The biosphere (from Greek βίος bíos "life" and σφαῖρα sphaira "sphere") also known as the ecosphere (from Greek οἶκος oîkos "environment" and σφαῖρα), is the worldwide sum of all ecosystems. It can also be termed the zone of life on Earth, a closed system (apart from solar and cosmic radiation and heat from the interior
of the Earth), and largely self-regulating. By the most general biophysiological definition, the biosphere is the global ecological system integrating all living beings and their relationships, including their interaction with the elements of the lithosphere, geosphere, hydrosphere, and atmosphere. The biosphere is postulated to have evolved, beginning with a process of biopoiesis (life created naturally from non-living matter, such as simple organic compounds) or biogenesis (life created from living matter), at least some 3.5 billion years ago.
In a general sense, biospheres are any closed, self-regulating
systems containing ecosystems. This includes artificial biospheres such
as Biosphere 2 and BIOS-3, and potentially ones on other planets or moons.
Origin and use of the term
A beach scene on Earth, simultaneously showing the lithosphere (ground), hydrosphere (ocean) and atmosphere (air)
The term "biosphere" was coined by geologist Eduard Suess in 1875, which he defined as the place on Earth's surface where life dwells.
Geochemists define the biosphere as being the total sum of living organisms (the "biomass" or "biota"
as referred to by biologists and ecologists). In this sense, the
biosphere is but one of four separate components of the geochemical
model, the other three being geosphere, hydrosphere, and atmosphere. When these four component spheres are combined into one system, it is known as the Ecosphere. This term was coined during the 1960s and encompasses both biological and physical components of the planet.
The Second International Conference on Closed Life Systems defined biospherics as the science and technology of analogs and models of Earth's biosphere; i.e., artificial Earth-like biospheres. Others may include the creation of artificial non-Earth biospheres—for example, human-centered biospheres or a native Martian biosphere—as part of the topic of biospherics.
Earth's biosphere
Age
Stromatolite fossil estimated at 3.2–3.6 billion years old
Every part of the planet, from the polar ice caps to the equator, features life of some kind. Recent advances in microbiology have demonstrated that microbes live deep beneath the Earth's terrestrial surface, and that the total mass of microbial life in so-called "uninhabitable zones" may, in biomass,
exceed all animal and plant life on the surface. The actual thickness
of the biosphere on earth is difficult to measure. Birds typically fly
at altitudes as high as 1,800 m (5,900 ft; 1.1 mi) and fish live as much
as 8,372 m (27,467 ft; 5.202 mi) underwater in the Puerto Rico Trench.
There are more extreme examples for life on the planet: Rüppell's vulture has been found at altitudes of 11,300 m (37,100 ft; 7.0 mi); bar-headed geese migrate at altitudes of at least 8,300 m (27,200 ft; 5.2 mi); yaks live at elevations as high as 5,400 m (17,700 ft; 3.4 mi) above sea level; mountain goats live up to 3,050 m (10,010 ft; 1.90 mi). Herbivorous animals at these elevations depend on lichens, grasses, and herbs.
Life forms live in every part of the Earth's biosphere, including soil, hot springs, inside rocks at least 19 km (12 mi) deep underground, the deepest parts of the ocean, and at least 64 km (40 mi) high in the atmosphere. Microorganisms, under certain test conditions, have been observed to survive the vacuum of outer space. The total amount of soil and subsurface bacterial carbon is estimated as 5 × 1017 g, or the "weight of the United Kingdom". The mass of prokaryote microorganisms—which includes bacteria and archaea, but not the nucleated eukaryote microorganisms—may be as much as 0.8 trillion tons of carbon (of the total biosphere mass, estimated at between 1 and 4 trillion tons). Barophilic marine microbes have been found at more than a depth of 10,000 m (33,000 ft; 6.2 mi) in the Mariana Trench, the deepest spot in the Earth's oceans. In fact, single-celled life forms have been found in the deepest part of the Mariana Trench, by the Challenger Deep, at depths of 11,034 m (36,201 ft; 6.856 mi).
Other researchers reported related studies that microorganisms thrive
inside rocks up to 580 m (1,900 ft; 0.36 mi) below the sea floor under
2,590 m (8,500 ft; 1.61 mi) of ocean off the coast of the northwestern United States, as well as 2,400 m (7,900 ft; 1.5 mi) beneath the seabed off Japan. Culturable thermophilic microbes have been extracted from cores drilled more than 5,000 m (16,000 ft; 3.1 mi) into the Earth's crust in Sweden, from rocks between 65–75 °C (149–167 °F). Temperature increases with increasing depth into the Earth's crust.
The rate at which the temperature increases depends on many factors,
including type of crust (continental vs. oceanic), rock type, geographic
location, etc. The greatest known temperature at which microbial life
can exist is 122 °C (252 °F) (Methanopyrus kandleri
Strain 116), and it is likely that the limit of life in the "deep
biosphere" is defined by temperature rather than absolute depth. On 20 August 2014, scientists confirmed the existence of microorganisms living 800 m (2,600 ft; 0.50 mi) below the ice of Antarctica.
According to one researcher, "You can find microbes everywhere —
they're extremely adaptable to conditions, and survive wherever they
are."
Our biosphere is divided into a number of biomes, inhabited by fairly similar flora and fauna. On land, biomes are separated primarily by latitude. Terrestrial biomes lying within the Arctic and Antarctic Circles are relatively barren of plant and animal life, while most of the more populous biomes lie near the equator.
Annual variation
On
land, vegetation appears on a scale from brown (low vegetation) to dark
green (lots of vegetation); at the ocean surface, phytoplankton are
indicated on a scale from purple (low) to yellow (high). This
visualization was created with data from satellites including SeaWiFS,
and instruments including the NASA/NOAA Visible Infrared Imaging
Radiometer Suite and the Moderate Resolution Imaging Spectroradiometer.
Artificial biospheres
Biosphere 2 in Arizona.
Experimental biospheres, also called closed ecological systems,
have been created to study ecosystems and the potential for supporting
life outside the earth. These include spacecraft and the following
terrestrial laboratories:
No
biospheres have been detected beyond the Earth; therefore, the
existence of extraterrestrial biospheres remains hypothetical. The rare Earth hypothesis suggests they should be very rare, save ones composed of microbial life only. On the other hand, Earth analogs may be quite numerous, at least in the Milky Way galaxy, given the large number of planets. Three of the planets discovered orbiting TRAPPIST-1 could possibly contain biospheres. Given limited understanding of abiogenesis, it is currently unknown what percentage of these planets actually develop biospheres.
Based on observations by the Kepler Space Telescope
team, it has been calculated that provided the probability of
abiogenesis is higher than 1 to 1000, the closest alien biosphere should
be within 100 light-years from the Earth.
It is also possible that artificial biospheres will be created during the future, for example on Mars. The process of creating an uncontained system that mimics the function of Earth's biosphere is called terraforming.
A biosignature (sometimes called chemical fossil or molecular fossil) is any substance – such as an element, isotope, molecule, or phenomenon – that provides scientific evidence of past or present life. Measurable attributes of life include its complex physical and chemical structures and also its utilization of free energy and the production of biomass and wastes. Due to its unique characteristics, a biosignature can be interpreted as having been produced by living organisms;
however, it is important that they not be considered definitive
because there is no way of knowing in advance which ones are universal
to life and which ones are unique to the peculiar circumstances of life
on Earth. Nonetheless, life forms are known to shed unique chemicals, including DNA, into the environment as evidence of their presence in a particular location.
The ancient record on Earth provides an opportunity to see what
geochemical signatures are produced by microbial life and how these
signatures are preserved over geologic time. Some related disciplines
such as geochemistry, geobiology, and geomicrobiology often use biosignatures to determine if living organisms are or were present in a sample. These possible biosignatures include: (a) microfossils and stromatolites; (b) molecular structures (biomarkers) and isotopic compositions of carbon, nitrogen and hydrogen in organic matter;
(c) multiple sulfur and oxygen isotope ratios of minerals; and (d)
abundance relationships and isotopic compositions of redox sensitive
metals (e.g., Fe, Mo, Cr, and rare earth elements).
For example, the particular fatty acids measured in a sample can indicate which types of bacteria and archaea live in that environment. Another example are the long-chain fatty alcohols with more than 23 atoms that are produced by planktonicbacteria. When used in this sense, geochemists often prefer the term biomarker. Another example is the presence of straight-chain lipids in the form of alkanes, alcohols an fatty acids with 20-36 carbon atoms in soils or sediments. Peat deposits are an indication of originating from the epicuticular wax of higher plants.
Life processes may produce a range of biosignatures such as nucleic acids, lipids, proteins, amino acids, kerogen-like material and various morphological features that are detectable in rocks and sediments.
Microbes
often interact with geochemical processes, leaving features in the rock
record indicative of biosignatures. For example, bacterial
micrometer-sized pores in carbonate rocks
resemble inclusions under transmitted light, but have distinct size,
shapes and patterns (swirling or dendritic) and are distributed
differently from common fluid inclusions. A potential biosignature is a phenomenon that may have been produced by life, but for which alternate abiotic origins may also be possible.
In astrobiology
Astrobiological exploration is founded upon the premise that biosignatures encountered in space will be recognizable as extraterrestrial life.
The usefulness of a biosignature is determined, not only by the
probability of life creating it, but also by the improbability of
nonbiological (abiotic) processes producing it.
Concluding that evidence of an extraterrestrial life form (past or
present) has been discovered, requires proving that a possible
biosignature was produced by the activities or remains of life.
As with most scientific discoveries, discovery of a biosignature will
require of evidence building up until no other explanation exists.
Possible examples of a biosignature might be complex organic molecules
and/or structures whose formation is virtually unachievable in the
absence of life. For example, cellular and extracellular morphologies, biomolecules in rocks, bio-organic molecular structures, chirality, biogenic minerals,
biogenic stable isotope patterns in minerals and organic compounds,
atmospheric gases, and remotely detectable features on planetary
surfaces, such as photosynthetic pigments, etc.
Categories
In general, biosignatures and habitable environment signatures can be grouped into ten broad categories:
Stable isotope patterns: Isotopic evidence or patterns that require biological processes.
Chemistry: Chemical features that require biological activity.
Organic matter: Organics formed by biological processes.
Microscopic structures and textures: Biologically formed cements, microtextures, microfossils, and films.
Macroscopic physical structures and textures: Structures that indicate microbial ecosystems, biofilms (e.g., stromatolites), or fossils of larger organisms.
Temporal variability: Variations in time of atmospheric gases,
reflectivity, or macroscopic appearance that indicate the presence of
life.
Surface reflectance features: Large-scale reflectance features due to biological pigments, which could be detected remotely.
Atmospheric gases: Gases formed by metabolic and/or aqueous processes, which may be present on a planet-wide scale.
Technosignatures: Signatures that indicate a technologically advanced civilization.
Chemical
No
single compound will prove life once existed. Rather, it will be
distinctive patterns present in any organic compounds showing a process
of selection. For example, membrane lipids
left behind by degraded cells will be concentrated, have a limited size
range, and comprise an even number of carbons. Similarly, life only
uses left-handed amino acids. Biosignatures need not be chemical, however, and can also be suggested by a distinctive magnetic biosignature.
On Mars, surface oxidants and UV radiation will have altered or destroyed organic molecules at or near the surface. One issue that may add ambiguity in such a search is the fact that, throughout Martian history, abiogenic organic-rich chondritic meteorites have undoubtedly rained upon the Martian surface. At the same time, strong oxidants in Martian soil along with exposure to ionizing radiation might alter or destroy molecular signatures from meteorites or organisms. An alternative approach would be to seek concentrations of buried crystalline minerals, such as clays and evaporites, which may protect organic matter from the destructive effects of ionizing radiation and strong oxidants.
The search for Martian biosignatures has become
more promising due to the discovery that surface and near-surface
aqueous environments existed on Mars at the same time when biological
organic matter was being preserved in ancient aqueous sediments on
Earth.
Morphology
Some researchers suggested that these microscopic structures on the Martian ALH84001 meteorite could be fossilized bacteria.
Another possible biosignature might be morphology
since the shape and size of certain objects may potentially indicate
the presence of past or present life. For example, microscopic magnetite crystals in the Martian meteoriteALH84001
were the longest-debated of several potential biosignatures in that
specimen because it was believed until recently that only bacteria could
create crystals of their specific shape. For example, the possible biomineral studied in the Martian ALH84001 meteorite includes putative microbial fossils,
tiny rock-like structures whose shape was a potential biosignature
because it resembled known bacteria. Most scientists ultimately
concluded that these were far too small to be fossilized cells.
A consensus that has emerged from these discussions, and is now seen as
a critical requirement, is the demand for further lines of evidence in
addition to any morphological data that supports such extraordinary
claims.
Currently, the scientific consensus is that "morphology alone cannot be
used unambiguously as a tool for primitive life detection." Interpretation of morphology is notoriously subjective, and its use alone has led to numerous errors of interpretation.
Atmospheric properties and composition
Methane (CH4) on Mars - potential sources and sinks.
The atmospheric properties of exoplanets are of particular
importance, as atmospheres provide the most likely observables for the
near future, including habitability indicators and biosignatures. Over
billions of years, the processes of life on a planet would result in a
mixture of chemicals unlike anything that could form in an ordinary
chemical equilibrium. For example, large amounts of oxygen and small amounts of methane are generated by life on Earth.
Also, an exoplanet's color —or reflectance spectrum— might give away the presence of vast colonies of life forms at its surface.
The presence of methane in the atmosphere of Mars indicates that there must be an active source on the planet, as it is an unstable gas.
Furthermore, current photochemical models cannot explain the presence
of methane in the atmosphere of Mars and its reported rapid variations
in space and time. Neither its fast appearance nor disappearance can be
explained yet.[26] To rule out a biogenic origin for the methane, a future probe or lander hosting a mass spectrometer will be needed, as the isotopic proportions of carbon-12 to carbon-14 in methane could distinguish between a biogenic and non-biogenic origin, similarly to the use of the δ13C standard for recognizing biogenic methane on Earth. 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. According to the scientists, "...low H2/CH4 ratios (less than approximately 40) indicate that life is likely present and active." The planned ExoMars Trace Gas Orbiter, launched in March 2016 to Mars, will study atmospheric trace gases and will attempt to characterize potential biochemical and geochemical processes at work.
Other scientists have recently reported methods of detecting hydrogen and methane in extraterrestrial atmospheres. Habitability indicators and biosignatures must be interpreted within a planetary and environmental context.
For example, the presence of oxygen and methane together could indicate
the kind of extreme thermochemical disequilibrium generated by life. Two of the top 14,000 proposed atmospheric biosignatures are dimethyl sulfide and chloromethane (CH 3Cl). An alternative biosignature is the combination of methane and carbon dioxide.
Indirect evidence
Scientific observations include the possible identification of biosignatures through indirect observation. For example, electromagnetic information through infrared radiation telescopes, radio-telescopes, space telescopes, etc. From this discipline, the hypothetical electromagnetic radio signatures that SETI
scans for would be a biosignature, since a message from intelligent
aliens would certainly demonstrate the existence of extraterrestrial
life.
The Viking missions
to Mars in the 1970s conducted the first experiments which were
explicitly designed to look for biosignatures on another planet. Each
of the two Viking landers carried three life-detection experiments which looked for signs of metabolism; however, the results were declared inconclusive.
Mars Science Laboratory
The Curiosity rover from the Mars Science Laboratory mission, with its Curiosity rover is currently assessing the potential past and present habitability of the Martian environment and is attempting to detect biosignatures on the surface of Mars.
Considering the MSL instrument payload package, the following classes
of biosignatures are within the MSL detection window: organism
morphologies (cells, body fossils, casts), biofabrics (including
microbial mats), diagnostic organic molecules, isotopic signatures,
evidence of biomineralization and bioalteration, spatial patterns in
chemistry, and biogenic gases. The Curiosity rover targets outcrops to maximize the probability of detecting 'fossilized' organic matter preserved in sedimentary deposits.
ExoMars rover
The 2016 ExoMars Trace Gas Orbiter (TGO) is a Mars telecommunications orbiter and atmospheric gas analyzer mission. It delivered the Schiaparelli EDM lander and then began to settle into its science orbit to map the sources of methane on Mars and other gases, and in doing so, will help select the landing site for the ExoMars rover to be launched in 2020.
The primary objective of the ExoMars rover mission is the search for
biosignatures on the surface and subsurface by using a drill able to
collect samples down to a depth of 2 metres (6.6 ft), away from the
destructive radiation that bathes the surface.
Mars 2020 Rover
The Mars 2020 rover, planned to launch in 2020, is intended to investigate an astrobiologically relevant ancient environment on Mars, investigate its surface geological processes and history, including the assessment of its past habitability, the possibility of past life on Mars, and potential for preservation of biosignatures within accessible geological materials. In addition, it will cache the most interesting samples for possible future transport to Earth.
Titan Dragonfly
The planned Dragonfly lander/aircraft to launch in 2025, would seek evidence of biosignatures on the organic-rich surface and atmosphere of Titan, as well as study its possible prebiotic primordial soup.
Schematic of the Viking Lander Biological Experiment System
The two Viking landers each carried four types of biologicalexperiments to the surface of Mars in 1976. These were the first Mars landers to carry out experiments to look for biosignatures of microbial 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.
The experiments
Four
experiments are presented here in the order in which they were carried
out by the two Viking landers. The biology team leader for the Viking
program was Harold P. Klein (NASA Ames).
Gas chromatograph — mass spectrometer
A gas chromatograph — mass spectrometer (GCMS) is a device that separates vapor components chemically via a gas chromatograph and then feeds the result into a mass spectrometer, which measures the molecular weight of each chemical. As a result, it can separate, identify, and quantify a large number of different chemicals. The GCMS (PI: Klaus Biemann,
MIT) was used to analyze the components of untreated Martian soil, and
particularly those components that are released as the soil is heated to
different temperatures. It could measure molecules present at a level
of a few parts per billion.
The GCMS measured no significant amount of organic molecules in the Martian soil. In fact, Martian soils were found to contain less carbon than lifeless lunar soils returned by the Apollo program.
This result was difficult to explain if Martian bacterial metabolism
was responsible for the positive results seen by the Labeled Release
experiment (see below). A 2011 astrobiology
textbook notes that this was the decisive factor due to which "For most
of the Viking scientists, the final conclusion was that the Viking missions failed to detect life in the Martian soil."
Experiments conducted in 2008 by the Phoenix lander discovered the presence of perchlorate
in Martian soil. The 2011 astrobiology textbook discusses the
importance of this finding with respect to the results obtained by Viking
as "while perchlorate is too poor an oxidizer to reproduce the LR
results (under the conditions of that experiment perchlorate does not
oxidize organics), it does oxidize, and thus destroy, organics at the
higher temperatures used in the Viking GCMS experiment. NASA
astrobiologist Chris McKay has estimated, in fact, that if Phoenix-like
levels of perchlorates were present in the Viking samples, the organic
content of the Martian soil could have been as high as 0.1% and still
would have produced the (false) negative result that the GCMS returned.
Thus, while conventional wisdom regarding the Viking biology
experiments still points to "no evidence of life", recent years have
seen at least a small shift toward "inconclusive evidence"."
According to a 2010 NASA press release: "The only organic
chemicals identified when the Viking landers heated samples of Martian
soil were chloromethane and dichloromethane
-- chlorine compounds interpreted at the time as likely contaminants
from cleaning fluids." According to a paper authored by a team led by Rafael Navarro-González of the National Autonomous University of Mexico,
"those chemicals are exactly what [their] new study found when a little
perchlorate -- the surprise finding from Phoenix -- was added to desert
soil from Chile containing organics and analyzed in the manner of the
Viking tests." However, the 2010 NASA press release also noted that:
"One reason the chlorinated organics found by Viking were interpreted as
contaminants from Earth was that the ratio of two isotopes of chlorine
in them matched the three-to-one ratio for those isotopes on Earth. The
ratio for them on Mars has not been clearly determined yet. If it is
found to be much different than Earth's, that would support the 1970s
interpretation." Biemann has written a commentary critical of the Navarro-González and McKay paper, to which the latter have replied; the exchange was published in December 2011.
Gas exchange
The gas exchange (GEX) experiment (PI: Vance Oyama,
NASA Ames) looked for gases given off by an incubated soil sample by
first replacing the Martian atmosphere with the inert gas Helium.
It applied a liquid complex of organic and inorganic nutrients and
supplements to a soil sample, first with just nutrients added, then with
water added too. Periodically, the instrument sampled the atmosphere of the incubation chamber and used a gas chromatograph to measure the concentrations of several gases, including oxygen, CO2, nitrogen, hydrogen, and methane.
The scientists hypothesized that metabolizing organisms would either
consume or release at least one of the gases being measured. The result
was negative.
Labeled release
The labeled release (LR) experiment (PI: Gilbert Levin, Biospherics Inc.) gave the most promise for exobiologists.
In the LR experiment, a sample of Martian soil was inoculated with a
drop of very dilute aqueous nutrient solution. The nutrients (7
molecules that were Miller-Urey products) were tagged with radioactive 14C. The air above the soil was monitored for the evolution of radioactive 14CO2 gas as evidence that microorganisms in the soil had metabolized
one or more of the nutrients. Such a result was to be followed with the
control part of the experiment as described for the PR below. The
result was quite a surprise, considering the negative results of the
first two tests, with a steady stream of radioactive gases being given
off by the soil immediately following the first injection. The
experiment was done by both Viking probes, the first using a sample from
the surface exposed to sunlight and the second probe taking the sample
from underneath a rock; both initial injections came back positive.
Subsequent injections a week later did not, however, elicit the same
reaction, and according to a 1976 paper by Levin and Patricia Ann Straat
the results were inconclusive. In 1997, Levin, Straat and Barry DiGregorio co-authored a book on the issue, titled Mars: The Living Planet.
A CNN
article from 2000 noted that "Though most of his peers concluded
otherwise, Levin still holds that the robot tests he coordinated on the
1976 Viking lander indicated the presence of living organisms on Mars." A 2006 astrobiology
textbook noted that "With unsterilized Terrestrial samples, though, the
addition of more nutrients after the initial incubation would then
produce still more radioactive gas as the dormant bacteria sprang into
action to consume the new dose of food. This was not true of the Martian
soil; on Mars, the second and third nutrient injections did not produce
any further release of labeled gas."
The 2011 edition of the same textbook noted that "Albet Yen of the Jet
Propulsion Laboratory has shown that, under extremely cold and dry
conditions and in a carbon dioxide atmosphere, ultraviolet light
(remember: Mars lacks an ozone layer, so the surface is bathed in
ultraviolet) can cause carbon dioxide to react with soils to produce
various oxidizers, including highly reactive superoxides (salts containing O2−)
When mixed with small organic molecules, superoxidizers readily oxidize
them to carbon dioxide, which may account for the LR result. Superoxide
chemistry can also account for the puzzling results seen when more
nutrients were added to the soil in the LR experiment; because life
multiplies, the amount of gas should have increased when a second or
third batch of nutrients was added, but if the effect was due to a
chemical being consumed in the first reaction, no new gas would be
expected. Lastly, many superoxides are relatively unstable and are
destroyed at elevated temperatures, also accounting for the
"sterilization" seen in the LR experiment."
In a 2002 paper published by Joseph Miller, he speculates that
recorded delays in the system's chemical reactions point to biological
activity similar to the circadian rhythm previously observed in terrestrial cyanobacteria.
On 12 April 2012, an international team including Levin and Straat published a peer reviewed paper suggesting the detection of "extant microbial life on Mars", based on mathematical speculation through cluster analysis of the Labeled Release experiments of the 1976 Viking Mission.
Pyrolytic release
The pyrolytic release (PR) experiment (PI: Norman Horowitz, Caltech) consisted of the use of light, water, and a carbon-containing atmosphere of carbon monoxide (CO) and carbon dioxide (CO2), simulating that on Mars. The carbon-bearing gases were made with carbon-14 (14C), a heavy, radioactiveisotope of carbon. If there were photosynthetic organisms present, it was believed that they would incorporate some of the carbon as biomass through the process of carbon fixation, just as plants and cyanobacteria
on earth do. After several days of incubation, the experiment removed
the gases, baked the remaining soil at 650 °C (1200 °F), and collected
the products in a device which counted radioactivity. If any of the 14C
had been converted to biomass, it would be vaporized during heating and
the radioactivity counter would detect it as evidence for life. Should
a positive response be obtained, a duplicate sample of the same soil
would be heated to "sterilize" it. It would then be tested as a control
and should it still show activity similar to the first response, that
was evidence that the activity was chemical in nature. However, a nil,
or greatly diminished response, was evidence for biology. This same
control was to be used for any of the three life detection experiments
that showed a positive initial result.
Scientific conclusions
Organic
compounds seem to be common, for example, on asteroids, meteorites,
comets and the icy bodies orbiting the Sun, so detecting no trace of any
organic compound on the surface of Mars came as a surprise. The GC-MS
was definitely working, because the controls were effective and it was
able to detect traces of the cleaning solvents that had been used to
sterilize it prior to launch. At the time, the total absence of organic material on the surface made the results of the biology experiments moot, since metabolism
involving organic compounds were what those experiments were designed
to detect. However, the general scientific community surmise that the
Viking's biological tests remain inconclusive.
Most researchers surmise that the results of the Viking biology
experiments can be explained by purely chemical processes that do not
require the presence of life, and the GC-MS results rule out life.
Despite the positive result from the Labeled Release experiment, a
general assessment is that the results seen in the four experiments are
best explained by oxidative chemical reactions with the Martian soil.
One of the current conclusions is that the Martian soil, being
continuously exposed to UV light from the Sun (Mars has no protective ozone layer), has built up a thin layer of a very strong oxidant.
A sufficiently strong oxidizing molecule would react with the added
water to produce oxygen and hydrogen, and with the nutrients to produce carbon dioxide (CO2).
On August 2008, the Phoenix lander detected perchlorate, a strong oxidizer when heated above 200 °C. This was initially thought to be the cause of a false positive LR result. However, results of experiments published in December 2010
propose that organic compounds "could have been present" in the soil
analyzed by both Viking 1 and 2, since NASA's Phoenix lander in 2008
detected perchlorate, which can break down organic compounds. The
study's authors found that perchlorate can destroy organics when heated
and produce chloromethane and dichloromethane
as byproduct, the identical chlorine compounds discovered by both
Viking landers when they performed the same tests on Mars. Because
perchlorate would have broken down any Martian organics, the question of
whether or not Viking found organic compounds is still wide open, as
alternative chemical and biological interpretations are possible.
In 2013, astrobiologist Richard Quinn at the Ames Center
conducted experiments in which perchlorates irradiated with gamma rays
seemed to reproduce the findings of the labeled-release experiment.
He concluded that neither hydrogen peroxide nor superoxide is required
to explain the results of the Viking biology experiments.
Controversy
Before the discovery of the oxidizer perchlorate
on Mars in 2008, some theories remained opposed to the general
scientific conclusion. An investigator suggested that the biological
explanation of the lack of detected organics by GC-MS could be that the
oxidizing inventory of the H2O2-H2O solvent well exceeded the reducing power of the organic compounds of the organisms.
It has also been argued that the Labeled Release (LR) experiment
detected so few metabolising organisms in the Martian soil, that it
would have been impossible for the gas chromatograph to detect them. This view has been put forward by the designer of the LR experiment, Gilbert Levin, who believes the positive LR results are diagnostic for life on Mars.
He and others have conducted ongoing experiments attempting to
reproduce the Viking data, either with biological or non-biological
materials on Earth. While no experiment has ever precisely duplicated
the Mars LR test and control results, experiments with hydrogen peroxide-saturated titanium dioxide have produced similar results.
While the majority of astrobiologists still conclude that the Viking biological experiments were inconclusive or negative, Gilbert Levin
is not alone in believing otherwise. The current claim for life on Mars
is grounded on old evidence reinterpreted in the light of recent
developments.
On 2006, scientist Rafael Navarro demonstrated that the Viking
biological experiments likely lacked sensitivity to detect trace
amounts of organic compounds. In a paper published in December 2010,
the scientists suggest that if organics were present, they would not
have been detected because when the soil is heated to check for
organics, perchlorate destroys them rapidly producing chloromethane and
dichloromethane, which is what the Viking landers found. This team also
notes that this is not a proof of life but it could make a difference in
how scientists look for organic biosignatures in the future. Results from the current Mars Science Laboratory mission and the under-development ExoMars program, may help settle this controversy.
On 2006, Mario Crocco went as far as proposing the creation of a new nomenclatural rank that classified some Viking results as 'metabolic' and therefore representative of a new form of life.
The taxonomy proposed by Crocco has not been accepted by the scientific
community, and the validity of Crocco's interpretation hinged entirely
on the absence of an oxidative agent in the Martian soil.
Critiques
James Lovelock
argued that the Viking mission would have done better to examine the
Martian atmosphere than look at the soil. He theorised that all life
tends to expel waste gases into the atmosphere, and as such it would be
possible to theorise the existence of life on a planet by detecting an
atmosphere that was not in chemical equilibrium.
He concluded that there was enough information about Mars' atmosphere
at that time to discount the possibility of life there. Since then, methane has been discovered in Mars' atmosphere at 10ppb, thus reopening this debate. Although in 2013 the Curiosity rover failed to detect methane at its location in levels exceeding 1.3ppb. later in 2013 and in 2014, measurements by Curiosity did detect methane, suggesting a time-variable source. The planned ExoMars Trace Gas Orbiter,
launched on March 2016, will implement this approach and will focus on
detection, characterization of spatial and temporal variation, and
localization of sources for a broad suite of atmospheric trace gases on
Mars and help determine if their formation is of biological or
geological origin. The Mars Orbiter Mission
is also attempting —since late 2014— to detect and map methane on Mars'
atmosphere. A press commentary argued that, if there was life at the
Viking lander sites, it may have been killed by the exhaust from the
landing rockets. That is not a problem for missions which land via an airbag-protected
capsule, slowed by parachutes and retrorockets, and dropped from a
height that allows rocket exhaust to avoid the surface. Mars Pathfinder's Sojourner rover and the Mars Exploration Rovers each used this landing technique successfully. The Phoenix Scout lander descended to the surface with retro-rockets, however, their fuel was hydrazine,
and the end products of the plume (water, nitrogen, and ammonia) were
not found to have affected the soils at the landing site.
Future missions
Urey design
The question of life on Mars
will probably not be resolved entirely until future missions to Mars
either conclusively demonstrate the presence of life on the planet,
identify the chemical(s) responsible for the Viking results, or both.
The Mars Science Laboratory mission landed the Curiosity rover on August 6, 2012, and its goals include investigation of the Martian climate, geology, and whether Mars could have ever supported life, including investigation of the role of water and planetary habitability. Astrobiology research on Mars will continue with the ExoMars Trace Gas Orbiter in 2016, ExoMars rover on 2018, and the Mars 2020 rover in 2020.
The Urey instrument was a funded study for sensitive organic compound detector, but has not been sent to Mars but was considered for ExoMars program of the 2000s
Proposed missions
The Biological Oxidant and Life Detection (BOLD) is a proposed Mars mission that would follow up the Viking soil tests by using several small impact landers. Another proposal is the Phoenix lander-based Icebreaker Life.
