Showing papers in "Astrobiology in 2007"
TL;DR: It is concluded that attempts at remote sensing of biosignatures and nonbiological markers from M star planets are important, not as tests of any quantitative theories or rational arguments, but instead because they offer an inspection of the residues from a Gyr-long biochemistry experiment in the presence of extreme environmental fluctuations.
Abstract: The changing view of planets orbiting low mass stars, M stars, as potentially hospitable worlds for life and its remote detection was motivated by several factors, including the demonstration of viable atmospheres and oceans on tidally locked planets, normal incidence of dust disks, including debris disks, detection of planets with masses in the 5–20 M⊕ range, and predictions of unusually strong spectral biosignatures We present a critical discussion of M star properties that are relevant for the longand short-term thermal, dynamical, geological, and environmental stability of conventional liquid water habitable zone (HZ) M star planets, and the advantages and disadvantages of M stars as targets in searches for terrestrial HZ planets using various detection techniques Biological viability seems supported by unmatched very long-term stability conferred by tidal locking, small HZ size, an apparent short-fall of gas giant planet perturbers, immunity to large astrosphere compressions, and several other fact
408 citations
Search for extraterrestrial intelligence1, University of California, Los Angeles2, University of California, Berkeley3, Carnegie Institution for Science4, British Antarctic Survey5, Goddard Space Flight Center6, Southwest Research Institute7, Ames Research Center8, Case Western Reserve University9, Georgia State University10, Met Office11, National Center for Science Education12, Space Telescope Science Institute13, University of Texas at Austin14, California Institute of Technology15, California Academy of Sciences16, Michigan State University17, University of Washington18
TL;DR: It is concluded that M dwarf stars may indeed be viable hosts for planets on which the origin and evolution of life can occur and it makes sense to include M dwarfs in programs that seek to find habitable worlds and evidence of life.
Abstract: Stable, hydrogen-burning, M dwarf stars make up about 75% of all stars in the Galaxy. They are extremely long-lived, and because they are much smaller in mass than the Sun (between 0.5 and 0.08 MSun), their temperature and stellar luminosity are low and peaked in the red. We have re-examined what is known at present about the potential for a terrestrial planet forming within, or migrating into, the classic liquid–surface–water habitable zone close to an M dwarf star. Observations of protoplanetary disks suggest that planet-building materials are common around M dwarfs, but N-body simulations differ in their estimations of the likelihood of potentially habitable, wet planets that reside within their habitable zones, which are only about one-fifth to 1/50th of the width of that for a G star. Particularly in light of the claimed detection of the planets with masses as small as 5.5 and 7.5 MEarth orbiting M stars, there seems no reason to exclude the possibility of terrestrial planets. Tidally locked synchron...
388 citations
TL;DR: It is suggested that larger and more massive terrestrial-type exoplanets may better protect their atmospheres against CMEs, because the larger cores of such exoplanet would generate stronger magnetic moments and their higher gravitational acceleration would constrain the expansion of their thermosphere-exosphere regions and reduce atmospheric escape.
Abstract: Atmospheric erosion of CO2 -rich Earth-size exoplanets due to coronal mass ejection (CME)-induced ion pick up within close-in habitable zones of active M-type dwarf stars is investigated. Since M s...
346 citations
TL;DR: It is shown that Earth-like exoplanets within close-in HZs should experience a continuous CME exposure over long periods of time, which may result in little or no magnetospheric protection of planetary atmospheres from a dense flow of CME plasma.
Abstract: Low mass M- and K-type stars are much more numerous in the solar neighborhood than solar-like G-type stars. Therefore, some of them may appear as interesting candidates for the target star lists of terrestrial exoplanet (i.e., planets with mass, radius, and internal parameters identical to Earth) search programs like Darwin (ESA) or the Terrestrial Planet Finder Coronagraph/Inferometer (NASA). The higher level of stellar activity of low mass M stars, as compared to solar-like G stars, as well as the closer orbital distances of their habitable zones (HZs), means that terrestrial-type exoplanets within HZs of these stars are more influenced by stellar activity than one would expect for a planet in an HZ of a solar-like star. Here we examine the influences of stellar coronal mass ejection (CME) activity on planetary environments and the role CMEs may play in the definition of habitability criterion for the terrestrial type exoplanets near M stars. We pay attention to the fact that exoplanets within HZs that ...
291 citations
TL;DR: The "near-infrared (NIR) end" of the red edge to trend from blue-shifted to reddest for snow algae, temperate algae, lichens, mosses, aquatic plants, and finally terrestrial vascular plants is found.
Abstract: Why do plants reflect in the green and have a “red edge” in the red, and should extrasolar photosynthesis be the same? We provide (1) a brief review of how photosynthesis works, (2) an overview of the diversity of photosynthetic organisms, their light harvesting systems, and environmental ranges, (3) a synthesis of photosynthetic surface spectral signatures, and (4) evolutionary rationales for photosynthetic surface reflectance spectra with regard to utilization of photon energy and the planetary light environment. We found the “near-infrared (NIR) end” of the red edge to trend from blue-shifted to reddest for (in order) snow algae, temperate algae, lichens, mosses, aquatic plants, and finally terrestrial vascular plants. The red edge is weak or sloping in lichens. Purple bacteria exhibit possibly a sloping edge in the NIR. More studies are needed on pigment–protein complexes, membrane composition, and measurements of bacteria before firm conclusions can be drawn about the role of the NIR reflectance. Pig...
285 citations
TL;DR: In this paper, a line-by-line radiative transfer model was used to calculate the incident spectral photon flux densities at the surface of the planet and under water, identifying bands of available photosynthetically relevant radiation and finding that photosynthetic pigments on planets around F2V stars may peak in absorbance in the blue, K2V in the red-orange, and M stars in the near-infrared, in bands at 0.93-1.1 μm, 1.4 μm and 1.8-2.5...
Abstract: As photosynthesis on Earth produces the primary signatures of life that can be detected astronomically at the global scale, a strong focus of the search for extrasolar life will be photosynthesis, particularly photosynthesis that has evolved with a different parent star. We take previously simulated planetary atmospheric compositions for Earth-like planets around observed F2V and K2V, modeled M1V and M5V stars, and around the active M4.5V star AD Leo; our scenarios use Earth's atmospheric composition as well as very low O2 content in case anoxygenic photosynthesis dominates. With a line-by-line radiative transfer model, we calculate the incident spectral photon flux densities at the surface of the planet and under water. We identify bands of available photosynthetically relevant radiation and find that photosynthetic pigments on planets around F2V stars may peak in absorbance in the blue, K2V in the red-orange, and M stars in the near-infrared, in bands at 0.93–1.1 μm, 1.1–1.4 μm, 1.5–1.8 μ m, and 1.8–2.5...
273 citations
TL;DR: The Antarctic permafrost habitat and biodiversity is described and provided a model for martian ecosystems and what may be the oldest microbial communities on Earth is observed.
Abstract: Antarctic permafrost soils have not received as much geocryological and biological study as has been devoted to the ice sheet, though the permafrost is more stable and older and inhabited by more microbes. This makes these soils potentially more informative and a more significant microbial repository than ice sheets. Due to the stability of the subsurface physicochemical regime, Antarctic permafrost is not an extreme environment but a balanced natural one. Up to 104 viable cells/g, whose age presumably corresponds to the longevity of the permanently frozen state of the sediments, have been isolated from Antarctic permafrost. Along with the microbes, metabolic by-products are preserved. This presumed natural cryopreservation makes it possible to observe what may be the oldest microbial communities on Earth. Here, we describe the Antarctic permafrost habitat and biodiversity and provide a model for martian ecosystems.
240 citations
TL;DR: Findings indicate that most lichenized fungal and algal cells can survive in space after full exposure to massive UV and cosmic radiation, conditions proven to be lethal to bacteria and other microorganisms.
Abstract: This experiment was aimed at establishing, for the first time, the survival capability of lichens exposed to space conditions. In particular, the damaging effect of various wavelengths of extraterrestrial solar UV radiation was studied. The lichens used were the bipolar species Rhizocarpon geographicum and Xanthoria elegans, which were collected above 2000 m in the mountains of central Spain and as endolithic communities inhabiting granites in the Antarctic Dry Valleys. Lichens were exposed to space in the BIOPAN-5 facility of the European Space Agency; BIOPAN-5 is located on the outer shell of the Earth-orbiting FOTON-M2 Russian satellite. The lichen samples were launched from Baikonur by a Soyuz rocket on May 31, 2005, and were returned to Earth after 16 days in space, at which time they were tested for survival. Chlorophyll fluorescence was used for the measurement of photosynthetic parameters. Scanning electron microscopy in back-scattered mode, low temperature scanning electron microscopy, and transmission electron microscopy were used to study the organization and composition of both symbionts. Confocal laser scanning microscopy, in combination with the use of specific fluorescent probes, allowed for the assessment of the physiological state of the cells. All exposed lichens, regardless of the optical filters used, showed nearly the same photosynthetic activity after the flight as measured before the flight. Likewise, the multimicroscopy approach revealed no detectable ultrastructural changes in most of the algal and fungal cells of the lichen thalli, though a greater proportion of cells in the flight samples had compromised membranes, as revealed by the LIVE/DEAD BacLight Bacterial Viability Kit. These findings indicate that most lichenized fungal and algal cells can survive in space after full exposure to massive UV and cosmic radiation, conditions proven to be lethal to bacteria and other microorganisms. The lichen upper cortex seems to provide adequate protection against solar radiation. Moreover, after extreme dehydration induced by high vacuum, the lichens proved to be able to recover, in full, their metabolic activity within 24 hours.
211 citations
TL;DR: It is shown that the time-varying hydrostatic head of a tidally forced ice shell may drive hydrothermal fluid flow through the seafloor, which can generate moderate but potentially important heat through viscous interaction with the matrix of porous seafl floor rock.
Abstract: We examine means for driving hydrothermal activity in extraterrestrial oceans on planets and satellites of less than one Earth mass, with implications for sustaining a low level of biological activity over geological timescales. Assuming ocean planets have olivine-dominated lithospheres, a model for cooling-induced thermal cracking shows how variation in planet size and internal thermal energy may drive variation in the dominant type of hydrothermal system—for example, high or low temperature system or chemically driven system. As radiogenic heating diminishes over time, progressive exposure of new rock continues to the current epoch. Where fluid-rock interactions propagate slowly into a deep brittle layer, thermal energy from serpentinization may be the primary cause of hydrothermal activity in small ocean planets. We show that the time-varying hydrostatic head of a tidally forced ice shell may drive hydrothermal fluid flow through the seafloor, which can generate moderate but potentially important heat ...
190 citations
TL;DR: The European Space Agency's ExoMars mission will seek evidence of organic compounds of biological and non-biological origin at the martian surface, and it is necessary to define and prioritize specific molecular targets for antibody development.
Abstract: The European Space Agency's ExoMars mission will seek evidence of organic compounds of biological and non-biological origin at the martian surface. One of the instruments in the Pasteur payload may be a Life Marker Chip that utilizes an immunoassay approach to detect specific organic molecules or classes of molecules. Therefore, it is necessary to define and prioritize specific molecular targets for antibody development. Target compounds have been selected to represent meteoritic input, fossil organic matter, extant (living, recently dead) organic matter, and contamination. Once organic molecules are detected on Mars, further information is likely to derive from the detailed distribution of compounds rather than from single molecular identification. This will include concentration gradients beneath the surface and gradients from generic to specific compounds. The choice of biomarkers is informed by terrestrial biology but is wide ranging, and nonterrestrial biology may be evident from unexpected molecular distributions. One of the most important requirements is to sample where irradiation and oxidation are minimized, either by drilling or by using naturally excavated exposures. Analyzing regolith samples will allow for the search of both extant and fossil biomarkers, but sequential extraction would be required to optimize the analysis of each of these in turn.
187 citations
TL;DR: The radiolytically processed surface of Europa could serve to maintain an oxidized ocean even if the surface oxidants are delivered only once every approximately 0.5 Gyr, which would be energetically hospitable for terrestrial marine macrofauna.
Abstract: Europa is a prime target for astrobiology. The presence of a global subsurface liquid water ocean and a composition likely to contain a suite of biogenic elements make it a compelling world in the search for a second origin of life. Critical to these factors, however, may be the availability of energy for biological processes on Europa. We have examined the production and availability of oxidants and carbon-containing reductants on Europa to better understand the habitability of the subsurface ocean. Data from the Galileo Near-Infrared Mapping Spectrometer were used to constrain the surface abundance of CO2 to 0.036% by number relative to water. Laboratory results indicate that radiolytically processed CO2-rich ices yield CO and H2CO3; the reductants H2CO, CH3OH, and CH4 are at most minor species. We analyzed chemical sources and sinks and concluded that the radiolytically processed surface of Europa could serve to maintain an oxidized ocean even if the surface oxidants (O2, H2O2, CO2, SO2, and SO4 2−) ar...
TL;DR: The results indicate that ultramafic-hosted systems are capable of supplying about twice as much chemical energy as analogous deep-sea hydrothermal systems hosted in basaltic rocks.
Abstract: Numerical models are employed to investigate sources of chemical energy for autotrophic microbial metabolism that develop during mixing of oxidized seawater with strongly reduced fluids discharged from ultramafic-hosted hydrothermal systems on the seafloor. Hydrothermal fluids in these systems are highly enriched in H2 and CH4 as a result of alteration of ultramafic rocks (serpentinization) in the subsurface. Based on the availability of chemical energy sources, inferences are made about the likely metabolic diversity, relative abundance, and spatial distribution of microorganisms within ultramafic-hosted systems. Metabolic reactions involving H2 and CH4, particularly hydrogen oxidation, methanotrophy, sulfate reduction, and methanogenesis, represent the predominant sources of chemical energy during fluid mixing. Owing to chemical gradients that develop from fluid mixing, aerobic metabolisms are likely to predominate in low-temperature environments (<20–30°C), while anaerobes will dominate higher-temperat...
TL;DR: A new model for water delivery to terrestrial planets in dynamically calm systems, with low-eccentricity or low-mass giant planets-such systems may be very common in the Galaxy is presented.
Abstract: The water content and habitability of terrestrial planets are determined during their final assembly, from perhaps 100 1,000-km “planetary embryos” and a swarm of billions of 1–10-km “planetesimals.” During this process, we assume that water-rich material is accreted by terrestrial planets via impacts of water-rich bodies that originate in the outer asteroid region. We present analysis of water delivery and planetary habitability in five high-resolution simulations containing about 10 times more particles than in previous simulations. These simulations formed 15 terrestrial planets from 0.4 to 2.6 Earth masses, including five planets in the habitable zone. Every planet from each simulation accreted at least the Earth's current water budget; most accreted several times that amount (assuming no impact depletion). Each planet accreted at least five water-rich embryos and planetesimals from the past 2.5 astronomical units; most accreted 10–20 water-rich bodies. We present a new model for water delivery to ter...
TL;DR: RR spectroscopy was shown to be a useful tool for the analysis and remote in situ detection of carotenoids from halophilic archaea without the need for large sample sizes and complicated extractions, which are required by analytical techniques such as high performance liquid chromatography and mass spectrometry.
Abstract: Recently, halite and sulfate evaporate rocks have been discovered on Mars by the NASA rovers, Spirit and Opportunity. It is reasonable to propose that halophilic microorganisms could have potentially flourished in these settings. If so, biomolecules found in microorganisms adapted to high salinity and basic pH environments on Earth may be reliable biomarkers for detecting life on Mars. Therefore, we investigated the potential of Resonance Raman (RR) spectroscopy to detect biomarkers derived from microorganisms adapted to hypersaline environments. RR spectra were acquired using 488.0 and 514.5 nm excitation from a variety of halophilic archaea, including Halobacterium salinarum NRC-1, Halococcus morrhuae, and Natrinema pallidum. It was clearly demonstrated that RR spectra enhance the chromophore carotenoid molecules in the cell membrane with respect to the various protein and lipid cellular components. RR spectra acquired from all halophilic archaea investigated contained major features at approximately 1000, 1152, and 1505 cm(-1). The bands at 1505 cm(-1) and 1152 cm(-1) are due to in-phase C=C (nu(1) ) and C-C stretching ( nu(2) ) vibrations of the polyene chain in carotenoids. Additionally, in-plane rocking modes of CH(3) groups attached to the polyene chain coupled with C-C bonds occur in the 1000 cm(-1) region. We also investigated the RR spectral differences between bacterioruberin and bacteriorhodopsin as another potential biomarker for hypersaline environments. By comparison, the RR spectrum acquired from bacteriorhodopsin is much more complex and contains modes that can be divided into four groups: the C=C stretches (1600-1500 cm(-1)), the CCH in-plane rocks (1400-1250 cm(-1)), the C-C stretches (1250-1100 cm(-1)), and the hydrogen out-of-plane wags (1000-700 cm(-1)). RR spectroscopy was shown to be a useful tool for the analysis and remote in situ detection of carotenoids from halophilic archaea without the need for large sample sizes and complicated extractions, which are required by analytical techniques such as high performance liquid chromatography and mass spectrometry.
TL;DR: Findings indicate that deep Precambrian Shield fracture waters contain some of the highest levels of dissolved H(2) ever reported and represent a potentially important energy-rich environment for subsurface microbial life.
Abstract: Dissolved H2 concentrations up to the mM range and H2 levels up to 9–58% by volume in the free gas phase are reported for groundwaters at sites in the Precambrian shields of Canada and Finland. Along with previously reported dissolved H2 concentrations up to 7.4 mM for groundwaters from the Witwatersrand Basin, South Africa, these findings indicate that deep Precambrian Shield fracture waters contain some of the highest levels of dissolved H2 ever reported and represent a potentially important energy-rich environment for subsurface microbial life. The δ 2H isotope signatures of H2 gas from Canada, Finland, and South Africa are consistent with a range of H2-producing water-rock reactions, depending on the geologic setting, which include both serpentinization and radiolysis. In Canada and Finland, several of the sites are in Archean greenstone belts characterized by ultramafic rocks that have under-gone serpentinization and may be ancient analogues for serpentinite-hosted gases recently reported at the Lost...
TL;DR: The current understanding of the controls upon the distribution of euendolithic microborings are reviewed and three lines of approach are proposed for testing their biogenicity: first, a geological setting that demonstrates a syngenetic origin for the eu endolithicmicroborings; second, microboring morphologies and distributions that are suggestive of biogenic behavior and distinct from ambient inclusion trails; and third, elemental and isotopic evidence suggestive of biological processing.
Abstract: Micron-sized cavities created by the actions of rock-etching microorganisms known as euendoliths are explored as a biosignature for life on early Earth and perhaps Mars. Rock-dwelling organisms can tolerate extreme environmental stresses and are excellent candidates for the colonization of early Earth and planetary surfaces. Here, we give a brief overview of the fossil record of euendoliths in both sedimentary and volcanic rocks. We then review the current understanding of the controls upon the distribution of euendolithic microborings and use these to propose three lines of approach for testing their biogenicity: first, a geological setting that demonstrates a syngenetic origin for the euendolithic microborings; second, microboring morphologies and distributions that are suggestive of biogenic behavior and distinct from ambient inclusion trails; and third, elemental and isotopic evidence suggestive of biological processing. We use these criteria and the fossil record of terrestrial euendoliths to outline potential environments and techniques to search for endolithic microborings on Mars.
TL;DR: Because the biological need for energy is universal, the energy balance construct also helps to constrain habitability in systems (e.g., those envisioned to use solvents other than water) for which little constraint currently exists.
Abstract: Habitability can be formulated as a balance between the biological demand for energy and the corresponding potential for meeting that demand by transduction of energy from the environment into biological process. The biological demand for energy is manifest in two requirements, analogous to the voltage and power requirements of an electrical device, which must both be met if life is to be supported. These requirements exhibit discrete (non-zero) minima whose magnitude is set by the biochemistry in question, and they are increased in quantifiable fashion by (i) deviations from biochemically optimal physical and chemical conditions and (ii) energy-expending solutions to problems of resource limitation. The possible rate of energy transduction is constrained by (i) the availability of usable free energy sources in the environment, (ii) limitations on transport of those sources into the cell, (iii) upper limits on the rate at which energy can be stored, transported, and subsequently liberated by biochemical mechanisms (e.g., enzyme saturation effects), and (iv) upper limits imposed by an inability to use "power" and "voltage" at levels that cause material breakdown. A system is habitable when the realized rate of energy transduction equals or exceeds the biological demand for energy. For systems in which water availability is considered a key aspect of habitability (e.g., Mars), the energy balance construct imposes additional, quantitative constraints that may help to prioritize targets in search-for-life missions. Because the biological need for energy is universal, the energy balance construct also helps to constrain habitability in systems (e.g., those envisioned to use solvents other than water) for which little constraint currently exists.
TL;DR: It is shown that two populations of chiral crystals of left and right hand cannot coexist in solution: one of the chiral populations disappears in an irreversible autocatalytic process that nurtures the other one.
Abstract: Chiral symmetry breaking occurs when a physical or chemical process spontaneously generates a large excess of one of the two enantiomers—left-handed (L) or right-handed (D)––with no preference as to which of the two enantiomers is produced. From the viewpoint of energy, these two enantiomers can exist with an equal probability, and inorganic processes that involve chiral products commonly yield a racemic mixture of both. The fact that biologically relevant molecules exist only as one of the two enantiomers is a fascinating example of complete symmetry breaking in chirality and has long intrigued the science community. The origin of this selective chirality has remained a fundamental enigma with regard to the origin of life since the time of Pasteur, some 140 years ago. Here, it is shown that two populations of chiral crystals of left and right hand cannot coexist in solution: one of the chiral populations disappears in an irreversible autocatalytic process that nurtures the other one. Final and complete c...
TL;DR: Field, petrographic, isotopic, and trace element evidence indicate that most carbonaceous matter represents sedimentary material that originated by biogenic processes in the Archean oceans and not by hydrothermal processes inThe subsurface.
Abstract: The 3.5-3.2 Ga old volcano-sedimentary succession of the Barberton greenstone belt (South Africa) is characterized by lithological units that are repeated in a regular manner. Komatiitic, basaltic, and dacitic volcanic and volcaniclastic sequences are capped by zones of silica enrichment, followed by bedded carbonaceous cherts. Stratiform and crosscutting carbonaceous chert veins are common in silica alteration zones and bedded cherts. A detailed field study of several chert horizons and chert veins that range in age from 3.47 to 3.30 Ga revealed the importance of syndepositional hydrothermal activity for their origin. Bedded cherts consist of silicified detrital and tuffaceous sediments that were deposited on the seafloor. Silicification took place at the sediment-water interface as a result of diffuse upflow of low-temperature hydrothermal fluids, which gave rise to the formation of impermeable chert caps. Fluid overpressure resulted in the breaching of the cap rocks at times. Chert veins contain angular host rock fragments, replace wall rocks, and show evidence of multiple vein fillings and in situ brecciation of earlier generations of vein fillings. They represent hydraulic fractures that were initiated by overpressuring of the hydrothermal system. The vein networks were infilled, partly by hydrothermal chert precipitates, and partly by still unconsolidated (not yet silicified) sedimentary material derived from overlying sedimentary horizons. Field, petrographic, isotopic, and trace element evidence indicate that most carbonaceous matter represents sedimentary material that originated by biogenic processes in the Archean oceans and not by hydrothermal processes in the subsurface.
TL;DR: Comparing calculated radiolytic H(2) production rates to estimates of net (organic-fueled) respiration at several Ocean Drilling Program (ODP) Leg 201 sites suggests that radiolysis gains importance as an electron donor source as net respiration and organic carbon content decrease.
Abstract: Radiolysis of water may provide a continuous flux of an electron donor (molecular hydrogen) to subsurface microbial communities. We assessed the significance of this process in anoxic marine sediments by comparing calculated radiolytic H2 production rates to estimates of net (organic-fueled) respiration at several Ocean Drilling Program (ODP) Leg 201 sites. Radiolytic H2 yield calculations are based on abundances of radioactive elements (uranium, thorium, and potassium), porosity, grain density, and a model of water radiolysis. Net respiration estimates are based on fluxes of dissolved electron acceptors and their products. Comparison of radiolytic H2 yields and respiration at multiple sites suggests that radiolysis gains importance as an electron donor source as net respiration and organic carbon content decrease. Our results suggest that radiolytic production of H2 may fuel 10% of the metabolic respiration at the Leg 201 site where organic-fueled respiration is lowest (ODP Site 1231). In sediments with ...
TL;DR: The observation of a well-defined rainbow scattering peak at the "primary rainbow" scattering angle is shown to be feasible with the proposed Terrestrial Planet Finder Coronograph mission in similar total integration times to those required for spectroscopic characterization.
Abstract: Current proposals for the characterization of extrasolar terrestrial planets rest primarily on the use of spectroscopic techniques. While spectroscopy is effective in detecting the gaseous components of a planet's atmosphere, it provides no way of detecting the presence of liquid water, the defining characteristic of a habitable planet. In this paper, I investigate the potential of an alternative technique for characterizing the atmosphere of a planet using polarization. By looking for a polarization peak at the “primary rainbow” scattering angle, it is possible to detect the presence of liquid droplets in a planet's atmosphere and constrain the nature of the liquid through its refractive index. Single scattering calculations are presented to show that a well-defined rainbow scattering peak is present over the full range of likely cloud droplet sizes and clearly distinguishes the presence of liquid droplets from solid particles such as ice or dust. Rainbow scattering has been used in the past to determine...
TL;DR: Low levels of acetonitrile have been interpreted within the context of a Hadean placer beach prebiotic framework to demonstrate the promise of investigating natural nuclear reactors as power production sites that might have assisted the origins of life on young rocky planets with a sufficiently differentiated crust/mantle structure.
Abstract: There are growing indications that life began in a radioactive beach environment. A geologic framework for the origin or support of life in a Hadean heavy mineral placer beach has been developed, based on the unique chemical properties of the lower-electronic actinides, which act as nuclear fissile and fertile fuels, radiolytic energy sources, oligomer catalysts, and coordinating ions (along with mineralogically associated lanthanides) for prototypical prebiotic homonuclear and dinuclear metalloenzymes. A four-factor nuclear reactor model was constructed to estimate how much uranium would have been required to initiate a sustainable fission reaction within a placer beach sand 4.3 billion years ago. It was calculated that about 1–8 weight percent of the sand would have to have been uraninite, depending on the weight percent, uranium enrichment, and quantity of neutron poisons present within the remaining placer minerals. Radiolysis experiments were conducted with various solvents with the use of uranium- a...
TL;DR: The mechanism for prebiotic pyrimidine-ring formation of monocyclic HCN-pentamers with ab initio electronic structure theory is studied, and the results primarily inform discussions of adenine synthesis in interstellar space.
Abstract: The question whether the nucleobases can be synthesized in interstellar space is of fundamental significance in considerations of the origin of life. Adenine is formally the HCN pentamer, and exper...
TL;DR: The tolerances that make tardigrades suitable for astrobiological studies and the reported radiation tolerance in other anhydrobiotic animals are reviewed, and the possible involvement of an efficient, but yet undocumented, mechanism for DNA repair is discussed.
Abstract: Exposure of living organisms to open space requires a high level of tolerance to desiccation, cold, and radiation. Among animals, only anhydrobiotic species can fulfill these requirements. The invertebrate phylum Tardigrada includes many anhydrobiotic species, which are adapted to survive in very dry or cold environmental conditions. As a likely by-product of the adaptations for desiccation and freezing, tardigrades also show a very high tolerance to a number of other, unnatural conditions, including exposure to ionizing radiation. This makes tardigrades an interesting candidate for experimental exposure to open space. This paper reviews the tolerances that make tardigrades suitable for astrobiological studies and the reported radiation tolerance in other anhydrobiotic animals. Several studies have shown that tardigrades can survive gamma-irradiation well above 1 kilogray, and desiccated and hydrated (active) tardigrades respond similarly to irradiation. Thus, tolerance is not restricted to the dry anhydrobiotic state, and I discuss the possible involvement of an efficient, but yet undocumented, mechanism for DNA repair. Other anhydrobiotic animals (Artemia, Polypedium), when dessicated, show a higher tolerance to gamma-irradiation than hydrated animals, possibly due to the presence of high levels of the protective disaccharide trehalose in the dry state. Tardigrades and other anhydrobiotic animals provide a unique opportunity to study the effects of space exposure on metabolically inactive but vital metazoans.
TL;DR: The model results suggest that such signals are robust, changing in the M star world atmospheric column due to GCR NOx effects by up to 20% compared to an M star run without GCR effects, and can therefore survive at least the effects of GCRs.
Abstract: Planets orbiting in the habitable zone of M dwarf stars are subject to high levels of galactic cosmic rays (GCRs), which produce nitrogen oxides (NOx) in Earth-like atmospheres. We investigate to what extent these NO(Mx) species may modify biomarker compounds such as ozone (O3) and nitrous oxide (N2O), as well as related compounds such as water (H2O) (essential for life) and methane (CH4) (which has both abiotic and biotic sources). Our model results suggest that such signals are robust, changing in the M star world atmospheric column due to GCR NOx effects by up to 20% compared to an M star run without GCR effects, and can therefore survive at least the effects of GCRs. We have not, however, investigated stellar cosmic rays here. CH4 levels are about 10 times higher on M star worlds than on Earth because of a lowering in hydroxyl (OH) in response to changes in the ultraviolet. The higher levels of CH4 are less than reported in previous studies. This difference arose partly because we used different biogenic input. For example, we employed 23% lower CH4 fluxes compared to those studies. Unlike on Earth, relatively modest changes in these fluxes can lead to larger changes in the concentrations of biomarker and related species on the M star world. We calculate a CH4 greenhouse heating effect of up to 4K. O3 photochemistry in terms of the smog mechanism and the catalytic loss cycles on the M star world differs considerably compared with that of Earth.
TL;DR: It is concluded that there are a limited number of reactions that can yield energy through formate reduction, in contrast to numerous formate oxidation reactions that could yield abundant energy for chemosynthetic microorganisms.
Abstract: Formate, a simple organic acid known to support chemotrophic hyperthermophiles, is found in hot springs of varying temperature and pH However, it is not yet known how metabolic strategies that use formate could contribute to primary productivity in hydrothermal ecosystems In an effort to provide a quantitative framework for assessing the role of formate metabolism, concentration data for dissolved formate and many other solutes in samples from Yellowstone hot springs were used, together with data for coexisting gas compositions, to evaluate the overall Gibbs energy for many reactions involving formate oxidation or reduction The result is the first rigorous thermodynamic assessment of reactions involving formate oxidation to bicarbonate and reduction to methane coupled with various forms of iron, nitrogen, sulfur, hydrogen, and oxygen for hydrothermal ecosystems We conclude that there are a limited number of reactions that can yield energy through formate reduction, in contrast to numerous formate oxid
TL;DR: The biologically destructive effects of atmospheric transit can generate entirely novel and improved endolithic habitats for organisms on the destination planetary body that survive the dispersal filter, and advances the understanding of how island biogeography works on the interplanetary scale.
Abstract: We launched a cryptoendolithic habitat, made of a gneissic impactite inoculated with Chroococcidiopsis sp., into Earth orbit. After orbiting the Earth for 16 days, the rock entered the Earth's atmosphere and was recovered in Kazakhstan. The heat of entry ablated and heated the rock to a temperature well above the upper temperature limit for life to below the depth at which light levels are insufficient for photosynthetic organisms (∼5 mm), thus killing all of its photosynthetic inhabitants. This experiment shows that atmospheric transit acts as a strong biogeographical dispersal filter to the interplanetary transfer of photosynthesis. Following atmospheric entry we found that a transparent, glassy fusion crust had formed on the outside of the rock. Re-inoculated Chroococcidiopsis grew preferentially under the fusion crust in the relatively unaltered gneiss beneath. Organisms under the fusion grew approximately twice as fast as the organisms on the control rock. Thus, the biologically destructive effects o...
TL;DR: The results show that two pathways exist for the abiotic fixation of nitrogen from the atmosphere into the crust: one via HNO and another via NO(2), which could represent fixation on a planet with liquid water (and hence would also be a source of nitrogen for the origin of life).
Abstract: Understanding the abiotic fixation of nitrogen is critical to understanding planetary evolution and the potential origin of life on terrestrial planets. Nitrogen, an essential biochemical element, is certainly necessary for life as we know it to arise. The loss of atmospheric nitrogen can result in an incapacity to sustain liquid water and impact planetary habitability and hydrological processes that shape the surface. However, our current understanding of how such fixation may occur is almost entirely theoretical. This work experimentally examines the chemistry, in both gas and aqueous phases, that would occur from the formation of NO and CO by the shock heating of a model carbon dioxide/nitrogen atmosphere such as is currently thought to exist on early terrestrial planets. The results show that two pathways exist for the abiotic fixation of nitrogen from the atmosphere into the crust: one via HNO and another via NO2. Fixation via HNO, which requires liquid water, could represent fixation on a planet wit...
TL;DR: The range of their size, ultrastructures, and isotopic signatures suggests that the morphology and geochemistry of the Wutai graphite discs were overprinted by metamorphism and their ultimate carbon source probably had diverse origins that included abiotic processes.
Abstract: Abundant graphite particles occur in amphibolite-grade quartzite of the Archean-Paleoproterozoic Wutai Metamorphic Complex in the Wutaishan area of North China. Petrographic thin section observations suggest that the graphite particles occur within and between quartzite clasts and are heterogeneous in origin. Using HF maceration techniques, the Wutai graphite particles were extracted for further investigation. Laser Raman spectroscopic analysis of a population of extracted graphite discs indicated that they experienced a maximum metamorphic temperature of 513 +/- 50 degrees C, which is consistent with the metamorphic grade of the host rock and supports their indigenicity. Scanning and transmission electron microscopy revealed that the particles bear morphological features (such as hexagonal sheets of graphite crystals) related to metamorphism and crystal growth, but a small fraction of them (graphite discs) are characterized by a circular morphology, distinct marginal concentric folds, surficial wrinkles, and complex nanostructures. Ion microprobe analysis of individual graphite discs showed that their carbon isotope compositions range from -7.4 per thousand to -35.9 per thousand V-PDB (Vienna Pee Dee Belemnite), with an average of -20.3 per thousand, which is comparable to bulk analysis of extracted carbonaceous material. The range of their size, ultrastructures, and isotopic signatures suggests that the morphology and geochemistry of the Wutai graphite discs were overprinted by metamorphism and their ultimate carbon source probably had diverse origins that included abiotic processes. We considered both biotic and abiotic origins of the carbon source and graphite disc morphologies and cannot falsify the possibility that some circular graphite discs characterized by marginal folds and surficial wrinkles represent deflated, compressed, and subsequently graphitized organic-walled vesicles. Together with reports by other authors of acanthomorphic acritarchs from greenschist-amphibolite-grade metamorphic rocks, this study suggests that it is worthwhile to examine carbonaceous materials preserved in highly metamorphosed rocks for possible evidence of ancient life.
TL;DR: To unify the general and specific modes of understanding life requires a frame of reference that is applicable across a broad spectrum of alternative biological models yet capable of incisive insight into terrestrial biology.
Abstract: 819 AKEY CHALLENGE IN ASTROBIOLOGY is to comprehend life and its interaction with the environment at a level sufficiently fundamental to embrace the alternative biochemistries that may be encountered in a search for life elsewhere (Baross et al., 2007). Life on Earth presents us with a single (albeit highly diversified) biochemical model around which to build this comprehension. This model is extremely valuable in providing an empirical starting point for understanding metabolic potential and environmental tolerance, and as a continuing “reality check” on whatever generalized concepts of life may be developed. Reference to this single example, however, also carries the risk of narrowing our sense of possibility—of leading us to define biochemistry, habitability, and biosignatures in terms so specific that they may exclude different forms of life. The benefits and risks of reference to terrestrial life are both increasing as the current revolution in molecular biology unfolds. As our understanding of terrestrial biology rapidly expands, we are presented with an ever more complete and refined model to which we can refer. But this advance in understanding is fueled by tools, approaches, and insights that (appropriately) are increasingly and remarkably specific with respect to terrestrial biochemistry. Research in astrobiology has much to gain by harnessing these tools and approaches but must simultaneously strive for breadth and generality in understanding life. The need for well-defined yet broadly applicable notions of habitability and biosignatures will become critical as astrobiological objectives are factored increasingly into space missions. The conception, parameterization, and instrumentation of these missions will require concrete determinations of where to look and what information to seek, and interpretation of the resultant data will require considerable plasticity in our conception of what constitutes evidence of life. Thus, we must not only pursue, in parallel, both a specific and general understanding of life and its functions, but also be continually prepared to unite these modes of understanding toward optimal development of astrobiological missions. To unify the general and specific modes of understanding life requires a frame of reference that is applicable across a broad spectrum of alternative biological models yet capable of incisive insight into terrestrial biology. Consideration of the biological relationship with energy offers one such frame of reference.