Astrobiology最新文献

To date, the Mars 2020 mission's deep-UV Raman and fluorescence instrument (Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals [SHERLOC]) has reported potential Raman detections of macromolecular carbon in data collected on the floor of Jezero crater and in Neretva Vallis, a valley incised through the Jezero crater rim and Margin Unit. The crater floor detection is associated with a collocated fluorescence signal that has been interpreted to indicate the presence of small aromatic molecules and/or cerium-bearing phosphates. Previous work has demonstrated that the potential macromolecular carbon detection is similar to data collected from abiotic macromolecular carbon in a martian meteorite. The work described here was performed to support the interpretation of this and any future possible SHERLOC macromolecular carbon detections by comparing the possible G-band to biologically produced macromolecular carbon (kerogen). We report the results of collocated, in situ deep UV Raman and fluorescence measurements of kerogen preserved within Neoarchean and Eocene carbonate microbialites collected with a SHERLOC analog instrument. Our results support the conclusion that SHERLOC has detected macromolecular carbon in Jezero crater that may be of an abiotic or biological origin and suggest that a carbonate mineral may be the source of the collocated fluorescence signal. These findings reinforce the possibility that samples collected during the Mars 2020 mission may hold compelling evidence of ancient microbial life on Mars and the importance of delivering the samples to Earth for laboratory analysis to determine whether the material is biological in origin.

The Mars Sample Return (MSR) Campaign aims to collect and transport to Earth samples of martian atmosphere contained in sample tubes onboard the Mars 2020 rover, Perseverance. Understanding and mitigating the potential impact of terrestrial noble gas contamination is critical to ensuring the scientific integrity of these samples. This study quantifies the desorption of terrestrial argon (⁴⁰Ar) and xenon (¹²⁹Xe) from the interior of a flight-like Mars 2020 sample tube under high vacuum and provides critical insights into the potential contamination risks for returned martian atmospheric samples. Our results show that desorption rates decrease exponentially with time over ∼19 months and that desorbed terrestrial ⁴⁰Ar and ¹²⁹Xe will contribute less than 0.01% and 0.1%, respectively, to the martian noble gas inventory within a sealed sample tube that consists of martian atmosphere at a pressure of ∼7 mbar. This study suggests that the Mars 2020 sample tubes are suitable for capturing and preserving atmospheric samples from Mars for future scientific investigation.

The joint National Aeronautics and Space Administration and European Space Agency Mars Sample Return (MSR) Campaign is a proposed multi-mission effort to bring selected geological samples from Mars to Earth for the purpose of scientific investigation. Significant parts of these investigations could be affected by Earth-sourced contamination that is either misinterpreted as having a martian origin or that masks a martian signal. The Mars 2020 Perseverance rover implemented strict contamination control requirements to limit contamination of the samples during sample collection. Contamination control and contamination knowledge requirements have not yet been established for the samples after they arrive on Earth. The MSR Sample Receiving Facility (SRF) Contamination Panel (SCP) was tasked with defining the terrestrial biological, organic, and inorganic contamination limits for martian samples during their residence inside the SRF. To reach our recommendations, the SCP studied (i) the previously proposed limits and rationale of the Organic Contamination Panel, (ii) cleanliness levels achieved for sampling hardware by the M2020 mission, (iii) recent improvements in analytical technology and detection limits, (iv) updated information regarding the organic content of martian samples (e.g., from the Sample Analysis at Mars instrument on the Curiosity rover and laboratory analyses of martian meteorites), and (v) information about the composition and geologic context of samples being collected by the Perseverance rover for return to Earth.

Martian rock and regolith samples are being collected and cached by NASA's Perseverance rover, with the goal of returning them to Earth as soon as the mid-2030s. Upon return, samples would be housed in a sample receiving facility under biological containment to prevent exposing Earth's biosphere to any potential biohazards that might be present. Samples could be released from high containment for scientific investigations if they are found to be safe or are sterilized. The Sample Safety Assessment Protocol Tiger Team (SSAP-TT) was convened by the Sample Receiving Project between August 2023 and August 2024 and tasked with the development of a Sample Safety Assessment Protocol (SSAP). The result of this work is a proposed three-step protocol, supported by Bayesian statistical hypothesis testing, to assess the risk as to whether returned samples contain modern martian biology that could represent a biohazard. The proposed protocol outlines procedures to determine whether the samples could be safely released from high containment without sterilization or require a "hold and review" step. This article presents the central concept of the SSAP approach-comparing returned samples to the abiotic baseline. Organic molecules, which exist throughout the solar system, can have either biotic or abiotic origins. However, biotically produced organic molecules exhibit distinct complexity, distribution, and abundance characteristics that differentiate them from those formed through abiotic chemistry. The proposed protocol would examine the organic inventory of returned samples by using multiple techniques, including morphological and spectral assessments, to determine whether any signals exceed the abiotic baseline; that is, whether the organic molecular inventory could be explained solely by abiotic chemical synthesis. This approach provides a rigorous, yet feasible, safety assessment protocol by using modern techniques while minimizing sample consumption. We also identify key areas for future research and development, which include detection limits and further characterization of the martian abiotic background.

The Mars Sample Return (MSR) Campaign aims to retrieve a set of carefully selected and documented samples collected by NASA's Perseverance rover in and around Jezero Crater on Mars and deliver this set to Earth for comprehensive laboratory analyses. To emphasize the immense scientific return of this unique collection, this work presents a Sample Science Traceability Matrix (SSTM), a systematic framework that aligns each sample with the MSR campaign's defined science objectives, subobjectives, and critical research questions. The SSTM explicitly connects prioritized goals-including geologic history, astrobiology, planetary evolution, and human exploration science-to each of the individual samples gathered in and around Jezero Crater on Mars. This matrix offers a structured, quantitative method to assess each sample's capacity to address key scientific questions, while highlighting synergies across the sample suite and showcasing the overall value of the collection. The SSTM provides a valuable tool for guiding future sample analyses and identifying the most impactful samples that could be collected in the future to complete the set collected by the Mars 2020 mission. It also supports the next phase of Mars sample science and informs strategies for future Mars exploration missions. Key Words: Mars Sample Return-Perseverance-Jezero Crater-Laboratory-Sample collection-Science goals. Astrobiology 25, 725-741.

The alteration of biomass into simpler molecular remnants is relevant for the search for ancient and extraterrestrial life, where identifying recurrent taphonomic pathways is crucial for the attribution of biogenicity to otherwise nonbiological molecules. This work evaluates the alteration of lipids-recalcitrant biomarkers derived from cell membranes-across a lithification gradient, from a biologically active microbial mat, through a lithifying mat, to a fully lithified microbialite. Lipids from these samples, obtained from the high-altitude, hypersaline lake of Pozo Bravo (Argentinean Andes), were analyzed at molecular and isotopic levels to reconstruct biological sources and assess preservation along a bio-to-geo transition. Lipids from the lithifying mat and microbialite retained molecular features from the soft microbial mat (e.g., cyano- and purple sulfur bacteria), albeit at lower concentrations and diversity. Moreover, our analysis revealed preferential alteration of labile structural features such as unsaturations, methyl-, and pentacyclic structures, which decreased by ≥91% from soft to lithifying mat and ≥68% from lithifying mat to microbialite. Saturated and linear chains were more resistant, decreasing by ≥64% and ≥29%, respectively. These findings highlight how lipid preservation varies during lithification; thus, they provide valuable insights for biogenicity assessments and can help guide future efforts aimed at detecting ancient life.

The McMurdo Dry Valleys may harbor diverse surface microbial communities, yet little is known about subsurface microorganisms in permafrost and their potential for paleoecological reconstruction. Here, we present microbial diversity and paleoecology from lower Wright Valley (7000- to 25,000-year-old) and Pearse Valley (>180,000-year-old) permafrost habitats in the McMurdo Dry Valleys. Using a new decontamination protocol, low-biomass extraction approaches, and 16S ribosomal RNA gene amplification sequencing, we assessed microbial community structure and diversity. The difference between surface and subsurface microbial communities at both lower Wright and Pearse valleys suggests the environmental conditions were different at the time of colonization. Microbial taxa identified in subsurface permafrost but not in the surface soil in both valleys indicate an ancient and isolated microbial community. In contrast, communities were not resolved at a high-elevation site in the stable upland zone, the Friis Hills (>6 Ma). The inability to identify DNA using amplicon sequencing in the Friis Hills is consistent with previous efforts to analyze high-elevation soils and permafrost, which suggests that microbial habitability is severely restricted in persistent cold, arid habitats. Therefore, utilizing other approaches may be necessary to analyze surface and subsurface permafrost on Earth, and perhaps Mars, where low-abundance microbial populations may be present.

Concentrated magnesium chloride brines are extreme environments that are inhospitable to life on Earth. The ionic strength of these brines significantly depresses water activity and concomitantly exerts significant chaotropic stress. Although these brines are largely considered sterile, the well-known preservative effects of magnesium chloride on certain biomolecules, such as DNA, confound life detection approaches and efforts to constrain precisely the habitable window of life on Earth. While the ability of these brines to preserve genetic material is well documented, the preservation of whole cells, which are generally thought to be preserved in magnesium chloride brines, is poorly described. This work explores the effects of long-term exposure of highly chaotropic magnesium chloride on viability, cell integrity, and DNA preservation in the model organisms Escherichia coli, Salinibacter ruber, Halobacterium salinarum, and Haloquadratum walsbyi. The selected halophiles are relevant for this study as they are abundant and globally distributed in brine environments, while E. coli was chosen to represent infall or transport of non-adapted cells. We observed unexpected resilience in E. coli, which survived in 4 M magnesium chloride for longer than the tested halophiles, and nonviable cells maintained structural whole-cell integrity for over 3 years. Whole S. ruber cells were also preserved in 4 M magnesium chloride, while the tested haloarchaea lost viability and completely degraded within hours of exposure. DNA from all tested strains was recovered from incubations after upwards of 3 years of exposure; it showed some signs of degradation but was nonetheless still amplifiable via polymerase chain reaction. Our work demonstrates that the preservation of whole cells in magnesium chloride brines is not universal. Considering the potential abundance of chaotropic brine environments within our solar system, understanding the limits of life and the preservation of biosignatures in these brines is critical to inform future life detection missions on Earth and beyond.

Future missions dedicated to the search for extant life on Mars will require a clear understanding of the organic biosignature degradation processes in the shallow icy subsurface. Galactic and solar cosmic rays constantly bombard the martian surface and transform and degrade organic biomolecules over time, eventually destroying chemical evidence of life. We conducted radiolysis experiments by exposing individual amino acids in H₂O-ice and silicate matrices and amino acids from dead Escherichia coli microorganisms in H₂O-ice to gamma radiation as a proxy for cosmic ray exposure on the martian surface. The rates of amino acid radiolytic degradation were determined. We found that amino acids in the surface ice on Mars would survive over 50 million years of cosmic ray exposure, which is far greater than the expected age of the current surface ice deposits on Mars. Amino acids from dead E. coli organic matter in H₂O-ice and isolated pure amino acids dissolved in H₂O-ice tend to degrade at similar rates. We found that amino acid radiolytic degradation rates increased with increasing ice temperature in both abiotic and biological amino acids. Montmorillonite did not provide additional protection against gamma radiation to amino acids. Based on our experiments, locations with pure ice or ice-dominated permafrost would be the best places to look for recently deposited amino acids on Mars and, thus, should be considered as a target sampling location for future Mars missions searching for extant life.