Earth is expected to have acquired a reduced proto-atmosphere enriched in H and CH 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 HO-derived oxidant radicals and the selective escape of hydrogen to space. During this time, atmospheric CH 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 CH and CH significantly suppress HO photolysis and subsequent CH oxidation during the photochemical evolution of a wet proto-atmosphere enriched in H and CH. As a result, nearly half of the initial CH converted to heavier organics along with the deposition of prebiotically essential molecules such as HCN and HCO 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.

Prebiotic synthesis of complex organic molecules in water-rich environments has been a long-standing challenge. In the modern deep sea, emission of liquid CO has been observed in multiple locations, which indicates the existence of benthic CO pools. Recently, a liquid/supercritical CO (ScCO) hypothesis has been proposed that a two-phase ScCO-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 ScCO-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 ScCO. Further, a glass window reactor was introduced for direct observation of the two-phase environment, where the escape of water into the ScCO 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 ScCO 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 ScCO-water two-phase environment to drive prebiotic nucleotide synthesis and likely induce condensation reactions of various organic and inorganic compounds in the deep-sea CO pool on Earth and potentially other ocean worlds.

Thiophene and two derivatives (2-methylthiophene and 3-methylthiophene) have been detected on the surface of Mars with the Sample Analysis at Mars instrument suite onboard NASA's Curiosity rover. Thiophene could serve as a secondary chemical biosignature since the secondary biosynthesis of thiophene is considered an important production pathway. However, it is critical to understand the abiotic formation and destruction of thiophene and its derivatives since these pathways could affect the molecules' stabilities on planetary surfaces over geological timescales. Here, we present the radiolytic destruction kinetics of thiophene, 2-methylthiophene, and 3-methylthiophene as single-component ices and when diluted in water ice at low temperatures. Using infrared spectroscopy, we determined the destruction rate constants and extrapolated our radiolytic half-lives to the surface of Mars, assuming the measured and modeled surface dose rates. We found that our rate constants strongly depend on temperature and presence of water ice. Based on our determined radiolytic half-life for thiophene under conditions most similar to those of thiophene groups in Martian macromolecules, we expect thiophene to be stable on the surface for significantly longer than the Martian surface exposure age of sites in Gale crater where thiophenes have been detected.

In this study, we calculated the travel times of a thermal probe that descends through Europa's ice shell. The ice column is simplified to a conductive layer. Using a cellular automaton model, the descent of the probe was simulated by tracking temperature changes, with cell interaction dictated by heat conduction and cell state transition rules determined by cell temperatures. Validation tests, including a soil column simulation, and comparison with experimental data, support the reliability of the model. Simulations were performed with 2 different cell sizes, 19 constant probe temperatures, and 5 ice thermal conductivities. A smaller cell size (mm) produced shorter travel times (between 22 days for a probe temperature and ∼4 years for ) than a larger cell size (m), which produced travel times between 27 years ( 600K) and ∼10 years ( 280K). The ice shell's thermal conductivity has a modest impact on descent times. The results are generally consistent with previous approaches that used more detailed probe engineering considerations. These results suggest that a probe relying solely on heat production may traverse Europa's conductive ice shell within a mission's timeframe.

We propose a novel approach to identify the origin of pyrite grains and distinguish biologically influenced sedimentary pyrite using combined sulfur isotope (δS) and trace element (TE) analyses. To classify and predict the origin of individual pyrite grains, we applied multiple machine-learning algorithms to coupled δS and TE data from pyrite grains formed from diverse sedimentary, hydrothermal, and metasomatic processes across geologic time. Our unsupervised classification algorithm, K-means++ cluster analysis, yielded six classes based on the formation environment of the pyrite: sedimentary, low temperature hydrothermal, medium temperature, polymetallic hydrothermal, high temperature, and large euhedral. We tested three supervised models (random forest [RF], Naïve Bayes, k-nearest neighbors), and RF outperformed the others in predicting pyrite formation type, achieving a precision (area under the ROC curve) of 0.979 ± 0.005 and an overall average class accuracy of 0.878 ± 0.005. Moreover, we found that coupling TE and δS data significantly improved the performance of the RF model compared with using either TE or δS data alone. Our data provide a novel framework for exploring sedimentary rocks that have undergone multiple hydrothermal, magmatic, and metamorphic alterations. Most significant, however, is the demonstrated potential for distinguishing between biogenic and abiotic pyrite in samples from early Earth. This approach could also be applied to the search for potential biosignatures in samples returned from Mars.

Understanding how organics degrade under galactic cosmic rays (GCRs) is critical as we search for traces of ancient life on Mars. Even if the planet harbored life early in its history, its surface rocks have been exposed to ionizing radiation for about four billion years, potentially destroying the vast majority of biosignatures. In this study, we investigated for the first time the impact of simulated GCRs (using gamma rays) on several types of lipid biosignatures (including hopane C, sterane C, alkanes, and fatty acids [FAs]) in both the presence and absence of salts (NaCl, KCl, and MgCl). We measured that the lipids degraded 6-20 times faster than amino acids in similar conditions; moreover, when irradiated in the presence of a salt substrate, degradation was at least 4-6 times faster than without salt, which suggests that salty environments that are often preferred targets for astrobiology warrant caution. We detected radiolytic by-products only for FAs-in the form of alkanes and aldehydes. These results expand our understanding of the degradation of organic molecules in Mars analog environments and underscore the urgent need to direct rover missions to sampling sites protected from GCRs, for example, sites on Mars that have been recently exposed by a wind scarp retreat or meteoritic impact.

A major unknown in the field of planetary protection is the degree to which natural atmospheric processes remove terrestrial microorganisms from robotic and crewed spacecraft that could potentially contaminate Mars (i.e., forward contamination). We present experiments in which we measured the removal rate of HA101 spores from aluminum surfaces under the bombardment of naturally rounded sand grains. To simulate grain impacts, we constructed a pneumatic sand-feed system and gun to accelerate grains to a desired speed, with independent control of impacting grain mass, flux, and angle. Spore counts of the resulting bombarded surfaces when using scanning electron microscopy indicate that although spores directly impacted by sand grains would likely be killed, those immediately adjacent to grain impacts might be released into the environment intact. The experiments demonstrate a linear relationship between the fractional dislodgement rate of spores and grain impact speed, which can be used to estimate input to microbial transport models (e.g., using numerical models of saltation). Even the slowest grain impacts (∼2.7 m/s) dislodged spores. Such slow events may be common and widespread on Mars, which suggests that microbial dislodgement by slow saltation near the surface is largely unavoidable.

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.

In Lammer et al. (2024), we defined Earth-like habitats (EHs) as rocky exoplanets within the habitable zone of complex life (HZCL) on which Earth-like N-O-dominated atmospheres with minor amounts of CO can exist, and derived a formulation for estimating the maximum number of EHs in the galaxy given realistic probabilistic requirements that have to be met for an EH to evolve. In this study, we apply this formulation to the galactic disk by considering only requirements that are already scientifically quantifiable. By implementing literature models for star formation rate, initial mass function, and the mass distribution of the Milky Way, we calculate the spatial distribution of disk stars as functions of stellar mass and birth age. For the stellar part of our formulation, we apply existing models for the galactic habitable zone and evaluate the thermal stability of nitrogen-dominated atmospheres with different CO mixing ratios inside the HZCL by implementing the newest stellar evolution and upper atmosphere models. For the planetary part, we include the frequency of rocky exoplanets, the availability of surface water and subaerial land, and the potential requirement of hosting a large moon by evaluating their importance and implementing these criteria from minima to maxima values as found in the scientific literature. We also discuss further factors that are not yet scientifically quantifiable but may be requirements for EHs to evolve. Based on such an approach, we find that EHs are relatively rare by obtaining plausible maximum numbers of and planets that can potentially host N-O-dominated atmospheres with maximum CO mixing ratios of 10% and 1%, respectively, implying that, on average, a minimum of rocky exoplanets in the HZCL are needed for 1 EH to evolve. The actual number of EHs, however, may be substantially lower than our maximum ranges since several requirements with unknown occurrence rates are not included in our model (, the origin of life, working carbon-silicate and nitrogen cycles); this also implies extraterrestrial intelligence (ETI) to be significantly rarer still. Our results illustrate that not every star can host EHs nor can each rocky exoplanet within the HZCL evolve such that it might be able to host complex animal-like life or even ETIs. The Copernican Principle of Mediocrity therefore cannot be applied to infer that such life will be common in the galaxy.

In this hypothesis article, we discuss the basic requirements of planetary environments where aerobe organisms can grow and survive, including atmospheric limitations of millimeter-to-meter-sized biological animal life based on physical limits and O, N, and CO toxicity levels. By assuming that animal-like extraterrestrial organisms adhere to similar limits, we define Earth-like habitats (EH) as rocky exoplanets in the habitable zone for complex life that host N-O-dominated atmospheres with minor amounts of CO, at which advanced animal-like life or potentially even extraterrestrial intelligent life can in principle evolve and exist. We then derive a new formula that can be used to estimate the maximum occurrence rate of such Earth-like habitats in the Galaxy. This contains realistic probabilistic arguments that can be fine-tuned and constrained by atmospheric characterization with future space and ground-based telescopes. As an example, we briefly discuss two specific requirements feeding into our new formula that, although not quantifiable at present, will become scientifically quantifiable in the upcoming decades due to future observations of exoplanets and their atmospheres. Key Words: Eta-Earth-Earth-like habitats-oxygenation time-nitrogen atmospheres-carbon dioxide-animal-like life. Astrobiology 24, 897-915.

Big impacts on the early Earth would have created highly reducing atmospheres that generated molecules needed for the origin of life, such as nitriles. However, such impactors could have been followed by collisions that were sufficiently big to vaporize the ocean and destroy any pre-existing life. Thus, a post-impact-reducing atmosphere that gives rise to life needs to be followed by a lack of subsequent sterilizing impacts for life to persist. We assume that prebiotic chemistry required a post-impact-reducing atmosphere. Then, using statistics for the impact history on Earth and the minimum impact mass needed to generate post-impact highly reducing atmospheres, we show that the median timing of impact-driven biopoiesis is favored early in the Hadean, ∼4.35 Ga. However, uncertainties are large because impact bombardment is stochastic, and so biopoiesis could have occurred between 4.45 and 3.9 Ga within 95% uncertainty. In an optimistic scenario for biopoiesis from post-impact-reducing atmospheres, we find that the origin of life is favorable in ∼90% of stochastic impact realizations. In our most pessimistic case, biopoiesis is still fairly likely (∼20% chance). This potentially bodes well for life on rocky exoplanets that have experienced an early episode of impact bombardment given how planets form.

Rocky planets may acquire a primordial atmosphere by the outgassing of volatiles from their magma ocean. The distribution of O between HO, CO, and CO in chemical equilibrium subsequently changes significantly with decreasing temperature. We consider here two chemical models: one where CH and NH are assumed to be irrevocably destroyed by photolysis and second where these molecules persist. In the first case, we show that CO cannot coexist with HO, since CO oxidizes at low temperatures to form CO and H. In both cases, H escapes from the thermosphere within a few 10 million years by absorption of stellar XUV radiation. This escape drives an atmospheric self-oxidation process, whereby rocky planet atmospheres become dominated by CO and HO regardless of their initial oxidation state at outgassing. HCN is considered a potential precursor of prebiotic compounds and RNA. Oxidizing atmospheres are inefficient at producing HCN by lightning. Alternatively, we have demonstrated that lightning-produced NO, which dissolves as nitrate in oceans, and interplanetary dust particles may be the main sources of fixed nitrogen in emerging biospheres. Our results highlight the need for origin-of-life scenarios where the first metabolism fixes its C from CO, rather than from HCN and CO.

The abundance of potentially habitable hypersaline environments in our solar system compels us to understand the impacts of high-salt matrices and brine dynamics on biosignature detection efforts. We identified and quantified organic compounds in brines from South Bay Salt Works (SBSW), where evapoconcentration of ocean water enables exploration of the impact of NaCl- and MgCl-dominated brines on the detection of potential biosignature molecules. In SBSW, organic biosignature abundance and distribution are likely influenced by evapoconcentration, osmolyte accumulation, and preservation effects. Bioluminescence assays show that adenosine triphosphate (ATP) concentrations are higher in NaCl-rich, low water activity () samples (<0.85) from SBSW. This is consistent with the accumulation and preservation of ATP at low as described in past laboratory studies. The water-soluble small organic molecule inventory was determined by using microchip capillary electrophoresis paired with high-resolution mass spectrometry (µCE-HRMS). We analyzed the relative distribution of proteinogenic amino acids with a recently developed quantitative method using CE-separation and laser-induced fluorescence (LIF) detection of amino acids in hypersaline brines. Salinity trends for dissolved free amino acids were consistent with amino acid residue abundance determined from the proteome of the microbial community predicted from metagenomic data. This highlights a tangible connection up and down the "-omics" ladder across changing geochemical conditions. The detection of water-soluble organic compounds, specifically proteinogenic amino acids at high abundance (>7 mM) in concentrated brines, demonstrates that potential organic biomarkers accumulate at hypersaline sites and suggests the possibility of long-term preservation. The detection of such molecules in high abundance when using diverse analytical tools appropriate for spacecraft suggests that life detection within hypersaline environments, such as evaporates on Mars and the surface or subsurface brines of ocean world Europa, is plausible and argues such environments should be a high priority for future exploration. Key Words: Salts-Analytical chemistry-Amino acids-Biosignatures-Capillary electrophoresis-Preservation. Astrobiology 24, 795-812.

The study of extremophilic microorganisms has sparked interest in understanding extraterrestrial microbial life. Such organisms are fundamental for investigating life forms on Saturn's icy moons, such as Enceladus, which is characterized by potentially habitable saline and alkaline niches. Our study focused on the salt-alkaline soil of the Al Wahbah crater in Saudi Arabia, where we identified microorganisms that could be used as biological models to understand potential life on Enceladus. The search involved isolating 48 bacterial strains, sequencing the genomes of two thermo-haloalkaliphilic strains, and characterizing them for astrobiological application. A deeper understanding of the genetic composition and functional capabilities of the two novel strains of provided valuable insights into their survival strategies and the presence of coding genes and pathways related to adaptations to environmental stressors. We also used mass spectrometry with a molecular network approach, highlighting various classes of molecules, such as phospholipids and nonproteinogenic amino acids, as potential biosignatures. These are essential features for understanding life's adaptability under extreme conditions and could be used as targets for biosignatures in upcoming missions exploring Enceladus' orbit. Furthermore, our study reinforces the need to look at new extreme environments on Earth that might contribute to the astrobiology field.

The emergence of life from nonlife, or abiogenesis, remains a fundamental question in scientific inquiry. In this article, we investigate the probability of the origin of life (per conducive site) by leveraging insights from Earth's environments. If life originated endogenously on Earth, its existence is indeed endowed with informative value, although the interpretation of the attendant significance hinges critically upon prior assumptions. By adopting a Bayesian framework, for an agnostic prior, we establish a direct connection between the number of potential locations for abiogenesis on Earth and the probability of life's emergence per site. Our findings suggest that constraints on the availability of suitable environments for the origin(s) of life on Earth may offer valuable insights into the probability of abiogenesis and the frequency of life in the universe.

The 2023-2032 Planetary Science and Astrobiology Decadal Survey prioritized the Uranus Orbiter and Probe (UOP) mission concept as the next priority flagship mission. The UOP concept includes scientific studies of the Uranian moon system. Although the Uranian moons differ greatly from the ocean worlds in the Jovian and Saturnian systems, the emerging hypothesis is that some of them could at least sustain thin, potentially concentrated, oceans. Herein, we make a case that these moons are important and interesting targets of astrobiological research. Studying these worlds would provide critical astrobiological data related to their habitability, including origin, evolution, and potential death, as well as the formation and evolution of ocean worlds more broadly. There is a strong need for research that connects astrobiology to modeling and experimentation to better characterize the possible conditions of these worlds, and this will be critical in formulating and maximizing the potential science that could be done by a Uranus flagship mission.

Europa and Enceladus are key targets to search for evidence of life in our solar system. However, the surface and shallow subsurface of both airless icy moons are constantly bombarded by ionizing radiation that could degrade chemical biosignatures. Therefore, sampling of icy surfaces in future life detection missions to Europa and Enceladus requires a clear understanding of the necessary ice depth where unaltered organic biomolecules might be present. We conducted radiolysis experiments by exposing individual amino acids in ices and amino acids from dead microorganisms in ices to gamma radiation to simulate conditions on these icy worlds. In the pure amino acid samples, glycine did not show a detectable decrease in abundance, whereas the abundance of isovaline decreased by 40% after 4 MGy of exposure. Amino acids in dead () organic matter exhibited a gradual decline in abundances with the increase of exposure dosage, although at much slower rates than individual amino acids. The majority of amino acids in dead samples demonstrated a step function decline as opposed to a gradual decline. After the initial drop in abundance with 1 MGy of exposure, those amino acids did not display further decreases in abundance after exposure up to 4 MGy. New radiolysis constants for isolated amino acids and amino acids in dead material for Europa/Enceladus-like conditions have been derived. Slow rates of amino acid destruction in biological samples under Europa and Enceladus-like surface conditions bolster the case for future life detection measurements by Europa and Enceladus lander missions. Based on our measurements, the "safe" sampling depth on Europa is ∼20 cm at high latitudes of the trailing hemisphere in the area of little impact gardening. Subsurface sampling is not required for the detection of amino acids on Enceladus-these molecules will survive radiolysis at any location on the Enceladus surface. If the stability of amino acids observed in organic materials is confirmed in other microorganisms, then the survival of amino acids from a potential biosphere in Europa ice would be significantly increased.

To support training and foster retention in the emerging field of astrobiology, NASA has funded opportunities for graduate students and early career scientists to develop a community, foster interdisciplinarity, increase confidence, and showcase career options. The design of these opportunities builds on research on factors that increase retention, including feeling competent, having autonomy and a sense of purpose, having a sense of identity, and being connected to others in the field. Findings are reported from retrospective studies of two NASA career-building opportunities, the Astrobiology Graduate Conference and the International Astrobiology Summer School held in Santander, Spain. We present evidence that attendees gain confidence by presenting to, and working with, their peers, and feel competent to express their ideas and interests and build relationships in the field that continue after the experiences. Many say that they feel less isolated and go on to present or publish with colleagues they meet. Their career options also expand by meeting potential colleagues from different disciplines. Based on the findings, participating in either of these long-running programs shows clear positive impact on early career astrobiology professionals.

In a previous experiment, we demonstrated the capability of flow cytometry as a potential life detection technology for icy moons using exogenous fluorescent stains (Wallace et al., 2023). In this companion experiment, we demonstrated the capability of flow cytometry to detect life using intrinsically fluorescent biomolecules in addition to exogenous stains. We used a method similar to our previous work to positively identify six classes of intrinsically fluorescent biomolecules: flavins, carotenoids, chlorophyll, tryptophan, NAD+, and NAD(P)H. We demonstrated the effectiveness of this method with six known organisms and known abiotic material and showed that the cytometer is easily able to distinguish the known organisms and the known abiotic material by using the intrinsic fluorescence of these six biomolecules. To simulate a life detection experiment on an icy moon lander, we used six natural samples with unknown biotic and abiotic content. We showed that flow cytometry can identify all six intrinsically fluorescent biomolecules and can separate the biotic material from the known abiotic material on scatter plots. The use of intrinsically fluorescent biomolecules in addition to exogenous stains will potentially cast a wider net for life detection on icy moons using flow cytometry.