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Thursday, September 27, 2018

Biosignature

From Wikipedia, the free encyclopedia

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.

In geomicrobiology

Electron micrograph of microfossils from a sediment core obtained by the Deep Sea Drilling Program

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 planktonic bacteria. 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:
  1. Stable isotope patterns: Isotopic evidence or patterns that require biological processes.
  2. Chemistry: Chemical features that require biological activity.
  3. Organic matter: Organics formed by biological processes.
  4. Minerals: Minerals or biomineral-phases whose composition and/or morphology indicate biological activity (e.g., biomagnetite).
  5. Microscopic structures and textures: Biologically formed cements, microtextures, microfossils, and films.
  6. Macroscopic physical structures and textures: Structures that indicate microbial ecosystems, biofilms (e.g., stromatolites), or fossils of larger organisms.
  7. Temporal variability: Variations in time of atmospheric gases, reflectivity, or macroscopic appearance that indicate the presence of life.
  8. Surface reflectance features: Large-scale reflectance features due to biological pigments, which could be detected remotely.
  9. Atmospheric gases: Gases formed by metabolic and/or aqueous processes, which may be present on a planet-wide scale.
  10. 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 meteorite ALH84001 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.

Robotic surface missions

The Viking missions to Mars
Carl Sagan with a model of the Viking lander

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.
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Viking lander biological experiments

From Wikipedia, the free encyclopedia
 
Schematic of the Viking Lander Biological Experiment System

The two Viking landers each carried four types of biological experiments 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, radioactive isotope 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.

In 2008, the Thermal and Evolved Gas Analyzer was operated at Mars, which could chemically analyze 8 samples.

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.
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ExoMars

From Wikipedia, the free encyclopedia
 
ExoMars
ЭкзоМарс
Image depicting the three spacecraft of the mission, an orbiter at left, lander at center, and rover at right, against a Martian landscape and sky.
Artist's illustration of ExoMars' Trace Gas Orbiter (left), Schiaparelli lander (middle), and rover (right)
Mission type Mars reconnaissance
Operator ESA · RFSA
Website exploration.esa.int/mars (ESA)
exomars.cosmos.ru (RFSA)
Mission duration Elapsed: 2 years, 6 months and 12 days
ExoMars insignia.png
ExoMars ESA mission insignia

ExoMars (Exobiology on Mars) is a two-part astrobiology project to search for evidence of life on Mars, a joint mission of the European Space Agency (ESA) and the Russian space agency Roscosmos. The first part, launched in 2016, placed a trace gas research and communication satellite into Mars orbit and released a stationary experimental lander (which crashed). The second part is planned to launch in 2020, and to land the ExoMars rover on the surface, supporting a science mission that is expected to last into 2022 or beyond.

ExoMars goals are to search for signs of past life on Mars, investigate how the Martian water and geochemical environment varies, investigate atmospheric trace gases and their sources and by doing so demonstrate the technologies for a future Mars sample return mission. The mission will search for ancient biosignatures of Martian life, employing several spacecraft elements to be sent to Mars on two launches.

The ExoMars Trace Gas Orbiter (TGO) and a test stationary lander called Schiaparelli were launched on 14 March 2016. TGO entered Mars orbit on 19 October 2016 and will proceed to map the sources of methane (CH
4) and other trace gases present in the Martian atmosphere that could be evidence for possible biological or geological activity. The TGO features four instruments and will also act as a communications relay satellite. The Schiaparelli experimental lander separated from TGO on 16 October and was maneuvered to land in Meridiani Planum, but it crashed on the surface of Mars. The landing was designed to test new key technologies to safely deliver the 2020 rover mission.
In 2020, a Roscosmos-built lander (ExoMars 2020 surface platform) is to deliver the ESA-built ExoMars Rover to the Martian surface. The rover will also include some Roscosmos built instruments. The second mission operations and communications will be led by ALTEC's Rover Control Centre in Italy.

History

An ExoMars rover as an exhibit at Gasometer Oberhausen, Germany

Since its inception, ExoMars has gone through several phases of planning with various proposals for landers, orbiters, launch vehicles, and international cooperation planning, such as the defunct 2009 Mars Exploration Joint Initiative (MEJI) with the United States. Originally, the ExoMars concept consisted of a large robotic rover being part of ESA's Aurora Programme as a Flagship mission and was approved by the European Space Agency ministers in December 2005. Originally conceived as a rover with a stationary ground station, ExoMars was planned to launch in 2011 aboard a Russian Soyuz Fregat rocket.

ExoMars begun in 2001 as part of the ESA Aurora program for the human exploration of Mars. That initial vision called for rover in 2009 and later a sample return mission. Another mission intended to support the Aurora program is a Phobos sample return mission. In December 2005, the different nations composing the ESA gave approval to the Aurora program and to ExoMars. Aurora is an optional program and each state is allowed to decide which part of the program they want to be involved in and to what extent (e.g. how much funds they want to put into the program). The Aurora program was initiated in 2002 with support of twelve nations: Austria, Belgium, France, Germany, Italy, the Netherlands, Portugal, Spain, Sweden, Switzerland, the United Kingdom and Canada.

In 2007, Canadian-based technology firm MacDonald Dettwiler and Associates Ltd. (MDA) was selected for a one-million-euro contract with EADS Astrium of Britain to design and build a prototype Mars rover chassis for the European Space Agency. Astrium was also contracted to design the final rover.

On July 2009 NASA and ESA signed the Mars Exploration Joint Initiative, which proposed to utilise an Atlas rocket launcher instead of a Soyuz, which significantly altered the technical and financial setting of the ExoMars mission. On 19 June, when the rover was still planned to piggyback the Mars Trace Gas Orbiter, it was reported that a prospective agreement would require that ExoMars lose enough weight to fit aboard the Atlas launch vehicle with a NASA orbiter.

Mars Astrobiology Explorer-Cacher (MAX-C) rover

Then the mission was combined with other projects to a multi-spacecraft mission divided over two Atlas V-launches: the ExoMars Trace Gas Orbiter (TGO) was merged into the project, piggybacking a stationary meteorological lander slated for launch in January 2016. It was also proposed to include a second rover, the MAX-C.

In August 2009 it was announced that the Russian Federal Space Agency (now Roscosmos) and ESA had signed a contract that included cooperation on two Mars exploration projects: Russia's Fobos-Grunt project and ESA's ExoMars. Specifically, ESA secured a Russian Proton rocket as a "backup launcher" for the ExoMars rover, which would include Russian-made parts.

On 17 December 2009, the ESA governments gave their final approval to a two-part Mars exploration mission to be conducted with NASA, confirming their commitment to spend €850 million ($1.23 billion) on missions in 2016 and 2018.

In April 2011, because of a budgeting crisis, a proposal was announced to cancel the accompanying MAX-C rover, and fly only one rover in 2018 that would be larger than either of the vehicles in the paired concept. One suggestion was that the new vehicle would be built in Europe and carry a mix of European and U.S. instruments. NASA would provide the rocket to deliver it to Mars and provide the sky crane landing system. Despite the proposed reorganisation, the goals of the 2018 mission opportunity would have stayed broadly the same.

Under the FY2013 Budget President Obama released on 13 February 2012, NASA terminated its participation in ExoMars due to budgetary cuts in order to pay for the cost overruns of the James Webb Space Telescope. With NASA's funding for this project completely cancelled, most of these plans had to be restructured.

On 14 March 2013, representatives of the ESA and the Russian space agency (Roscosmos), signed a deal in which Russia became a full partner. Roscosmos will supply both missions with Proton launch vehicles with Briz-M upper stages and launch services, as well as an additional entry, descent and landing module for the rover mission in 2018. Under the agreement, Roscosmos was granted three asking conditions:
  1. Roscosmos will contribute two Proton launch vehicles as payment for the partnership.
  2. The Trace Gas Orbiter payload shall include two Russian instruments that were originally developed for Fobos-Grunt.
  3. All scientific results must be intellectual property of the European Space Agency and the Russian Academy of Sciences (i.e. Roscosmos will have full access to research data).
ESA had originally cost-capped the ExoMars projects at €1 billion, (USD 1.3 billion) but the withdrawal of the U.S. space agency (NASA) and the consequent reorganisation of the ventures will probably add several hundred million euros to the sum so far raised. So on March 2012, member states instructed the agency's executive to look at how this shortfall could be made up. One possibility is that other science activities within ESA may have to step back to make ExoMars a priority. On September 2012 it was announced that new ESA members, Poland and Romania will be contributing up to €70 million to the ExoMars mission. ESA has not ruled out a possible partial return of NASA to the 2018 portion of ExoMars, albeit in a relatively minor role.

Russia's financing of ExoMars could be partially covered by insurance payments of 1.2 billion rubles ($40.7 million USD) for the loss of Fobos-Grunt, and reassigning funds for a possible coordination between the Mars-NET and ExoMars projects. On 25 January 2013, Roscosmos fully funded the development of the scientific instruments to be flown on the first launch, the Trace Gas Orbiter (TGO).

As of March 2014, the lead builder of the ExoMars rover, the British division of Airbus Defence and Space, had started procuring critical components, but the 2018 rover mission was still short by more than 100 million euros, or $138 million. The wheels and suspension system are paid by the Canadian Space Agency and are being manufactured by MDA Corporation in Canada.

Status

A prototype of the ExoMars Rover at the 2015 Cambridge Science Festival

In January 2016 it was announced that the financial situation of the 2018 mission 'might' require a 2-year delay. Italy is the largest contributor to ExoMars, and the UK is the mission's second-largest financial backer.

The rover was scheduled to launch in 2018 and land on Mars in early 2019, but in May 2016 ESA announced that the launch would occur in 2020 due to delays in European and Russian industrial activities and deliveries of the scientific payload.

2016 first spacecraft launch

The spacecraft containing ExoMars Trace Gas Orbiter (TGO) and Schiaparelli launched on 14 March 2016 (Livestream began at 08:30 GMT [03:30 AM EDT]). Four rocket burns occurred in the following 10 hours before the descent module and orbiter were released. Signal from the Orbiter was successfully received at 21:29 GMT of the same day, which confirmed that the launch was fully successful and the spacecraft is on its way to Mars. Shortly after separation from the probes, the Briz-M upper booster stage possibly exploded a few kilometers away, however apparently without damaging the orbiter or lander. The spacecraft, which housed the Trace Gas Orbiter and the Schiaparelli lander, took its nominal orbit towards Mars and was seemingly in working order. Over the next two weeks, controllers continued to check and commission its systems, including the power, communications, startrackers, and guidance and navigation system.

Mission objectives

The scientific objectives, in order of priority, are:
  • to search for possible biosignatures of past Martian life.
  • to characterise the water and geochemical distribution as a function of depth in the shallow subsurface.
  • to study the surface environment and identify hazards to future manned missions to Mars.
  • to investigate the planet's subsurface and deep interior to better understand the evolution and habitability of Mars.
  • achieve incremental steps ultimately culminating in a sample return flight.
The technological objectives to develop are:
  • landing of large payloads on Mars.
  • to exploit solar electric power on the surface of Mars.
  • to access the subsurface with a drill able to collect samples down to a depth of 2 metres (6.6 ft)
  • to develop surface exploration capability using a rover.

Mission profile

ExoMars is a joint programme of the European Space Agency (ESA) and the Russian space agency Roscosmos. According to current plans, the ExoMars project will comprise four spacecraft: two stationary landers, one orbiter and one rover. All mission elements will be sent in two launches using two heavy-lift Proton rockets.

Contributing agency First launch in 2016 Second launch in 2020
Roscosmos logo ru.svg Proton rocket Proton rocket
Two instrument packages for the TGO Russian-built landing system and surface science platform will deliver the rover to the surface. Russia will provide various scientific instruments for the lander and rover.
ESA ExoMars Trace Gas Orbiter ExoMars rover, and various scientific instruments on the rover
Schiaparelli EDM lander

The two landing modules and the rover will be sterilised in order not to contaminate the planet with Earth life forms. Cleaning will require a combination of sterilising methods, including ionising radiation, UV radiation, and chemicals such as ethyl and isopropyl alcohol.

First launch (2016)

Trace Gas Orbiter

The Trace Gas Orbiter (TGO) is a Mars telecommunications orbiter and atmospheric gas analyzer mission that was launched on 14 March 2016. The spacecraft arrived in the Martian orbit in October 2016. It delivered the ExoMars Schiaparelli EDM lander and then proceed to map the sources of methane on Mars and other gases, and in doing so, help select the landing site for the ExoMars rover to be launched in 2020. The presence of methane in Mars' atmosphere is intriguing because its likely origin is either present-day life or geological activity. Upon the arrival of the rover in 2021, the orbiter would be transferred into a lower orbit where it would be able to perform analytical science activities as well as provide the Schiaparelli EDM lander and ExoMars rover with telecommunication relay. NASA provided an Electra telecommunications relay and navigation instrument to ensure communications between probes and rovers on the surface of Mars and controllers on Earth. The TGO would continue serving as a telecommunication relay satellite for future landed missions until 2022.

Schiaparelli EDM lander

Model of the ExoMars Schiaparelli EDL Demonstrator Module (EDM). During its descent it returned 600 MB of data, but it did not achieve a soft landing.

The Entry, Descent and Landing Demonstrator Module (EDM) called Schiaparelli, was intended to provide the European Space Agency (ESA) and Russia's Roscosmos with the technology for landing on the surface of Mars. It was launched together with the ExoMars Trace Gas Orbiter (TGO) on 14 March 2016 and was scheduled to land softly on 19 October 2016. No signal indicating a successful landing was received, and on 21 October 2016 NASA released a Mars Reconnaissance Orbiter image showing what appears to be the lander crash site. The lander was equipped with a non-rechargeable electric battery with enough power for four sols. The soft landing should have taken place on Meridiani Planum during the dust storm season, which would have provided a unique chance to characterise a dust-loaded atmosphere during entry and descent, and to conduct surface measurements associated with a dust-rich environment.

Once on the surface, it was to measure the wind speed and direction, humidity, pressure and surface temperature, and determine the transparency of the atmosphere. It carried a surface payload, based on the proposed meteorological DREAMS (Dust Characterisation, Risk Assessment, and Environment Analyser on the Martian Surface) package, consists of a suite of sensors to measure the wind speed and direction (MetWind), humidity (MetHumi), pressure (MetBaro), surface temperature (MarsTem), the transparency of the atmosphere (Optical Depth Sensor; ODS), and atmospheric electrification (Atmospheric Radiation and Electricity Sensor; MicroARES). The DREAMS payload was to function for 2 or 3 days as an environmental station for the duration of the EDM surface mission after landing.

Second launch (2020)

Russian landing system

The second mission, scheduled for launch in July 2020, will have an 1800 kg Russian-built landing platform system derived from the 2016 Schiaparelli EDM lander, to place the ExoMars rover on the surface of Mars. This lander platform will be built 80% by the Russian company Lavochkin, and 20% by ESA. Lavochkin will produce most of the landing system's hardware, while ESA will handle elements such as the guidance, radar and navigation systems. Lavochkin's current landing strategy is to use two parachutes; one will open while the module is still moving at supersonic speed, and another will deploy once the probe has been slowed down to subsonic velocity. The heat shield will eventually fall away from the entry capsule to allow the ExoMars rover, riding its retro-rocket-equipped lander, to come for a soft landing on legs or struts. The surface platform lander will then deploy ramps for the rover to drive down.

Critics have stated that while Russian expertise may be sufficient to provide a launch vehicle, it does not currently extend to the critical requirement of a landing system for Mars.

Surface platform

After landing on Mars in 2021, the rover will descend from the platform via a ramp. The platform is expected to image the landing site, monitor the climate, investigate the atmosphere, analyse the radiation environment, study the distribution of any subsurface water at the landing site, and perform geophysical investigations of the internal structure of Mars. Following a March 2015 request for the contribution of scientific instruments for the landing system, there will be four instruments; the two European-led instruments selected are:
  • the Lander Radioscience experiment (LaRa) will study the internal structure of Mars, and will make precise measurements of the rotation and orientation of the planet by monitoring two-way Doppler frequency shifts between the surface platform and Earth. It will also detect variations in angular momentum due to the redistribution of masses, such as the migration of ice from the polar caps to the atmosphere.
  • the HABIT (HabitAbility: Brine, Irradiation and Temperature) package will investigate the amount of water vapour in the atmosphere, daily and seasonal variations in ground and air temperatures, and the UV radiation environment.
  • two Russian-led instruments will monitor pressure and humidity, UV radiation and dust, the local magnetic field and plasma environment.
The platform is expected to operate for at least one Earth year, and its instruments might be powered by a radioisotope thermoelectric generator to provide long-term power.

Rover

An early design ExoMars rover test model at the
ILA 2006 in Berlin
 
The ExoMars rover will land in 2021 and is designed to navigate autonomously across the Martian surface.

















Instrumentation will consist of the exobiology laboratory suite, known as "Pasteur analytical laboratory" to look for signs of biomolecules and biosignatures from past life. Among other instruments, the rover will also carry a 2-metre (6.6 ft) sub-surface core drill to pull up samples for its on-board laboratory. The rover will have a mass of about 207 kg (456 lb).

The ExoMar's rover includes the Pasteur instrument suite, including the Mars Organic Molecule Analyzer (MOMA), MicrOmega-IR, and the Raman Laser Spectrometer (RLS). Examples of external instruments on the rover include:

Landing site selection

Oxia Planum, near the equator, is the selected landing site for its potential to preserve biosignatures and smooth surface

A primary goal when selecting the rover's landing site is to identify a particular geologic environment, or set of environments, that would support —now or in the past— microbial life. The scientists prefer a landing site with both morphologic and mineralogical evidence for past water. Furthermore, a site with spectra indicating multiple hydrated minerals such as clay minerals is preferred, but it will come down to a balance between engineering constraints and scientific goals.
Engineering constraints call for a flat landing site in a latitude band straddling the equator that is only 30° latitude from top to bottom because the rover is solar-powered and will need best sunlight exposure. The landing module carrying the rover will have a landing ellipse that measures about 105 km by 15 km. Scientific requirements include landing in an area with 3.6 billion years old sedimentary rocks that are a record of the past wet habitable environment. The year before launch, the European Space Agency will make the final decision. By March 2014, the long list was:

Following additional review by an ESA-appointed panel, four sites, all of which are located relatively near the equator, were formally recommended in October 2014 for further detailed analysis:


On 21 October 2015, Oxia Planum was reported to be the preferred landing site for the ExoMars rover.

The delay of the rover mission to 2020 from 2018 meant that Oxia Planum was no longer the only favourable landing site due to changes in the possible landing ellipse. Both Mawrth Vallis and Aram Dorsum, surviving candidates from the previous selection, could be reconsidered. ESA convened further workshops to re-evaluate the three remaining options and in March 2017 selected two sites to study in detail;

The final selection is scheduled to occur approximately a year before launch.
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