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8.2B: Martian Biosignatures - Biology

8.2B: Martian Biosignatures - Biology


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Learning Objectives

  • Describe biosignatures

A biosignature is any substance – such as an element, isotope, molecule, or phenomenon – that provides scientific evidence of past or present life. It is important to understand that while the presence of these substances or events could be a result of past or present life, they are not definitive evidence and should not be treated as such. Scientists determine the significance of a biosignature not only by examining the probability of life creating it, but mostly by the improbability of abiotic processes producing it.

Martian Biosignatures

On Earth, normal mammalian functioning has produced a fog of chemicals that is not replicated by any chemical process. This fog is made up of large amounts of oxygen and small amounts of methane. This mixture of gases has also been observed in the atmosphere of the planet Mars. Due to scientific thought that this fog cannot be formed by a chemical process, logic concludes that there must be some source of life on the planet.

Scientists feel it is necessary to explore their hypotheses, so in the 1970s there were two American probes called Viking I and II that were sent to Mars to explore for life. The probes took images of the planet while in orbit and also while actually on the surface of Mars. The Viking landers carried three life-detection experiments that looked for signs of metabolism. Unfortunately, the imaging and life-detection results were inconclusive. There are plans for future missions to Mars, the Mars Science Laboratory and ExoMars, which will not only search for biosignatures but try to detect habitable environments as well.

Key Points

  • A biosignature is any substance – such as an element, isotope, molecule, or phenomenon – that provides scientific evidence of past or present life.
  • On Earth, normal mammalian functioning has produced a fog of chemicals that is not replicated by any chemical process. This mixture of gases has also been observed in the atmosphere of the planet Mars.
  • In the 1970s there were two American probes called Viking I and II that were sent to Mars to explore the planet for life. The Viking landers carried three life-detection experiments that looked for signs of metabolism, but the imaging and life-detection results were inconclusive.
  • There are plans for future missions to Mars to search for more evidence of biosignatures and habitable environments for life.

Key Terms

  • biosignature: Any measurable phenomenon that indicates the presence of life.
  • metabolism: The complete set of chemical reactions that occur in living cells.
  • abiotic: Nonliving, inanimate, characterized by the absence of life; of inorganic matter.

Hungarian Researchers Claim to See Biosignatures in Martian Meteorite

ALH-77005, a Martian rock found in Antarctica, contains numerous mineralized ‘biosignatures,’ including coccoidal, filamentous structures and organic material, according to a team of scientists from Hungary.

An artist’s impression of habitable Mars. Image credit: Daein Ballard / CC BY-SA 3.0.

“Our work is important to a broad audience because it integrates planetary, earth, biological, chemical, and environmental sciences and will be of interest to many researchers in those fields,” said team leader Dr. Ildiko Gyollai, a researcher at the HAS Research Centre for Astronomy and Earth Sciences in Budapest.

“The research will also be of interest to planetologists, experts of meteorite and astrobiology as well as researchers of the origin of life, and to the general public since it offers an example of a novel aspect of microbial mediation in stone meteorites.”

Thin section of ALH-77005: poikilitic texture of olivine with pyroxene cumulate grains, the studied melt pocket (rectangle) mostly composed of olivine. Image credit: Gyollai et al, doi: 10.1515/astro-2019-0002.

The achondrite meteorite ALH-77005 was found partially imbedded in the ice at the Allan Hills site in South Victoria Land during the Japanese National Institute of Polar Research mission in 1977-1978.

It had a rounded shape and its surface was partially ablated and roughly-polished by wind-blown ice.

Its age is estimated to be about 175 million years, with an exposure to cosmic rays of about 3 million years.

Thin section of ALH-77005 in plane polarized light: the area studied by FTIR spectroscopy is marked by rectangle, where the strong putative microbially mediated alteration was observed. Image credit: Gyollai et al, doi: 10.1515/astro-2019-0002.

Dr. Gyollai and co-authors analyzed a thin section of ALH-77005 by optical and FTIR-ATR microscopy.

They were able to detect the presence of coccoidal and filamentous structures (probably built by iron-oxidizing microbes) organic material biogenic minerals, like ferrihydrite, goethite, and hematite.

“The other signatures for biogenicity of ALH-77005 are strong negative δ13C, enrichment of iron, manganese, phosphorus, zinc in shock melt support scenario,” the researchers said.

“Our study proposes presence of microbial mediation on Mars.”

The team’s paper was published online in the journal Open Astronomy.

Ildikó Gyollai et al. 2019. Mineralized biosignatures in ALH-77005 Shergottite – Clues to Martian Life? Open Astronomy 28 (1): 32-39 doi: 10.1515/astro-2019-0002


Contents

The term was first proposed by the Russian (Soviet) astronomer Gavriil Tikhov in 1953. [28] Astrobiology is etymologically derived from the Greek ἄστρον , astron, "constellation, star" βίος , bios, "life" and -λογία , -logia, study. The synonyms of astrobiology are diverse however, the synonyms were structured in relation to the most important sciences implied in its development: astronomy and biology. A close synonym is exobiology from the Greek Έξω , "external" Βίος, bios, "life" and λογία, -logia, study. The term exobiology was coined by molecular biologist and Nobel Prize winner Joshua Lederberg. [29] Exobiology is considered to have a narrow scope limited to search of life external to Earth, whereas subject area of astrobiology is wider and investigates the link between life and the universe, which includes the search for extraterrestrial life, but also includes the study of life on Earth, its origin, evolution and limits.

Another term used in the past is xenobiology, ("biology of the foreigners") a word used in 1954 by science fiction writer Robert Heinlein in his work The Star Beast. [31] 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. [32]

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. [33] [34] 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. [35] 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. [2] [36] In 1971, NASA funded the search for extraterrestrial intelligence (SETI) to search radio frequencies of the electromagnetic spectrum for interstellar communications transmitted by extraterrestrial life outside the Solar System. NASA's Viking missions to Mars, launched in 1976, included three biology experiments designed to look for metabolism of present life on Mars.

Advancements in the fields of astrobiology, observational astronomy and discovery of large varieties of extremophiles with extraordinary capability to thrive in the harshest environments on Earth, have led to speculation that life may possibly be thriving on many of the extraterrestrial bodies in the universe. [12] 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, [37] [38] water being considered an essential precursor to the development of carbon-based life. [39]

Missions specifically designed to search for current life on Mars were the Viking program and Beagle 2 probes. The Viking results were inconclusive, [40] and Beagle 2 failed minutes after landing. [41] A future mission with a strong astrobiology role would have been the Jupiter Icy Moons Orbiter, designed to study the frozen moons of Jupiter—some of which may have liquid water—had it not been cancelled. In late 2008, the Phoenix lander probed the environment for past and present planetary habitability of microbial life on Mars, and researched the history of water there.

The European Space Agency's astrobiology roadmap from 2016, identified five main research topics, and specifies several key scientific objectives for each topic. The five research topics are: [42] 1) Origin and evolution of planetary systems 2) Origins of organic compounds in space 3) Rock-water-carbon interactions, organic synthesis on Earth, and steps to life 4) Life and habitability 5) Biosignatures as facilitating life detection.

In November 2011, NASA launched the Mars Science Laboratory mission carrying the Curiosity rover, which landed on Mars at Gale Crater in August 2012. [43] [44] [45] The Curiosity rover is currently probing the environment for past and present planetary habitability of microbial life on Mars. On 9 December 2013, NASA reported that, based on evidence from Curiosity studying Aeolis Palus, Gale Crater contained an ancient freshwater lake which could have been a hospitable environment for microbial life. [46] [25]

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. [47] Meanwhile, NASA launched the Mars 2020 astrobiology rover and sample cacher for a later return to Earth.

Planetary habitability Edit

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. [48] 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. [49] [50] Some researchers posit environments of water-ammonia mixtures as possible solvents for hypothetical types of biochemistry. [51]

A third assumption is to focus on planets orbiting Sun-like stars for increased probabilities of planetary habitability. [52] 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. [53] The long lifetimes of red dwarfs could allow the development of habitable environments on planets with thick atmospheres. This is significant, as red dwarfs are extremely common. (See Habitability of red dwarf systems).

Since Earth is the only planet known to harbor life, there is no evident way to know if any of these simplifying assumptions are correct.

Communication attempts Edit

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, [54] [55] scientist Stephen Hawking warned against it, suggesting that aliens might simply raid Earth for its resources and then move on. [56]

Elements of astrobiology Edit

Astronomy Edit

Most astronomy-related astrobiology research falls into the category of extrasolar planet (exoplanet) detection, the hypothesis being that if life arose on Earth, then it could also arise on other planets with similar characteristics. To that end, a number of instruments designed to detect Earth-sized exoplanets have been considered, most notably NASA's Terrestrial Planet Finder (TPF) and ESA's Darwin programs, both of which have been cancelled. NASA launched the Kepler mission in March 2009, and the French Space Agency launched the COROT space mission in 2006. [57] [58] 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, [59] is using computer modeling to generate a wide variety of virtual planets to see what they would look like if viewed by TPF or Darwin. It is hoped that once these missions come online, their spectra can be cross-checked with these virtual planetary spectra for features that might indicate the presence of life.

An estimate for the number of planets with intelligent communicative extraterrestrial life can be gleaned from the Drake equation, essentially an equation expressing the probability of intelligent life as the product of factors such as the fraction of planets that might be habitable and the fraction of planets on which life might arise: [60]

N = R ∗ × f p × n e × f l × f i × f c × L

  • 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, [61] but some applications of the formula had been taken literally and related to simplistic or pseudoscientific arguments. [62] 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) [63] may have greatly increased the probability of intelligent life arising on our planet. [64] [65]

Biology Edit

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. [62] 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. [66]

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. [67] 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. [67] 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. [68] 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. [69] [70] 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, [71] the bacterium Bacillus safensis, [72] Deinococcus radiodurans, [72] Bacillus subtilis, [72] yeast Saccharomyces cerevisiae, [72] seeds from Arabidopsis thaliana ('mouse-ear cress'), [72] as well as the invertebrate animal Tardigrade. [72] 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. [73]

Jupiter's moon, Europa, [70] [74] [75] [76] [77] [78] and Saturn's moon, Enceladus, [79] [80] 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. [68]

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. [81] [82] [83] 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." [81]

More than 20% of the carbon in the universe may be associated with polycyclic aromatic hydrocarbons (PAHs), possible starting materials for the formation of life. PAHs seem to have been formed shortly after the Big Bang, are widespread throughout the universe, and are associated with new stars and exoplanets. [84] PAHs are subjected to interstellar medium conditions and are transformed through hydrogenation, oxygenation and hydroxylation, to more complex organics—"a step along the path toward amino acids and nucleotides, the raw materials of proteins and DNA, respectively". [85] [86]

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 to find patterns that could be detected through observation of exoplanets. [87] [88]

Astroecology Edit

Astroecology concerns the interactions of life with space environments and resources, in planets, asteroids and comets. On a larger scale, astroecology concerns resources for life about stars in the galaxy through the cosmological future. Astroecology attempts to quantify future life in space, addressing this area of astrobiology.

Experimental astroecology investigates resources in planetary soils, using actual space materials in meteorites. [89] The results suggest that Martian and carbonaceous chondrite materials can support bacteria, algae and plant (asparagus, potato) cultures, with high soil fertilities. The results support that life could have survived in early aqueous asteroids and on similar materials imported to Earth by dust, comets and meteorites, and that such asteroid materials can be used as soil for future space colonies. [89] [90]

On the largest scale, cosmoecology concerns life in the universe over cosmological times. The main sources of energy may be red giant stars and white and red dwarf stars, sustaining life for 10 20 years. [89] [91] 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. [92]

Astrogeology Edit

Astrogeology is a planetary science discipline concerned with the geology of celestial bodies such as the planets and their moons, asteroids, comets, and meteorites. The information gathered by this discipline allows the measure of a planet's or a natural satellite's potential to develop and sustain life, or planetary habitability.

An additional discipline of astrogeology is geochemistry, which involves study of the chemical composition of the Earth and other planets, chemical processes and reactions that govern the composition of rocks and soils, the cycles of matter and energy and their interaction with the hydrosphere and the atmosphere of the planet. Specializations include cosmochemistry, biochemistry and organic geochemistry.

The fossil record provides the oldest known evidence for life on Earth. [93] By examining the fossil evidence, paleontologists are able to better understand the types of organisms that arose on the early Earth. Some regions on Earth, such as the Pilbara in Western Australia and the McMurdo Dry Valleys of Antarctica, are also considered to be geological analogs to regions of Mars, and as such, might be able to provide clues on how to search for past life on Mars.

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. [62] [94] 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. [94] Although terrestrial life and life that might arise independently of Earth are expected to use many similar, if not identical, building blocks, they also are expected to have some biochemical qualities that are unique. If life has had a comparable impact elsewhere in the Solar System, the relative abundances of chemicals key for its survival—whatever they may be—could betray its presence. Whatever extraterrestrial life may be, its tendency to chemically alter its environment might just give it away. [95]

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. [94] 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. [94] The four most likely candidates for life in the Solar System are the planet Mars, the Jovian moon Europa, and Saturn's moons Titan [96] [97] [98] [99] [100] and Enceladus. [80] [101]

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. [39] 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. [102] [103] At the Martian low temperatures and low pressure, liquid water is likely to be highly saline. [104] As for Europa and Enceladus, large global oceans of liquid water exist beneath these moons' icy outer crusts. [75] [96] [97] 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. [105] 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. [106] The presence of the minerals may have been the result of a collision with an asteroid or comet according to the scientists. [106] Additionally, on 27 June 2018, astronomers reported the detection of complex macromolecular organics on Enceladus [107] 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". [80] [101]

Another planetary body that could potentially sustain extraterrestrial life is Saturn's largest moon, Titan. [100] Titan has been described as having conditions similar to those of early Earth. [108] 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. [109] Some scientists think it possible that these liquid hydrocarbons might take the place of water in living cells different from those on Earth. [110] [111] 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. [112]

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. [113] 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. [114]

Measuring the ratio of hydrogen and methane levels on Mars may help determine the likelihood of life on Mars. [115] [116] According to the scientists, ". low H2/CH4 ratios (less than approximately 40) indicate that life is likely present and active." [115] Other scientists have recently reported methods of detecting hydrogen and methane in extraterrestrial atmospheres. [117] [118]

Complex organic compounds of life, including uracil, cytosine and thymine, have been formed in a laboratory under outer space conditions, using starting chemicals such as pyrimidine, found in meteorites. Pyrimidine, like polycyclic aromatic hydrocarbons (PAHs), is the most carbon-rich chemical found in the universe. [119]

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.

The systematic search for possible life outside Earth is a valid multidisciplinary scientific endeavor. [120] 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, [121] [122] and may be present even if there are no biological cells. [123]

Research outcomes Edit

As of 2019 [update] , no evidence of extraterrestrial life has been identified. [126] 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. [127] [128] [129]

Yamato 000593, the second largest meteorite from Mars, was found on Earth in 2000. At a microscopic level, spheres are found in the meteorite that are rich in carbon compared to surrounding areas that lack such spheres. The carbon-rich spheres may have been formed by biotic activity according to some NASA scientists. [130] [131] [132]

On 5 March 2011, Richard B. Hoover, a scientist with the Marshall Space Flight Center, speculated on the finding of alleged microfossils similar to cyanobacteria in CI1 carbonaceous meteorites in the fringe Journal of Cosmology, a story widely reported on by mainstream media. [133] [134] However, NASA formally distanced itself from Hoover's claim. [135] 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." [136]

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. [137] [138] 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. [137] [139] According to one of the researchers, "You can find microbes everywhere—they're extremely adaptable to conditions, and survive wherever they are." [137] 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. [140] [141] These finds expand the potential habitability of certain niches of other planets.

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. [142] [143] On 7 June 2018, NASA announced a cyclical seasonal variation in atmospheric methane, which may be produced by geological or biological sources. [144] [145] [146] The European ExoMars Trace Gas Orbiter is currently measuring and mapping the atmospheric methane.

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.

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. [147]

Extremophiles Edit

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, although evolution also puts some restrictions on their use as analogues to extraterrestrial life. [148]

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. [149]

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.

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. [150] The result was inconclusive, [151] and is still disputed by some scientists. [152] [153] [154] [155]

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. [156] 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. [157]

EXPOSE is a multi-user facility mounted in 2008 outside the International Space Station dedicated to astrobiology. [158] [159] EXPOSE was developed by the European Space Agency (ESA) for long-term spaceflights that allow exposure of organic chemicals and biological samples to outer space in low Earth orbit. [160]

The Mars Science Laboratory (MSL) mission landed the Curiosity rover that is currently in operation on Mars. [161] It was launched 26 November 2011, and landed at Gale Crater on 6 August 2012. [45] Mission objectives are to help assess Mars' habitability and in doing so, determine whether Mars is or has ever been able to support life, [162] collect data for a future 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.

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. [163] 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. [163]

ExoMars is a robotic mission to Mars to search for possible biosignatures of Martian life, past or present. This astrobiological mission is currently under development by the European Space Agency (ESA) in partnership with the Russian Federal Space Agency (Roscosmos) it is planned for a 2022 launch. [164] [165] [166]

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. [167] 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. [168] [169]

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. [170] [171] It will also aid in the selection of future landing sites. [172] [173]

Proposed concepts Edit

Icebreaker Life is a lander mission that proposed for NASA's Discovery Program for the 2021 launch opportunity, [174] 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. [175] [176] One of the key goals of the Icebreaker Life mission is to test the hypothesis that the ice-rich ground in the polar regions has significant concentrations of organics due to protection by the ice from oxidants and radiation.

Journey to Enceladus and Titan

Journey to Enceladus and Titan (JET) is an astrobiology mission concept to assess the habitability potential of Saturn's moons Enceladus and Titan by means of an orbiter. [177] [178] [179]

Enceladus Life Finder

Enceladus Life Finder (ELF) is a proposed astrobiology mission concept for a space probe intended to assess the habitability of the internal aquatic ocean of Enceladus, Saturn's sixth-largest moon. [180] [181]

Life Investigation For Enceladus

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. [182] [183] [184]

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. [185] 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. [186]

Explorer of Enceladus and Titan

Explorer of Enceladus and Titan (E 2 T) is an orbiter mission concept that would investigate the evolution and habitability of the Saturnian satellites Enceladus and Titan. The mission concept was proposed in 2017 by the European Space Agency. [187]


Will We Soon Have Proof of Martian Life?

Did Mars ever have life? Among space fans, that question probably rivals “what’s for dinner?” in popularity. But unlike your eating options, the question of martian life is hard to address. Frankly, it’s a honking challenge to search for organisms that are far, far away and may have died out long, long ago.

But here’s the good news: Scientists will soon have a hi-tech ally in the Martian hunt. On 30 July, the Mars 2020 Mission will be lofted into space from Cape Canaveral, carrying the Perseverance rover. This one-ton robot will trundle around the Red Planet looking for places where biology may have existed. Additionally, it will collect interesting samples that can eventually be returned to Earth for deeper analysis.

Until now, the most ambitious search for Martians was the Viking expedition during the mid-1970s. Two landers, bristling with instruments, conducted several experiments looking for life – including microbial life. Many among the public were disappointed when the Viking biology team concluded that the landers had found no compelling evidence for life. But given the limited sensitivity of the instruments and the fact that they were stuck on the small patch of real estate where they landed, it would be brash to conclude that the entire planet is, or was, always sterile.

The new search will be better in several respects. Perseverance is outfitted with more sensitive instruments, armed with the knowledge gained by decades’ worth of orbiter observations, and has the tremendous advantage of mobility. It also has a different strategy: Rather than look for existing life on Mars, it will try to find evidence for organisms that lived in the planet’s salad days, billions of years ago when it was a wetter, better place. After all, no matter what the history of life on Mars might be, there will be more dead organisms than living ones.

Of course, no rover can survey all 36 billion acres of martian turf. So the key to finding proof of erstwhile Red Planet residents is to know the territory, says Adrian Brown, a former SETI Institute principal investigator and now a scientist at Plancius Research in Maryland.

“We want to look in places where we think there once was liquid water, and not just pools that sat around for a few months or years, but larger bodies that existed for a really long time,” he says.

Consequently, Perseverance will direct its attentions to a 30-mile diameter feature known as Jezero crater. This crater was gouged out by a meteor billions of years ago, and eventually served as a catch basin for two rivers. For millions of years, Jezero crater, watered by these rivers, existed as Jezero lake. As everyone knows, lakes on Earth house myriad small organisms, so maybe the same was true on Mars. The remains of these former inhabitants might still be present in the dried out mud of the crater’s floor.

In fact, clues to their presence may have already been detected from above. Brown notes that a spectrometer on the Mars Reconnaissance Orbiter found evidence for carbonate compounds in Jezero crater. On Earth, carbonates are produced by small, water-dwelling animals, like corals or foraminifera. The wallboard in your house is composed of such dead critters.

While one cannot be certain that the carbonates found spectroscopically by Brown and his colleagues are due to life, it’s a gun with more than a whiff of smoke. Indeed, it’s such a compelling clue to the possibility of life on Mars that NASA has gambled $2 billion to send Perseverance on its way. It will start its search after its arrival in February.

There’s more to Perseverance’s mission then simply inspecting carbonates. Another possible discovery, one that would make every astrobiologist’s day, would be finding layered rock features known as stromatolites – the structural remains of generations of bacteria that lived and died atop one another.

These would be evidence of biology, not just chemistry. Single-celled organisms don’t leave much in the way of fossils they’re short on bones and teeth. But stromatolites are macroscopic and hard. In northwestern Australia, in the arid Pilbara craton, one can find rock outcrops of Earthly stromatolites that date back 3-1/2 billion years, among the earliest evidence for terrestrial biology. Maybe something similar can be found in Jezero crater.

The race to detect martian biosignatures won’t end with the Mars 2020 mission. NASA has developed a plan to return the rocks collected by Perseverance using robotic systems and a rocket. The hope is for the samples from Jezero to be in terrestrial labs by 2031. Then, with the world’s best instruments pulling them apart atom-by-atom, we will get our best chance yet to find signs of life from another planet.

“For centuries, scientists have been banging their heads trying to learn if Mars ever blossomed with biology,” notes Brown. “This experiment may be our best chance to find it so far. And of course it would be both important and exciting to learn that Earth is not the only planet in our solar system to have cooked up life.”


An Astrobiology Strategy for the Exploration of Mars (2007)

Life as we know it (i.e., terran life, as discussed in Chapter 1) is based on organic chemistry and is constructed of carbonaceous compounds. These organic materials are pervasive in Earth&rsquos crust and constitute an extensive chemical and isotopic record of past life that far exceeds what is recorded by visible fossils. 1 The ubiquity of coal, organic-rich black shales, and petroleum hydrocarbons, for example, is one manifestation of life&rsquos activities that extends deep into the geological record and can be used to observe past biological activity and events. 2 In fact, biogenic organic matter is so ubiquitous and overwhelming in its abundance that it is exceedingly difficult to identify organic compounds and organic matter of unambiguously nonbiological origin. The notable exceptions are organic compounds in meteorites and synthetics. 3

Experience with studies of terrestrial materials suggests that of all the various life-detection techniques available, analysis of carbon chemistry is the first among equals. Imaging and other life-detection techniques are important and will always be part and parcel of planetary exploration, but few would assert that any single methodology provides a more robust way to find extraterrestrial life than organic analysis. Accordingly, the prime emphasis here is on chemical methods for life detection. However, organic analysis alone is insufficient to detect life. The results from an ensemble of all of the relevant methodologies, combined with considerations of geological and environmental plausibility, will likely provide the best evidence for the presence or absence of life in a sample.

Although all of the assumed characteristics of hypothetical martian life forms discussed in Chapter 1 can inform and guide the overall search for biosignatures, the assumption concerning the key role likely to be played by organic chemistry will prove to be particularly important. This assumption implies that martian organisms would produce and use a wide range of small molecules and organic polymers that could serve as chemical biosignatures in their intact or fragmentary states. But to apply this knowledge for remote sensing experiments on Mars or other planetary bodies, astrobologists need to distinguish reliably between biological molecules and those that are nonbiological in origin. The following discussion identifies specific features that distinguish abiotic compounds from compounds or patterns produced by present-day life on Earth. To address the past geocentric focus, the discussion

considers some generic features that could not be generated abiologically and that would be the foundation of a sound approach to the recognition of nonterran life.

ABIOTIC CHEMISTRY

Abiotic chemistry, both organic and inorganic, provides important information about the pathways that might have led toward an origin of life. Unfortunately, there is in origin-of-life scenarios no consensus about the synthesis of organics on early Earth or elsewhere, and so astrobiologists cannot search for a specific chemistry. Among the models suggested as possibly relevant for the origin of life are atmospheric electric discharges, as proposed by Miller and Urey, 4 which have been shown to synthesize a range of organic compounds, including amino acids, from mixtures of methane, ammonia, and water. Discharge experiments yield few organic compounds when carried out in the kinds of oxidized gas mixtures of carbon dioxide thought to have predominated on early Mars. Additional processes that might have contributed to the inventory of organic compounds on early Mars include those associated with the transient effects of bolide impacts 5 and, more importantly, a variety of mineral-catalyzed chemical reactions including water-rock reactions (e.g., serpentinization) and Strecker, Fischer-Tropsch, and FeS-driven organic synthesis. 6 Water-rock reactions produce copious amounts of hydrogen that could lead to the subsurface formation of hydrocarbons from carbon dioxide and have also been shown to reduce nitrogen to ammonia, 7 both of which could make their way to planetary surfaces. Strecker synthesis is the reaction of ammonia, hydrogen cyanide, and aldehydes to give amino acids and related products. Fischer-Tropsch chemistry is the mineral-catalyzed high-temperature reaction of carbon monoxide and hydrogen to give hydrocarbons. FeS-driven organic synthesis, first proposed by Wächtershäuser, 8 , 9 has been experimentally demonstrated for only a relatively limited set of syntheses.

It is safe to assume that organic compounds that might have contributed to the prebiotic potential of the planet could have been synthesized elsewhere in the solar system or in interstellar space and then carried to the surface of Mars via carbonaceous chondrites and interplanetary dust particles. Since there is no consensus about the past history of prebiotic processes on Mars, it is more constructive to first consider the availability of the elements that constitute organic matter.

Carbon. C is found as gaseous carbon dioxide in the martian atmosphere, as carbon dioxide ice, and as carbonate minerals. Carbonates have been found in small amounts in martian meteorites but have not been detected in significant quantities by orbital remote sensing techniques or in chemical analyses of the martian regolith by landers.

Hydrogen. H is present as water ice and vapor and in hydrated minerals, and may be present within the crust as liquid water. The high D/H ratios of martian water show that Mars has lost a fraction of its water to space from the upper atmosphere. Because of the low atmospheric pressure, liquid water is not stable at the surface of modern Mars. The polar ice caps are thought to contain significant quantities of water ice, and the Gamma Ray Spectrometer on the Mars Odyssey spacecraft has detected significant quantities of subsurface hydrogen, presumably in the form of water ice. 10 Thus, the abundance of hydrogen would not have hindered life on Mars at any time in its history.

Nitrogen. N is poorly retained by the inner planets owing to its volatility and stability as N2 and also to the relative instability and solubility of its involatile forms. Currently, 2.7 percent of the martian atmosphere is nitrogen. Although nitrogen is crucial for life, it may be rare on Mars. 11 The observed ratio of 15 N/ 14 N suggests that a large fraction of the planet&rsquos nitrogen inventory has been lost to space. No measurements have yet identified nitrogen stored in surface or subsurface minerals.

Oxygen. O is present in H2O and CO2, in oxides and sulfate minerals on the highly oxidized surface, and in silicates and other minerals within the crust.

Phosphorus. Phosphate minerals are actually more abundant in meteorites than in most igneous rocks on Earth. Volatile compounds of phosphorus (phosphorus pentoxide and phosphine) are rare, making phosphate minerals more valuable as sources of phosphorus for organisms than other biotic elements with common volatile forms.

Sulfur. S is very abundant as sulfates at the martian surface, and sulfides are common accessory minerals in martian meteorites and, presumably, the martian crust. Isotopic measurements suggest that sulfur species are also present in the martian atmosphere. 12

Other metals. Metal ions such as are required by biological systems&mdashMg, Ca, Na, K, and transition elements&mdashare abundant in martian surface rocks and, presumably, in subsurface rocks as well.

TERRAN BIOSIGNATURES AND POTENTIAL MARTIAN BIOSIGNATURES

Molecular Biosignatures

The carbon chemistry of terran organisms is well understood. Researchers have detailed knowledge of the metabolic and reproductive machinery of many living organisms and can recognize the residual chemicals long after life has expired. Chemistry provides many tools for identifying extant and fossil carbon-based life on Earth and, potentially, throughout the universe.

At the most basic level, researchers can examine the elemental composition of bulk organic matter preserved on Mars or in returned Mars samples as an indicator of biogenicity. On Earth, all organisms are composed largely of the six elements&mdashC, H, N, O, P, and S&mdashwhose abundances are discussed above and in Chapter 2. Their proportions vary between organisms and ecosystems. 13 Mechanisms and pathways involved in preservation can change these ratios for example, N and P decline significantly during fossilization. Nevertheless, the discovery in a Mars sediment sample of organic matter with significant abundances of N, O, P, and S would indicate a similarity to biological material on Earth. The relative scarcity of N (see previous section) combined with the key role it plays in biological processes suggests that organic nitrogen compounds would be an important potential biosignature. 14

Organic geochemists coined the term &ldquobiological marker compound&rdquo or &ldquobiomarker&rdquo to describe individual organic compounds that serve as molecular biosignatures. 15 &ndash 17 Biomarkers comprise a spectrum of biomolecules spanning those that are present in living systems (biomarkers for extant life), structurally-related fossil derivatives that have been preserved in sediments (biomarkers for past life), or complex chemicals that have generic traits characteristic of biology but for which no precursor organism is known (sometimes called orphan biomarkers). The last set could include molecules derived from unrecognized terran life (present or past) or extraterrestrial life.

Biomolecules commonly show a huge diversity of chemical structures. However, unambiguous identification of something as chemically complex and biology-specific as DNA, a protein, a phospholipid, a steroid, or even a select set of small molecules would be difficult to refute as a successful life-detection experiment. Such a set of select small molecules might include some of the 20 protein amino acids in large excess over their nonprotein counterparts, some sugars, or a select group of fatty acids such as might be found in the polar lipids of contemporary organisms. While nucleic acids, proteins, carbohydrates, and intermediary metabolites are essential components of life, and obviously potential molecular biosignatures, compounds in these classes are rapidly recycled by other living systems and are chemically fragile. On Earth, they are not known for their ability to survive intact over geological timescales.

Lipids and structural biopolymers are biologically essential classes of compounds renowned for their stability under harsh environmental conditions. 18 Hydrocarbons, for example, are a class of lipid known to be stable on Earth over billion-year timescales. 19 , 20 Furthermore, their chemical structures can be as diagnostic for biology as those of amino acids or other biomolecules. Thermodynamic arguments suggest that the lower temperatures on Mars would aid in the preservation of hydrocarbons. The specific empirical evidence for this comes from observations of petroleum deposits on Earth: high-temperature reservoirs show enhanced hydrocarbon cracking (i.e., more gas and gasoline-grade hydrocarbons) compared to equivalent low-temperature reservoirs.

Several important molecular biosignatures result from the propensity of molecules containing just a few carbon atoms to exist in different chemical and structural configurations, known as isomers. In other words, isomers are molecules having the same number of atoms of each element (i.e., their chemical formulas are the same), but exhibiting different connectivities between, and/or spatial arrangements of, their constituent atoms. In the simplest of cases, isomers of the same compound might be chemically identical but differ in their ability to rotate polarized light (e.g., the chirality of amino acids, as described in Box 3.1). In more complex examples, the connectivity and

spatial arrangements of atoms in organic molecules might give rise to compounds with very different chemical and physical characteristics (e.g., the diastereoisomers and structural isomers described in Boxes 3.2 and 3.3, respectively). All of these properties can unambiguously indicate biological origins because living systems frequently make use of just one of the multiple isomers that can exist for any given molecule. 21 , 22

Another important set of molecular biosignatures can be identified, based on the observation that all known organisms utilize a universal subset of small metabolites as generic building blocks for constructing biomass and more complex biomolecules. 23 The 20 amino acids of proteins, the four nucleotides of DNA, and the acetate precursor of most lipids are prime examples of generic building blocks. This simple fact, so fundamental to life on Earth, leads to patterns in the molecules of life and in the molecular remains of past life. This is in stark contrast to organic compounds produced in abiotic processes, which have structures and distributions with distinctly different patterns more likely to reflect thermodynamic controls. For any class of organic compounds, biosynthesis results in recurring patterns, readily recognizable to organic chemists. Detection of particular patterns (e.g., biomolecules with a preference for even or odd numbers of carbon atoms, as described in Box 3.4) and recurring themes (e.g., families of related molecules with a limited subset of all the possible numbers of carbon atoms, as described in Box 3.5) in small to moderate-sized organic molecules could lead to the validation of biosignatures for both terran and, possibly, nonterran life.

Taken together, these various chemical characteristics have led researchers to identify the following generic molecular biosignatures for carbon-based life:

Diastereoisomeric preference (see Box 3.2),

Structural isomer preference (see Box 3.3),

Repeating structural subunits or atomic ratios (see Box 3.4), and

Uneven distribution patterns or clusters of structurally related compounds (see Box 3.5).

In summary, any family of organic molecules common to Earthly life (e.g., lipids) if discovered on Mars would be important biological markers. However, at a more basic level, patterns of carbon number, or limited isomer distributions, or, isotopic composition (see next section), consistent with synthesis from small, repeating precursor molecules may point the way to the detection of extraterrestrial life be it terran or non-terran in its biological architecture.

Isotopic Biosignatures

The elements that are most important in organic chemistry all have multiple isotopes. The isotopic patterns of these elements and, increasingly, of transition metals can constitute biosignatures in terran samples. This is the case because kinetically controlled isotopic fractionations are common in biology and can be significant and dominant over equilibrium fractionation. Although geological processes fractionate these isotopes, biological processes tend to produce different, and sometimes diagnostic, effects. For example, enzymes involved in carbon fixation, methanogenesis, methane oxidation, sulfate reduction, and denitrification impose significant fractionations between precursor and product for carbon, hydrogen, sulfur, and nitrogen. Depletions or enrichments of certain isotopes from expected values can be used as biosignatures. However, such fractionations can reveal biological activity only if all the various components of a system are available for measurement and open system behavior has operated.

No fractionations will be observed if all of a precursor is converted to a product, regardless of whether equilibrium or kinetic fractionations operate. Furthermore, for an isotopic biosignature to be sound, the components of the system must be preserved intact without subsequent fractionation by physical or chemical processes. A myth commonly perpetuated is that a C-isotopic signature in organic carbon compounds of &minus20&permil to &minus80&permil is diagnostic of biology irrespective of any other factor. The 13 C-composition in organic compounds can be a biosignature only if the isotopic composition of the precursor carbon source is also known and, importantly, if the pedigree of the materials is also consistent with biological processes. These issues have made biological interpretations of

An important property of carbon compounds is that the same atoms can bond to each other in the same manner while assuming different configurations in space. The different three-dimensional arrangements of organic molecules having the same chemical and structural formulas can lead to a number of important properties relevant to the study of biomarkers. One of these properties is chirality. That is, some molecules have their component atoms arranged in two different spatial configurations that are mirror images of each other. If the mirror images are not superimposable one upon the other, then the molecule is said to be chiral and its two structural forms are called enantiomers (Figure 3.1.1).

The vast preponderance of biologically formed chiral compounds are synthesized exclusively as one or the other enantiomer for example, right-handed sugars and left-handed amino acids are the norm in biological systems. This phenomenon is known as homochirality. Some organisms, bacteria for example, may synthesize the same chiral compound in different enantiomeric forms. Once the organism dies, and its biochemicals are released into the environment, their chiral purity may or may not persist depending on the relative stability of the chemical bonds in the enantiomers. Various natural chemical processes can lead to racemization, the formation of mixtures of the two enantiomers. Although racemization may result in loss or corruption of a biological signature, the rate at which it happens can also have a practical application, such as in the dating of fossil organic matter using the degree of amino acid racemization. Amino acids with a slight chiral excess of, presumably, abiotic origin occur in meteorites. 1 , 2 Nevertheless, biology is the most likely source of compounds that occur purely or predominantly as one enantiomer.

Enantiomeric excess can be detected in a number of ways. Chiral compounds are optically active. That is, they rotate the plane of polarized light passing through them when in solution. Direct observation of optical activity is cumbersome. Biochemical detection of enantiomeric excess is possible, but the methodologies are generally specific to individual compounds or compound types. The most widely applicable and sensitive techniques involve indirect measurement through gas chromatography or gas chromatography-mass spectrometry.

1 J.R. Cronin and S. Pizzarello, &ldquoEnantiomeric Excesses in Meteoritic Amino Acids,&rdquo Science 275:951-955, 1997.

2 M.H. Engel and S.A. Macko, eds., Organic Geochemistry Principles and Applications, Plenum Press, New York, 1993.

FIGURE 3.1.1 The atoms in the &alpha-amino acid alanine can assume two different configurations in three-dimensional space. The two forms, L-alanine and D-alanine, are called enantiomers because they are non-superimposable mirror images of each other. Abiotic processes produce equal mixtures of both L and D enantiomers, but terran life preferentially uses the L or D form. For example, most organisms on Earth make exclusive use of the L form of &alpha-amino acids. Chemical bonds oriented out of and into the plane of the page are shown as solid or dashed wedges, respectively. Courtesy of Roger E. Summons, Massachusetts Institute of Technology.

Diastereomeric Preference

Diastereomeric preference is another manifestation of the ability of atoms in certain molecules to assume different orientations in space. If the two spatial arrangements of atoms are not mirror images of each other, then the different molecular forms are known as diastereomers or diastereoisomers (Figure 3.2.1). Unlike enantiomers, diastereoisomers have different physical and chemical properties and can be separated by chromatography or other processes that exploit subtle differences in polarity. Simple sugars are good examples of diastereoisomers and the more complex the molecule, the more possibilities there are to form diastereomers. Thus, for example, the steroid cholesterol (see Figure 3.2.2) can exist in 256 different structural configurations, but living systems make use of only one of them. 1

1 K.E. Peters, J.M. Moldowan, and C.C. Walters, The Biomarker Guide, Cambridge University Press, 2004.

FIGURE 3.2.1 The ability of atoms in organic molecules to assume multiple configurations in three-dimensional space is demonstrated by these three forms of tartaric acid. Structures A and B and A and C are superimposable mirror images of each other and so are termed diastereomers. Structures B and C are non-superimposable mirror images of each other and are, thus, enantiomers (see Box 3.1). Courtesy of Roger E. Summons, Massachusetts Institute of Technology.

FIGURE 3.2.2 Structure of cholesterol with its eight asymmetric carbon atoms identified with their position number. Theoretically, this compound could exist in as many as 256 (2 8 ) possible stereoisomers, and yet biosynthesis produces only the one illustrated.

carbon, nitrogen, or sulfur isotopic data in Archean sediments, for example, subject to debate. 24 &ndash 27 Although not likely to yield unambiguous biosignatures in the near future, isotopic analyses of martian sediments and atmospheric gases will be important for discerning their evolution and for establishing comparative data, as they do on Earth. Identification of a suite of supporting isotopic data in a reaction pathway, and its environmental context, is the most effective approach to identifying an isotopic biosignature. Elucidation of the isotopic systematics of

Structural Isomers

The propensity of carbon compounds to exist with multiple ring systems and unsaturations means that the generic organic compound CpHqNrOsPtSu, can assume an enormous variety of possible structures, known as structural isomers. 1 Despite the potential for variety, researchers observe that naturally synthesized biochemicals fall into patterns, and the number of known compounds is but a small subset of what is chemically feasible. Moreover, the biomolecule may be the thermodynamically least favored structure within a set of possible isomers if this aspect enhances its functional capacity.

Structural isomers are readily separated using chromatography. In many, but not all cases, their mass spectra are also distinctive. As with other forms of isomerism, combinatorial instruments such as gas chromatographs-mass spectrometers and liquid chromatographs-mass spectrometers provide the most sensitive and diagnostic tools for trace analysis.

1 E.L. Eliel, S.H. Wilen, and L.N. Mander, Stereochemistry of Organic Compounds, Wiley, New York, 1994.

the C-cycle on Earth has been underway for more than 50 years, and much remains to be understood. 28 , 29 An added complication for studies of Mars is the unknown degree to which nonbiological atmospheric processes fractionate isotopes.

An example of an isotopic biomarker that might be used in the search for life on Mars is the 18 O/ 16 O ratio in phosphates. 30 Phosphorus in the form of phosphates (PO4 3&ndash ) is utilized in genetic material and cell membranes, and as a cofactor and energy-transporting molecule in terran biology. On Earth, the ultimate source of PO4 3&ndash is apatite that is dissolved, biologically processed, and redeposited as various sedimentary PO4 3&ndash phases and as biogenic calcium phosphate deposits (phosphorites). Biologically processed PO4 3&ndash on Earth has a strong biotic O-isotopic signature that is highly evolved from abiotic apatite baseline values. On Mars, evolution of the 18 O/ 16 O ratios in phosphates from this abiotic baseline could be used as a biomarker. Furthermore, the 18 O/ 16 O ratio of PO4 3&ndash records temperature and high-temperature exchange reactions with water, also making PO4 3&ndash a potential indicator of past hydrothermal activity on Mars. 31

An additional example of an isotopic effect concerns the tendency in biological processes for large molecules to be synthesized by the repeated addition of subunits of two or five carbon atoms (see Box 3.4). The lipid building blocks acetate (C2) and isopentenyl pyrolphosphate (C5) are, for example, isotopically inhomogeneous. Acetate provides one of the best examples because it shows very significant differences in the 13 C contents of its methyl and carboxyl carbons. 32 The most overt consequences are isotopic ordering in fatty acids and a major isotopic difference between acetogenic and polyisoprenoid lipids. In a single organism, the isotopic differences between acetogenic and polyisoprenoid lipids depend on how many of the polyisoprenoid carbon atoms arise from acetate versus carbohydrate metabolism. 33

Morphological Biosignatures

Morphological biosignatures represent the class of objects that can be interpreted as indicative of life based on their size, shape distribution, and provenance. Features of interest occur at both the macroscopic (e.g., stromatolites and microbially induced sedimentary structures) and the microscopic (e.g., microfossils) scale. If they were discovered on Mars, macroscale morphological features such as stromatolites, although being the subject of some contention as a definitive indicator of biogenicity, 34 would prove to be highly desirable targets for further study and/or sample return. 35 &ndash 37

Subunits and Building Blocks of Complex Organic Molecules

Virtually all biomolecules are constructed from a limited number of generic subunits or building blocks, the best-known examples being proteins and nucleic acids. Lipids, which are formed from only two basic building blocks, are polymers of either acetate or isopentenyldiphosphate precursors. The final products lack a hydrolyzable functionality (e.g., peptide linkages) at the point where subunits join, and, unlike other proteins and nucleic acids, lipids cannot be depolymerized.

A classic example of lipids are those that are found in membrane lipid bilayers of bacteria and eukarya and are made up of fatty acids esterified to glycerol. The most common fatty acids are all-acetate products and thus have even carbon numbers (e.g., C14, C16, C18, and C20). Odd-carbon-numbered members, generally synthesized from a non-acetyl starter, exist but are less abundant. Extension of fatty acid chain length proceeds by the addition of further acetate units. Terminating and modifying reactions such as desaturation, reduction, or decarboxylation yield common intermediate-molecular-weight series of products such as the plant and algal waxes made up of even-numbered alcohols (e.g., C26, C28, C30, C32) and odd-numbered hydrocarbons (e.g., C25, C27, C29, C31).

An additional illustration of the building-block principle is displayed by the terpenoids. These polymers of &Delta3-isopentenyldiphosphate have somewhat more complex origins and much more complex structures (Figure 3.4.1). As a result of isoprenoid biosynthesis and its evolution over geological time, terran life contains an enormous array of complex molecules related through their C5 architecture. The multiplicity of isoprenoid biosynthetic pathways, their distribution across different phylogenetic groups, their requirement, or otherwise, for molecular oxygen, and the types of post-synthesis modification are generally held to provide a powerful biosignature of evolutionary origins. For example, the molecules resulting from the pathway shown in Figure 3.4.1 are highly diagnostic of biosynthesis because, individually, they exhibit many features of biosynthesis (e.g., carbon number, chirality, and subsets of isomers).

Crocetane, 2,6,10-trimethyl-7-(3-methylbutyl)-dodecane, squalene, and biphytane are irregularly branched compounds, whereas phytane, labdane, and kaurane are regular and are constructed from four head-tail linked isoprene units. These compounds also illustrate how different structures can be diagnostic for specific physiologies (phytol and farnesol for photosynthesis, phytane for various archaea, crocetane for methanotrophy) or specific organisms (2,6,10-trimethyl-7-(3-methylbutyl)-dodecane for diatoms biphytane for crenarchaeota labdane and kaurane for conifers).

1 G. Ourisson and P. Albrecht, &ldquoHopanoids. 1. Geohopanoids: The Most Abundant Natural Products on Earth?,&rdquo Accounts of Chemical Research 25:398-402, 1992.

Cameras and spectral imagers on previous, continuing, and planned life-detection missions to Mars are capable of identifying structures and objects ranging from the macroscopic to the minuscule that, on Earth, are considered visible signatures for past or present biological activity. Such objects and structures include intact microbes, metazoa and metaphytes, stromatolites, microbial mats, and other large-scale structures composed of aggregates of cells, as well as component parts of multicellular organisms such as cysts, pollen, embryos, organs, and so on. On Earth, these objects are pervasive in surface environments and in the deep subsurface and leave no doubt about how abundant and tenacious life is. Researchers can also, to a degree, visually identify in Earth&rsquos sediments a rich fossil life extending in age to more than 2 billion years. So far, no such visible &ldquobiological&rdquo objects have been convincingly identified on Mars or in martian meteorites. If life exists, or existed in the past, on Mars or other

FIGURE 3.4.1 Structures of some regular, irregular, and cyclic C2O (diterpenoid) and C3O (triterpenopid), and C4O (tetraterpenoid) hydrocarbons that have been identified in sediments and that illustrate a variety of biosynthetic patterns based on repeating five-carbon subunits (after J.M. Hayes, &ldquoFractionation of Carbon and Hydrogen Isotopes in Biosynthetic Processes,&rdquo Reviews in Mineralogy and Geochemistry 43: 225-277, 2001).

planetary bodies, the evidence has not been forthcoming. In many respects, the search for martian life mirrors the search for the earliest life on Earth and faces similar obstacles. Attempting to reconstruct terran life&rsquos history back into deep time, researchers are confronted by the problem of a record made increasingly cryptic by the geochemical and geological processes that continually re-surface Earth and modify the rock record.

Poor preservation and ambiguity about what constitutes a biosignature have confounded the search for visible evidence of early microbial life on Earth 38 &ndash 45 and in the martian meteorite ALH 84001 in particular. 46 Related reports, and some of the controversies stemming from them, teach researchers that drawing an inference of biogenicity based on morphology is fraught with difficulties. If the feature being observed is demonstrably syngenetic with the host rock and displays a limited size (length and width) distribution, shows evidence of cellular

Clusters and Uneven Distribution Patterns of Structurally Related Compounds

The biosynthesis of large organic molecules from smaller molecules, as discussed in Box 3.4, leads to wider consequences, evidence of which can, in principle, be used as biomarkers. The synthesis of lipids by organisms, for example, from C2 or C5 building blocks creates clusters of compounds that differ by n C2 (acetogenic lipids) or n C5 (polyisoprenoids) units, where n is a positive interger. In a typical sample of terrestrial lipids, researchers find, for example, a predominance of even-carbon-numbered fatty acids odd-carbon-numbered hydrocarbons in leaf wax C15, C20, and C25 acyclic isoprenoids C20 and C30 cyclic terpenoids including steroids and C40 carotenoids. Subsets of these traits are even identifiable in highly altered or processed materials such as petroleum, where n-alkanes may exhibit preferences for odd-over-even or even-over-odd carbon numbers. Clusters of carbon numbers have the potential to be biosignatures because they indicate biosynthesis from universal building blocks.

In addition to obvious patterns of related compounds differing by two or five carbon atoms, the action of repeated addition of C2 or C5 subunits leads to an additional important biosignature. Functional biochemicals, such as lipids, have a tendency to show clusterings of related compounds at discrete molecular weight ranges. Examples of clusters seen include the following:

C15-C17 and C25-C33, respectively, for hydrocarbons associated with, for example, bacteria and plants

C26-C30 for the sterols associated with most eukaryotes

C30 for the triterpenoids associated with plants and bacteria and

C20, C25, C30, and C40 for lipids associated with archaea.

An additional biomarker related to clustering and isotopic fractionation is described in the subsection &ldquoIsotopic Biosignatures.&rdquo

A factor complicating the use of these biosignatures is the fact that most samples of biologically produced organic matter come from organisms that exist in complex ecosystems. The volatile components of a microbial mat, for example, will show compound classes with carbon numbers distributed roughly as described above and in Box 3.4. Similarly, the lipids in biofilms from hydrothermal vents display an uneven-carbon-number distribution. 1 The geological record is replete with additional examples. 2 Moreover, the C25-C30 fraction might contain more material than the C15-C20 fraction. This &ldquolumpiness&rdquo is in stark contrast to what is seen in assemblages of molecules made abiotically. 3 , 4 The Fischer-Tropsch process used to synthesize hydrocarbons, for example, creates molecules with an exponential distribution of sizes, with C1 > C2 > C3 > C4, and so on, falling away to almost zero by C30. Similarly, the amino acids seen in meteorites exhibit more C1 than C2 than C3 than C4 and so on. 5 - 8

1 L.L. Jahnke, W. Eder, R. Huber, J.M. Hope, K.U. Hinrichs, J.M. Hayes, D.J. Des Marais, S.L. Cady, and R.E. Summons, &ldquoSignature Lipids and Stable Carbon Isotope Analyses of Octopus Spring Hyperthermophilic Communities Compared to those of Aquificales Representatives,&rdquo Applied and Environmental Microbiology 67:5179-5189, 2001.

2 K.E. Peters, J.M. Moldowan, and C.C. Walters, The Biomarker Guide, Cambridge University Press, Cambridge, U.K., 2004.

3 See, for example, B. Sherwood Lollar, T.D. Westgate, J.A. Ward, G.F. Slater, and G. Lacrampe-Couloume, &ldquoAbiogenic Formation of Alkanes in the Earth&rsquos Crust as a Minor Source for Global Hydrocarbon Reservoirs,&rdquo Nature 416:522-524, 2002.

4 See, for example, M. Allen, B. Sherwood-Lollar, B. Runnegar, D.Z. Oehler, J.R. Lyons, C.E. Manning, and M.E. Summers, &ldquoIs Mars Alive?,&rdquo Eos 87:433 and 439, 2006.

5 M.A. Sephton, &ldquoOrganic Compounds in Carbonaceous Meteorites,&rdquo Natural Products Reports 19:292-311, 2002.

6 M.A. Sephton, C.T. Pillinger, and I. Gilmour, &ldquoAromatic Moieties in Meteoritic Macromolecular Materials: Analyses by Hydrous Pyrolysis and 13 C of Individual Compounds,&rdquo Geochimica et Cosmochimica Acta 64:321-328, 2000.

7 M.A. Sephton, C.T. Pillinger, and I. Gilmour &ldquoPyrolysis-Gas Chromatography&ndashIsotope Ratio Mass Spectrometry of Macromolecular Material in Meteorites,&rdquo Planetary Space Science 47:181-187, 2001.

8 M.A. Sephton, G.D. Love, J.S. Watson, A.B. Verchovsky, I.P. Wright, C.E. Snape, and I. Gilmour, &ldquoHydropyrolysis of Insoluble Carbonaceous Matter in the Murchison Meteorite: New Insights into Its Macromolecular Structure.&rdquo Geochimica et Cosmochimica Acta 68:1385-1393, 2004.

degradation, or is part of a discernable population that occurs in discrete phases within the samples on Earth that are relevant to the context of the sample, then further investigation is warranted. 47 The debates on early life and ALH 84001 (see Chapter 2) have shown that morphology must be combined with both chemistry and context to enable unambiguous detection of life. However, morphology is extremely valuable for detecting targets of interest for further investigation, particularly macroscopic structures such as stromatolites, microbial mats, and other large-scale aggregates created by communities of microorganisms.

Mineralogical and Inorganic Chemical Biosignatures

The mineralogy and chemistry of Earth materials can constitute a biosignature in some systems where organisms either accelerate or inhibit reactions that are thermodynamically possible. In addition, organisms can change the chemistry of rocks, fluids, and gases through the processes of secretion, assimilation, and electron transfer, sometimes creating mineralogical or chemical gradients that differ from those that would be established in an abiotic environment. Although there are a few examples of mineralogical biosignatures on Earth that unambiguously identify a biotic origin (e.g., coccoliths and diatoms), these are not likely to be applicable to Mars. 48 Most other types of inorganic chemical biosignatures can provide only indirect evidence of the presence of life and would thus most likely constitute supporting evidence accompanying other more diagnostic criteria. Examples of inorganic biosignatures are discussed below.

Biota can affect the identity of phases manifested in the rock record. For example, some bacteria transform mackinawite to greigite (sulfides), 49 and some fungi promote the formation of weddellite (Ca oxalate) in soils. These effects are related to the biological ability to nucleate minerals onto organic templates, or to the production of organic ligands that solubilize elements, affect growth mechanisms, or precipitate as salts. The inclusion of organic molecules or micronutrient impurities in mineral precipitates could also conceivably be indicative of biological activity.

Physical properties of minerals might also yield indirect, albeit ambiguous, evidence of biological processes. For example, the size distribution of precipitates might indirectly suggest a biotic origin, given that many mineralogical by-products of metabolism are nanocrystalline because they are formed under conditions of high oversaturation. 50 Surface etching or crystal habit, which can be affected by biological exudates or biofilm formation, might also be indirect indicators of biota. Biological phenomena can also be inferred in some cases from the characteristics of aggregations of minerals. Of possible interest for Mars is aggregation characteristic of Fe minerals precipitated by bacteria. For example, both the size distribution and the aggregation of magnetite crystals have been posited as biosignatures, 51 , 52 although these characteristics have also been attributed to abiotic processes, 53 thus pointing out the ambiguous nature of mineralogical properties as biosignatures.

Gradients in the concentration of elements recorded in Earth materials can also be diagnostic of biological phenomena. A well-known manifestation of elemental gradients driven by biological processes is certain soil horizons in which the exudation of organic complexants mobilizes elements and produces patterns indicative of the presence of biota. 54 The formation of gradients in the concentration of elements at the meter scale in soil horizons and at the micron scale on mineral surfaces or in endolithic communities might thus be important. 55 &ndash 57 The assimilation of trace elements at a low concentration by microorganisms or the sequestration of toxic elements into biologically mediated precipitates could also create distributions of trace elements that record the prior presence of biota in regolith or sedimentary environments.

Anomalies in the concentration of phosphorus have also been suggested as possible biomarkers that could be used in the search for life on Mars. 58 Phosphorus as PO4 3&ndash is utilized in a wide variety of biological processes and material. The ultimate source of PO4 3&ndash on Earth is igneous apatite, which is biologically processed and redeposited as biogenic calcium phosphates (phosphorites). On Earth, PO4 3&ndash is adsorbed strongly to iron- and aluminum-oxides and oxyhydroxides under aqueous conditions. Phosphorus phases found in martian soils, sedimentary environments, and in association with the abundant iron oxides on Mars might be a good target in a search for phosphorus as a biosignature. Additionally, patterns of phosphorous concentration could be used to guide the search for potential PO4 3&ndash biosignatures and other kinds of fossils.

Based on such considerations, past and present approaches to Mars astrobiological exploration have heavily emphasized instrument packages capable of detecting the chemical signatures of life, especially carbon compounds, isotopic signatures, and various other products of metabolism. The 2001 workshop on biosignatures organized by the NASA Biomarker Task Force established comprehensive objectives for developing a better understanding of biosignatures. Unfortunately, though, the results of the task group&rsquos deliberations were never published in full. 59 Because they represent an important starting point for future discussions, those objectives are reproduced in Appendix C.

REFERENCES

1. J.J. Brocks and R.E. Summons, &ldquoSedimentary Hydrocarbons, Biomarkers for Early Life,&rdquo pp. 65-115 in Treatise in Geochemistry (H.D. Holland and K. Turekian, eds.), 2003 K.E. Peters, J.M. Moldowan, and C.C. Walters, The Biomarker Guide, Cambridge University Press, Cambridge, 2004.

2. See, for example, A.H. Knoll, R.E. Summons, J.R. Waldbauer and J.E. Zumberge, &ldquoSuccessions in Biological Primary Productivity in the Oceans&rdquo in The Evolution of Photosynthetic Organisms in the Oceans (P. Falkwoski and A.H. Knoll eds), in press K.E. Peters, J.M. Moldowan and C.C Walters, The Biomarker Guide, Cambridge University Press, Cambridge, 2004.

3. See, for example, A.I. Rushdi and B.R.T. Simoneit, &ldquoLipid Formation by Aqueous Fischer-Tropsch-Type Synthesis over a Temperature Range of 100 to 400°C,&rdquo Origins of Life and Evolution of Biospheres 31:103-118, 2004 J.D. Pasteris and B. Wopenka, &ldquoLaser&ndashRaman Spectroscoy (Communication Arising): Images of the Earth&rsquos Earliest Fossils?&rdquo Nature 420:476-477, 2002 B. Sherwood Lollar, T.D. Westgate, J.A. Ward, G.F. Slater, and G. Lacrampe-Couloume, &ldquoAbiogenic Formation of Alkanes in the Earth&rsquos Crust as a Minor Source for Global Hydrocarbon Reservoirs,&rdquo Nature 416:522-524, 2002 T.M. McCollom, and J.S. Seewald, &ldquoCarbon Isotope Composition of Organic Compounds Produced by Abiotic Synthesis under Hydrothermal Conditions,&rdquo Earth and Planetary Science Letters 243:74-84, 2006.

4. S.L. Miller, &ldquoProduction of Some Organic Compounds under Possible Primitive Earth Conditions, Journal of the American Chemical Society 7:2351, 1955.

5. J.A. Kasting, &ldquoBolide Impacts and the Oxidation State of Carbon in the Earth&rsquos Early Atmosphere,&rdquo Origins of Life and Evolution of the Biosphere 20:199-231, 1990.

6. See, for example, R.M. Hazen &ldquoLife&rsquos Rocky Start,&rdquo Scientific American 284(4):76-85, 2001.

7. J.A. Brandes, N.Z. Boctor, G.D. Cody, B.A. Cooper, R.M. Hazen, and H.S. Yoder, &ldquoAbiotic Nitrogen Reduction on the Early Earth,&rdquo Nature 395:365-367, 1998.

8. G. Wächtershäuser, &ldquoBefore Enzymes and Templates: Theory of Surface Metabolism,&rdquo Microbiology Review 52:452-484, 1988.

9. G. Wächtershäuser, &ldquoEvolution of the First Metabolic Cycles,&rdquo Proceedings of the National Academy of Sciences 87:200-204, 1990.

10. W.V. Boynton, W.C. Feldman, S.W. Squyres, T.H. Prettyman, J. Brückner, L.G. Evans, R.C. Reedy, R. Starr, J.R. Arnold, D.M. Drake, P.A.J. Englert, A.E. Metzger, I. Mitrofanov, J.I. Trombka, C. d&rsquoUston, H. Wänke, O. Gasnault, D.K. Hamara, D.M. Janes, R.L. Marcialis, S. Maurice, I. Mikheeva, G.J. Taylor, R. Tokar, and C. Shinohara, &ldquoDistribution of Hydrogen in the Near Surface of Mars: Evidence for Subsurface Ice Deposits,&rdquo Science 297:81-85, 2002.

11. D.G. Capone, R. Popa, B. Flood, K.H. Nealson, &ldquoGeochemistry. Follow the Nitrogen,&rdquo. Science 312:708-709, 2006.

12. J. Farquhar, J. Savarino, T.L. Jackson, M.H. Thiemens, &ldquoEvidence of Atmospheric Sulphur in the Martian Regolith from Sulphur Isotopes in Meteorites,&rdquo Nature 404:50-52, 2000.

13. P.G. Falkowski and C.S. Davis, &ldquoNatural Proportions,&rdquo Nature 431:131, 2004.

14. D.G. Capone, R. Popa, B. Flood, K.H. Nealson, &ldquoGeochemistry. Follow the Nitrogen,&rdquo. Science 312:708-709, 2006.

15. G. Eglinton and M. Calvin &ldquoChemical Fossils,&rdquo Scientific American 261:32-43, 1967.

16. M.H. Engel and S.A. Macko, eds., Organic Geochemistry Principles and Applications, Plenum Press, New York, 1993.

17. K.E. Peters, J.M. Moldowan, and C.C. Walters,. The Biomarker Guide, Cambridge University Press, Cambridge, 2004.

18. M.H. Engel and S.A. Macko, eds., Organic Geochemistry Principles and Applications, Plenum Press, New York, 1993.

19. J.J. Brocks and R.E. Summons, &ldquoSedimentary Hydrocarbons, Biomarkers for Early Life,&rdquo pp. 65-115 in Treatise in Geochemistry (H.D. Holland and K. Turekian, eds.), 2003.

20. K.E. Peters, J.M. Moldowan, and C.C. Walters,. The Biomarker Guide, Cambridge University Press, Cambridge, 2004.

21. K.E. Peters, J.M. Moldowan, and C.C. Walters, The Biomarker Guide, Cambridge University Press, Cambridge, 2004.

22. E.L. Eliel, S.H. Wilen, and L.N. Mander, Stereochemistry of Organic Compounds, Wiley, New York, 1994.

23. See, for example, N.A. Campbell and J.B. Reece, Biology (7th edition), Benjamin Cummings, 2004.

24. See, for example, S.J. Mojzsis, G. Arrhenius, K.D. McKeegan, T.M. Harrison, A.P. Nutman, and C.R. Friend, &ldquoEvidence for Life on Earth Before 3,800 Million Years Ago,&rdquo Nature 384:55-59, 1996.

25. M.A. van Zuilen, K. Mathew, B. Wopenka, A. Lepland, K. Marti, and G. Arrhenius, &ldquoNitrogen and Argon Isotopic Signatures in Graphite from the 3.8-Ga-old Isua Supracrustal Belt, Southern West Greenland,&rdquo Geochimica et Cosmochimica Acta 69:1241-1252, 2005.

26. Y. Ueno, H. Yurimoto, H. Yoshioka, T. Komiya, and S. Maruyama, &ldquoIon Microprobe Analysis of Graphite from ca. 3.8 Ga Metasediments, Isua Supracrustal Belt, West Greenland: Relationship between Metamorphism and Carbon Isotopic Composition,&rdquo Geochimica et Cosmochimica Acta 66:1257-1268, 2002.

27. Y. Shen, R. Buick and D.E. Canfield &ldquoIsotopic Evidence for Microbial Sulphate Reduction in the Early Archaean Era,&rdquo Nature 410:77-81, 2001.

28. H. Craig, &ldquoThe Geochemistry of the Stable Carbon Isotopes of Carbon,&rdquo Geochimica et Cosmochimica Acta 3:53-92, 1953.

29. J.M. Hayes and J.R. Waldbauer, &ldquoThe Carbon Cycle and Associated Redox Processes through Time,&rdquo Philosophical Transactions of the Royal Society B: Biological Science 361:931-950, 2006.

30. R.E. Blake, J.C. Alt, and A.M. Martini, &ldquoOxygen Isotope Ratios of PO4 &ndash : An Inorganic Indicator of Enzymatic Activity and P Metabolism and a New Biomarker in the Search for Life,&rdquo Proceedings of the National Academy of Sciences, Astrobiology Special Feature 98:2148-2153, 2001.

31. R.E. Blake, J.C. Alt, and A.M. Martini, &ldquoOxygen Isotope Ratios of PO4 &ndash : An Inorganic Indicator of Enzymatic Activity and P Metabolism and a New Biomarker in the Search for Life,&rdquo Proceedings of the National Academy of Sciences, Astrobiology Special Feature 98:2148-2153, 2001.

32. J.M. Hayes, &ldquoFractionation of Carbon and Hydrogen Isotopes in Biosynthetic Processes,&rdquo Reviews in Mineralogy and Geochemistry 43:225-277, 2001.

33. J.M. Hayes, &ldquoFractionation of Carbon and Hydrogen Isotopes in Biosynthetic Processes,&rdquo Reviews in Mineralogy and Geochemistry 43:225-277, 2001.

34. J.M. Garcia-Ruiz, S.T. Hyde, A.M. Carnerup, A.G. Christy, M.J. Van Kranendonk, and N.J. Welham, &ldquoSelf-Assembled Silica-Carbonate Structures and Detection of Ancient Microfossils,&rdquo Science 302:1194-1197, 2003.

35. H.J. Hofmann, K. Grey, A.H. Hickman, and R. Thorpe, &ldquoOrigin of 3.45 Ga Coniform Stromatolites in Warrawoona Group, Western Australia,&rdquo Geological Society of America Bulletin 111:1256-1262, 1999.

36. S.L. Cady, J.D. Farmer, J.P. Grotzinger, J.W. Schopf, and A. Steele, &ldquoMorphological Biosignatures and the Search for Life on Mars,&rdquo Astrobiology 3:351-368, 2003.

37. A.C. Allwood, M.R. Walter, B.S. Kamber, C.P. Marshall, and I.W. Burch, &ldquoStromatolite Reef from the Early Archaean Era of Australia,&rdquo Nature 441:714-718, 2006.

38. See, for example, D.R. Lowe, &ldquoAbiological Origin of Described Stromatolites Older than 3.2 Ga,&rdquo Geology 22:387-390, 1994.

39. J.P. Grotzinger and A.H. Knoll, &ldquoStromatolites in Precambrian Carbonates Evolutionary Mileposts or Environmental Dipsticks,&rdquo Annual Reviews of Earth and Planetary Sciences 27:313-358, 1999.

40. H.J. Hofmann, K. Grey, A.H. Hickman, and R. Thorpe, &ldquoOrigin of 3.45 Ga Coniform Stromatolites in Warrawoona. Group, Western Australia.&rdquo Geological Society of America Bulletin 111:1256-1262, 1999.

41. M.D. Brasier, O.R. Green, A.P. Jephcoat, A.K. Kleppe, M.J. Van Kranendonk, J.F. Lindsay, A. Steele, and N.V. Grassineau, &ldquoQuestioning the Evidence for Earth&rsquos Oldest Fossils,&rdquo Nature 416:76-81, 2002.

42. J.W. Schopf, &ldquoMicrofossils of the Early Archaean Apex Chert: New Evidence of the Antiquity of Life,&rdquo Science 260:640-646, 1993.

43. J.W. Schopf, &ldquoAre the Oldest Fossils Cyanobacteria?,&rdquo pp. 23-61 in Evolution of Microbial Life Society for General Microbiology Symposium 54 (D. McL. Roberts, P. Sharp, G. Alderson, and M. Collins, eds.), Cambridge University Press, Cambridge, 1996.

44. J.W. Schopf, A.B. Kudryavtsev, D.G. Agresti, T.J. Wdowiak, and A.D. Czaja, &ldquoLaser Raman Imagery of Earth&rsquos Earliest Fossils,&rdquo Nature 416:73-76, 2002. J.M. Garcia-Ruiz, S.T. Hyde, A.M. Carnerup, V. Christy, M.J. Van Kranendonk, and N.J. Welham, &ldquoSelf-Assembled Silica-Carbonate Structures and Detection of Ancient Microfossils,&rdquo Science 302:1194-1197, 2003.

45. S.M. Awramik and K. Grey, &ldquoStromatolites: Biogenicity, Biosignatures, and Bioconfusion,&rdquo pp. 227-235 in Astrobiology and Planetary Missions (R. B. Hoover, G.V. Levin, A.Y. Rozanov, G.R. Gladstone, eds.), Proceedings of the SPIE, Volume 5906, 2005.

46. D.S. McKay, E.K. Gibson, Jr., K.L. Thomas-Keprt, H. Vali, C.S. Romanek, S.J. Clemett, X.D.F. Chillier, C.R. Maechling, and R.N. Zare, &ldquoSearch for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH 84001,&rdquo Science 273:924-930, 1996.

47. J.W. Schopf, &ldquoThe Oldest Fossils and What they Mean,&rdquo pp. 29-63 in J.W. Schopf (ed.), Major Events in the History of Life, Jones and Bartlett Publishers, Boston, Mass., 1992.

48. J.F. Banfield, J.W. Moreau, C.S. Chan, S.A. Welch, and B. Little, &ldquoMineralogical Biosignatures and the Search for Life on Mars,&rdquo Astrobiology 1:447-465, 2001.

49. M.B. McNeil and B. Little, &ldquoMackinawite Formation during Microbial Corrosion,&rdquo Journal of Corrosion 46:599-600, 1990.

50. J.F. Banfield, J.W. Moreau, C.S. Chan, S.A. Welch, and B. Little, &ldquoMineralogical Biosignatures and the Search for Life on Mars,&rdquo Astrobiology 1:447-465, 2001.

51. K.L Thomas-Keprta, D.A. Bazylinski, J.L. Kirschvink, S.J. Clemett, D.S. McKay, S.J. Wentworth, H. Vali, E.K. Gibson, and C.S. Romanek, &ldquoElongated Prismatic Magnetite Crystals in ALH 84001 Carbonate Globules: Potential Martian Magnetofossils,&rdquo Geochimica et Cosmochimica Acta 64:4049-4081, 2000.

52. K.L. Thomas-Keprta, S.J. Clemett, D.A. Bazylinski, J.L. Kirschvink, D.S. McKay, S.J. Wentworth, H. Vali, E.K. Gibson, Jr., M.F. McKay, and C.S. Romanek, &ldquoTruncated Hexa-Octahedral Magnetite Crystals in ALH 84001: Presumptive Biosignatures,&rdquo Proceedings of the National Academy of Sciences 98:2164-2169, 2001.

53. See, for example, A.H. Treiman, &ldquoSubmicron Magnetite Grains and Carbon Compounds in Martian Meteorite ALH 84001: Inorganic, Abiotic Formation by Shock and Thermal Metamorphism,&rdquo Astrobiology 3:369-392, 2003.

54. A. Neaman, J. Chorover, and S.L. Brantley, &ldquoElement Mobility Patterns Record Organic Ligands in Soils on Early Earth,&rdquo Geology 33(2):117-120, 2005.

55. B. Kalinowski, L. Liermann, S.L. Brantley, A. Barnes, and C.G. Pantano, &ldquoX Ray Photoelectron Evidence for Bacteria-Enhanced Dissolution of Hornblende,&rdquo Geochimica et Cosmochimica Acta 64:1331-1343, 2000.

56. A. Neaman, J. Chorover, and S.L. Brantley, &ldquoElement Mobility Patterns Record Organic Ligands in Soils on Early Earth,&rdquo Geology 33(2):117-120, 2005.

57. H.J. Sun and E.I. Friedmann, &ldquoGrowth on Geological Time Scales in the Antarctic Cryptoendolithic Microbial Community,&rdquo Geomicrobiology Journal 16:193-202, 1999.

58. G. Weckwerth and M. Schidlowski, &ldquoPhosphorus as a Potential Guide in the Search for Extinct Life on Mars,&rdquo Advances in Space Research 15:185-191, 1995.

59. Michael Meyer, NASA Science Mission Directorate, personal communication, 2006.


8.2B: Martian Biosignatures - Biology

Spring and evaporite deposits are considered two of the most promising environments for past habitability on Mars and preservation of biosignatures. Manitoba, Canada hosts the East German Creek (EGC) hypersaline spring complex, and the post impact evaporite gypsum beds of the Lake St. Martin (LSM) impact. The EGC complex has microbial mats, sediments, algae and biofabrics, while endolithic communities are ubiquitous in the LSM gypsum beds. These communities are spectrally detectable based largely on the presence of a chlorophyll absorption band at 670 nm however, the robustness of this feature under Martian surface conditions was unclear. Biological and biology-bearing samples from EGC and LSM were exposed to conditions similar to the surface of present day Mars (high UV flux, 100 mbar, anoxic, CO 2 rich) for up to 44 days, and preservation of the 670 nm chlorophyll feature and chlorophyll red-edge was observed. A decrease in band depth of the 670 nm band ranging from

16 to 80% resulted, with correlations seen in the degree of preservation and the spatial proximity of samples to the spring mound and mineral shielding effects. The spectra were deconvolved to Mars Exploration Rover (MER) Pancam and Mars Science Laboratory (MSL) Mastcam science filter bandpasses to investigate the detectability of the 670 nm feature and to compare with common mineral features. The red-edge and 670 nm feature associated with chlorophyll can be distinguished from the spectra of minerals with features below

1000 nm, such as hematite and jarosite. However, distinguishing goethite from samples with the chlorophyll feature is more problematic, and quantitative interpretation using band depth data makes little distinction between iron oxyhydroxides and the 670 nm chlorophyll feature. The chlorophyll spectral feature is observable in both Pancam and Mastcam, and we propose that of the proposed EXOMARS Pancam filters, the PHYLL filter is best suited for its detection.


8.2B: Martian Biosignatures - Biology

On Mars, groundwater discharge, heated by geological processes at depth, represents a likely late-stage reservoir of liquid water available for biological activity. Photo-geological observations of the Martian surface support geologically, relatively young groundwater discharge via sapping and/or fault-controlled springs. Our approach to the investigation of the possible biological potential of such reservoirs has been to characterize analogous, terrestrial spring systems. Our study site is a fault-driven, mesophilic, sulfur spring system between the Hayward and Calaveras faults in California. We have examined hydro-geological variables, nutrient availability for microbial metabolism, differences in extant community structure, and the seasonal changes associated with these variables. The springs under study also precipitate calcite and form large mounds, offering the potential to evaluate the preservation of biosignatures. The geochemistry and isotopic composition (2H/18O) of spring waters indicate that the various springs discharge waters represent differing amounts of mixing between deeper, connate water with shallow meteoric inputs. Clone libraries of 16S rDNA and fluorescence in situ hybridization experiments suggest that oxidation of sulfur compounds by Epsilon- and Gammaproteobacteria is a significant process occurring in the springs, and lipid analyses support these observations. While the studied springs undergo seasonal shifts in their respective geochemistries, only the microbial community at one of the springs elicits a commensurate seasonal variation. During the dry season, the community at this spring shifts to a red, plaque-like biofilm and iron-cycling organisms from the Alphaproteobacteria class increase significantly in their relative abundance within the community. Preliminary chemical analysis of the calcite accretions indicates abundant organic carbon, and thus, suggests a possible record of prior microbial ecosystems. On-going investigations of recalcitrant lipid species such as bacteriohopanepolyols (BHPs), in both extant biology as well as the accreted calcite, is underway and should provide insight to the taphonomic processes affecting the viability of lipid biosignatures. Results emphasize the role of local geophysical history in spring microbial community structure and productivity.


Astrobiology and Ocean Worlds

NASA’s astrobiology program addresses three fundamental questions: How does life begin and evolve? Is there life beyond Earth and, if so, how can we detect it? What is the future of life on Earth and in the universe? Our researchers include experts in a wide range of fields to address these pivotal questions, providing a comprehensive, integrated understanding of biological, geological, chemical, planetary, and cosmic phenomena. A focus on ocean worlds – bodies with substantial, stable liquid on their surface or subsurface – requires additional expertise in fields such as oceanography and marine biology, and opens up the key places in our solar system and beyond where life is most likely to exist.

Research in the Astrobiology and Ocean Worlds group spans a wide range of topics, including: assessment of the habitability of the solar system’s planets and moons employment of morphology, chemistry and mineralogy to assess biosignatures, in particular on Jupiter’s moon Europa, Saturn’s moons Enceladus and Titan, and Neptune’s moon Triton development of instruments and methodologies to detect extinct or extant life finding correlations between carbon isotopic compositions and microstructures and/or taxonomy in microfossils assessment of the theories for life’s origins, including the alkaline hydrothermal vent model and understanding the geological history of Mars as it relates to habitability.

Additionally, we work on all aspects of future ocean worlds and astrobiology-based missions, from the development of long-term mission concepts to working on the instruments and methods that will perform the analysis of robotic platforms, and developing the hardware for future missions. JPL researchers are currently involved in a variety of missions and mission concepts, such as the Mars Science Laboratory (aka Curiosity), Mars 2020, Europa Clipper, and Dragonfly. We are also heavily involved in planning future Mars and Outer Planets Missions.


Summary

Based on the contents of this review and the research covered herein, we propose to establish a microfossil atlas, covering all known aspects of the ecology of volcanic habitats on Earth, including prevalent information about trace and body fossils of prokaryotic and eukaryotic nature. To accomplish this, we need the combining results from all working areas as reviewed above including information about microbial morphology, organic microfossil-content (biomarkers) and elemental and isotopic content of igneous-dwelling fossils and their associated biominerals. A first rough classification will primarily be based on morphology, but biomarkers and relevant isotopic fractionations will be added to this scenario to enhance the classification and to make it taxonomically robust. A combination of biomarkers and isotopes will make it possible to discriminate between groups of microorganisms based on metabolisms such as, for example, methanogens and methanotrophs. Species discrimination is made possible by the presence and detection of different lipids and δ 13 C values within fossil microorganisms and/or associated biominerals (in the case of δ 13 C) (Drake et al., 2015). Ultimately, however, the volcanic microfossil atlas will be classified based on taxonomy. Rough discriminations between prokaryotes and eukaryotes will be possible, as will hopefully also more precise classifications, down to class-level.

The current payloads of NASAs Mars 2020 and the ExoMars missions are capable of analyzing structures 㱠 μm, possibly somewhat smaller. Therefore, both missions will be able to target larger biogenic structures from volcanic rocks, such as mm-sized mineralized fungal mycelia, or larger microstromatolites in open vesicles. The ExoMars cameras with a resolution of 8 μm/px has a greater chance of identifying small features and individual hyphae, as seen in Figure 5B, but in turn, the NASA mission has the possibility of collecting samples for later ex situ investigation on Earth- a resolution of 15 μm/pixel may therefore be sufficient to resolve enough features to select samples with a high probability of containing biosignatures. Our hope with establishing a volcanic microfossil atlas is that it may act as a complement to more established sedimentary-based fossil charts by providing a robust assessment of microbial diversity in the igneous oceanic crust. Apart from providing general guidelines for microfossil studies on Earth, we envision the atlas to specifically aid in the search for relevant target-sites for planetary missions, such as the NASA Mars mission 2020 and ExoMars.


8.2B: Martian Biosignatures - Biology

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