Astrobiology最新文献

The search for life in the solar system often focuses on water and on environments where habitable conditions exist, persistently or occasionally. In this search, dry permafrost (ice-free frozen soil) has received minimal attention. It was previously proposed that within martian dry permafrost the water activity ($a_w$, an essential property for habitability) could be enhanced by diurnal thermal cycles and water desorption from soil grains, but the details remain unexplored. We examined $a_w$ in dry soil (which contained only vapor and adsorbed water) through experiments and numerical simulations and contrasted the results with a habitability threshold for terrestrial organisms ($a_w\hspace{0.17em}\hspace{0.17em}>\hspace{0.17em}\hspace{0.17em}0.6$). We found that heating cycles in a soil raised $a_w$. As water vapor desorbs from warming soil grains, it diffuses toward cooler adjacent soil, where a fraction of this incoming vapor enhances the local $a_w$. In laboratory tests with loess and clay soils, we observed $a_w$ to increase by 0.06-0.12. Extrapolating from laboratory to permafrost conditions by using numerical simulations, we found that some Antarctic soils can be boosted periodically into a habitable range. In contrast, the current martian climate is too dry or cold for this $a_w$-enhancement process to impact habitability. However, high-obliquity periods on Mars are analogous to the Antarctic case.

The mode of tectonics that governed early Earth is controversial. This makes it challenging to infer surface environments relevant to the origin of life. The majority of the literature published in the past two decades was inclined to favor the appearance of plate tectonics sometime around the mid-Archean (∼3 Ga), with the operation of stagnant lid convection (or its variants) dominant in the earlier part of Earth's history. However, the available and increasing geological record from early Earth is actually equivocal, and there is no theoretical basis to prefer stagnant lid convection over plate tectonics. In fact, such a delayed onset of plate tectonics would inhibit the emergence of life in the Archean, let alone in the Hadean. On the contrary, rapid plate tectonics in the early Hadean, enabled by the fractional crystallization of a magma ocean, could quickly transform inclement young Earth into a habitable planet, with formation of multiple surface environments potentially conducive to abiogenesis.

Exoplanet habitability remains a challenging field due to the large distances separating Earth from other stars. Using insights from biology and astrophysics, we studied the habitability of M dwarf exoplanets by modeling their surface temperature and flare ultraviolet (UV) and X-ray doses using the martian atmosphere as a shielding model. Analyzing the Proxima Centauri and TRAPPIST-1 systems, our models suggest that Proxima b and TRAPPIST-1 e are likeliest to have temperatures compatible with surface liquid water, as well as tolerable radiation environments. Results of the modeling were used as a basis for microbiology experiments to assess spore survival and germination of the melanin-rich fungus Aspergillus niger to exoplanet-like radiation (UV-C and X-rays). Results showed that A. niger spores can endure superflare events on M dwarf planets when shielded by a Mars-like atmosphere or by a thin layer of soil or water. Melanin-deficient spores suspended in a melanin-rich solution showed higher survival rates and germination efficiency when compared to melanin-free solutions. Overall, the models developed in this work establish a framework for microbiological research in habitability studies. Finally, we showed that A. niger spores can survive harsh radiation conditions of simulated exoplanets, which also emphasizes the importance of multifunctional molecules like melanins in radiation shielding beyond Earth.

The notion of liquid water beneath the ice layer at the south polar layered deposits (SPLD) of Mars is an interesting possibility given the implications for astrobiology and possible human habitation. A body of liquid water located at a depth of 1.5 km has been inferred from radar data in the South Polar Cap. However, the high temperatures that would facilitate the existence of liquid water or brine at that depth are not consistent with estimations of heat flow that are based on the lithosphere's flexure. Attempts to reconcile both issues have been inconclusive or otherwise unsuccessful. Here, we analyze the possible role(s) of subsurface ammonia and/or methanol in maintaining water in a liquid state at subsurface temperatures that are compatible with the lithosphere strength. Our results indicate that the presence of these compounds at the base of the SPLD can reconcile the existence of liquid water with previous estimations of surface heat flow.

This work examines the impact of high temperatures from celestial shock events on the stability of sulfates found on Mars (gypsum) and those expected to be present (syngenite and görgeyite). Raman spectroscopy, a cutting-edge technique in space exploration, was used to track their stability. Specifically, a Renishaw inVia^™ micro-Raman confocal spectrometer was coupled with an external Linkam THMS600/HF600 temperature-controlled stage to monitor the sample temperature while measuring the main Raman band positions of the sulfates and those of water molecules in these salts across temperatures ranging from 313 to 673 K. Results showed a shift toward lower wavenumbers with increasing temperature for all compounds, up to each compound's inflection temperature, where phase transformations occurred. The linear trends identified in this study provide valuable insights for interpreting data from space missions equipped with Raman instruments and understanding Earth-based measurements. These trends enable the estimation of Raman band wavenumbers at specific temperatures, as well as the determination of the temperature at which a given spectrum was acquired. Additionally, the research demonstrated that the three heated salts fully rehydrated after at least 1 month under standard environmental conditions (23°C, 1 atm, and ∼80% relative humidity). This finding on reversibility is crucial for interpreting time-dependent results, such as characterizing meteorites that contain evaporite salts.

Since the discovery of perchlorates in martian soils, astrobiologists have been curious if and how life could survive in these low-water, high-salt environments. Perchlorates induce chaotropic and oxidative stress but can also confer increased cold tolerance in some extremophiles. Though bacterial survival has been demonstrated at subzero temperatures and in perchlorate solution, proteomic analysis of cells growing in an environment like martian regolith brines-perchlorate with subzero temperatures-has yet to be demonstrated. By defining biosignatures of survival and growth in perchlorate-amended media at subzero conditions, we move closer to understanding the mechanisms that underlie the feasibility of life on Mars. Colwellia psychrerythraea str. 34H (Cp34H), a marine psychrophile, was exposed to perchlorate ions in the form of a diluted Phoenix Mars Lander Wet Chemistry Laboratory solution at -1°C and -5°C. At both temperatures in perchlorate-amended media, Cp34H grew at reduced rates. Mass spectrometry-based proteomics analyses revealed that proteins responsible for mitigating effects of oxidative and chaotropic stress increased, while cellular transport proteins decreased. Cumulative protein signatures suggested modifications to cell-cell or cell-surface adhesion properties. These physical and biochemical traits could serve as putative identifiable biosignatures for life detection in martian environments.

In the search for life on Mars, evaluating the biogenicity of morphological structures may be important, as they can provide a primary independent line of evidence for past life and can be used to target areas to focus further analyses. However, our experience with terrestrial materials indicates that the deleterious effects of diagenetic processes regularly make the assessment, and even detection, of microfossils and other microscopic biosignatures challenging. To improve our understanding of these effects on Mars, we collected samples that contained sheath-shaped extracellular structures produced by iron-oxidizing bacteria (FeOB) from a Mars analog circumneutral iron deposit and subjected them to artificial maturation by hydrous pyrolysis. Simulated diagenesis induced a phase change in the mineralogy of the structures, from ferrihydrite to crystalline iron oxides. We found that conditions associated with the onset of this phase change were correlated with the start of significant degradation of the extracellular structures. Our results reveal the sensitivity of remains of FeOB to diagenesis, which provides insights for improved targeting of astrobiological missions to areas on Mars that are most conducive to morphological biosignature preservation. Additionally, these results compel increased scrutiny of FeOB-like purported biosignatures if their mineralogy is dominated by crystalline iron oxides.

The intense debate about the presence of methane in the martian atmosphere has stimulated the study of methanogenic species that are adapted to terrestrial habitats that resemble martian environments. We examined the environmental conditions, energy sources, and ecology of terrestrial methanogens that thrive in deep crystalline fractures, subsea hypersaline lakes, and subglacial water bodies, considered analogs of a hypothetical habitable martian subsurface. We combined this information with recent data on the distribution of buried water/ice and radiogenic elements on Mars, and with models of the subsurface thermal regime of this planet, we identified a 4.3-8.8 km-deep regolith habitat at the midlatitude location of Acidalia Planitia that might fit the requirements for hosting putative martian methanogens analogous to the methanogenic families, Methanosarcinaceae and Methanomicrobiaceae.

The vegetation red edge of terrestrial plants is a key biosignature for the detection of life on Earth-like habitable exoplanets. Although water is essential for plants, an excess of water can limit the distribution of terrestrial vegetation. On planets with extensive water coverage and limited land, floating vegetation on the water's surface could serve as a crucial indicator of life. This study examined the spectral reflectance of floating plants across various scales, from individual leaves to lake-wide vegetation coverage. Our comparisons between individual leaves revealed that the red edge of floating plants was equivalent to or even more pronounced than that of terrestrial plants. Although water can reduce plant reflectance, the naturally low reflectance of water enhances the detection sensitivity for floating vegetation. Our observations of seasonal changes, such as the proliferation of floating plants in summer and their decline in winter, revealed significant variations in lake reflectance. By analyzing satellite images of lakes and marshes over a 5-year period, we confirmed that these seasonal variations in reflectance reliably indicated the presence of floating vegetation. The seasonal signal showed robustness to the effects of clouds, which pose another challenge on water-rich planets. We propose that floating vegetation be considered alongside, or even in place of, terrestrial vegetation in the search for extraterrestrial life.

Digitate siliceous sinter deposits are common in geothermal environments. They form via evaporation and precipitation of cooling silica-rich fluids and passive microbial templating. Increasing interest in these "finger-like" microstromatolitic sinters is related to their morphological and mineralogical resemblance to opaline silica-rich rocks discovered by NASA's Spirit rover in the Columbia Hills, Gusev crater, Mars. However, these terrestrial deposits remain understudied, specifically in terms of biosignature content and long-term preservation potential. In this study, six digitate, opaline (opal-A) sinter deposits were collected from five Taupō Volcanic Zonegeothermal fields, and their lipid biosignatures were investigated as Mars analogs. Samples were collected in pools and discharge channels of varied temperatures, pH, and water chemistries, with spicular to nodular morphologies. Results revealed the presence of biomarkers from unsilicified and silicified communities populating the hot spring sinters, including lipids from terrigenous plants, algae, and bacteria. Although DNA sequencing suggests that the composition and diversity of microbial communities are correlated with temperature, pH, and water chemistry of the springs, these environmental parameters did not seem to affect lipid recovery. However, the morphology of the sinters did play a role in lipid yield, which was higher in the finest, needle-like spicules in comparison to the broad, knobby sinters. The capability of current Mars flight mission techniques such as pyrolysis-gas chromatography-mass spectrometry to detect lipid biomarkers was also evaluated from a subset of samples in a pilot study under flight conditions. The early preservation of lipids in the studied sinters and their detection using flight-like techniques suggest that martian siliceous deposits are strong candidates for the search for biosignatures on Mars.