Astrobiology

"Exobiology" redirects here. It is not to be confused with Xenobiology.
For the journal, see Astrobiology (journal).
Nucleic acids may not be the only biomolecules in the Universe capable of coding for life processes.[1]

Astrobiology is the study of the origin, evolution, distribution, and future of life in the universe: extraterrestrial life and life on Earth. Astrobiology addresses the question of whether life exists beyond Earth, and how humans can detect it if it does[2] (the term exobiology is similar but more specific—it covers the search for life beyond Earth, and the effects of extraterrestrial environments on living things[3]).

Astrobiology makes use of physics, chemistry, astronomy, biology, molecular biology, ecology, planetary science, geography, and geology to investigate the possibility of life on other worlds and help recognize biospheres that might be different from that on Earth.[4] The origin and early evolution of life is an inseparable part of the discipline of astrobiology.[5] Astrobiology concerns itself with interpretation of existing scientific data; given more detailed and reliable data from other parts of the universe, the roots of astrobiology itself—physics, chemistry and biology—may have their theoretical bases challenged. 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 and evolution 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.[6][7][8]

The chemistry of life 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.[9][10][11] According to the panspermia hypothesis, microscopic life—distributed by meteoroids, asteroids and other small Solar System bodies—may exist throughout the universe.[12] According to research published in August 2015, very large galaxies may be more favorable to the creation and development of habitable planets than smaller galaxies, like the Milky Way galaxy.[13] Nonetheless, Earth is the only place in the universe known to harbor life.[14][15] Estimates of habitable zones around other stars,[16][17] along with the discovery of hundreds of extrasolar planets and new insights into the extreme habitats here on Earth, suggest that there may be many more habitable places in the universe than considered possible until very recently.[18][19][20]

Current studies on the planet Mars by the Curiosity and Opportunity rovers are now searching for evidence of ancient life as well as plains related to ancient rivers or lakes that may have been habitable.[21][22][23][24] 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 on Mars.

Overview

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 Joshua Lederberg.[25] 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.

It is not known whether life elsewhere in the universe would utilize cell structures like those found on Earth. (Chloroplasts within plant cells shown here.)[26]

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.[27] 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.[28]

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.[29] 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.[2][30] 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.

In June 2014, the John W. Kluge Center of the Library of Congress held a seminar focusing on astrobiology. Panel members (L to R) Robin Lovin, Derek Malone-France, and Steven J. Dick

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 its 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.[31]

Missions specifically designed to search for current life on Mars were the Viking program and Beagle 2 probes. The Viking results were inconclusive,[32] and Beagle 2 failed minutes after landing.[33] 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 to research the history of water there.

In November 2011, NASA launched the Mars Science Laboratory mission carrying the Curiosity rover, which landed on Mars at Gale Crater in August 2012.[34][35][36] 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.[37][38]

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 2018.[39] While NASA is developing the Mars 2020 astrobiology rover and sample cacher for a later return to Earth.

Methodology

Planetary habitability

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.[40] 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 just at an 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 arbitrarily 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.[41] Some researchers posit environments of ammonia, or more likely, water-ammonia mixtures as possible solvents for hypothetical types of biochemistry.[42]

A third assumption is to focus on planets orbiting Sun-like stars for increased probabilities of planetary habitability.[43] 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.[44] 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 the simplifying assumptions are correct.

Communication attempts

The illustration on the Pioneer plaque

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,[45][46] scientist Stephen Hawking has warned against it, suggesting that aliens might simply raid Earth for its resources and then move on.

Elements of astrobiology

Astronomy

Main article: Astronomy
Artist's impression of the extrasolar planet OGLE-2005-BLG-390Lb orbiting its star 20,000 light-years from Earth; this planet was discovered with gravitational microlensing.
The NASA Kepler mission, launched in March 2009, searches for extrasolar planets.

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.[47][48] 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,[49] 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:[50]

N = R^{*} ~ \times ~ f_{p} ~ \times ~ n_{e} ~ \times ~ f_{l} ~ \times ~ f_{i} ~ \times ~ f_{c} ~ \times ~ L

where:

However, whilst the rationale behind the equation is sound, it is unlikely that the equation will be constrained to reasonable error limits any time soon. The first term, N, 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. 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. Drake originally formulated the equation merely as an agenda for discussion at the Green Bank conference,[51] but some applications of the formula had been taken literally and related to simplistic or pseudoscientific arguments.[52] 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)[53] may have greatly increased the probability of intelligent life arising on our planet.[54][55]

Biology

Hydrothermal vents are able to support extremophile bacteria on Earth and may also support life in other parts of the cosmos.

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.[52]

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.[56] 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.[56] These creatures thrive despite having no access to sunlight, and it was soon discovered that they comprise an entirely independent ecosystem. Instead of plants, the basis for this 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. 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.

Extremophiles, organisms able to survive in extreme environments, are a core research element for astrobiologists. Such organisms include biota which are able to survive several kilometers below the ocean's surface near hydrothermal vents and microbes that thrive in highly acidic environments.[57] It is now known that extremophiles 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.[58] It 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,[59] the bacterium Bacillus safensis,[60] Deinococcus radiodurans,[60] Bacillus subtilis,[60] yeast Saccharomyces cerevisiae,[60] seeds from Arabidopsis thaliana ('mouse-ear cress'),[60] as well as the invertebrate animal Tardigrade.[60]

Jupiter's moon, Europa,[58][61][62][63][64][65] and Saturn's moon, Enceladus,[66][67] are now considered the most likely locations for extant extraterrestrial life in the Solar System.

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 matter ("amorphous organic solids with a mixed aromatic-aliphatic structure") that could be created naturally, and rapidly, by stars.[68][69][70] 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."[68] In September 2012, NASA scientists reported that polycyclic aromatic hydrocarbons (PAHs), subjected to interstellar medium conditions, 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".[71][72]

More than 20% of the carbon in the universe may be associated with 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.[73]

Astroecology

Main article: Astroecology

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.[74] 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.[74][75]

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.[74][74][76] 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.[77]

Astrogeology

Astrogeology is a planetary science discipline concerned with the geology of the 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.[78] 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.[52][79] 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.[79] 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.[80]

Life in the Solar System

Europa, due to the ocean that exists under its icy surface, might host some form of microbial life.

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.[79] 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.[79] The three most likely candidates for life in the Solar System are the planet Mars, the Jovian moon Europa, and Saturn's moon Titan.[81][82][83][84][85] More recently, Saturn's moon Enceladus may be considered a likely candidate as well.[67][86]

Mars, Enceladus and Europa are considered likely candidates in the search for life primarily because they may have liquid water, a molecule essential for life as we know it for its use as a solvent in cells.[31] Water on Mars is found 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.[87][88] At the Martian low temperatures and low pressure, liquid water is likely to be highly saline.[89] As for Europa, liquid water likely exists beneath the moon's icy outer crust.[62][81][82] 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.[90] 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.[91] The presence of the minerals may have been the result of a collision with an asteroid or comet according to the scientists.[91]

Another planetary body that could potentially sustain extraterrestrial life is Saturn's largest moon, Titan.[85] Titan has been described as having conditions similar to those of early Earth.[92] On its surface, scientists have discovered the first liquid lakes outside Earth, but they seem to be composed of ethane and/or methane, not water.[93] Some scientists think it possible that these liquid hydrocarbons might take the place of water in living cells different from those on Earth.[94][95] After Cassini data was studied, it was reported on March 2008 that Titan may also have an underground ocean composed of liquid water and ammonia.[96] Additionally, Saturn's moon Enceladus may have an ocean below its icy surface[97] 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".[67][86]

Measuring the ratio of hydrogen and methane levels on Mars may help determine the likelihood of life on Mars.[98][99] According to the scientists, "...low H2/CH4 ratios (less than approximately 40) indicate that life is likely present and active."[98] Other scientists have recently reported methods of detecting hydrogen and methane in extraterrestrial atmospheres.[100][101]

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), the most carbon-rich chemical found in the universe.[102]

Rare Earth hypothesis

Main article: 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, but rather that life is more than likely to be found on innumerable other worlds.

The anthropic principle states that fundamental laws of the universe work specifically in a way that life would be possible. The anthropic principle supports the Rare Earth Hypothesis by arguing the overall elements that are needed to support life on Earth are so fine-tuned that it is nearly impossible for another just like it to exist by random chance (note that these terms are used by scientists in a different way from the vernacular conception of them).

Research

The systematic search for possible life outside Earth is a valid multidisciplinary scientific endeavor.[103] 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.[104]

Research outcomes

Asteroid(s) may have transported life to Earth.

As of 2015, 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.[105][106][107]

Yamato 000593 is the second largest meteorite from Mars, and 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.[108][109][110]

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.[111][112] However, NASA formally distanced itself from Hoover's claim.[113] 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."[114]

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.[115][116] Other researchers reported related studies that microbes thrive inside rocks up to 1900 feet below the sea floor under 8500 feet of ocean off the coast of the northwestern United States.[115][117] According to one of the researchers, "You can find microbes everywhere — they're extremely adaptable to conditions, and survive wherever they are."[115] These finds expand the potential habitability of certain niches of other planets.

Methane

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.[118][119] The Curiosity rover will perform precision measurements of oxygen and carbon isotope ratios in carbon dioxide (CO2) and methane (CH4) in the atmosphere of Mars in order to distinguish between a geochemical and a biological origin.[120][121][122]

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 observing 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.[123]

Missions

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

Carl Sagan posing with a model of the Viking Lander.

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 to look 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.[124] The result was inconclusive,[125] and is still disputed by some scientists.[126][127][128][129]

Beagle 2

Main article: Beagle 2
Replica of the 33.2 kg Beagle-2 lander
Mars Science Laboratory rover concept artwork

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.[130]

EXPOSE

Main article: EXPOSE

EXPOSE is a multi-user facility mounted in 2008 outside the International Space Station dedicated to astrobiology.[131][132] EXPOSE was developed by the European Space Agency (ESA) for long-term spaceflights that allows to expose organic chemicals and biological samples to outer space in low Earth orbit.[133]

Mars Science Laboratory

The Mars Science Laboratory (MSL) mission landed a rover that is currently in operation on Mars.[134] It was launched 26 November 2011, and landed at Gale Crater on 6 August 2012.[36] Mission objectives are to help assess Mars' habitability and in doing so, determine whether Mars is or has ever been able to support life,[135] 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.

ExoMars rover

Main article: ExoMars rover
ExoMars rover model

ExoMars 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.[136][137][138]

Red Dragon

Red Dragon is a planned series low-cost Mars lander missions that will utilize the SpaceX Falcon Heavy launch vehicle, and a modified Dragon V2 capsule to enter the Martian atmosphere and land using retrorockets. The lander's primary mission would be a technology demonstration, and to search for evidence of life on Mars (biosignatures), past or present. The concept had been meant to compete for funding on 2012/2013 as a NASA Discovery mission.[139][140] On April 2016, SpaceX announced that they will proceed with the mission, with technical support from NASA, to be launched with a Falcon Heavy rocket in 2018.[141] These Mars missions will also be pathfinders for the much larger SpaceX Mars colonization architecture that will be announced in September 2016.[142]

Mars 2020

Main article: Mars 2020

The 'Mars 2020' rover mission is a concept under development by NASA with a possible launch in 2020. It is intended to 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.[143] 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.[144][145]

Proposed concepts

Icebreaker Life

Main article: Icebreaker Life

Icebreaker Life is a lander mission that is being proposed for NASA's Discovery Program for the 2018 launch opportunity.[146] If selected and funded, the stationary lander would be a near copy of the successful 2008 Phoenix and it would carry 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.[147][148] 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

Journey to Enceladus and Titan (JET) is an orbiter astrobiology mission concept to assess the habitability potential of Saturn's moons Enceladus and Titan.[149][150][151]

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.[152][153]

Life Investigation For Enceladus

Life Investigation For Enceladus (LIFE) is a proposed astrobiology sample-return mission concept for Enceladus. 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 Titan upper atmosphere.[154][155][156]

Europa Multiple-Flyby Mission

Europa Multiple-Flyby Mission is a mission planned by NASA for a 2025 launch that will conduct detailed reconnaissance of Jupiter's moon Europa and will investigate whether the icy moon could harbor conditions suitable for life.[157][158] It will also aid in the selection of future landing sites.[159][160]

See also

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