2909387. PMID 20217844.
- Livio, Mario (15 February 2017). "Winston Churchill's essay on alien life found". Nature. 542 (7641): 289–291. Bibcode:2017Natur.542..289L. doi:10.1038/542289a. PMID 28202987.
- De Freytas-Tamura, Kimiko (15 February 2017). "Winston Churchill Wrote of Alien Life in a Lost Essay". The New York Times. Retrieved 18 February 2017.
- Grinspoon 2004
- Steven J. Dick & James E. Strick (2004). The Living Universe: NASA and the Development of Astrobiology. New Brunswick, NJ: Rutgers University Press.
- Parker, T.; Clifford, S. M.; Banerdt, W. B. (2000). "Argyre Planitia and the Mars Global Hydrologic Cycle" (PDF). Lunar and Planetary Science. XXXI: 2033. Bibcode:2000LPI....31.2033P.
- Heisinger, H.; Head, J. (2002). "Topography and morphology of the Argyre basin, Mars: implications for its geologic and hydrologic history". Planet. Space Sci. 50 (10–11): 939–981. Bibcode:2002P&SS...50..939H. doi:10.1016/S0032-0633(02)00054-5.
- Tyson, Peter (4 January 2004). "Life's Little Essential". NOVA. PBS.
- Klein HP & Levin GV (1 October 1976). "The Viking Biological Investigation: Preliminary Results". Science. 194 (4260): 99–105. Bibcode:1976Sci...194...99K. doi:10.1126/science.194.4260.99. PMID 17793090.
- Amos, Jonathan (16 January 2015). "Lost Beagle2 probe found 'intact' on Mars". BBC. Retrieved 16 January 2015.
- AstRoMap European Astrobiology Roadmap. Gerda Horneck, Nicolas Walter, Frances Westall, John Lee Grenfell, William F. Martin, Felipe Gomez, Stefan Leuko, Natuschka Lee, Silvano Onofri, Kleomenis Tsiganis, Raffaele Saladino, Elke Pilat-Lohinger, Ernesto Palomba, Jesse Harrison, Fernando Rull, Christian Muller, Giovanni Strazzulla, John R. Brucato, Petra Rettberg, and Maria Teresa Capria. Astrobiology. Volume 16, Number 3, 2016. doi:10.1089/ast.2015.1441
- Webster, Guy; Brown, Dwayne (22 July 2011). "NASA's Next Mars Rover To Land At Gale Crater". NASA JPL. Retrieved 22 July 2011.
- Chow, Dennis (22 July 2011). "NASA's Next Mars Rover to Land at Huge Gale Crater". Space.com. Retrieved 22 July 2011.
- Amos, Jonathan (22 July 2011). "Mars rover aims for deep crater". BBC News. Archived from the original on 22 July 2011. Retrieved 22 July 2011.
- Chang, Kenneth (9 December 2013). "On Mars, an Ancient Lake and Perhaps Life". The New York Times. Retrieved 9 December 2013.
- "Second ExoMars mission moves to next launch opportunity in 2020" (Press release). European Space Agency. 2 May 2016. Retrieved 2 May 2016.
- "Polycyclic Aromatic Hydrocarbons: An Interview With Dr. Farid Salama". Astrobiology Magazine. 2000. Archived from the original on 20 June 2008. Retrieved 20 October 2008.
- Astrobiology. Macmillan Science Library: Space Sciences. 2006. Retrieved 20 October 2008.
- Penn State (19 August 2006). "The Ammonia-Oxidizing Gene". Astrobiology Magazine. Retrieved 20 October 2008.
- "Stars and Habitable Planets". Sol Company. 2007. Archived from the original on 1 October 2008. Retrieved 20 October 2008.
- "M Dwarfs: The Search for Life is On". Red Orbit & Astrobiology Magazine. 29 August 2005. Retrieved 20 October 2008.
- Sagan, Carl. Communication with Extraterrestrial Intelligence. MIT Press, 1973, 428 pgs.
- "You Never Get a Seventh Chance to Make a First Impression: An Awkward History of Our Space Transmissions". Lightspeed Magazine. March 2011. Retrieved 13 March 2015.
- "Stephen Hawking: Humans Should Fear Aliens". Huffington Post. 25 June 2010. Retrieved 27 May 2017.
- "Kepler Mission". NASA. 2008. Archived from the original on 31 October 2008. Retrieved 20 October 2008.
- "The COROT space telescope". CNES. 17 October 2008. Archived from the original on 8 November 2008. Retrieved 20 October 2008.
- "The Virtual Planet Laboratory". NASA. 2008. Retrieved 20 October 2008.
- Ford, Steve (August 1995). "What is the Drake Equation?". SETI League. Archived from the original on 29 October 2008. Retrieved 20 October 2008.
- Amir Alexander. "The Search for Extraterrestrial Intelligence: A Short History – Part 7: The Birth of the Drake Equation".
- "Astrobiology". Biology Cabinet. 26 September 2006. Archived from the original on 12 December 2010. Retrieved 17 January 2011.
- Horner, Jonathan; Barrie Jones (24 August 2007). "Jupiter: Friend or foe?". Europlanet. Archived from the original on 2 February 2012. Retrieved 20 October 2008.
- Jakosky, Bruce; David Des Marais; et al. (14 September 2001). "The Role of Astrobiology in Solar System Exploration". NASA. SpaceRef.com. Retrieved 20 October 2008.
- Bortman, Henry (29 September 2004). "Coming Soon: "Good" Jupiters". Astrobiology Magazine. Retrieved 20 October 2008.
- [Living at the Extremes: Extremophiles and the Limits of Life in a Planetary Context.] N. Merino, H.S. Aronson, D. Bojanova, J. Feyhl-Buska, et al. - EarthArXiv. February, 2019.
- Chamberlin, Sean (1999). "Black Smokers and Giant Worms". Fullerton College. Retrieved 11 February 2011.
- Trixler, F (2013). "Quantum tunnelling to the origin and evolution of life". Current Organic Chemistry. 17 (16): 1758–1770. doi:10.2174/13852728113179990083. PMC 3768233. PMID 24039543.
- Carey, Bjorn (7 February 2005). "Wild Things: The Most Extreme Creatures". Live Science. Retrieved 20 October 2008.
- Cavicchioli, R. (Fall 2002). "Extremophiles and the search for extraterrestrial life" (PDF). Astrobiology. 2 (3): 281–92. Bibcode:2002AsBio...2..281C. CiteSeerX 10.1.1.472.3179. doi:10.1089/153110702762027862. PMID 12530238.
- Young, Kelly (10 November 2005). "Hardy lichen shown to survive in space". New Scientist. Retrieved 17 January 2019.
- The Planetary Report, Volume XXIX, number 2, March/April 2009, "We make it happen! Who will survive? Ten hardy organisms selected for the LIFE project, by Amir Alexander
- Hashimoto, T.; Kunieda, T. (2017). "DNA Protection protein, a novel mechanism of radiation tolerance: Lessons from Tardigrades". Life. 7 (2): 26. doi:10.3390/life7020026. PMC 5492148. PMID 28617314.
- "Jupiter's Moon Europa Suspected of Fostering Life". Daily University Science News. 2002. Retrieved 8 August 2009.
- Weinstock, Maia (24 August 2000). "Galileo Uncovers Compelling Evidence of Ocean on Jupiter's Moon Europa". Space.com. Retrieved 20 October 2008.
- Cavicchioli, R. (Fall 2002). "Extremophiles and the search for extraterrestrial life". Astrobiology. 2 (3): 281–92. Bibcode:2002AsBio...2..281C. CiteSeerX 10.1.1.472.3179. doi:10.1089/153110702762027862. PMID 12530238.
- David, Leonard (7 February 2006). "Europa Mission: Lost in NASA Budget". Space.com. Retrieved 8 August 2009.
- "Clues to possible life on Europa may lie buried in Antarctic ice". Marshal Space Flight Center. NASA. 5 March 1998. Archived from the original on 31 July 2009. Retrieved 8 August 2009.
- Lovett, Richard A. (31 May 2011). "Enceladus named sweetest spot for alien life". Nature. doi:10.1038/news.2011.337. Retrieved 3 June 2011.
- Kazan, Casey (2 June 2011). "Saturn's Enceladus Moves to Top of "Most-Likely-to-Have-Life" List". The Daily Galaxy. Retrieved 3 June 2011.
- Chow, Denise (26 October 2011). "Discovery: Cosmic Dust Contains Organic Matter from Stars". Space.com. Retrieved 26 October 2011.
- ScienceDaily Staff (26 October 2011). "Astronomers Discover Complex Organic Matter Exists Throughout the Universe". ScienceDaily. Retrieved 27 October 2011.
- Kwok, Sun; Zhang, Yong (26 October 2011). "Mixed aromatic–aliphatic organic nanoparticles as carriers of unidentified infrared emission features". Nature. 479 (7371): 80–3. Bibcode:2011Natur.479...80K. doi:10.1038/nature10542. PMID 22031328.
- Hoover, Rachel (21 February 2014). "Need to Track Organic Nano-Particles Across the Universe? NASA's Got an App for That". NASA. Retrieved 22 February 2014.
- Staff (20 September 2012). "NASA Cooks Up Icy Organics to Mimic Life's Origins". Space.com. Retrieved 22 September 2012.
- Gudipati, Murthy S.; Yang, Rui (1 September 2012). "In-Situ Probing of Radiation-Induced Processing of Organics in Astrophysical Ice Analogs—Novel Laser Desorption Laser Ionization Time-Of-Flight Mass Spectroscopic Studies". The Astrophysical Journal Letters. 756 (1): L24. Bibcode:2012ApJ...756L..24G. doi:10.1088/2041-8205/756/1/L24.
- Mautner, Michael N. (2002). "Planetary bioresources and astroecology. 1. Planetary microcosm bioessays of Martian and meteorite materials: soluble electrolytes, nutrients, and algal and plant responses". Icarus. 158 (1): 72–86. Bibcode:2002Icar..158...72M. doi:10.1006/icar.2002.6841.
- Mautner, Michael N. (2002). "Planetary resources and astroecology. Planetary microcosm models of asteroid and meteorite interiors: electrolyte solutions and microbial growth. Implications for space populations and panspermia" (PDF). Astrobiology. 2 (1): 59–76. Bibcode:2002AsBio...2...59M. doi:10.1089/153110702753621349. PMID 12449855.
- Mautner, Michael N. (2005). "Life in the cosmological future: Resources, biomass and populations" (PDF). Journal of the British Interplanetary Society. 58: 167–180. Bibcode:2005JBIS...58..167M.
- Mautner, Michael N. (2000). Seeding the Universe with Life: Securing Our Cosmological Future (PDF). Washington D. C. ISBN 978-0-476-00330-9.
- "Fossil Succession". U.S. Geological Survey. 14 August 1997. Archived from the original on 14 October 2008. Retrieved 20 October 2008.
- Pace, Norman R. (30 January 2001). "The universal nature of biochemist ry". Proceedings of the National Academy of Sciences of the USA. 98 (3): 805–808. Bibcode:2001PNAS...98..805P. doi:10.1073/pnas.98.3.805. PMC 33372. PMID 11158550.
- Marshall, Michael (21 January 2011). "Telltale chemistry could betray ET". New Scientists. Retrieved 22 January 2011.
- Tritt, Charles S. (2002). "Possibility of Life on Europa". Milwaukee School of Engineering. Archived from the original on 9 June 2007. Retrieved 20 October 2008.
- Friedman, Louis (14 December 2005). "Projects: Europa Mission Campaign". The Planetary Society. Archived from the original on 20 September 2008. Retrieved 20 October 2008.
- David, Leonard (10 November 1999). "Move Over Mars – Europa Needs Equal Billing". Space.com. Retrieved 20 October 2008.
- Than, Ker (28 February 2007). "New Instrument Designed to Sift for Life on Mars". Space.com. Retrieved 20 October 2008.
- Than, Ker (13 September 2005). "Scientists Reconsider Habitability of Saturn's Moon". Science.com. Retrieved 11 February 2011.
- Lovett, Richard A. (31 May 2011). "Enceladus named sweetest spot for alien life". Nature. doi:10.1038/news.2011.337. Retrieved 3 June 2011.
- "NASA Images Suggest Water Still Flows in Brief Spurts on Mars". NASA. 2006. Archived from the original on 16 October 2008. Retrieved 20 October 2008.
- "Water ice in crater at Martian north pole". European Space Agency. 28 July 2005. Archived from the original on 23 September 2008. Retrieved 20 October 2008.
- Landis, Geoffrey A. (1 June 2001). "Martian Water: Are There Extant Halobacteria on Mars?". Astrobiology. 1 (2): 161–164. Bibcode:2001AsBio...1..161L. doi:10.1089/153110701753198927. PMID 12467119.
- Kruszelnicki, Karl (5 November 2001). "Life on Europa, Part 1". ABC Science. Retrieved 20 October 2008.
- Cook, Jia-Rui c. (11 December 2013). "Clay-Like Minerals Found on Icy Crust of Europa". NASA. Retrieved 11 December 2013.
- Postberg, Frank; et al. (27 June 2018). "Macromolecular organic compounds from the depths of Enceladus". Nature. 558 (7711): 564–568. Bibcode:2018Natur.558..564P. doi:10.1038/s41586-018-0246-4. PMC 6027964. PMID 29950623.
- "Titan: Life in the Solar System?". BBC – Science & Nature. Retrieved 20 October 2008.
- Britt, Robert Roy (28 July 2006). "Lakes Found on Saturn's Moon Titan". Space.com. Archived from the original on 4 October 2008. Retrieved 20 October 2008.
- National Research Council (2007). The Limits of Organic Life in Planetary Systems. Washington, DC: The National Academies Press. p. 74. doi:10.17226/11919. ISBN 978-0-309-10484-5.
- McKay, C. P.; Smith, H. D. (2005). "Possibilities for methanogenic life in liquid methane on the surface of Titan". Icarus. 178 (1): 274–276. Bibcode:2005Icar..178..274M. doi:10.1016/j.icarus.2005.05.018.
- Lovett, Richard A. (20 March 2008). "Saturn Moon Titan May Have Underground Ocean". National Geographic News. Archived from the original on 24 September 2008. Retrieved 20 October 2008.
- Oze, Christopher; Jones, Camille; Goldsmith, Jonas I.; Rosenbauer, Robert J. (7 June 2012). "Differentiating biotic from abiotic methane genesis in hydrothermally active planetary surfaces". PNAS. 109 (25): 9750–9754. Bibcode:2012PNAS..109.9750O. doi:10.1073/pnas.1205223109. PMC 3382529. PMID 22679287.
- Staff (25 June 2012). "Mars Life Could Leave Traces in Red Planet's Air: Study". Space.com. Retrieved 27 June 2012.
- Brogi, Matteo; Snellen, Ignas A. G.; de Krok, Remco J.; Albrecht, Simon; Birkby, Jayne; de Mooij, Ernest J. W. (28 June 2012). "The signature of orbital motion from the dayside of the planet t Boötis b". Nature. 486 (7404): 502–504. arXiv:1206.6109. Bibcode:2012Natur.486..502B. doi:10.1038/nature11161. PMID 22739313.
- Mann, Adam (27 June 2012). "New View of Exoplanets Will Aid Search for E.T." Wired. Retrieved 28 June 2012.
- Marlaire, Ruth (3 March 2015). "NASA Ames Reproduces the Building Blocks of Life in Laboratory". NASA. Retrieved 5 March 2015.
- "NASA Astrobiology: Life in the Universe". Archived from the original on 23 March 2008. Retrieved 13 March 2015.
- Griffin, Dale Warren (14 August 2013). "The Quest for Extraterrestrial Life: What About the Viruses?". Astrobiology. 13 (8): 774–783. Bibcode:2013AsBio..13..774G. doi:10.1089/ast.2012.0959. PMID 23944293.
- Berliner, Aaron J.; Mochizuki, Tomohiro; Stedman, Kenneth M. (2018). "Astrovirology: Viruses at Large in the Universe". Astrobiology. 18 (2): 207–223. Bibcode:2018AsBio..18..207B. doi:10.1089/ast.2017.1649. PMID 29319335.
- Janjic, Aleksandar (2018). "The Need for Including Virus Detection Methods in Future Mars Missions". Astrobiology. 18 (12): 1611–1614. Bibcode:2018AsBio..18.1611J. doi:10.1089/ast.2018.1851.
- Cofield, Calla; Chou, Felicia (25 June 2018). "NASA Asks: Will We Know Life When We See It?". NASA. Retrieved 26 June 2018.
- Staff (25 June 2018). "UCR team among scientists developing guidebook for finding life beyond earth - Major series of review articles outlines past, present, and future of searching for life on other planets". University of California - Riverside. Retrieved 26 June 2018.
- No, NASA Hasn't Found Alien Life. Mike Wall, Space. 26 June 2017.
- Crenson, Matt (6 August 2006). "Experts: Little Evidence of Life on Mars". Associated Press. Archived from the original on 16 April 2011. Retrieved 8 March 2011.
- McKay DS; Gibson E. K.; Thomas-Keprta K. L.; Vali H.; Romanek C. S.; Clemett S. J.; Chillier X. D. F.; Maechling C. R.; Zare R. N. (1996). "Search for past life on Mars: Possible relic biogenic activity in Martian meteorite ALH84001". Science. 273 (5277): 924–930. Bibcode:1996Sci...273..924M. doi:10.1126/science.273.5277.924. PMID 8688069.
- McKay David S.; Thomas-Keprta K. L.; Clemett, S. J.; Gibson, E. K. Jr; Spencer L.; Wentworth S. J. (2009). Hoover, Richard B.; Levin, Gilbert V.; Rozanov, Alexei Y.; Retherford, Kurt D. (eds.). "Life on Mars: new evidence from martian meteorites". Proc. SPIE. Proceedings of SPIE. 7441 (1): 744102. Bibcode:2009SPIE.7441E..02M. doi:10.1117/12.832317. Retrieved 8 March 2011.
- Webster, Guy (27 February 2014). "NASA Scientists Find Evidence of Water in Meteorite, Reviving Debate Over Life on Mars". NASA. Retrieved 27 February 2014.
- White, Lauren M.; Gibson, Everett K.; Thomnas-Keprta, Kathie L.; Clemett, Simon J.; McKay, David (19 February 2014). "Putative Indigenous Carbon-Bearing Alteration Features in Martian Meteorite Yamato 000593". Astrobiology. 14 (2): 170–181. Bibcode:2014AsBio..14..170W. doi:10.1089/ast.2011.0733. PMC 3929347. PMID 24552234.
- Gannon, Megan (28 February 2014). "Mars Meteorite with Odd 'Tunnels' & 'Spheres' Revives Debate Over Ancient Martian Life". Space.com. Retrieved 28 February 2014.
- Tenney, Garrett (5 March 2011). "Exclusive: NASA Scientist Claims Evidence of Alien Life on Meteorite". Fox News. Archived from the original on 6 March 2011. Retrieved 6 March 2011.
- Hoover, Richard B. (2011). "Fossils of Cyanobacteria in CI1 Carbonaceous Meteorites: Implications to Life on Comets, Europa, and Enceladus". Journal of Cosmology. 13: xxx. Archived from the original on 8 March 2011. Retrieved 6 March 2011.
- Sheridan, Kerry (7 March 2011). "NASA shoots down alien fossil claims". ABC News. Retrieved 7 March 2011.
- Tyson, Neil deGrasse (23 July 2001). "The Search for Life in the Universe". Department of Astrophysics and Hayden Planetarium. NASA. Archived from the original on 21 July 2011. Retrieved 7 March 2011.
- Choi, Charles Q. (17 March 2013). "Microbes Thrive in Deepest Spot on Earth". LiveScience. Retrieved 17 March 2013.
- Glud, Ronnie; Wenzhöfer, Frank; Middleboe, Mathias; Oguri, Kazumasa; Turnewitsch, Robert; Canfield, Donald E.; Kitazato, Hiroshi (17 March 2013). "High rates of microbial carbon turnover in sediments in the deepest oceanic trench on Earth". Nature Geoscience. 6 (4): 284–288. Bibcode:2013NatGe...6..284G. doi:10.1038/ngeo1773.
- Oskin, Becky (14 March 2013). "Intraterrestrials: Life Thrives in Ocean Floor". LiveScience. Retrieved 17 March 2013.
- Vladimir A. Krasnopolsky (February 2005). "Some problems related to the origin of methane on Mars". Icarus. 180 (2): 359–367. Bibcode:2006Icar..180..359K. doi:10.1016/j.icarus.2005.10.015.
- "PFS Results". Planetary Fourier Spectrometer. Archived from the original on 2 May 2013.
- Brown, Dwayne; Wendel, JoAnna; Steigerwald, Bill; Jones, Nancy; Good, Andrew (7 June 2018). "Release 18-050 - NASA Finds Ancient Organic Material, Mysterious Methane on Mars". NASA. Retrieved 7 June 2018.
- NASA (7 June 2018). "Ancient Organics Discovered on Mars - video (03:17)". NASA. Retrieved 7 June 2018.
- Wall, Mike (7 June 2018). "Curiosity Rover Finds Ancient 'Building Blocks for Life' on Mars". Space.com. Retrieved 7 June 2018.
- Than, Ker (24 April 2007). "Major Discovery: New Planet Could Harbor Water and Life". Space.com. Archived from the original on 15 October 2008. Retrieved 20 October 2008.
- Di Donato, Paola; Romano, Ida; Mastascusa, Vincenza; Poli, Annarita; Orlando, Pierangelo; Pugliese, Mariagabriella; Nicolaus, Barbara (March 2018). "Survival and Adaptation of the Thermophilic Species Geobacillus thermantarcticus in Simulated Spatial Conditions". Origins of Life and Evolution of Biospheres. 48 (1): 141–158. Bibcode:2018OLEB...48..141D. doi:10.1007/s11084-017-9540-7. ISSN 0169-6149.
- Chambers, Paul (1999). Life on Mars; The Complete Story. London: Blandford. ISBN 978-0-7137-2747-0.
- Levin, G and P. Straaf. 1976. Viking Labeled Release Biology Experiment: Interim Results. Science: 194. 1322–1329.
- Bianciardi, Giorgio; Miller, Joseph D.; Straat, Patricia Ann; Levin, Gilbert V. (March 2012). "Complexity Analysis of the Viking Labeled Release Experiments". IJASS. 13 (1): 14–26. Bibcode:2012IJASS..13...14B. doi:10.5139/IJASS.2012.13.1.14. Archived from the original on 15 April 2012. Retrieved 15 April 2012.
- Klotz, Irene (12 April 2012). "Mars Viking Robots 'Found Life'". Discovery News. Retrieved 16 April 2012.
- Paepe, Ronald (2007). "The Red Soil on Mars as a proof for water and vegetation" (PDF). Geophysical Research Abstracts. 9 (1794). Archived from the original (PDP) on 13 June 2011. Retrieved 2 May 2012.
- "Beagle 2 : the British led exploration of Mars". Archived from the original on 4 March 2016. Retrieved 13 March 2015.
- Elke Rabbow; Gerda Horneck; Petra Rettberg; Jobst-Ulrich Schott; Corinna Panitz; Andrea L'Afflitto; Ralf von Heise-Rotenburg; Reiner Willnecker; Pietro Baglioni; Jason Hatton; Jan Dettmann; René Demets; Günther Reitz (9 July 2009). "EXPOSE, an Astrobiological Exposure Facility on the International Space Station – from Proposal to Flight" (PDF). Orig Life Evol Biosph. 39 (6): 581–98. Bibcode:2009OLEB...39..581R. doi:10.1007/s11084-009-9173-6. PMID 19629743. Archived from the original (PDF) on 10 January 2014. Retrieved 8 July 2013.
- Karen Olsson-Francis; Charles S. Cockell (23 October 2009). "Experimental methods for studying microbial survival in extraterrestrial environments" (PDF). Journal of Microbiological Methods. 80 (1): 1–13. doi:10.1016/j.mimet.2009.10.004. PMID 19854226. Archived from the original (PDF) on 18 September 2013. Retrieved 31 July 2013.
- "EXPOSE – home page". Centre national d'études spatiales (CNES). Archived from the original on 15 January 2013. Retrieved 8 July 2013.
- "Name NASA's Next Mars Rover". NASA/JPL. 27 May 2009. Archived from the original on 22 May 2009. Retrieved 27 May 2009.
- "Mars Science Laboratory: Mission". NASA/JPL. Retrieved 12 March 2010.
- Early Tanpopo mission results show microbes can survive in space. American Geophysical Union - Geospace. Larry O'Hanlon. 19 May 2017.
- Amos, Jonathan (15 March 2012). "Europe still keen on Mars missions". BBC News. Retrieved 16 March 2012.
- Svitak, Amy (16 March 2012). "Europe Joins Russia on Robotic ExoMars". Aviation Week. Retrieved 16 March 2012.
- Selding, Peter B. de (15 March 2012). "ESA Ruling Council OKs ExoMars Funding". Space News. Retrieved 16 March 2012.
- Cowing, Keith (21 December 2012). "Science Definition Team for the 2020 Mars Rover". NASA. Science Ref. Retrieved 21 December 2012.
- "Science Team Outlines Goals for NASA's 2020 Mars Rover". Jet Propulsion Laboratory. NASA. 9 July 2013. Retrieved 10 July 2013.
- "Mars 2020 Science Definition Team Report – Frequently Asked Questions" (PDF). NASA. 9 July 2013. Retrieved 10 July 2013.
- "Europa Clipper". Jet Propulsion Laboratory. NASA. November 2013. Archived from the original on 13 December 2013. Retrieved 13 December 2013.
- Kane, Van (26 May 2013). "Europa Clipper Update". Future Planetary Exploration. Retrieved 13 December 2013.
- Pappalardo, Robert T.; S. Vance; F. Bagenal; B.G. Bills; D.L. Blaney; D.D. Blankenship; W.B. Brinckerhoff; et al. (2013). "Science Potential from a Europa Lander" (PDF). Astrobiology. 13 (8): 740–773. Bibcode:2013AsBio..13..740P. doi:10.1089/ast.2013.1003. hdl:1721.1/81431. PMID 23924246.
- Senske, D. (2 October 2012), "Europa Mission Concept Study Update", Presentation to Planetary Science Subcommittee (PDF), retrieved 14 December 2013
- Christopher P. McKay; Carol R. Stoker; Brian J. Glass; Arwen I. Davé; Alfonso F. Davila; Jennifer L. Heldmann; et al. (5 April 2013). "The Icebreaker Life Mission to Mars: A Search for Biomolecular Evidence for Life". Astrobiology. 13 (4): 334–353. Bibcode:2013AsBio..13..334M. doi:10.1089/ast.2012.0878. PMID 23560417.
- Choi, Charles Q. (16 May 2013). "Icebreaker Life Mission". Astrobiology Magazine. Retrieved 1 July 2013.
- C. P. McKay; Carol R. Stoker; Brian J. Glass; Arwen I. Davé; Alfonso F. Davila; Jennifer L. Heldmann; et al. (2012). "THE ICEBREAKER LIFE MISSION TO MARS: A SEARCH FOR BIOCHEMICAL EVIDENCE FOR LIFE". Concepts and Approaches for Mars Exploration (PDF). Lunar and Planetary Institute. Retrieved 1 July 2013.
- Sotin, C.; Altwegg, K.; Brown, R.H.; et al. (2011). JET: Journey to Enceladus and Titan (PDF). 42nd Lunar and Planetary Science Conference. Lunar and Planetary Institute.
- Kane, Van (3 April 2014). "Discovery Missions for an Icy Moon with Active Plumes". The Planetary Society. Retrieved 9 April 2015.
- Matousek, Steve; Sotin, Christophe; Goebel, Dan; Lang, Jared (18–21 June 2013). JET: Journey to Enceladus and Titan (PDF). Low Cost Planetary Missions Conference. California Institute of Technology. Archived from the original (PDF) on 4 March 2016. Retrieved 10 April 2015.
- Lunine, J.I.; Waite, J.H.; Postberg, F.; Spilker, L. (2015). Enceladus Life Finder: The search for life in a habitable moon (PDF). 46th Lunar and Planetary Science Conference (2015). Houston, Texas.: Lunar and Planetary Institute.
- Clark, Stephen (6 April 2015). "Diverse destinations considered for new interplanetary probe". Space Flight Now. Retrieved 7 April 2015.
- Tsou, Peter; Brownlee, D.E.; McKay, Christopher; Anbar, A.D.; Yano, H. (August 2012). "LIFE: Life Investigation For Enceladus A Sample Return Mission Concept in Search for Evidence of Life". Astrobiology. 12 (8): 730–742. Bibcode:2012AsBio..12..730T. doi:10.1089/ast.2011.0813. PMID 22970863.
- Tsou, Peter; Anbar, Ariel; Atwegg, Kathrin; Porco, Carolyn; Baross, John; McKay, Christopher (2014). "LIFE – Enceladus Plume Sample Return via Discovery" (PDF). 45th Lunar and Planetary Science Conference. Retrieved 10 April 2015.
- Tsou, Peter (2013). "LIFE: Life Investigation For Enceladus – A Sample Return Mission Concept in Search for Evidence of Life". Jet Propulsion Laboratory. Archived from the original (.doc) on 1 September 2015. Retrieved 10 April 2015.
- Sotin, C.; Hayes, A.; Malaska, M.; Nimmo, F.; Trainer, M.; Mastrogiuseppe, M.; et al. (20–24 March 2017). OCEANUS: A New Frontiers orbiter to study Titan's potential habitability (PDF). 48th Lunar and Planetary Science Conference. The Woodlands, Texas.
- Tortora, P.; Zannoni, M.; Nimmo, F.; Mazarico, E.; Iess, L.; Sotin, C.; Hayes, A.; Malaska, M. (23–28 April 2017). Titan gravity investigation with the Oceanus mission. 19th EGU General Assembly, EGU2017. Egu General Assembly Conference Abstracts. 19. p. 17876. Bibcode:2017EGUGA..1917876T.
- Mitri, Giuseppe; Postberg, Frank; Soderblom, Jason M.; Tobie, Gabriel; Tortora, Paolo; Wurz, Peter; et al. (2017). "Explorer of Enceladus and Titan (E2T): Investigating the habitability and evolution of ocean worlds in the Saturn system". American Astronomical Society. Retrieved 16 September 2017.
- The International Journal of Astrobiology, published by Cambridge University Press, is the forum for practitioners in this interdisciplinary field.
- Astrobiology, published by Mary Ann Liebert, Inc., is a peer-reviewed journal that explores the origins of life, evolution, distribution, and destiny in the universe.
- Catling, David C. (2013). Astrobiology: A Very Short Introduction. Oxford: Oxford University Press. ISBN 978-0-19-958645-5.
- Cockell, Charles S. (2015). Astrobiology: Understanding Life in the Universe. NJ: Wiley-Blackwell. ISBN 978-1-118-91332-1.
- Kolb, Vera M., ed. (2015). Astrobiology: An Evolutionary Approach. Boca Raton: CRC Press. ISBN 978-1-4665-8461-7.
- Dick, Steven J.; James Strick (2005). The Living Universe: NASA and the Development of Astrobiology. Piscataway, NJ: Rutgers University Press. ISBN 978-0-8135-3733-7.
- Grinspoon, David (2004) . Lonely planets. The natural philosophy of alien life. New York: ECCO. ISBN 978-0-06-018540-4.
- Mautner, Michael N. (2000). Seeding the Universe with Life: Securing Our Cosmological Future (PDF). Washington D. C. ISBN 978-0-476-00330-9.
- Jakosky, Bruce M. (2006). Science, Society, and the Search for Life in the Universe. Tucson: University of Arizona Press. ISBN 978-0-8165-2613-0.
- Lunine, Jonathan I. (2005). Astrobiology. A Multidisciplinary Approach. San Francisco: Pearson Addison-Wesley. ISBN 978-0-8053-8042-2.
- Gilmour, Iain; Mark A. Sephton (2004). An introduction to astrobiology. Cambridge: Cambridge Univ. Press. ISBN 978-0-521-83736-1.
- Ward, Peter; Brownlee, Donald (2000). Rare Earth: Why Complex Life is Uncommon in the Universe. New York: Copernicus. ISBN 978-0-387-98701-9.
- Chyba, C. F.; Hand, K. P. (2005). "ASTROBIOLOGY: The Study of the Living Universe". Annual Review of Astronomy and Astrophysics. 43 (1): 31–74. Bibcode:2005ARA&A..43...31C. doi:10.1146/annurev.astro.43.051804.102202.
- D. Goldsmith, T. Owen, The Search For Life in the Universe, Addison-Wesley Publishing Company, 2001 (3rd edition). ISBN 978-1891389160
- Spanish Centro de Astrobiología
- UK Centre for Astrobiology
- Astrobiology Research at The Library of Congress
- Astrobiology Survey – An introductory course on astrobiology
- Astrobiology Magazine Exploring Solar System and Beyond