The term "astroecology" was first applied in the context of performing studies in actual meteorites to evaluate their potential resources favorable to sustaining life. Early results showed that meteorite/asteroid materials can support microorganisms, algae and plant cultures under Earth's atmosphere and supplemented with water.
Several observations suggest that diverse planetary materials,
similar to meteorites collected on Earth, could be used as agricultural
soils, as they provide nutrients to support microscopic life when
supplemented with water and an atmosphere.
Experimental astroecology has been proposed to rate planetary materials
as targets for astrobiology exploration and as potential biological
in-situ resources. The biological fertilities of planetary materials can be assessed by measuring water-extractable electrolyte nutrients. The results suggest that carbonaceous asteroids and Martian basalts can serve as potential future resources for substantial biological populations in the Solar System.
Analysis of the essential nutrients (C, N, P, K) in meteorites yielded information for calculating the amount of biomass that can be constructed from asteroid resources. For example, carbonaceous asteroids are estimated to contain about 1022 kg potential resource materials, and laboratory results suggest that they could yield a biomass on the order of 6·1020 kg, about 100,000 times more than biological matter presently on Earth.
Cultures on simulated asteroid/meteorite materials
To
quantify the potential amounts of life in biospheres, theoretical
astroecology attempts to estimate the amount of biomass over the
duration of a biosphere. The resources, and the potential time-integrated biomass were estimated for planetary systems, for habitable zones around stars, and for the galaxy and the universe. Such astroecology calculations suggest that the limiting elements nitrogen and phosphorus in the estimated 1022 kg carbonaceous asteroids could support 6·1020 kg biomass for the expected five billion future years of the Sun, yielding a future time-integrated BIOTA (BIOTA, Biomass Integrated Over Times Available, measured in kilogram-years) of 3·1030 kg-years in the Solar System a hundred thousand times more than life on Earth to date. Considering biological requirements of 100 W kg−1 biomass, radiated energy about red giant stars and white and red dwarf stars could support a time-integrated BIOTA up to 1046 kg-years in the galaxy and 1057 kg-years in the universe.
Such astroecology considerations quantify the immense potentials of future life in space, with commensurate biodiversity and possibly, intelligence. Chemical analysis of carbonaceous chondrite meteorites show that they contain extractable bioavailable water, organic carbon, and essential phosphate, nitrate and potassium nutrients.
The results allow assessing the soil fertilities of the parent
asteroids and planets, and the amounts of biomass that they can sustain.
Laboratory experiments showed that material from the Murchison meteorite,
when ground into a fine powder and combined with Earth's water and air,
can provide the nutrients to support a variety of organisms including
bacteria (Nocardia asteroides), algae, and plant cultures such as potato and asparagus.
The microorganisms used organics in the carbonaceous meteorites as the
carbon source. Algae and plant cultures grew well also on Mars
meteorites because of their high bio-available phosphate contents. The Martian materials achieved soil fertility ratings comparable to productive agricultural soils. This offers some data relating to terraforming of Mars.
Terrestrial analogues of planetary materials are also used in
such experiments for comparison, and to test the effects of space
conditions on microorganisms.
The biomass that can be constructed from resources can be
calculated by comparing the concentration of elements in the resource
materials and in biomass (Equation 1). A given mass of resource materials (mresource) can support mbiomass, X of biomass containing element X (considering X as the limiting nutrient), where cresource, X is the concentration (mass per unit mass) of element X in the resource material and cbiomass, X is its concentration in the biomass.
(1)
Assuming that 100,000 kg biomass supports one human, the
asteroids may then sustain about 6e15 (six million billion) people,
equal to a million Earths (a million times the present population). Similar materials in the comets could support biomass and populations about one hundred times larger. Solar energy
can sustain these populations for the predicted further five billion
years of the Sun. These considerations yield a maximum time-integrated BIOTA of 3e30 kg-years in the Solar System. After the Sun becomes a white dwarf star, and other white dwarf stars, can provide energy for life much longer, for trillions of eons. (Table 2)
Effects of wastage
Astroecology
also concerns wastage, such as the leakage of biological matter into
space. This will cause an exponential decay of space-based biomass as given by Equation (2), where M (biomass 0) is the mass of the original biomass, k is its rate of decay (the fraction lost in a unit time) and biomass t is the remaining biomass after time t.
Equation 2:
Integration from time zero to infinity yields Equation (3) for the total time-integrated biomass (BIOTA) contributed by this biomass:
Equation 3:
For example, if 0.01% of the biomass is lost per year, then the time-integrated BIOTA will be 10,000. For the 6·1020 kg biomass constructed from asteroid resources, this yields 6·1024 kg-years of BIOTA
in the Solar System. Even with this small rate of loss, life in the
Solar System would disappear in a few hundred thousand years, and the
potential total time-integrated BIOTA of 3·1030 kg-years under the main-sequence Sun would decrease by a factor of 5·105, although a still substantial population of 1.2·1012 biomass-supported humans could exist through the habitable lifespan of the Sun.
The integrated biomass can be maximized by minimizing its rate of
dissipation. If this rate can be reduced sufficiently, all the
constructed biomass can last for the duration of the habitat and it pays
to construct the biomass as fast as possible. However, if the rate of
dissipation is significant, the construction rate of the biomass and its
steady-state amounts may be reduced allowing a steady-state biomass and
population that lasts throughout the lifetime of the habitat.
An issue that arises is whether we should build immense amounts
of life that decays fast, or smaller, but still large, populations that
last longer. Life-centered biotic ethics suggests that life should last
as long as possible.
Galactic ecology
If life reaches galactic proportions, technology should be able to access all of the materials resources, and sustainable life will be defined by the available energy. The maximum amount of biomass about any star is then determined by the energy requirements of the biomass and by the luminosity of the star.
For example, if 1 kg biomass needs 100 Watts, we can calculate the
steady-state amounts of biomass that can be sustained by stars with
various energy outputs. These amounts are multiplied by the life-time of
the star to calculate the time-integrated BIOTA over the life-time of the star. Using similar projections, the potential amounts of future life can then be quantified.
For the Solar System from its origins to the present, the current 1015 kg biomass over the past four billion years gives a time-integrated biomass (BIOTA) of 4·1024 kg-years. In comparison, carbon, nitrogen, phosphorus and water in the 1022 kg asteroids allows 6·1020 kg biomass that can be sustained with energy for the 5 billion future years of the Sun, giving a BIOTA of 3·1030 kg-years in the Solar System and 3·1039 kg-years about 1011 stars in the galaxy. Materials in comets could give biomass and time-integrated BIOTA a hundred times larger.
The Sun will then become a white dwarf star, radiating 1015 Watts that sustains 1e13 kg biomass for an immense hundred million trillion (1020) years, contributing a time-integrated BIOTA of 1033 years. The 1012 white dwarfs that may exist in the galaxy during this time can then contribute a time-integrated BIOTA of 1045 kg-years. Red dwarf stars with luminosities of 1023 Watts and life-times of 1013 years can contribute 1034 kg-years each, and 1012 red dwarfs can contribute 1046 kg-years, while brown dwarfs can contribute 1039 kg-years of time-integrated biomass (BIOTA) in the galaxy. In total, the energy output of stars during 1020 years can sustain a time-integrated biomass of about 1045 kg-years in the galaxy. This is one billion trillion (1020) times more life than has existed on the Earth to date. In the universe, stars in 1011 galaxies could then sustain 1057 kg-years of life.
Directed panspermia
The astroecology results above suggest that humans can expand life in the galaxy through space travel or directed panspermia.
The amounts of possible life that can be established in the galaxy, as
projected by astroecology, are immense. These projections are based on
information about 15 billion past years since the Big Bang,
but the habitable future is much longer, spanning trillions of eons.
Therefore, physics, astroeclogy resources, and some cosmological
scenarios may allow organized life to last, albeit at an ever slowing
rate, indefinitely. These prospects may be addressed by the long-term extension of astroecology as cosmoecology.
Nucleic acids may not be the only biomolecules in the Universe capable of coding for life processes.
Astrobiology, formerly known as exobiology, is an interdisciplinary scientific field concerned with the origins, early evolution, distribution, and future of life in the universe. Astrobiology considers the question of whether extraterrestrial life exists, and if it does, how humans can detect it.
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.
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 Greekἄστρον, 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.
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.)
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.
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 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 microbiallife 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.
The European Space Agency is currently collaborating with the Russian Federal Space Agency (Roscosmos) and developing the ExoMars astrobiology rover, which was scheduled to be launched in July 2020, but was postponed to 2022. Meanwhile, NASA launched 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.
Since Earth is the only planet known to harbor life, there is no evident way to know if any of these simplifying assumptions are correct.
Research on communication with extraterrestrial intelligence (CETI)
focuses on composing and deciphering messages that could theoretically
be understood by another technological civilization. Communication
attempts by humans have included broadcasting mathematical languages,
pictorial systems such as the Arecibo message and computational approaches to detecting and deciphering 'natural' language communication. The SETI program, for example, uses both radio telescopes and optical telescopes to search for deliberate signals from an extraterrestrial intelligence.
While some high-profile scientists, such as Carl Sagan, have advocated the transmission of messages, scientist Stephen Hawking warned against it, suggesting that aliens might simply raid Earth for its resources and then move on.
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'sDarwin 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 communicativeextraterrestrial 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:
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 limits of
error any time soon. The problem with the formula is that it is not used
to generate or support hypotheses because it contains factors that can never be verified. The first term, R*, number of stars, is generally constrained within a few orders of magnitude. The second and third terms, fp, stars with planets and fe,
planets with habitable conditions, are being evaluated for the star's
neighborhood. Drake originally formulated the equation merely as an
agenda for discussion at the Green Bank conference, but some applications of the formula had been taken literally and related to simplistic or pseudoscientific arguments. Another associated topic is the Fermi paradox, which suggests that if intelligent life is common in the universe, then there should be obvious signs of it.
Another active research area in astrobiology is planetary system formation. It has been suggested that the peculiarities of the Solar System (for example, the presence of Jupiter as a protective shield) may have greatly increased the probability of intelligent life arising on our planet.
Biology cannot state that a process or phenomenon, by being
mathematically possible, has to exist forcibly in an extraterrestrial
body. Biologists specify what is speculative and what is not. The discovery of extremophiles,
organisms able to survive in extreme environments, became a core
research element for astrobiologists, as they are important to
understand four areas in the limits of life in planetary context: the
potential for panspermia,
forward contamination due to human exploration ventures, planetary
colonization by humans, and the exploration of extinct and extant
extraterrestrial life.
Until the 1970s, life was thought to be entirely dependent on energy from the Sun. Plants on Earth's surface capture energy from sunlight to photosynthesize
sugars from carbon dioxide and water, releasing oxygen in the process
that is then consumed by oxygen-respiring organisms, passing their
energy up the food chain. Even life in the ocean depths, where sunlight cannot reach, was thought to obtain its nourishment either from consuming organic detritus rained down from the surface waters or from eating animals that did. The world's ability to support life was thought to depend on its access to sunlight. However, in 1977, during an exploratory dive to the Galapagos Rift in the deep-sea exploration submersible Alvin, scientists discovered colonies of giant tube worms, clams, crustaceans, mussels, and other assorted creatures clustered around undersea volcanic features known as black smokers.
These creatures thrive despite having no access to sunlight, and it was
soon discovered that they comprise an entirely independent ecosystem.
Although most of these multicellular lifeforms need dissolved oxygen
(produced by oxygenic photosynthesis) for their aerobic cellular
respiration and thus are not completely independent from sunlight by
themselves, the basis for their food chain is a form of bacterium that derives its energy from oxidization of reactive chemicals, such as hydrogen or hydrogen sulfide,
that bubble up from the Earth's interior. Other lifeforms entirely
decoupled from the energy from sunlight are green sulfur 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."
In October 2020, astronomers proposed the idea of detecting life
on distant planets by studying the shadows of trees at certain times of
the day.
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.
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 to 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 maybe—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
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.
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.
Discussion on where in the Solar System
life might occur was limited historically by the understanding that
life relies ultimately on light and warmth from the Sun and, therefore,
is restricted to the surfaces of planets. The four most likely candidates for life in the Solar System are the planet Mars, the Jovian moon Europa, and Saturn's moons Titan, and Enceladus.
Mars, Enceladus and Europa
are considered likely candidates in the search for life primarily
because they may have underground liquid water, a molecule essential for
life as we know it for its use as a solvent in cells.
Water on Mars is found frozen in its polar ice caps, and newly carved
gullies recently observed on Mars suggest that liquid water may exist,
at least transiently, on the planet's surface. At the Martian low temperatures and low pressure, liquid water is likely to be highly saline. As for Europa and Enceladus, large global oceans of liquid water exist beneath these moons' icy outer crusts. This water may be warmed to a liquid state by volcanic vents on the ocean floor, but the primary source of heat is probably tidal heating. On 11 December 2013, NASA reported the detection of "clay-like minerals" (specifically, phyllosilicates), often associated with organic materials, on the icy crust of Europa. The presence of the minerals may have been the result of a collision with an asteroid or comet according to the scientists. Additionally, on 27 June 2018, astronomers reported the detection of complex macromolecular organics on Enceladus
and, according to NASA scientists in May 2011, "is emerging as the most
habitable spot beyond Earth in the Solar System for life as we know
it".
Another planetary body that could potentially sustain extraterrestrial life is Saturn's largest moon, Titan. Titan has been described as having conditions similar to those of early Earth. On its surface, scientists have discovered the first liquid lakes outside Earth, but these lakes seem to be composed of ethane and/or methane, not water. Some scientists think it possible that these liquid hydrocarbons might take the place of water in living cells different from those on Earth.
After Cassini data were studied, it was reported in March 2008 that
Titan may also have an underground ocean composed of liquid water and ammonia.
Phosphine has been detected in the atmosphere of the planet Venus. There are no known abiotic processes on the planet that could cause its presence.
Given that Venus has the hottest surface temperature of any planet in
the solar system, Venusian life, if it exists, is most likely limited to
extremophile microorganisms that float in the planet's upper atmosphere, where conditions are almost Earth-like.
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.
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
What biosignatures does life produce?
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 largestmeteorite 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 fringeJournal of Cosmology, a story widely reported on by mainstream media. However, NASA formally distanced itself from Hoover's claim. According to American astrophysicist Neil deGrasse Tyson:
"At the moment, life on Earth is the only known life in the universe,
but there are compelling arguments to suggest we are not alone."
Extreme environments on Earth
On 17 March 2013, researchers reported that microbial life forms thrive in the Mariana Trench, the deepest spot on the Earth.
Other researchers reported that microbes thrive inside rocks up to
1,900 feet (580 m) below the sea floor under 8,500 feet (2,600 m) of
ocean off the coast of the northwestern United States.
According to one of the researchers, "You can find microbes
everywhere—they're extremely adaptable to conditions, and survive
wherever they are." Evidence of perchlorates have been found throughout the solar system, and specifically on Mars. Dr. Kennda Lynch discovered the first known instance of perchlorates and perchlorates-reducing microbes in a paleolake in Pilot Valley, Utah. 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. On 7 June 2018, NASA announced a cyclical seasonal variation in atmospheric methane, which may be produced by geological or biological sources. The European ExoMars Trace Gas Orbiter is currently measuring and mapping the atmospheric methane.
Planetary systems
It is possible that some exoplanets may have moons with solid
surfaces or liquid oceans that are hospitable. Most of the planets so
far discovered outside the Solar System are hot gas giants thought to be
inhospitable to life, so it is not yet known whether the Solar System,
with a warm, rocky, metal-rich inner planet such as Earth, is of an
aberrant composition. Improved detection methods and increased
observation time will undoubtedly discover more planetary systems, and
possibly some more like ours. For example, NASA's Kepler Mission seeks to discover Earth-sized planets around other stars by measuring minute changes in the star's light curve as the planet passes between the star and the spacecraft. Progress in infrared astronomy and submillimeter astronomy has revealed the constituents of other star systems.
Planetary habitability
Efforts to answer questions such as the abundance of potentially habitable planets in habitable zones and chemical precursors have had much success. Numerous extrasolar planets have been detected using the wobble method and transit method, showing that planets around other stars
are more numerous than previously postulated. The first Earth-sized
extrasolar planet to be discovered within its star's habitable zone is Gliese 581 c.
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 on
earth. During the course of evolution, extremophiles have developed
various strategies to survive the different stress conditions of
different extreme environments. These stress responses could also allow
them to survive in harsh space conditions.
Thermophilic species G. thermantarcticus is a good example of a
microorganism that could survive space travel. It is a bacterium of the
spore-forming genus Bacillus. The formation of spores allows for it to
survive extreme environments while still being able to restart cellular
growth. It is capable of effectively protecting its DNA, membrane and
proteins integrity in different extreme conditions (desiccation,
temperatures up to -196 °C, UVC and C-ray radiation...). It is also able
to repair the damage produced by space environment.
By understanding how extremophilic organisms can survive the
Earth's extreme environments, we can also understand how microorganisms
could have survived space travel and how the panspermia hypothesis could
be possible.
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.
Norman Horowitz was the chief of the Jet Propulsion Laboratory bioscience section for the Mariner and Viking
missions from 1965 to 1976. Horowitz considered that the great
versatility of the carbon atom makes it the element most likely to
provide solutions, even exotic solutions, to the problems of survival of
life on other planets. However, he also considered that the conditions found on Mars were incompatible with carbon based life.
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.
The Mars Science Laboratory (MSL) mission landed the Curiosityrover 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.
Tanpopo
The Tanpopo mission is an orbital astrobiology experiment investigating the potential interplanetary transfer of life, organic compounds, and possible terrestrial particles in the low Earth orbit. The purpose is to assess the panspermia
hypothesis and the possibility of natural interplanetary transport of
microbial life as well as prebiotic organic compounds. Early mission
results show evidence that some clumps of microorganism can survive for
at least one year in space.
This may support the idea that clumps greater than 0.5 millimeters of
microorganisms could be one way for life to spread from planet to
planet.
Mars 2020 successfully landed its rover Perseverance in Jezero Crater on 18 February 2021. It will investigate environments on Mars relevant to astrobiology, investigate its surface geological processes and history, including the assessment of its past habitability and potential for preservation of biosignatures and biomolecules within accessible geological materials.
The Science Definition Team is proposing the rover collect and package
at least 31 samples of rock cores and soil for a later mission to bring
back for more definitive analysis in laboratories on Earth. The rover
could make measurements and technology demonstrations to help designers
of a human expedition understand any hazards posed by Martian dust and demonstrate how to collect carbon dioxide (CO2), which could be a resource for making molecular oxygen (O2) and rocket fuel.
Europa Clipper
Europa Clipper is a mission planned by NASA for a 2025 launch that will conduct detailed reconnaissance of Jupiter's moon Europa and will investigate whether its internal ocean could harbor conditions suitable for life. It will also aid in the selection of future landing sites.
Proposed concepts
Icebreaker Life
Icebreaker Life is a lander mission that proposed for NASA's Discovery Program for the 2021 launch opportunity,
but it was not selected for development. It would have had a stationary
lander that would be a near copy of the successful 2008 Phoenix
and it would have carried an upgraded astrobiology scientific payload,
including a 1-meter-long core drill to sample ice-cemented ground in the
northern plains to conduct a search for organic molecules and evidence of current or past life on Mars. One of the key goals of the Icebreaker Life mission is to test the hypothesis that the ice-rich ground in the polar regions has significant concentrations of organics due to protection by the ice from oxidants and radiation.
Life Investigation For Enceladus (LIFE) is a proposed astrobiology sample-return mission concept. The spacecraft would enter into Saturn
orbit and enable multiple flybys through Enceladus' icy plumes to
collect icy plume particles and volatiles and return them to Earth on a
capsule. The spacecraft may sample Enceladus' plumes, the E ring of Saturn, and the upper atmosphere of Titan.
Oceanus is an orbiter proposed in 2017 for the New Frontiers mission No. 4. It would travel to the moon of Saturn, Titan, to assess its habitability. Oceanus' objectives are to reveal Titan's organic chemistry, geology, gravity, topography, collect 3D reconnaissance data, catalog the organics and determine where they may interact with liquid water.
