Astrobiology is the study of the
origin,
evolution, distribution, and
future of
life in the
universe:
extraterrestrial life and
life on Earth. This
interdisciplinary field encompasses the search for habitable environments in our
Solar System and
habitable planets outside our Solar System, the search for evidence of prebiotic chemistry, laboratory and field research into the origins and early evolution of life on Earth, and studies of the potential for life to adapt to challenges on
Earth and in
outer space.
[2] Astrobiology addresses the
question of whether life exists beyond Earth, and how humans can detect it if it does.
[3] (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.)
[4]
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 the biosphere on Earth.
[5][6] 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.
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.
[7][8][9] According to the
panspermia hypothesis, microscopic life—distributed by
meteoroids,
asteroids and other
small Solar System bodies—may exist throughout the universe.
[10] Nonetheless, Earth is the only place in the universe known to harbor life.
[11][12] Although more than 99 percent of all species that ever lived on the planet are estimated to be extinct,
[13][14] there are currently 10–14 million
species of
life on the Earth.
[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. On 4 November 2013, astronomers reported, based on
Kepler space mission data, that there could be as many as 40 billion
Earth-sized planets orbiting in the
habitable zones of
sun-like stars and
red dwarf stars within the
Milky Way Galaxy.
[18][19] 11 billion of these estimated planets may be orbiting sun-like stars.
[20] The nearest such planet may be 12 light-years away, according to the scientists.
[18][19]
It has been proposed that
viruses are likely to be encountered on other life-bearing planets.
[21] Efforts to discover current or past
life on Mars is an active area of research. On 24 January 2014, NASA reported that
current studies on the planet
Mars by the
Curiosity and
Opportunity rovers will now be searching for evidence of ancient life, including a
biosphere based on
autotrophic,
chemotrophic and/or
chemolithoautotrophic microorganisms, as well as ancient water, including
fluvio-lacustrine environments (
plains related to ancient
rivers or
lakes) that may have been
habitable.
[22][23][24][25] The search for evidence of
habitability,
taphonomy (related to
fossils), and
organic carbon on the planet
Mars is now a primary
NASA objective.
[22]
Overview
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]
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
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 first coined by molecular biologist
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. Exobiology as a term has tended to be replaced by astrobiology.
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; Exobiology research is now one of four elements of NASA's current Astrobiology Program.
[3][30] In 1971, NASA funded the
Search for Extra-Terrestrial Intelligence (SETI) to search radio frequencies of the electromagnetic spectrum for
signals being 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
possible signs of present
life on Mars. The
Mars Pathfinder lander in 1997 carried a scientific payload intended for exopaleontology in the hopes of finding microbial fossils entombed in the rocks.
[31]
In the 21st century, astrobiology is a focus of a growing number of NASA and
European Space Agency Solar System exploration missions. The first European workshop on astrobiology took place in May 2001 in Italy,
[32] and the outcome was the
Aurora programme.
[33] Currently, NASA hosts the
NASA Astrobiology Institute and a growing number of universities in the United States (e.g.,
University of Arizona,
Penn State University,
Montana State University – Bozeman,
University of Washington, and
Arizona State University),
[34] Britain (e.g., The
University of Glamorgan,
Buckingham University),
[35] Canada, Ireland, and Australia (e.g., The
University of New South Wales)
[36] now offer graduate degree programs in astrobiology. The
International Astronomical Union regularly organizes international conferences through its Bioastronomy Commission.
[37]
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.
[38]
Missions specifically designed to search for life include the
Viking program and
Beagle 2 probes, both directed to Mars. The Viking results were inconclusive,
[39] and Beagle 2 failed to transmit from the surface and is assumed to have crashed.
[40] 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 (MSL) rover, nicknamed
Curiosity, which
landed on Mars at
Gale Crater in August 2012.
[41][42][43] 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.
[44][45]
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.
[46]
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 to assume that the vast majority of life forms in our galaxy are based on
carbon chemistries, as are all life forms on Earth.
[47] 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 a useful assumption, 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.
[48] Some researchers posit environments of
ammonia, or more likely, water-ammonia mixtures.
[49]
A third assumption is to focus on
sun-like
stars. This comes from the idea of
planetary habitability.
[50] Very big stars have relatively short lifetimes, meaning that life would not likely 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.
[51] Without a thick
atmosphere, one side of the planet would be perpetually baked and the other perpetually frozen. In 2005, the question was brought back to the attention of the
scientific community, as the long lifetimes of
red dwarfs could allow some biology on planets with thick atmospheres. This is significant, as red dwarfs are extremely common.
It is estimated that 10% of the stars in our galaxy are sun-like; there are about a thousand such stars within 100 light-years of our
Sun. These stars would be useful
primary targets for interstellar listening. 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
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
extraterrestrial intelligence.
While some high-profile scientists, such as
Carl Sagan, have advocated the transmission of messages,
[52][53] scientist
Stephen Hawking has warned against it, suggesting that aliens might simply raid Earth for its resources and then move on.
[54]
Elements of astrobiology
Astronomy
Most astronomy-related astrobiological 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. Additionally, NASA has launched the
Kepler mission in March 2009, and the
French Space Agency has launched the
COROT space mission in 2006.
[55][56] There are also several less ambitious ground-based efforts underway. (See
exoplanet).
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,
[57] 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. The
photometry temporal variability of extrasolar planets may also provide clues to their surface and atmospheric properties.
An estimate for the number of planets with
intelligent 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:
[58]
where:
- 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 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 sun's neighborhood. The problem with the formula is that it is not usable to generate or support
hypotheses because it contains units that can never be verified. Drake originally formulated the equation merely as an agenda for discussion at the Green Bank conference,
[59] but some applications of the formula had been taken literally and related to simplistic or
pseudoscientific arguments.
[60] 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. This is the purpose of projects like
SETI, which tries to detect signs of radio transmissions from intelligent extraterrestrial civilizations.
Another active research area in astrobiology is
planetary system formation. It has been suggested that the peculiarities of our
Solar System (for example, the presence of
Jupiter as a protective shield)
[61] may have greatly increased the probability of intelligent life arising on our planet.
[62][63] No firm conclusions have been reached so far.
Biology
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.
[60]
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, and are then eaten by oxygen-respiring animals, 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.
[64] A 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.
[64] These creatures thrive despite having no access to sunlight, and it was soon discovered that they comprise an entirely independent food chain. 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 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.
[65] It is now known that extremophiles thrive in ice, boiling water, acid, 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.
[66] 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. According to astrophysicist Dr. Steinn Sigurdsson, "There are viable bacterial spores that have been found that are 40 million years old on Earth - and we know they're very hardened to radiation."
[67] Some organisms able to withstand exposure to the vacuum and radiation of space include the lichen fungi
Rhizocarpon geographicum and
Xanthoria elegans,
[68] the bacterium
Bacillus safensis,
[69] Deinococcus radiodurans,
[69] Bacillus subtilis,
[69] yeast
Saccharomyces cerevisiae,
[69] seeds from
Arabidopsis thaliana ('mouse-ear cress'),
[69] as well as the invertebrate animal
Tardigrade.
[69] On 29 April 2013, scientists in Rensselaer Polytechnic Institute, funded by
NASA, reported that, during
spaceflight,
microbes (like
Pseudomonas aeruginosa) seem to adapt to the
space environment in ways "not observed on Earth" and can increase in "
virulence".
[70] On 27 June 2011, it was reported that a new
E. coli bacterium was produced from an engineered
DNA in which approximately 90% of its
thymine was replaced with the synthetic building block 5-chlorouracil, a substance "toxic to other organisms".
[71][72]
Jupiter's moon,
Europa,
[66][73][74][75][76][77] and Saturn's moon,
Enceladus,
[78][79] 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 by a variety of means.
In October 2011, scientists found that 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.
[80][81][82] As one of the scientists noted, "
Coal and
kerogen are products of life and it took a long time for them to form ... How do stars make such complicated organics under seemingly unfavorable conditions and [do] it so rapidly?"
[80] Further, the 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."
[80] In September 2012,
NASA scientists reported that
polycyclic aromatic hydrocarbons (PAHs), subjected to
interstellar medium (ISM) 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".
[83][84] Further, as a result of these transformations, the PAHs lose their
spectroscopic signature which could be one of the reasons "for the lack of PAH detection in
interstellar ice grains, particularly the outer regions of cold, dense clouds or the upper molecular layers of
protoplanetary disks."
[83][84]
On 29 August 2012, and in a world first, astronomers at
Copenhagen University reported the detection of a specific sugar molecule,
glycolaldehyde, in a distant star system. The molecule was found around the
protostellar binary
IRAS 16293-2422, which is located 400 light years from Earth.
[85][86] Glycolaldehyde is needed to form
ribonucleic acid, or
RNA, which is similar in function to
DNA. This finding suggests that complex organic molecules may form in stellar systems prior to the formation of planets, eventually arriving on young planets early in their formation.
[87]
On 21 February 2014,
NASA announced a
greatly upgraded database for tracking
polycyclic aromatic hydrocarbons (PAHs) in the
universe. According to scientists, 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.
[88]
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.
[89] 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.
[89][90]
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 10
20 years.
[89][89][91] Astroecologists suggest that their mathematical models may quantify the immense potential amounts of future life in space, allowing a comparable expansion in biodiversity, potentially leading to diverse intelligent life-forms.
[92]
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.
[93] 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.
Consistent with the above, the earliest evidence for
life on Earth are
graphite found to be
biogenic in 3.7 billion-year-old
metasedimentary rocks discovered in
Western Greenland[94] and
microbial mat fossils found in 3.48 billion-year-old
sandstone discovered in
Western Australia.
[95][96] Nonetheless,
several studies suggest that life on Earth may have started even earlier, as early as 4.25 billion years ago according to one study.
[97][98][99]
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.
[100] The likelihood that life throughout the universe is probably carbon-based is encouraged 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.
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.
[60][100] Indeed, it seems likely that the basic building blocks of life anywhere will be similar to our own, in the generality if not in the detail.
[100] 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.
[101]
Thought on where in the Solar System life might occur was limited historically by the belief that life relies ultimately on light and warmth from the Sun and, therefore, is restricted to the surfaces of planets.
[100] The three most likely candidates for life in the Solar System are the planet
Mars, the Jovian moon
Europa, and Saturn's moon
Titan.
[102][103][104][105][106] More recently, Saturn's moon
Enceladus may be considered a likely candidate as well.
[79][107] This speculation of likely candidates of life is primarily based on the fact that (in the cases of Mars and Europa) the
planetary bodies may have liquid water, a molecule essential for life as we know it, for its use as a
solvent in cells.
[38]
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.
[108][109] At the Martian low temperatures and low pressure, liquid water is likely to be highly saline.
[110] As for Europa, liquid water likely exists beneath the moon's icy outer crust.
[74][102][103] This water may be warmed to a liquid state by volcanic vents on the ocean floor (an especially intriguing theory considering the various types of extremophiles that live near Earth's volcanic vents), but the primary source of heat is probably
tidal heating.
[111] 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.
[112] The presence of the minerals may have been the result of a collision with an
asteroid or
comet according to the scientists.
[112]
Another
planetary body that could potentially sustain extraterrestrial life is
Saturn's largest moon,
Titan.
[106] Titan has been described as having conditions similar to those of early Earth.
[113] 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.
[114] 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.
[115] Additionally, Saturn's moon
Enceladus may have an ocean below its icy surface
[116] 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".
[79][107]
On 26 April 2012, scientists reported that
lichen survived and showed remarkable results on the
adaptation capacity of
photosynthetic activity within the
simulation time of 34 days under
Martian conditions in the Mars Simulation Laboratory (MSL) maintained by the
German Aerospace Center (DLR).
[117][118] In June, 2012, scientists reported that measuring the ratio of
hydrogen and
methane levels on Mars may help determine the likelihood of
life on Mars.
[119][120] According to the scientists, "...low H
2/CH
4 ratios (less than approximately 40) indicate that life is likely present and active."
[119] Other scientists have recently reported methods of detecting hydrogen and methane in
extraterrestrial atmospheres.
[121][122]
On 11 August 2014, astronomers released studies, using the
Atacama Large Millimeter/Submillimeter Array (ALMA) for the first time, that detailed the distribution of
HCN,
HNC,
H2CO, and
dust inside the
comae of
comets C/2012 F6 (Lemmon) and
C/2012 S1 (ISON).
[123][124]
Rare Earth hypothesis
This hypothesis states that based on astrobiological findings, multi-cellular life forms found on Earth may actually be more of a rarity than scientists initially assumed. 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). However,
Stephen Jay Gould compared the claim that the universe is fine-tuned for the benefit of our kind of life to saying that sausages were made long and narrow so that they could fit into modern
hot dog buns, or saying that ships had been invented to house
barnacles.
[125][126]
Research
The systematic search for possible life outside Earth is a valid multidisciplinary scientific endeavor.
[127] The
University of Glamorgan, UK, started just such a degree in 2006,
[35] and the American government funds the
NASA Astrobiology Institute. However, characterization of non-Earth life is unsettled; hypotheses and predictions as to its existence and origin vary widely, but at the present, the development of theories to inform and support the exploratory search for life may be considered astrobiology's most concrete practical application.
Biologist Jack Cohen and
mathematician Ian Stewart, amongst others, consider
xenobiology separate from astrobiology. Cohen and Stewart stipulate that astrobiology is the search for Earth-like life outside our Solar System and say that xenobiologists are concerned with the possibilities open to us once we consider that life need not be carbon-based or oxygen-breathing, so long as it has the defining
characteristics of life. (See
carbon chauvinism).
Research outcomes
Asteroid(s) may have transported life to
Earth.
As of 2014
[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, Chief Scientist for Astrobiology at
NASA's
Johnson Space Center, as well as other scientists, to contain
microfossils of extraterrestrial origin; this interpretation is controversial.
[128][129][130]
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 NASA scientists.
[131][132][133]
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.
[134][135] However, NASA formally distanced itself from Hoover's claim.
[136][137][138] 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."
[139]
- Extreme environments on Earth
On 17 March 2013, researchers reported data that suggested
microbial life forms thrive in the
Mariana Trench, the deepest spot on the Earth.
[140][141] 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.
[140][142] According to one of the researchers,"You can find microbes everywhere — they're extremely adaptable to conditions, and survive wherever they are."
[140]
- Methane
In 2004, the spectral signature of
methane was detected in the Martian atmosphere by both Earth-based telescopes as well as by the
Mars Express probe. 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.
[143][144] The
Mars Science Laboratory rover will perform precision measurements of oxygen and carbon
isotope ratios in carbon dioxide (CO
2) and methane (CH
4) in the
atmosphere of Mars in order to distinguish between a
geochemical and a
biological origin.
[145][146][147]
- Planetary systems
It is possible that some planets, like the gas giant
Jupiter in our
Solar System, may have moons with solid surfaces or liquid oceans that are more hospitable. Most of the planets so far discovered outside our Solar System are hot gas giants thought to be inhospitable to life, so it is not yet known whether our 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. Infrared searches have detected belts of dust and
asteroids around distant stars, underpinning the formation of planets.
- 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, which was found using
radial velocity.
[148]
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 to Mars, and the
Cassini probe to
Saturn's moon
Titan hope to further explore the possibilities of life on other
planets in our
Solar System.
Viking program
Carl Sagan posing with a model of the Viking Lander.
The two
Viking spacecraft 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
biosignatures of
life on Mars. The landers used a robotic arm to put 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.
[149] The result was inconclusive,
[150] and is still disputed by some scientists.
[151][152][153][154]
Beagle 2
Replica of the 33.2 kg
Beagle-2 lander
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. All contact with it was lost upon its entry into the atmosphere.
[155]
EXPOSE
EXPOSE was a multi-user facility mounted in 2008 outside the
International Space Station dedicated to astrobiology.
[156][157] EXPOSE was developed by the
European Space Agency (ESA) for
long-term spaceflights that allowed to expose
organic chemicals and biological samples to
outer space for one and a half years in
low Earth orbit.
[158] Somewhat related, on August 20, 2014, Russian cosmonauts claimed to have found sea
plankton living on the
outside window surfaces of the
International Space Station and have been unable to explain how it got there.
[159][160]
Mars Science Laboratory
The Mars Science Laboratory (MSL) mission landed a
rover that is currently in operation on
Mars.
[161] It was launched 26 November 2011, and landed at
Gale Crater on 6 August 2012.
[43] Mission objectives are to help assess Mars'
habitability and in doing so, determine whether Mars is or has ever been able to support
life,
[162] collect data for a future
manned 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
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) with collaboration by the
Russian Federal Space Agency (Roscosmos); it is planned for a 2018 launch.
[163][164][165]
Mars 2020 rover mission
The 'Mars 2020 rover mission' is a concept under study by NASA with a possible launch in 2020. It is intended to investigate astrobiologically relevant environments on Mars, investigate its surface
geological processes and history, including the assessment of its past
habitability and potential for preservation of
biosignatures within accessible geological materials.
[166] The Science Definition Team is proposing the rover collect and package as many as 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 (CO
2), which could be a resource for making oxygen (O
2) and
rocket fuel. Improved precision landing technology that enhances the scientific value of robotic missions also will be critical for eventual human exploration on the surface.
[167][168]
Red Dragon
Red Dragon is a proposed concept for a low-cost
Mars lander mission that would utilize a
SpaceX Falcon Heavy launch vehicle, and a modified
Dragon capsule to
enter the
Martian atmosphere. The lander's primary mission would be to search for evidence of
life on Mars (
biosignatures), past or present. The concept had been scheduled to propose for funding on 2012/2013 as a
NASA Discovery mission, for launch in 2018.
[169][170]
Icebreaker Life
Icebreaker Life is a lander mission that is being proposed for NASA's
Discovery Program for the 2018 launch opportunity.
[171] 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 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.
[172][173] 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.
Europa Clipper
Europa Clipper is a mission concept under study by NASA that would conduct detailed reconnaissance of
Jupiter's moon
Europa and would investigate whether the icy moon could harbor conditions suitable for
life.
[174][175] It would also aid in the selection of future
landing sites.
[176][177]
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