Ebook: Perspectives in Astrobiology

Astrobiology is the multi-disciplinary field devoted to the investigation of the origin; physical, chemical and environmental limitations; and the distribution in space and time of life on Earth and in the Cosmos. Astrobiology seeks an answer to one of the most fundamental of all questions: — Is Life Restricted to Planet Earth or is Life a Cosmic Imperative? Understanding the characteristics, properties, habits, and diversity of living organisms on Earth is crucial to determining where and how to search for evidence of life elsewhere. New techniques and methodologies must be developed in order to determine a suitable suite of valid biomarkers that is needed to facilitate the differentiation of abiotic processes from true signatures of life. This is crucial to establishing the criteria needed to properly evaluate potential biosignatures in ancient Earth rocks and in a wide variety of Astromaterials (e.g., meteorites, interstellar dust particles and samples returned from future space flight missions to comets, asteroids and Mars).

It is well known that microbial extremophiles (e.g., prokaryotes such as archaea and bacteria) were the first life forms to appear on Earth. They are also the most abundant and the most widely distributed life forms on our planet. Extremophiles inhabit deep ice, deep crustal rocks, hydrothermal vents, permafrost and the deepest layers of the Antarctic Ice Sheet, deep marine sediments, acidic brines and hypersaline, alkaline lakes and lagoons. They live in the most hostile environments of our planet, growing wherever there is a source of water, energy, and carbon compounds and represent good analogs for life forms that may be present elsewhere in the Solar System Their life processes result in the production of biominerals, chiral amino acids, biological fractionation of stable isotopes, macromolecular fossils, chemical biosignatures and microfossils.

This "Perspectives in Astrobiology" volume includes papers treating a wide variety of these topics. They range from considerations of relict microbial communities of extreme environments (e.g. hydrotherms, hypersaline lagoons, and soda lakes and the subglacial Antarctic ice sheet) to complex organic molecules such as sugars under prebiotic conditions, biominerals and biotic and abiotic framboidal microstructures in Earth rocks, the processes of mineralization and fossilization of cyanobacteria, and biomarkers and microfossils detected in carbonaceous meteorites. Other papers discuss the use of stable isotopes and their biological fractionation as a baseline for evaluating extraterrestrial evidence and the use of chirality and composition of indigenous amino acids for differentiating between terrestrial and extraterrestrial organic matter in Astromaterials. Also treated in this volume are geomorph parallels, sediment patterns, and cyclicities in permafrost sediments of Earth and Mars; the survival of bacteria in space, eclipsing binaries, and advanced DNA and protein chip technology for future robotic missions to search for life in the Solar System.

Richard B. Hoover, Alexei Yu. Rozanov, Roland Paepe

The origin of framboidal structures and their mineralogical composition (on the example of black shales of the Upper Permian of the Barents Sea Shelf and the Cambrian of the Siberian Platform) are discussed. The biogenic origin of framboids is confirmed.

The main types of biomorphic microstructures from black carbonate shales of the Sinsk formation (Lower Cambrian) of the Siberian platform are described. The possibility of the thin-layered part of the Sinsk formation being the vitally buried remains of cyanobacterial mat is discussed.

The subglacial Antarctic Ice Sheet is one of the lesser known and most inaccessible places for direct sampling. This work, conducted under the Italian Program for Antarctic Research (PNRA), aims to use results of multicomponent analyses and observations on ice proximal marine sediment cores (Ross Sea, Glomar Challenger Area) as the key data set to introduce the first general sedimentary model(s) into an Astrobiology Roadmap scientific scheme, i.e.: Goals 5 and 8. Information obtained from Antarctic glacigenic sediments can provide plausible models for planetary bodies where glacial ice fields and related processes occur (i.e., the ice caps of Mars, and Jupiter's icy moon, Europa). The analyses (C_org and biogenic opal) on diamicton mud grains (MDGS) suggest that fine-grained ice rafted detritus (IRD) can retain their original depositional settings, e.g.; exposed outcrops and subglacial lake basins, after incorporation within marine sediment and provide some information on the Antarctic interiors eroded by the East and West Antarctic Ice Sheets. Finally, observations on marine and ice cores suggest that MDGS and silt-clay aggregates are more widely distributed throughout Antarctic sediments and glacial fields than previously believed. The most reliable models should consider potential sources and processes/mechanisms explaining high-C_org content in subglacially-derived material. Ice proximal marine sediment would provide an unlimited amount of MDGS samples as continent-derived material as test sources of organics of use in astrobiological research.

Photoelectric observations of the DR Vulpeculae (Vul) have been carried out in B and V colors at the Ege University Observatory and new light curves have been obtained for the system. The O-C diagram was analyzed using all reliable timings found in the literature and new values for the elements of the apsidal motion was computed. The apsidal motion period of the system, 36.8 yr, was obtained.

Several hundred amino acids have been identified in organisms, but only 20 are the building blocks of proteins. As best as can be determined from the fossil record, the 20 protein amino acids have never varied with respect to structure or stereochemistry. Approximately 55 amino acids that have yet to be discovered in modern terrestrial organisms have been identified in carbonaceous meteorites. If life originated on Earth, a fundamental question that remains to be answered is what were the source(s) and mechanisms of formation of the amino acids that preceded life? Laboratory simulation experiments have not resulted in the synthesis of all of the protein amino acids. Also, these experiments always produce racemic amino acids (D/L=1), whereas life as we know it is based almost exclusively on L-amino acids. The alternative to laboratory synthesis has been investigations of ancient rocks, terrestrial and extraterrestrial. Given that life is ubiquitous on present-day Earth and no rock is an entirely closed system, the challenge has been to distinguish ancient, indigenous amino acids from those more recently introduced via contact with the Earth's biosphere. Amino acids in Precambrian rocks are not easily distinguished from modern overprints. However, amino acids in carbonaceous meteorites with short residence times on Earth provide a unique opportunity to begin to assess what the Earth's organic inventory may have been like prior to life's origin. In addition to numerous exotic amino acids, several of the common protein amino acids essential for life occur in the Murchison and Orgueil meteorites. More importantly, these amino acids exhibit the L-enantiomer excess that was, arguably, a necessary precondition for the origin of life. The stable isotope composition of amino acids in the Murchison meteorite confirms their authenticity. It is hypothesized that at least some of the starting materials for life on Earth may have been introduced by impact events. There are several protein amino acids that occur in all living organisms on Earth but have not been synthesized in the laboratory by abiotic mechanisms and have not been detected in carbonaceous meteorites. It is suggested that the presence of these amino acids, e.g.: Phe, Lys, His, and Arg, on other planetary bodies would by evidence for the existence of life as we know it.

Mineralization (silification, carbonatization and phosphatization) of cyanobacteria was studied in nature and laboratory. The increase in concentration of silica, phosphates, carbonates, and calcium leads to a sequence of morphological changes of cyanobacteria: (1) Formation of slime sheath, (2) formation of isolated globules of minerals on the slime sheath, (3) mineralization of slime sheaths of viable cells, and (4) mineralization of trichomes of dead cells.

The detection of biominerals, chemical biomarkers, and putative microfossils in the Mars meteorite, ALH84001, stimulated the newly emerging fields of astrobiology and bacterial paleontology. The debate triggered by the ALH84001 results highlighted the importance of developing methodologies for recognizing chemical biomarkers, biominerals, and microfossils in living and fossilized bacteria in ice, permafrost, rocks, meteorites, and other astromaterials prior to the return of samples from comets, asteroids, and Mars. Comparative studies of the chemical, mineral, and morphological biomarkers in living and fossil microorganisms are essential to developing the expertise needed to differentiate biogenic forms from abiotic microstructures and to recognize indigenous biosignatures and distinguish them from recent biological contaminants.

At the NASA Marshall Space Flight Center (MSFC) and the Paleontological Institute of the Russian Academy of Sciences (PIN/RAS), ultrahigh-resolution imaging and x-ray elemental analysis has been carried out on living bacteria, ancient microbes cryopreserved in ice and permafrost, biominerals and microfossils in a wide variety of rocks and meteorites. The environmental scanning electron microscope (ESEM) and field emission scanning electron microscope (FESEM) studies have resulted in the detection of a large number of indigenous biomarkers and lithified or carbonized microfossils found embedded in situ in the rock matrix of carbonaceous meteorites. Many of these forms are similar to microfossils and biominerals seen in living and fossil magnetotactic bacteria and cyanobacteria from hypersaline soda lakes; phosphorites of Khubsugul, Mongolia; and high carbon rocks of the Siberian and Russian Platforms. Some of the assemblages of microfossils in carbonaceous meteorites exhibit consistent consortia and microbial ecosystems. Many of the forms are large and extremely complex, exhibiting recognizable nanostructures, such as flagella, spines, biofilms, apical cells, and reproductive stages (trichomes, spores, akinetes, and hormogonia) such as are known in modern Nostocacean cyanobacteria. A review of the prior studies and recently obtained images, and x-ray data from a wide variety of carbonaceous meteorites and terrestrial rocks is provided herein.

The concept of interplanetary transfer of life requires that organisms cope with three major challenges: (1) The escape process, (2) the long-term stay in space, and (3) the landing process. The first step involves hypervelocity impact by comets or asteroids under strong or moderate shock metamorphism of the ejected microbe-bearing rock fragment. Experiments have shown that bacterial spores can survive such a simulated meteorite impact. The second step deals with the ability of microorganisms to withstand the complex interplay of the parameters of space, e.g.: vacuum, ultraviolet and ionizing radiation, and temperature extremes, when traveling in space over extended periods of time. Experiments in space, such as on board of Apollo, Spacelab 1 (SL 1), the Long Duration Exposure Facility (LDEF), FOTON, MIR, and the European Retrievable Carrier (EURECA), as well as at space simulation facilities on ground, have given the following five results: (1) Extraterrestrial solar UV radiation is a thousand times more efficient than UV at the surface of the Earth and kills 99% of bacterial spores within a few seconds; (2) space vacuum increases the UV sensitivity of the spores; (3) although spores survive extended periods of time in space vacuum (up to 6 yr) genetic changes occur, such as increased mutation rates; (4) after 6 yr in space, up to 70% of bacterial spores survived if protected against solar UV radiation and dehydration; (5) spores could escape a hit of a cosmic HZE particle, e.g.: iron ion, for up to 1 Ma. Calculations using radiative transfer models for cosmic rays and biological data from accelerator experiments have shown that a meteorite layer of 1 m or more effectively protects bacterial spores against galactic cosmic radiation for 1 Ma or more. It is concluded that radiation-resistant microbes could survive a journey from one planet to another in our solar system if they are located inside a meteorite thus shielded against cosmic radiation. However, viable transport between solar systems seems to be not possible, assuming impact ejection as the first step. The last step, capture and landing on a planet depends very much on the atmospheric conditions and the size of the meteorite. So far, few experiments have been done to investigate the effects of the landing process.

This work reports the selectivity of urazole, a prebiotic mimic of uracil, in its reactions with sugars. This selectivity is important in prebiotic synthesis of nucleosides. Urazole makes metal complexes. Thus, it may exist on meteorites as such. Some physical properties of urazole-metal complexes that are relevant for their recovery from the meteorites have been investigated.

An overview of biomineralization processes is given with special respect to phosphatic minerals formation. The properties of Phanerozoic apatite biominerals are described as being a biosignature of high rank.

Development of astrobotanical research by G.A. Tikhov in the Kazakh Academy of Sciences is considered seminal to today's astrobiology.

Many earlier attempts for computing periodicities in Mars sediment time series have proved to be quite successful with the Expert System for Spectral Analysis on continental deposits (ExSpect)—Matlab or Autoregressive (AR) Power Spectral Density (PSD) Cyclicity Calculation Method. The method was originally worked out for sediments interfering with datable key beds and fossil soil levels in recent (<2.4 Ma) deposits on Earth. Numerous time-bound (climatic) parameters could therefore be used on earth sediments. Since neither Mars sediment nor Mars sediment-boundary ages are known, recourse was taken to the relative sediment deposit genetic rate (RSDGR) as a standard parameter. For the purpose of the NATO-ASI course, different types of Mars images were selected, e.g.: images from sole ice layers of polar caps and images of sediment series performing permafrost features (cryoturbation levels, ice blocks, solifluction layers, etc.) indicating Earth-like climatic changes. The very latter can thus offer suggestions about the time-range that such climate changes may then represent. The results of cycle computation made on this basis were compared to those made for similar deposits on Earth. Since both were revealed to be highly comparable, the idea of cyclicities in resonance for both planets is hereby suggested and represented in tables at the end of this paper.

Morphological comparison of biomorphs in astrobiology is a commonly applied methodology to putative bacterial structures found in meteorites from Mars and other extraterrestrial bodies. Geomorphs, such as landscapes, sediment patterns and sequences, red soils, and permafrost polygonal patterns as surficial features, found on Earth find their perfect equivalents on the images of Mars. Moreover, landscape, sediment, red soil and permafrost analogues show a similar geographical distribution on Mars, and they are perfectly comparable to the distribution displayed on Earth. Since the habitat of life is confined to these surface geomorphs of Earth and Mars, it becomes a complex effort to disentangle whether or not the genesis of these geomorph parallels reflect a common evolution on Mars and Earth. These similarities in the shaping and weathering of the surface crusts of both planets point to comparable processes at some point in time and leave the question of why this common evolution is presumed not to continue today.

Rock coatings are ubiquitous in arid regions of the world. Amino acids in desert varnish coatings have been measured, and other organic compounds have been considered as chemical biosignatures in coatings. Understanding the mechanisms of formation or rock coatings and identifying their active and fossil biosignatures will provide useful methods for contrasting biotic and abiotic systems on Earth and other planetary bodies.

At the beginning of biological evolution before a protective ozone layer had developed in the atmosphere, high intensities of energy-rich solar ultraviolet (UV) radiation could reach Earth's surface. Today, the full spectrum of solar UV radiation is only experienced in space, where other important space parameters, such as vacuum, cosmic radiation, temperature extremes, and microgravity, influence survival and genetic stability. To reach a better understanding of the processes leading to the origin, evolution, and distribution of life on Earth, several in-space experiments have been performed with microorganisms. The ability of resistant life forms, such as bacterial spores, to survive high doses of extraterrestrial solar UV—alone or in combination with other space parameters, e.g.: vacuum—was investigated. The protective effects of organic as well as inorganic substances, such as artificial or real meteorite material, were determined in satellite experiments, on the Space Shuttle, and on the MIR station. It could be shown that thin layers of inorganic material are able to protect spores against the deleterious effects of the energy-rich UV radiation and that they are able to survive under these conditions for very long periods of time in space. With different cut-off filters, the effect of an increasing atmospheric ozone layer on the solar spectrum was simulated as it had occurred on Earth ˝2 Ga ago, and the resulting changes in the DNA damage inducing potential of solar UV radiation was investigated. Extraterrestrial solar UV radiation was found to have a thousand times higher biological effectiveness than UV radiation filtered by stratospheric ozone concentrations found today on Earth.

Bacterial paleontology is rather young branch of paleontological studies. Bacteria and microbes in general could be perfectly preserved as fossils. The major part of the sedimentary rocks formed in the photic zone of epicontinental basins of the past originated under the influence of microorganisms. Studies have increased the number of minerals known to be formed with the participation of microorganisms to >100. Bacterial-paleontological data accompanied by the data on the first origin of eukaryotes, metazoans, etc. significantly enrich the knowledge of evolution of the biosphere and reveal a long period of transitional biosphere prior to the appearance of the typical eukaryotic biosphere of the modern type. The bacterial-paleontological data on carbonaceus chondrites make a foundation for the possibility of the presence of extraterrestrial bacterial life.

Based on the currently known paleontological and biogeochemical record of life in the oldest terrestrial sediments and moderate extrapolations thereof, it may be stated with fair confidence that microbial (prokaryotic and archaeoprokaryotic) ecosystems had been prolific on early Earth from at least 3.8 Ga ago. While the information encoded in the oldest record (>3.5 Ga) is commonly impaired by a metamorphic overprint, the evidence for the existence of life at times <3.5 Ga seems so firmly established as to be virtually unassailable. This holds for both the morphological (cellular) record and the biogeochemical data. Specifically, the ¹³C/¹²C signature of fossil organic carbon conveys a remarkably consistent signal of biologically mediated (enzymatic) carbon isotope fractionations over ∼ 4 Ga of recorded geological history, suggesting an extreme degree of evolutionary conservatism in the biochemistry of (photo) autotrophic carbon fixation.

Postulating a universality of biological principles in analogy to the proven universality of the laws of physics and chemistry, it may be reasonably expected that the principal properties of extraterrestrial life are similar to those that characterize Earth-bound biology. Hence, the record of life preserved in Earth‘s oldest sediments should provide a sound baseline for the interpretation of extraterrestrial analogues. Inter alia, enzymatic reactions in exobiological systems ought to be beset with isotopic fractionations resembling those in earthly biochemistry, with ¹³C/¹²C values eventually retrieved from Martian rocks likely to constrain current conjectures on the existence of former life on Mars.

The electric discharges in gas-dusty atmospheres—the atmosphere of earlier Earth, the atmosphere of comets, and dust storms on Mars—can serve not only as an energy source for the formation of composite organic compounds, but also, in the case of gas-dusty atmospheres in dusty plasma of electric discharges, the formation of ordered structures of charged dust components is also possible. The dusty plasma with a high electronic temperature passes into a plasma-crystalline state with particular restrictions of initial scatter of grain velocities in a wide enough pressure range of gas component and discharge parameters. During these discharges, micron-sized particles can be charged up to magnitudes of 10⁴ of the elementary charge. These particles interact with the discharge plasma and form ordered structures. Scatter of particle velocities, at which this may occur, suggests the atmosphere of comets, 1 cm s⁻¹; high layers of an Earth's atmosphere, 100 cm s⁻¹; and dust devils on Mars, 250 cm s⁻¹.

Life depends on liquid water (H₂O), a suite of biogenic elements (carbon (C), oxygen (O), nitrogen (N), sulfur (S), and phosphorous (P)), and a useful source of free energy. All these requirements are on Titan, the largest satellite of Saturn. An internal liquid ocean and a dense atmosphere determine a great exobiological significance of the satellite. The putative ocean may have a chemical composition useful to living organisms, which could maintain different biogeochemical cycles there.

New data on the composition and genesis of volkonskoite, based on mineralogical investigation of the mineral are presented. Scanned electron microscopy (SEM) investigations of volkonskoites reveal an undoubted role of bacterial activity in volkonskoite formation, i.e.: in the chromium (Cr) concentration and fixation in the smectite structure.

A key issue in astrobiology research is and has been to understand the origin, evolution, and distribution of life on Earth and in the solar system. Consequently, crucial to this endeavor is the identification of means to unambiguously detect evidence of life. Efforts to identify traces of life in extraterrestrial materials or even remotely on the surface of Mars have resulted in ambiguous, non-conclusive information. As a result it has recently been proposed to pursue alternative technologies to help answer the challenge presented by the search for life elsewhere in the solar system. Here, an integration of approaches and technologies, developed and applied mainly in molecular biology and biotechnology with astrobiology is introduced. The overall concept behind this approach is to employ molecular biology tools and biotechnology to detect specific astrobiologically and geobiologically relevant target molecules. This is based around the high sensitivity, specificity, and affinity of proteins (antibodies) or DNA/RNA aptamers to a series of target molecules that define extinct and extant terrestrial life or prebiotic components. Advanced DNA and protein chip technology can be utilized to allow thousands of multiple tests in a single analytical step. Microarray technology can be combined with microfluidics to ultimately achieve high sensitivity and specificity in a lightweight automated device designed for solar system exploration.

Ancient phosphorites are one of the best model objects for studying fossilized microorganisms, their morphology and sizes, types of preservation, conditions of burial, and products of life activity. These studies help understanding and interpreting biomorphic structures of the astromaterials.

The study of possible astrobiological and ancient terrestrial biospheres might be based on an actualistic approach. The survey of relict microbial communities and their habitats demonstrates that actualistic microbial biogeochemistry might bring light to the early biosphere if it is based on the data from Earth sciences, which comprise the larger system. Scaling is the key issue for systemic approach. The general features of relict microbial communities, which are found in extreme habitats such as hydrotherms, hypersaline lagoons, and soda lakes, are described. They conform to demands for autonomous communities with closed biogeochemical cycles. Incomplete cycling leads to the biogeochemical succession. Reconstruction from actualistic studies should be based on the firm base of the paleorecord, obtained by geologists from the field studies of ancient rocks and the empirical data of microbial paleontology.