Astrobiology最新文献

Nontraditional stable isotopes of bioactive metals emerged as novel proxies for reconstructing the biogeochemical cycling of metals, which serve as cofactors in major metabolic pathways. The fractionation of metal isotopes between ambient fluid and microorganisms is ultimately recorded in authigenic minerals, such as carbonates, which makes them potentially more reliable than standard biomarkers in organic matter. Stromatolitic carbonates are geochemical archives that allow for the study of the long-term interplay of the biosphere, atmosphere, and hydrosphere through deep time, with the unique potential to investigate early life environments and the evolution of the metallome. The present study uses stromatolites from the ∼2.95-billion-year-old Pongola Supergroup (S. Africa) as a field laboratory for combined in situ trace metal mapping and layer-specific, novel stable metal isotope compositions to infer early Earth microbial metal cycling via phototrophic and chemo-litho-autotrophic metabolisms. Quantitative in situ trace element maps reveal intrinsic biosedimentary enrichments of nickel (Ni), cadmium (Cd), phosphorus (P), iron (Fe), and manganese (Mn) in stromatolitic laminae. In contrast, barium (Ba) shows a more homogeneous distribution. Authigenic carbonates from pristine stromatolite laminae show distinct δ¹³⁸Ba and δ¹¹²Cd fractionation above detrital background and bulk silicate Earth values, but opposing correlation with trace metal concentrations. Authigenic δ⁶⁰Ni values overlap with Mesoarchean diamictite compositions. Nickel isotopic compositions in authigenic stromatolitic carbonates, potentially a new proxy for methanogenic metal uptake, do not show any proof of the presence of this metabolism in the samples of this study. Meanwhile, Cd isotopic compositions in authigenic carbonates follow typical Rayleigh-type isotope fractionation; that is, the isotopic composition of Cd evolves to heavy values close to modern surface compositions. Correlations of δ¹¹²Cd with the micronutrients copper (Cu), molybdenum (Mo), and P, at positively fractionated carbon (C) isotopes (δ¹³C ∼+2‰), argue for active photosynthesis in the Pongola microbial habitat. We show that Ba isotopes can be used to infer carbonate precipitation rates similar to modern microbial carbonates. Thus, the combination of Cd and Ni isotopes has the unique potential as novel isotope biomarkers for the biochemical sedimentary record of early Earth where traditional lipid biomarkers are not applicable due to the incomplete preservation of organic matter.

Life is a complex, dynamic chemical system that requires a dense fluid solvent in which to take place. A common assumption is that the most likely solvent for life is liquid water, and some researchers argue that water is the only plausible solvent. However, a persistent theme in astrobiological research postulates that other liquids might be cosmically common and could be solvents for the chemistry of life. In this article, we present a new framework for the analysis of candidate solvents for life, and we deploy this framework to review substances that have been suggested as solvent candidates. We categorize each solvent candidate through the following four criteria: occurrence, solvation, solute stability, and solvent chemical functionality. Our semiquantitative approach addresses all the requirements for a solvent not only from the point of view of its chemical properties but also from the standpoint of its biochemical function. Only the protonating solvents fulfill all the chemical requirements to be a solvent for life, and of those only water and concentrated sulfuric acid are also likely to be abundant in a rocky planetary context. Among the nonprotonating solvents, liquid CO₂ stands out as a planetary solvent, and its potential as a solvent for life should be explored. We conclude with a discussion of whether it is possible for a biochemistry to change solvents as an adaptation to radical changes in a planet's environment. Our analysis provides the basis for prioritizing future experimental work to explore potential complex chemistry on other planets. Key Words: Habitability-Alternative solvents for life-Alternative biochemistry. Astrobiology 24, 1231-1256.

Amino acids have been identified in extraterrestrial materials such as meteorites and returned samples from asteroids and comets. Some of these amino acids or their precursors may have formed on icy interstellar dust grains or at a later phase when these grains became incorporated into larger parent bodies. In this work, we simulated parent body aqueous alteration of the residues from irradiated interstellar ice analogs in the presence of relevant minerals (pulverized serpentinite and Allende meteorite). We tracked the change in amino acid abundances as a function of hydrothermal processing time and examined how these differed based on the presence of minerals. We find that the presence of minerals and their mineralogy can have a significant impact on the formation and destruction of amino acids during simulated aqueous alteration experiments.

Life on Earth relies on mechanisms to store heritable information and translate this information into cellular machinery required for biological activity. In all known life, storage, regulation, and translation are provided by DNA, RNA, and ribosomes. Life beyond Earth, even if ancestrally or chemically distinct from life as we know it, may utilize similar structures: it has been proposed that charged linear polymers analogous to nucleic acids may be responsible for storage and regulation of genetic information in nonterran biochemical systems. We further propose that a ribosome-like structure may also exist in such a system, due to the evolutionary advantages of separating heritability from cellular machinery. In this study, we use a solid-state nanopore to detect DNA, RNA, and ribosomes, and we demonstrate that machine learning can distinguish between biomolecule samples and accurately classify new data. This work is intended to serve as a proof of principal that such biosignatures (i.e., informational polymers or translation apparatuses) could be detected, for example, as part of future missions targeting extant life on Ocean Worlds. A negative detection does not imply the absence of life; however, the detection of ribosome-like structures could provide a robust and sensitive method to seek extant life in combination with other methods. Key Words: RNA world-Darwinian evolution-Nucleic acids-Agnostic life detection. Astrobiology 24, 1257-1274.

If ocean-derived materials are present at Europa's surface, they would represent accessible records of ocean chemistry and habitability, but such materials would be further processed by Europa's harsh radiation environment. In this study, saturated fatty acids were precipitated onto a Europa-relevant hydrated magnesium sulfate and exposed to gamma radiation doses up to 2 MGy at -196°C. Alkane chains, with carbon numbers one less than those of the starting fatty acids, were the most abundant radiolysis products in solvent and thermal extracts analyzed by gas chromatography mass spectrometry. Detections of monounsaturated fatty acids and combined radiolysis products were attributed to the experiment's Europa-like parameters. Additionally, elevated concentrations of shorter-chain saturated fatty acids suggest that gamma radiation induced charge remote fragmentation of the alkyl chains of some starting fatty acids under these experimental conditions. Quantitation of fatty acid concentrations in the irradiated samples enabled the calculation of a radiolysis constant that indicated exposure to a 5 MGy dose of gamma radiation would have resulted in a ∼90% loss of the initial fatty acid population. The samples were further studied by Raman spectroscopy and laser desorption and ionization mass spectrometry, which characterized the distribution of fatty acids and their radiolysis products on sulfate surfaces. The substantial loss of starting fatty acids typically seen with increasing radiation dose, along with the remarkable diversity of radiolysis products identified, suggests that the detection of fatty acids in irradiated sulfate deposits on Europa will be challenged by rapid destruction of any initial fatty acid populations and scrambling of their residual signals by a myriad of organic radiolysis products. If missions to Europa encounter sulfate deposits, targeting minimally irradiated units may still enable the detection of surviving fatty acid signatures that could inform about Europa's subsurface chemistry and habitability.

Oxidative weathering is a major source of bio-essential micronutrients on Earth today; however, this flux would have been muted on the early Earth or on Mars, where atmospheric O_2(g) levels were very low. Here, we explore the hypothesis that nitrogen oxides generated by lightning in an anoxic atmosphere could have elevated pyrite oxidation levels under otherwise anoxic conditions. We performed spark discharge experiments in the presence of pyrite powder and three different gas mixtures, including 80% N_2(g) with 20% CO_2(g), 95% N_2(g) with 5% CO_2(g), and modern air. Experiments were run for 30 min, and we tracked the production of NO_(g), dissolved nitrate and nitrite, pH, dissolved sulfate, and total dissolved iron. Our results reveal increasing production of nitrogen oxides with increasing CO_2(g) and O_2(g) levels, which is consistent with previous studies. Dissolved iron and sulfate also increase, indicating that the nitrogen oxides are able to oxidize pyrite abiotically. Extrapolating these data to global conditions suggests that this mechanism was probably insignificant on a global scale on the early Earth; however, in thunderstorm-prone areas, such as in the modern tropics where lightning rates may locally be over 10 times above the global average, lightning could have rivalled abiotic pyrite oxidation by Archean O₂ levels. The lightning contribution would have been highest during time periods with elevated CO_2(g), which makes it a potentially important contributor to local release of sulphur, iron, and bio-essential micronutrients on prebiotic land surfaces or on other planets with anoxic CO₂-rich atmospheres.

Prebiotic synthesis of complex organic molecules in water-rich environments has been a long-standing challenge. In the modern deep sea, emission of liquid CO₂ has been observed in multiple locations, which indicates the existence of benthic CO₂ pools. Recently, a liquid/supercritical CO₂ (ScCO₂) hypothesis has been proposed that a two-phase ScCO₂-water environment could lead to efficient dehydration and condensation of organics. To confirm this hypothesis, we conducted a nucleoside phosphorylation reaction in a hydrothermal reactor creating ScCO₂-water two-phase environment. After 120 h of uridine, cytosine, guanosine, and adenosine phosphorylation at 68.9°C, various nucleoside monophosphates (NMPs), nucleotide diphosphates, and carbamoyl nucleosides were produced. The addition of urea enhanced the overall production of phosphorylated species with 5'-NMPs, the major products that reached over 10% yield. As predicted, phosphorylation did not proceed in the fully aqueous environment without ScCO₂. Further, a glass window reactor was introduced for direct observation of the two-phase environment, where the escape of water into the ScCO₂ phase was observed. These results are similar to those of a wet-dry cycle experiment simulating the terrestrial hot spring environment, indicating that the presence of ScCO₂ can create a comparatively dry condition in the deep sea. In addition, the high acidity present in the aqueous phase further supports nucleotide synthesis by enabling the release of orthophosphate from the hydroxyapatite mineral solving the phosphate problem. Thus, the present study highlights the potential of the unique ScCO₂-water two-phase environment to drive prebiotic nucleotide synthesis and likely induce condensation reactions of various organic and inorganic compounds in the deep-sea CO₂ pool on Earth and potentially other ocean worlds.

Standard definitions of habitability assume that life requires the presence of planetary gravity wells to stabilize liquid water and regulate surface temperature. Here, the consequences of relaxing this assumption are evaluated. Temperature, pressure, volatile loss, radiation levels, and nutrient availability all appear to be surmountable obstacles to the survival of photosynthetic life in space or on celestial bodies with thin atmospheres. Biologically generated barriers capable of transmitting visible radiation, blocking ultraviolet, and sustaining temperature gradients of 25-100 K and pressure differences of 10 kPa against the vacuum of space can allow habitable conditions between 1 and 5 astronomical units in the solar system. Hence, ecosystems capable of generating conditions for their own survival are physically plausible, given the known capabilities of biological materials on Earth. Biogenic habitats for photosynthetic life in extraterrestrial environments would have major benefits for human life support and sustainability in space. Because the evolution of life elsewhere may have followed very different pathways from that on Earth, living habitats could also exist outside traditional habitable environments around other stars, where they would have unusual yet potentially detectable biosignatures.

Cross Crater is a 65-km impact crater located in the Noachian highlands of the Terra Sirenum region of Mars. Geochemical modeling has indicated that alunite detected on the southwest wall of Cross Crater could have been formed by a fumarole upwelling into Cross Crater Lake and could indicate that an environment favorable to the development of life may have existed several billion years ago. Alunite did not form when Noachian precipitation reacted with basalt nor when the sediments and groundwater resulting from this reaction were reacted with a fumarole. Only when Cross Crater Lake water was equilibrated with sulfuric acid, thought to be a major component of the atmosphere in the Hesperian, following reaction with fumarole groundwater, did alunite precipitate from solution. Kaolinite, silica, or an Al-smectite such as montmorillonite also formed. The proximity of Cross Crater to the Tharsis volcanic region relative to Columbus crater, where alunite has also been detected, may have resulted in larger amounts of magmatic water input to the lake from sources along fractures that extend westward from Tharsis. This could explain the more extensive deposit of alunite at Cross Crater relative to Columbus crater.

Earth is expected to have acquired a reduced proto-atmosphere enriched in H₂ and CH₄ through the accretion of building blocks that contain metallic Fe and/or the gravitational trapping of surrounding nebula gas. Such an early, wet, reduced atmosphere that covers a proto-ocean would then ultimately evolve toward oxidized chemical compositions through photochemical processes that involve reactions with H₂O-derived oxidant radicals and the selective escape of hydrogen to space. During this time, atmospheric CH₄ could be photochemically reprocessed to generate not only C-bearing oxides but also organics. However, the branching ratio between organic matter formation and oxidation remains unknown despite its significance on the abiotic chemical evolution of early Earth. Here, we show via numerical analyses that UV absorptions by gaseous hydrocarbons such as C₂H₂ and C₃H₄ significantly suppress H₂O photolysis and subsequent CH₄ oxidation during the photochemical evolution of a wet proto-atmosphere enriched in H₂ and CH₄. As a result, nearly half of the initial CH₄ converted to heavier organics along with the deposition of prebiotically essential molecules such as HCN and H₂CO on the surface of a primordial ocean for a geological timescale order of 10-100 Myr. Our results suggest that the accumulation of organics and prebiotically important molecules in the proto-ocean could produce a soup enriched in various organics, which might have eventually led to the emergence of living organisms.