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 how humans can detect it
if it does.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.
== Overview ==
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.
== 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. 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.
=== 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 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 ===
==== 
Astronomy ====
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
=
R
∗
×
f
p
×
n
e
×
f
l
×
f
i
×
f
c
×
L
{\displaystyle N=R^{*}~\times ~f_{p}~\times
~n_{e}~\times ~f_{l}~\times ~f_{i}~\times
~f_{c}~\times ~L}
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 civilizationsHowever,
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 ====
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 ====
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 ====
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 three 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, liquid water
likely exists beneath the moon's icy outer
crust. 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.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 was studied, it was reported on March
2008 that Titan may also have an underground
ocean composed of liquid water and ammonia.
Additionally, Saturn's moon Enceladus may
have an ocean below its icy surface 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".
On 27 June 2018, astronomers reported the
detection of complex macromolecular organics
on Enceladus.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.
== Research ==
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.
=== Research outcomes ===
As of 2019, 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 EarthOn 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.
MethaneIn 2004, the spectral signature of
methane (CH4) 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 June 7, 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 systemsIt 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.
=== Extremophiles ===
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 in earth. During
the course of evolution, extremophiles have
developed different 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.
== 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
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.
EXPOSEEXPOSE 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 LaboratoryThe 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.
TanpopoThe 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 2020Mars 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 ClipperEuropa 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.
=== Proposed concepts ===
Icebreaker LifeIcebreaker 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 TitanJourney to Enceladus
and Titan (JET) is an astrobiology mission
concept to assess the habitability potential
of Saturn's moons Enceladus and Titan by means
of an orbiter.
Enceladus Life FinderEnceladus 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 EnceladusLife 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.
OceanusOceanus is an orbiter proposed in 2017
for the New Frontiers mission #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 TitanExplorer 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.
== See also
