Astrobiology最新文献

The Life Detection Knowledge Base (LDKB) is a community webtool developed to test and evaluate strategies to search for evidence of life beyond Earth, with an emphasis on recognizing potential false-positive and false-negative results. As part of the LDKB framework, we developed a taxonomy of potential biosignatures. The taxonomy brings together a broad array of life-detection strategies into a common and systematic structure that allows for equitable evaluations based on a specific set of criteria, chosen to assess the likelihood of false-positive and false-negative interpretations. The taxonomy is also a tool to organize life-detection strategies in a way that streamlines their infusion into robotic spaceflight missions. This article describes the structure of the taxonomy and its functional qualities. Two accompanying articles detail the overall LDKB framework and the set of criteria used to evaluate potential biosignatures.

The Life Detection Knowledge Base (LDKB) is part of the Life Detection Forum suite of web tools developed for life detection mission planners. This article details the development of one of its categories of biosignatures, the Structure category. The Structure category includes physical attributes of objects and their spatial relationships (e.g., orientation). Initial population of the LDKB Structure category performed during a Content Development Group (CDG) phase resulted in the selection of six high-priority biosignature themes for content development: crystal habits, microtunnels, Mesa Depression Relief structures (a sedimentary surface morphology), laminations, spheroids, and filaments. In populating content, it was concluded that environmental considerations are crucial to recognize structural biosignatures remotely for planetary exploration when not known a priori. CDG activity also revealed knowledge and technology gaps in identifying structural biosignatures. This included gaps in research on biological prevalence of structural features due to a lack of research on these topics and a gap in technologies for in situ surface imaging of potential structural biosignatures. In addition, the implementation of two functionalities in the tool (i.e., linking multiple lines of evidence within entries and including images to represent physical biosignature attributes) resulted directly from CDG activity. These improvements enhance the LDKB's ability to serve as a comprehensive repository for data on true biosignatures and their abiotic counterparts.

The Life Detection Knowledge Base (LDKB; https://lifedetectionforum.com/ldkb) is a community-owned web resource that is designed to facilitate the infusion of astrobiology knowledge and expertise into the conceptualization and design of life detection missions. The aim of the LDKB is to gather and organize diverse knowledge from a range of fields into a common reference frame to support mission science risk assessment, specifically in terms of the potential for false positive and false negative results when pursuing a particular observation strategy. Within the LDKB, knowledge sourced from the primary scientific literature is organized according to (1) a taxonomic classification scheme in which potential biosignatures are defined at a uniform level of granularity that corresponds to observable physical or chemical quantities, qualities, or states; (2) a set of four standard assessment criteria, uniformly applied to each potential biosignature, that target the factors that contribute to false positive and false negative potential; and (3) a discourse format that utilizes customizable, user-defined "arguments" to represent the essential aspects of relevant scientific literature in terms of their specific bearing on one of the four assessment criteria, and thereby on false positive and false negative potential. By mapping available and newly emerging knowledge into this standardized framework, we can identify areas where the current state of knowledge supports a well-informed science risk assessment as well as critical knowledge gaps where focused research could help flesh out and mature promising life detection approaches.

Astrobiology and the search for evidence of life beyond Earth are now key drivers for planetary science and astronomy missions. Efforts are underway to establish evaluative frameworks to interpret potential signs of life in returned data. However, there is a need for a "before-the-fact" system to assess mission science risk and the potential false negative and false positive results. The Life Detection Knowledge Base (LDKB) is a community-owned web tool that organizes the scientific literature and enables discourse and evaluation of potential biosignatures (defined to the same level of granularity) relative to a set of standard criteria. This article details the development of draft criteria and their utilization as an organizing basis for the LDKB and their vetting by the astrobiology community via two workshops. We report the incorporation of community feedback to generate a finalized set of criteria, which delineate contributing factors to the potential for false negative or false positive results in the search for evidence of life within and beyond our solar system.

Previous models of microbial survival on the moon do not directly consider the permanently shadowed regions (PSRs). These regions shield their interiors from many of the biocidal factors encountered in space flight, such as UV irradiation and high temperatures, and this shielding reduces the rate at which microbial spores become nonviable. We applied the Lunar Microbial Survival Model (LMS, Schuerger et al., 2019) to the environment found inside PSRs at two craters targeted for exploration by the Artemis missions, Shackleton and Faustini. The model produced rates of reduction of -0.0815 and -0.0683 logs per lunation, respectively, which implies that it would take 30.0 years for Shackleton and 30.8 years for Faustini to accumulate a single Sterility Assurance Level of -12 logs of reduction. The lunar PSRs are therefore one of the least biocidal environments in the solar system and would preserve viable terrestrial microbial contamination for decades.

Atmospheric organic hazes are widespread across various planetary bodies and have significant effects on both the surface and atmosphere. In this study, we investigate the optical and hygroscopic properties of organic hazes formed through photochemical processes. The hazes were generated from the irradiation of mixtures that contained molecular nitrogen (N₂), methane (CH₄), hydrogen sulfide (H₂S), and varying amounts of carbon dioxide (CO₂) to mimic early Earth-like conditions. In the absence of CO₂, the photochemical haze absorbed radiation at 405 nm. In contrast, the incorporation of CO₂ into the precursor gas mixtures resulted in hazes with reduced absorption at 405 nm. This decrease in absorption was due to the formation of non-absorbing inorganic salts and/or a change in organic composition; however, the exact composition is not fully known. Further, we observed that these hazes exhibited varying tendencies to uptake water, with non-CO₂ hazes showing no water uptake, while CO₂ hazes could absorb water and increase in size. Consequently, under humid conditions, the increased size of the haze enhanced its ability to scatter light and would thus promote cooling of a planetary atmosphere. Both the change in refractive indices and the increased hygroscopicity would contribute to greater cooling effects with higher CO₂ levels. In addition, the ability of the haze to uptake water would facilitate the particles acting as cloud condensation nuclei, potentially leading to the wet deposition of nutrients to a planet's surface that could help facilitate the emergence of life.

Carbonaceous chondrite (CC) meteorites are fragments of planetesimals that hold clues about the early solar system's organic matter. Amino acids are key to life on Earth; thus their study from extraterrestrial samples may help identify signs of prebiotic chemistry and life on other planets and may reveal how life as we know it began. This study analyzed amino acid concentrations and distributions in 42 CC samples, including returned samples from asteroids Ryugu and Bennu, to investigate the relationship between amino acid composition and parent body processes. We performed a statistical analysis of the amino acid molecular distributions and abundances in the context of meteoritic hydrogen, carbon, nitrogen, and carbonate total contents to explore the links between these organic species and thermal and aqueous processing experienced in the parent bodies. We also evaluated whether meteoritic amino acid ratios can be used as anti-biosignatures, and we re-evaluated the links between l-isovaline enantiomeric excesses and parent body aqueous alteration. While some trends were observed, correlations between amino acid distributions and alteration proxies (H, C, N, carbonates, enantiomeric excess) were generally weak, which indicates the need for larger sample sets. Thermal metamorphism correlated with lower amino acid and elemental [hydrogen (H), carbon (C), and nitrogen (N)] abundances, consistent with diverse parent bodies or localized processing. Ryugu samples exhibited significant amino acid variations despite similar bulk elemental compositions due to parent body heterogeneity. No strong statistical correlations were found between amino acid concentrations and H, C, or N content, which diminishes the reliability of predictions of amino acid abundances based solely on observed elemental abundances. While Ryugu and Bennu may share a common, Ceres-like parent body, observed differences in chemical composition suggest diverse evolutionary pathways. Finally, principal component analysis of amino acid and elemental data revealed distinct groupings that place Ryugu samples in a potentially unique subgroup and Bennu within the C2-ung chondrite group. These findings underscore the need for further study of such materials, especially given our discovery of their distinct nature, and emphasizes the insights gleaned from the ability to analyze returned asteroid samples.

The search for traces of life can be based on the detection of specific signatures produced by microorganisms on sedimentary rocks. Microbially induced sedimentary structures (MISSs) develop under specific physicochemical conditions that are likely to have potentially existed on Mars during the Noachian period. We designed an experiment under controlled laboratory conditions to explore the wide range variability in biogeomorphological responses of clay-sand substrates to the development of biological mats-including microbial mats-of different strains and biomasses, and an abiotic control. A 3D picture dataset based on the experiment was built using multi-image photogrammetry. Visual observations were combined with multivariate statistics on computed topographical variables to interpret the diversity in the resulting biotic and abiotic mud cracks. Finally, an artificial intelligence (AI) classifier based on convolutional neural networks was trained with the data. The resulting model predicted accurately not only the biotic-abiotic differences but also the differences between strains and biomasses of biotic treatments. Its results outperformed the blind human classification, even using only grayscale pictures. Class Activation Maps showed that AI followed several decision paths, not always like those of the human expert. Next steps are proposed for application of these models to ex situ biogeomorphological structures (fossil and modern MISS) on Earth's surface, to ultimately transpose them to a martian context.

Many of the recently discovered Earth-like exoplanets are hosted by M and F stars, stars that emit intense UVC, especially during a flare. We studied whether such planets are nevertheless habitable by irradiating a desert lichen, Clavascidium lacinulatum, with 254-nm 55 W/m² UVC nonstop for 3 months in the laboratory. Only 50% of its algal photobiont cells were inactivated. To put this in perspective, we used the same setup to challenge the photobiont cells but grown in pure culture, and Deinococcus radiodurans, the most radiation-resistant bacterium on Earth. Entire monolayers of hundreds of cells were inactivated in just 60 s. Further studies indicated that the cortex of the lichen was rendered UVC-opaque by deposits of phenolic secondary metabolites in its interstices. The lichen was injured only because, while most photochemical reactive oxygen species were quenched, photochemical ozone was not. We conclude that UVC-intense exoplanets are not necessarily uninhabitable to photosynthetic organisms.