Astrobiology最新文献

Chemical and geological processes on prebiotic Earth are believed to have resulted in the emergence of life through the increasing organization and functionality of organic molecules. This primer provides an overview of some key abiotic chemical and physical processes that could have contributed to life's building blocks (amino acids, nucleotides, fatty acids, and monosaccharides) becoming more ordered during the early stages in the origin of life. The processes considered include polymerization, intramolecular folding, multimolecular assembly, and chemical evolution through various selective mechanisms. Our goal is to provide an accessible, high-level synopsis of these key general concepts for a diverse audience.

The presence of major volatile elements-carbon, hydrogen, nitrogen, and sulfur-on Earth is critical for establishing life. The origin of these life-essential volatile elements (LEVEs) on Earth has been studied for many years. Here, we present a brief compilation of the prevailing ideas regarding volatile delivery to Earth and evaluate their origins, strengths, and weaknesses. Motivated by the fact that one model of LEVE delivery is via a giant impactor to Earth, we subsequently present a geochemical model aimed at understanding the possible volatile inventory and fractionation between the core, the silicate magma ocean (MO), and the atmosphere of a Mars-mass embryo. We looked at various end-member accretion scenarios of the embryo and their influence on the embryo's LEVE budget and the LEVE ratios. We varied various chemical (initial concentration of volatiles in the undifferentiated bodies and the oxygen fugacity [fO₂] of geochemical fractionation) and physical parameters (silicate-mass fraction of the accreting bodies, MO depth) to observe their effects on the absolute and relative LEVE budgets of the embryo. Our results show that an oxidizing condition (logf O₂ ≥ IW-1 [Iron-Wüstite]) is critical in establishing the relative LEVE budget of the embryo's MO, closer to that of present-day bulk silicate Earth. Furthermore, the accretion of larger bodies to form the Mars-mass embryo results in the closest match of the LEVE ratios to that of the present-day bulk silicate Earth (BSE). However, the absolute LEVE budget of the MO of Mars-mass embryo is depleted by at least 1-2 orders of magnitude compared with the BSE under all model calculation scenarios. In contrast, the CI-chondrite-normalized LEVE budget of the embryos's core, in many of the scenarios, especially from the reduced (e.g., IW-2) bodies, overlaps or exceeds the present-day BSE estimate. We argue that for a Mars-mass, differentiated embryo, the cores provide a better prospect for LEVE delivery to proto-Earth, through core breakups and subsequent mixing in the MO or solid mantle. Future studies need to better assess whether the fractional retention of core materials in the silicate reservoir can match the present-day BSE LEVE budgets and how such a process compares with the LEVE delivery via less-processed primitive asteroids.

The scientific study of the origins of life is a deep pursuit that exists at the intersection of multiple disciplines. Prebiotic chemistry focuses on understanding how biopolymer building blocks such as amino acids, nucleotides, and sugars emerged and how their chemical and structural space evolves toward life (chemical evolution). Simulation experiments have been essential for exploring plausible pathways for the origin of building blocks under early Earth conditions and planetary environments. Key examples include the seminal Miller-Urey experiment and the polymerization of hydrogen cyanide. Research highlights the role of environmental cycles and geochemistry in shaping the prebiotic chemical space. These processes facilitated the condensation and stabilization of biopolymer precursors, particularly in terrestrial or small-pond scenarios. Noncanonical building blocks, including triazines and alternative amino acids, may have contributed to proto-biopolymer formation. This expands our understanding of chemical evolution. Despite significant progress, challenges remain, particularly in understanding nucleoside formation and the transition to modern biopolymers. This review provides a general overview of the prebiotic formation of biopolymer building blocks and examines both classic and seminal experiments and recent experimental approaches. Insights provided by extraterrestrial samples, such as carbonaceous meteorites and asteroids, also contribute to offering a comprehensive perspective on abiogenesis.

The abundance and distribution of stable isotopes of an element in a substance can provide insights regarding the source, synthesis, and environmental history of that substance. Because isotopic discrimination during chemical reactions can be unique to specific chemical pathways or environmental conditions, isotopic patterns within a substance or between related substances may provide insights into their formation. Biosynthetic pathways can create isotopic patterns that differ from patterns that arise from abiotic processes, but this is not universally true. Isotope patterns are signatures of chemical reactions, so they require additional context to be used as biosignatures. The framework of the Life Detection Knowledge Base discussed herein is used to convey arguments that support or challenge the utility of isotopic patterns for life detection. Examples of carbon and sulfur isotopic patterns in organic materials and minerals are presented to indicate how the life detection criteria "prevalence" and "signal strength" can be applied. In future work, more abiotic processes that might create false-positive life detection claims must be characterized. A broader range of microbial communities, taxa, and biomolecules should be explored for isotopic patterns. Additional elements also warrant investigation as potential isotopic biosignatures and environmental indicators. Studies of sedimentary macromolecular organic matter should be expanded further to provide deeper insights into isotopic abundance patterns.

Laminae are millimeter-scale features in rocks created by physiochemical processes that can be influenced by the presence and activities of communities of organisms that occur as biofilms and microbial mats. The structure and composition of laminae reflect the processes involved in their formation and can be preserved in the rock record over geologic time; however, diagenetic and metamorphic alteration can lead to the loss of primary information and confusion over the interpretation of their origins. As potential records of ancient life, laminae can preserve evidence of microbial activity over billions of years of Earth's history. On planetary bodies such as Mars, laminae in sedimentary rocks are common and represent significant features of interest that can record habitable conditions (e.g., the presence of liquid water) at the time of their formation. Here we review the significance of laminae as targets for astrobiological exploration. We discuss common mechanisms by which laminae form in natural environments on Earth, present arguments and evidence for laminae as potential biosignatures, and describe how such information is presented in the NASA Life Detection Knowledge Base.

Our understanding of crystalline structures within terrestrial planetary analog environments can shed light on how these features can be interpreted on rocky planets and icy moons in our solar system. The ability to distinguish biogenic and abiotic components within the mineral, crystal, and structural features allows us to inform future life detection missions, science payloads, and instrument measurement resolutions. Moreover, having these terrestrial reference measurements in a review format allows the measurement rationale to be understood in the context of mission concepts and geomicrobiological assessment of life in extreme environments. From 2020 to 2022, this team contributed to NASA's Center for Life Detection, Life Detection Knowledge Base, where structural features in crystalline and crystal-centric sample analyses were reviewed and assessed for biogenic preservation potential. This article highlights the scientific rationale and astrobiological sample assessment of evaluation for crystal habits as a possible biosignature. This is to illustrate true and false positives of the standards of evidence for minerals and their associated crystal habits. Moreover, we illustrate how these efforts contribute to the overall assessment of this type of morphological evidence in extant and extinct life detection campaigns.

Modern Great Salt Lake, UT, United States, is what remains after the extensive evaporation of Pleistocene Lake Bonneville, which makes this site an appropriate analog to ancient martian lacustrine systems. Today, evaporite minerals surround the lake, including recently precipitated displacive gypsum selenite crystals. Our hypothesis was that hydrated clay solid inclusions within the gypsum would support microbial life with water and nutrients, while the mineral encasement would provide protection from ultraviolet light and temperature fluctuations. Our data demonstrate a complex microbial community that thrives in the clay-rich inclusions within the gypsum crystals. This mineral microbiome includes archaea and fungi, but most notably an immense number of bacterial species from the phyla Actinobacteria, Bacteroidetes, Firmicutes, and Proteobacteria. Evidence of primary producers (cyanobacteria and microalgae) that have the capacity for diverse metabolisms suggests the possibility of an entombed ecosystem with trophic levels, energy currencies, and connected metabolisms. X-ray diffraction analyses of the sediment in which the gypsum formed show the clay fraction mostly comprises discrete and randomly interstratified illite and smectite, along with lesser amounts of kaolinite and chlorite. The methods developed here for modern gypsum can be extended to studies of ancient minerals on Earth as well as hydrated sulfate minerals on Mars.

Cyanobacteria possess unique biological properties and the ability to perform life-sustaining processes, which make them useful for applications in space exploration and colonization. Their potential use in bioelectricity and fuel production has garnered significant interest. This study explores the effects of ionizing radiation on the cyanobacterium Limnospira indica used in bioelectrodes. This is an important consideration as radiation levels in space are significantly higher than those experienced on Earth with its protective atmosphere and magnetosphere. In an approximate space radiation simulation setting, using gamma radiation, living cells of L. indica strain PCC 8005 (formerly known as Arthrospira sp.) were interfaced as bioelectrodes with boron-doped diamond (BDD)-coated and fluorine-doped tin oxide (FTO)-coated glass substrates and exposed to ⁶⁰Co gamma rays at an acute dose rate of 136 Gy.h^-1 for up to 14 h; electrogenic abilities (i.e., respiration current in the dark) were measured by chronoamperometry. Limnospira indica-based bioelectrodes did not exhibit statistically significant changes in current generation even under high doses of 1.9 kGy gamma rays as compared with non-exposed bioelectrodes. Under radiation, bare FTO electrodes performed better than BDD electrodes, but negative gamma-induced effects in bare BDD electrodes were mitigated by cyanobacteria. The stable current generation under high-dose highlights the potential of biophotoelectrochemical and biophotovoltaic cells in radiation-intensive environments and applications in space.

Numerous studies have demonstrated that ultraviolet radiation in the C wavelength range produced by light-emitting diodes (UVC-LEDs) is effective for disinfection (i.e., inactivation of vegetative bacteria and viruses). However, there are few efficacy data available to confirm its use as a sterilization technique (complete inactivation of bacterial spores). The present study evaluated the use of UVC-LED to achieve the sterilization of stainless-steel surfaces as a function of UVC dose and several other variables. Spores of Bacillus atrophaeus and two strains of Bacillus pumilus were used as indicator microorganisms. Results showed that the microorganism, spore loading, and inoculation method all affected whether complete inactivation was achieved. Under the tested conditions, sterilization of stainless-steel surfaces was achieved using UV-LED with doses that ranged from ∼4500 to 21,000 mJ/cm², and if spore deposition was low enough to prevent clumping and subsequent shielding. We found that spore deposition in which sterilization was achieved ranged from 2.9 to 6.2 log₁₀ colony-forming units/cm² and depended primarily on the microorganism/strain. Shielding of UV radiation diminished efficacy and may have also occurred from the presence of foreign material.