Astrobiology最新文献

A primary aim of current and future space exploration missions is the detection and identification of chemical and biological indicators of life, namely biomarkers, on Mars. The Mars Sample Return NASA-ESA program will bring to Earth samples of martian soil, acquired from up to 7 cm depth. The ESA Rosalind Franklin rover will search for signs of life in the subsurface (down to a depth of 2 meters), given the highly radioactive conditions on Mars' surface, which are not ideal for life as we know it and for the preservation of its traces. In the frame of the Lichens and Fungi Experiment, small fragments of Antarctic sandstones colonized by cryptoendolithic microbial communities were exposed to space and simulated martian conditions in low Earth orbit for 18 months, aboard the EXPOSE-E payload. Through the use of Raman and infrared spectroscopies, as well as a metabolomic approach, we aimed to detect organic compounds in a quartz mineral matrix. The results show that pigments, such as melanin, carotenoids, and chlorophyll, lipids, and amino acids, maintained their stability within minerals under simulated martian conditions in space, which makes them ideal biomarkers for the exploration of putative life on Mars.

Because a range of silica minerals can precipitate from water, the analysis of silica mineral phases is important for astrobiological exploration. In this context, poorly crystalline opaline minerals that contain intracrystalline water are commonly accepted indicators of the presence of water in the geological past. However, opaline minerals are not the only silica phases that are evidence of past interaction with water. Water may become incorporated within crystalline quartz as silanol (Si-OH)-hydroxyl groups present as structural defects within a crystal lattice. Raman spectroscopy is a highly reliable method for detecting mineral composition, and it can also detect silanol. By analyzing Raman spectra from various silica gemstones and rocks, we found that 52 out of 71 quartz samples contain silanol. However, silanol was not universally present across all samples. Microcrystalline quartz and samples in which silica phases had replaced other minerals tended to display the highest levels of silanol, whereas macrocrystalline quartz exhibited the lowest values, as indicated by the Sil_prop parameter. In addition, we observed instances where quartz-hosted silanol and carbonaceous materials were codetected, which suggests the potential for Raman to be used to detect both carbonaceous organic matter and water, and therefore potential indications of both life and habitability.

The early start to life naively suggests that abiogenesis is a rapid process on Earth-like planets. However, if evolution typically takes ∼4 Gyr to produce intelligent life-forms like us, then the limited lifespan of Earth's biosphere (∼5-6 Gyr) necessitates an early (and possibly highly atypical) start to our emergence-an example of the weak anthropic principle. Our previously proposed objective Bayesian analysis of Earth's chronology culminated in a formula for the minimum odds ratio between the fast and slow abiogenesis scenarios (relative to Earth's lifespan). Timing from microfossils (3.7 Gya) yields 3:1 odds in favor of rapid abiogenesis, whereas evidence from carbon isotopes (4.1 Gya) gives 9:1, both below the canonical threshold of "strong evidence" (10:1). However, the recent result of a 4.2 Gya LUCA pushes the odds over the threshold for the first time (nominally 13:1). In fact, the odds ratio is >10:1 for all possible values of the biosphere's ultimate lifespan and speculative hypotheses of ancient civilizations. For the first time, we formally have strong evidence that favors the hypothesis that life rapidly emerges in Earth-like conditions (although such environments may themselves be rare).

Europa, Jupiter's second Galilean moon, is believed to host a subsurface ocean in contact with a rocky mantle, where hydrothermal activity may drive the synthesis of organic molecules. Among these possible organic molecules, abiotic synthesis of aromatic amino acids is unlikely, so their detection on planetary surfaces such as Europa suggests that they could be considered a potential biosignature. Fluorescence from aromatic amino acids, with characteristic emissions in the 200-400 nm wavelength range, can be induced by a laser and may be detectable where ocean material has been relatively recently emplaced on Europa's surface, as indicated by geologically young terrain and surface features. However, surface bombardment by charged particles from the jovian magnetosphere and solar ultraviolet (UV) radiation degrades organic molecules and limits their longevity. We model radiolysis and photolysis of aromatic amino acids embedded in ice. Our model shows dependencies on hemispheric and latitudinal patterns of charged particle bombardment and ice phase. We demonstrate that such molecules contained within freshly deposited ice in high-latitude regions on the surface of Europa are detectable using laser-induced UV fluorescence, even from an orbiting spacecraft.

We explore a hypothesis in which the detection of classes of lipid-like molecules with similar abundance-averaged lengths would constitute a biosignature for other worlds. This is based on the functional requirements of membrane molecules: they must have enough hydrophobic length to not diffuse away from the membrane, be capped by one or two hydrophilic polar groups, and also maintain a semipermeable membrane. Our hypothesis is that once membrane thickness is set in a biological system, it is very difficult to modify it, due to the necessity to redesign all the other associated molecules; the membrane thickness will be constant across all molecular classes that constitute membranes resulting from a common ancestor. In such a scenario, similar thickness values would thus constitute a biosignature and cross-correlate between different molecular classes. We tested this hypothesis by developing a simple method to use modeled lengths of lipid-like molecules to estimate the thicknesses of membranes formed by these molecules. We examined abundance patterns of four different classes of membrane molecules used by terrestrial life: fatty acids, glycerol dialkyl glycerol tetraether lipids, carotenoids, and ladderanes from microbial isolates and environmental samples, as well as abiotic samples of fatty acids. We found that the modeled cell membrane thicknesses from each of these molecular classes were similar and gave results consistent with the observed values. From these results, we propose that our approach provides a framework to identify potential membrane component molecules as an agnostic biosignature. The power of our approach is that our method enables multiple molecular classes to be compared and provides increasing confidence of a biological detection.

Searching for extraterrestrial life and supporting human life in space are traditionally regarded as separate challenges. However, there are significant benefits to an approach that treats them as different aspects of the same essential problem: How can we conceptualize life beyond our home planet?

Venus has become a target of astrobiological interest because it is physically accessible to direct exploration, unlike exoplanets. So far this interest has been motivated not by the explicit expectation of finding life but rather by a desire to understand the limits of biology. The venusian surface is sterilizing, but the cloud deck includes regions with temperatures and pressures conventionally considered compatible with life. However, the venusian clouds are thought to consist of concentrated sulfuric acid. To determine if any fundamental features of life as we understand them here on Earth could in principle exist in these extreme solvent conditions, we tested several simple lipids for resistance to solvolysis and their ability to form structures in concentrated sulfuric acid. We find that single-chain saturated lipids with sulfate, alcohol, trimethylamine, and phosphonate head groups are resistant to sulfuric acid degradation at room temperature. Furthermore, we find that they form stable higher-order structures typically associated with lipid membranes, micelles, and vesicles. Finally, results from molecular dynamics simulations suggest a molecular explanation for the observed robustness of the lipid structures formed in concentrated sulfuric acid. We conclude with implications for the study of Venus as a target of experimental astrobiology.

There is a crucial need to identify reliable imprints of microbe-mineral interactions to quantify the contribution of microorganisms to chemical weathering and detect traces of life in the geological record. Yet conventional methods based on qualitative descriptions of supposedly bioinduced etching features have often proven equivocal. Here, calcite dissolution experiments were carried out at various solution compositions, in the presence or absence of cyanobacterial biofilms. Nanoscale chemical and crystallographic characterizations failed to detect any distinctive biogenicity feature. Conversely, high-elevation regions at the calcite surface were detected through statistical characterizations of the microtopography, which made microbially weathered surfaces quantitatively distinguishable from their abiotic counterparts. Interestingly, the high-elevation regions that formed beneath clusters of microbial cells are at odds with the etching features that resemble cell morphologies and are usually sought as bioweathering markers. Atomic-scale stochastic simulations of the dissolution process suggested that these regions resulted from a local increase in fluid saturation state at the biofilm-mineral contact, which led to a localized reduction in dissolution rates. Overall, this study offers a new avenue for the nondestructive identification of bioweathering signatures in natural settings.

The moons of Jupiter and Saturn, such as Europa and Enceladus, are strong candidates for the search for life outside of Earth. Together with the use of direct observational methods, physical and chemical processes that take place on icy moons may be studied on planetary field analogs, that is, on similar reachable locations on Earth. Fieldwork performed on planetary field analogs can test protocols and technology that may be applied on future space missions to extraterrestrial environments. The Arctic is a strong candidate for such studies. This study assesses a spectroscopic protocol for biosignature detection in the Arctic, as a proxy to icy moons. Samples of ice and the water underneath were collected by our team in different locations at and nearby Hudson Bay, Canada, and spectroscopic analysis detected the presence of humic acid in all the samples. On the contrary, biosignatures such as amino acids and β-carotene may have been present in concentrations below the limit of detection of the equipment used. With proper optimization, it will be possible to implement this simple protocol that relies on lightweight equipment in future space missions to icy moons.