Astrobiology

Advances in planetary and space sciences, astrobiology, and life and cognitive sciences, combined with developments in communication theory, bioneural computing, machine learning, and big data analysis, create new opportunities to explore the probabilistic nature of alien life. Brought together in a multidisciplinary approach, they have the potential to support an integrated and expanded Search for Extraterrestrial Intelligence (SETI ¹ ), a search that includes looking for life as we do not know it. This approach will augment the odds of detecting a signal by broadening our understanding of the evolutionary and systemic components in the search for extraterrestrial intelligence (ETI), provide more targets for radio and optical SETI, and identify new ways of decoding and coding messages using universal markers. Key Words: SETI—Astrobiology—Coevolution of Earth and life—Planetary habitability and biosignatures. Astrobiology 16, 661–676.

Bacteria are important organisms for space missions due to their increased pathogenesis in microgravity that poses risks to the health of astronauts and for projected synthetic biology applications at the space station. We understand little about the effect, at the molecular systems level, of microgravity on bacteria, despite their significant incidence. In this study, we proposed a systems biology pipeline and performed an analysis on published gene expression data sets from multiple seminal studies on Pseudomonas aeruginosa and Salmonella enterica serovar Typhimurium under spaceflight and simulated microgravity conditions. By applying gene set enrichment analysis on the global gene expression data, we directly identified a large number of new, statistically significant cellular and metabolic pathways involved in response to microgravity. Alteration of metabolic pathways in microgravity has rarely been reported before, whereas in this analysis metabolic pathways are prevalent. Several of those pathways were found to be common across studies and species, indicating a common cellular response in microgravity. We clustered genes based on their expression patterns using consensus non-negative matrix factorization. The genes from different mathematically stable clusters showed protein-protein association networks with distinct biological functions, suggesting the plausible functional or regulatory network motifs in response to microgravity. The newly identified pathways and networks showed connection with increased survival of pathogens within macrophages, virulence, and antibiotic resistance in microgravity. Our work establishes a systems biology pipeline and provides an integrated insight into the effect of microgravity at the molecular systems level. Key Words: Systems biology—Microgravity—Pathways and networks—Bacteria. Astrobiology 16, 677–689.

The oxidation of molecular hydrogen (H₂) is thought to be a major source of metabolic energy for life in the deep subsurface on Earth, and it could likewise support any extant biosphere on Mars, where stable habitable environments are probably limited to the subsurface. Faulting and fracturing may stimulate the supply of H₂ from several sources. We report the H₂ content of fluids present in terrestrial rocks formed by brittle fracturing on fault planes (pseudotachylites and cataclasites), along with protolith control samples. The fluids are dominated by water and include H₂ at abundances sufficient to support hydrogenotrophic microorganisms, with strong H₂ enrichments in the pseudotachylites compared to the controls. Weaker and less consistent H₂ enrichments are observed in the cataclasites, which represent less intense seismic friction than the pseudotachylites. The enrichments agree quantitatively with previous experimental measurements of frictionally driven H₂ formation during rock fracturing. We find that conservative estimates of current martian global seismicity predict episodic H₂ generation by Marsquakes in quantities useful to hydrogenotrophs over a range of scales and recurrence times. On both Earth and Mars, secondary release of H₂ may also accompany the breakdown of ancient fault rocks, which are particularly abundant in the pervasively fractured martian crust. This study strengthens the case for the astrobiological investigation of ancient martian fracture systems. Key Words: Deep biosphere—Faults—Fault rocks—Seismic activity—Hydrogen—Mars. Astrobiology 16, 690–702.

The analysis of the organic compounds present in the martian regolith is essential for understanding the history and habitability of Mars, as well as studying the signs of possible extant or extinct life. To date, pyrolysis, the only technique that has been used to extract organic compounds from the martian regolith, has not enabled the detection of unaltered native martian organics. The elevated temperatures required for pyrolysis extraction can cause native martian organics to react with perchlorate salts in the regolith and possibly result in the chlorohydrocarbons that have been detected by in situ instruments. Supercritical carbon dioxide (SCCO2) extraction is an alternative to pyrolysis that may be capable of delivering unaltered native organic species to an in situ detector. In this study, we report the SCCO2 extraction of unaltered coronene, a representative polycyclic aromatic hydrocarbon (PAH), from martian regolith simulants, in the presence of 3 parts per thousand (ppth) sodium perchlorate. PAHs are a class of nonpolar molecules of astrobiological interest and are delivered to the martian surface by meteoritic infall. We also determined that the extraction efficiency of coronene was unaffected by the presence of perchlorate on the regolith simulant, and that no sodium perchlorate was extracted by SCCO2. This indicates that SCCO2 extraction can provide de-salted samples that could be directly delivered to a variety of in situ detectors. SCCO2 was also used to extract trace native fluorescent organic compounds from the martian regolith simulant JSC Mars-1, providing further evidence that SCCO2 extraction may provide an alternative to pyrolysis to enable the delivery of unaltered native organic compounds to an in situ detector on a future Mars rover. Key Words: Biomarkers—Carbon dioxide—In situ measurement—Mars—Search for Mars’ organics. Astrobiology 16, 703–714.

We developed a prototype instrument called the Standoff Biofinder, which can quickly locate biological material in a 500 cm² area from a 2 m standoff distance with a detection time of 0.1 s. All biogenic materials give strong fluorescence signals when excited with UV and visible lasers. In addition, the luminescence decay time of biogenic compounds is much shorter (<100 ns) than the micro- to millisecond decay time of transition metal ions and rare-earth ions in minerals and rocks. The Standoff Biofinder takes advantage of the short lifetime of biofluorescent materials to obtain real-time fluorescence images that show the locations of biological materials among luminescent minerals in a geological context. The Standoff Biofinder instrument will be useful for locating biological material during future NASA rover, lander, and crewed missions. Additionally, the instrument can be used for nondestructive detection of biological materials in unique samples, such as those obtained by sample return missions from the outer planets and asteroids. The Standoff Biofinder also has the capacity to detect microbes and bacteria on space instruments for planetary protection purposes. Key Words: Standoff Biofinder—Luminescence—Time-resolved fluorescence—Biofluorescence—Planetary exploration—Planetary protection—Noncontact nondestructive biodetection. Astrobiology 16, 715–729.

Nitrogen is an essential nutrient for all life on Earth and possibly elsewhere. Burial of nitrogen bound to organic matter constitutes the major flux of nitrogen into sediments today, which has led to the inference that nitrogen enrichments in sedimentary rocks may be a biosignature. However, abiotic processes such as lightning or volcanism can fix atmospheric N₂ and contribute to sedimentary nitrogen burial in the absence of life. It is therefore uncertain whether observed nitrogen enrichments of up to 430 ppm in Paleoarchean metasedimentary biotite grains are indeed biogenic. This study seeks to address that problem with a numerical model. The NH₄ ⁺ concentration of an abiotic ocean is modeled as a function of source fluxes, pH-dependent NH₃ volatilization, and equilibrated adsorption of NH₄ ⁺ onto clay particles. The results suggest that the observed nitrogen concentrations in Paleoarchean biotite can only be reconciled with purely abiotic processes if the ocean was more acidic (pH <6) and/or if the source fluxes from lightning and volcanism were at least an order of magnitude higher (≥10¹² mol/yr) than previously thought. The bulk of the nitrogen is thus most likely of biological origin. While this does not necessitate a particular metabolism such as biological N₂ fixation, the data provide evidence of nitrogen utilization back to 3.8 Gyr. Nitrogen abundances could thus provide useful information in extraterrestrial missions. Key Words: Early Earth—Biosignatures—Nitrogen fixation. Astrobiology 16, 730–735.