Life Detection Missions Research Articles

UV Irradiation and Near Infrared Characterization of Laboratory Mars Soil Analog Samples

The search for molecular biosignatures at the surface of Mars is complicated by an intense irradiation in the mid- and near- ultraviolet (UV) spectral range for several reasons: (i) many astrobiologically relevant molecules are electronically excited by efficient absorption of UV radiation and rapidly undergo photochemical reactions; (ii) even though the penetration depth of UV radiation is limited, aeolian erosion continually exposes fresh material to radiation; and (iii) UV irradiation generates strong oxidants such as perchlorates that can penetrate deep into soils and cause subsurface oxidative degradation of organics. As a consequence, it is crucial to investigate the effects of UV radiation on organic molecules embedded in mineral matrices mimicking the martian soil, in order to validate hypotheses about the nature of the organic compounds detected so far at the surface of Mars by the NASA Mars Science Laboratory’s Curiosity rover, as well as organics that will be possibly found by the next rover missions Mars 2020 (NASA) and ExoMars 2022 (ESA-Roscosmos). In addition, studying the alteration of possible molecular biosignatures in the martian environment will help to redefine the molecular targets for life detection missions and devise suitable detection methods. Here we report the results of mid- and near-UV irradiation experiments of Mars soil analog samples obtained adsorbing relevant organic molecules on a clay mineral that is quite common on Mars, i.e. montmorillonite, doped with 1 wt% of magnesium perchlorate. Specifically, we chose to investigate the photostability of a plausible precursor of the chlorohydrocarbons detected on Mars by the Curiosity rover, namely phthalic acid, along with the biomarkers of extant life L-phenylalanine and L-glutamic acid, which are proteomic amino acids, and adenosine 5’-monophosphate, which is a nucleic acid component. We monitored the degradation of these molecules adsorbed on montmorillonite through in situ spectroscopic analysis, investigating the reflectance properties of the samples in the Near InfraRed spectral region. Such spectroscopic characterization of molecular alteration products provides support for two upcoming robotic missions to Mars that will employ NIR spectroscopy to look for molecular biosignatures, through the instruments SuperCam on board Mars 2020, ISEM, Ma_Miss and MicrOmega on board ExoMars 2022.

Microbial community distribution in variously altered basalts: Insights into astrobiology sample site selection

Terrestrial basalts represent ecological niches with the potential for microbial habitability. However, little is known about how gradients of physical and/or chemical alteration may impact the distribution of microbial biomass. If higher abundances of microbial biomass are associated with identifiable alteration features, i.e. visible from orbit or rover, such features would become an important tool in the selection of astrobiology targets for future life-detection missions. To examine the associations between biomass abundance and basalt alteration processes, phospholipid fatty acid (PLFA) biomarker analysis was used to assess microbial biomass in basalts representative of distinct alteration categories collected from two Mars analog environments: Craters of the Moon National Monument and Preserve (COTM), Idaho and Hawai’i Volcanoes National Park (HVNP), Hawai’i. PLFA concentration was generally highest in field categorized high-temperature alteration (syn-emplacement) samples within COTM, particularly in those that may have also undergone some degree of secondary weathering. Basalts from HVNP showed more variable biomass abundances, with no clear trend between altered and unaltered basalts. HVNP fumarolic deposits (both active and relict fumarole-associated samples) generally had consistently detectable biomass although temperature appeared to play a role in abundance with the highest temperate fumarole exhibiting the lowest PLFA concentration. PLFA profiles generally had high proportions of brPLFA (including biomarkers of iron- and sulfur-reducing bacteria) and indicated that basalts were dominated by heterotrophic organisms, however no clear trend between community biomarker composition and alteration type or extent was identified. Heterogeneity of microbial biomass within terrestrial basalts suggests that identification of distinct alteration features clearly linked to high biomass (biomarkers) is challenging but that altered materials (syn-emplacement and secondary weathering) and fumarolic deposits are likely promising astrobiology targets.

Towards Determining Biosignature Retention in Icy World Plumes.

With the discovery of the persistent jets of water being ejected to space from Enceladus, an understanding of the effect of the space environment on potential organisms and biosignatures in them is necessary for planning life detection missions. We experimentally determine the survivability of microbial cells in liquid medium when ejected into vacuum. Epifluorescence microscopy, using a lipid stain, and SEM imaging were used to interrogate the cellular integrity of E. coli after ejected through a pressurized nozzle into a vacuum chamber. The experimental samples showed a 94% decrease in visible intact E. coli cells but showed a fluorescence residue in the shape of the sublimated droplets that indicated the presence of lipids. The differences in the experimental conditions versus those expected on Enceladus should not change the analog value because the process a sample would undergo when ejected into space was representative. E. coli was selected for testing although other cell types could vary physiologically which would affect their response to a vacuum environment. More testing is needed to determine the dynamic range in concentration of cells expected to survive the plume environment. However, these results suggest that lipids may be directly detectable evidence of life in icy world plumes.

0.25 Ga Salt Deposits Preserve Signatures of Habitable Conditions and Ancient Lipids.

Polygonal features in a ∼250 million-year-old Permian evaporitic deposit were investigated for their geological and organic content to test the hypothesis that they could preserve the signature of ancient habitable conditions and biological activity. Investigations on evaporitic rock were carried out as part of the MIne Analog Research (MINAR) project at Boulby Mine, the United Kingdom. The edges of the polygons have a higher clay content and contain higher abundances of minerals such as quartz and microcline, and clays such as illite and chlorite, compared with the interior of polygons, suggesting that the edges were preferred locations for the accumulation of weathering products during their formation. The mineral content and its strontium isotope ratio suggest that the material is from continental weathering at the borders of the Permian Zechstein Sea. The edges of the polygons contain material with mean δ13C and δ15N values of -20.8 and 5.3, respectively. Lipids, including alkanes and hopanes, were extracted from the interior and edges of the polygons, which are inferred to represent organic material entrained in the evaporites when they were formed. The presence of long-chain alkanes (C20-C35) that lack a carbon preference, low abundances of C23-C29 hopanes, and lack of marine, evaporitic, or thermal maturity indicators show that lipid biomarkers were, at least in part, potentially derived from a continental source and have not undergone significant thermal maturation since deposition. Lipid extractions using weak acids revealed significantly more lipids than those without acid, potentially indicating that encapsulation was not the only type of preservation mechanism occurring in Boulby salts. These data demonstrate the potential for ancient evaporites and their polygons to preserve information on local geological conditions, ancient habitability, and evidence of life. The data show that analogous martian evaporitic deposits are good targets for future life detection missions and the investigation of ancient martian habitability.

SpectroCube: a European 6U nanosatellite spectroscopy platform for astrobiology and astrochemistry

SpectroCube is a CubeSat-based miniaturized in-situ space exposure platform for astrochemistry and astrobiology research. Within a 6 unit (6U, with 1U corresponding to 10 cm x 10 cm x 10 cm) nanosatellite structure, an infrared spectrometer is interfaced with a sample handling system to measure photochemical changes of organic molecules, representing important biomarkers for the detection of life in our solar system and beyond. Monitoring degradation profiles and photochemical reaction kinetics of such biomarkers allows to identify suitable search targets for current and future planetary exploration and life-detection missions. SpectroCube is designed to be launched into a highly elliptical orbit around Earth and therefore allows to expose samples to higher solar UV and energetic particle radiation levels than previous exposure platforms in low Earth orbit, as for example on the International Space Station. In-situ data will be telemetered back to Earth and compared with solar and planetary simulation experiments in ground-based laboratory. We here present the design of SpectroCube, the scientific payload and its subsystems. We demonstrate that with the miniaturisation potential of infrared spectroscopy it is possible to fit the entire optical setup plus a sample handling system for up to 60 individually contained and hermetically sealed samples within less than half of the volume of a 6U CubeSat structure. Therefore, the remaining volume can be entirely used for additional subsystems such as attitude control, propulsion, fuel, onboard computer and telemetry.The design of the scientific payload is based on a commercial off-the-shelf miniaturised Fourier-transform spectrometer consisting of an infrared light source, an interferometer and infrared detector units. The mechanical robustness and suitability of such a system for space applications was assessed. Shock and vibration testing of the mechanically most sensitive unit, the interferometer, was performed and revealed that with adequate damping the spectroscopic performance can be maintained. Additional measurements of test samples conducted with the selected commercial off-the-shelf spectrometer candidate showed that the spectroscopic range, resolution and sensitivity is capable to monitor in situ the photochemical kinetics of important classes of organic molecules and biomarkers for astrobiology and astrochemistry research.

Are we There Yet? Understanding Interplanetary Microbial Hitchhikers using Molecular Methods.

Since the early time of spacetravel, planetary bodies undergoing chemical or biological evolution have beenof particular interest for life detection missions. NASA's and ESA's PlanetaryProtection offices ensure responsible exploration of the solar system and aimat avoiding inadvertent contamination of celestial bodies with biomolecules oreven living organisms. Lifeforms that have the potential to colonize foreign planetarybodies could be a threat to the integrity of science objectives of lifedetection missions. While standard requirements forassessing the cleanlinessof spacecraft are still based on cultivation approaches, several molecularmethods have been applied in the past to elucidate the full breadth of(micro)organisms that can be found on spacecraft and in cleanrooms, where thehardware is assembled. Here, we review molecular assays that have been appliedin PlanetaryProtection research and list their significant advantages anddisadvantages. By providing a comprehensive summary of the latest molecularmethods yet to be applied in this researcharea, this article will not only aidin designing technological roadmaps for future Planetary Protection endeavorsbut also help other disciplines in environmental microbiology that deal withlow biomass samples.

The Search for Chiral Asymmetry as a Potential Biosignature in our Solar System.

The search for evidence of extraterrestrial life in our Solar System is currently guided by our understanding of terrestrial biology and its associated biosignatures. The observed homochirality in all life on Earth, that is, the predominance of "left-handed" or l-amino acids and "right-handed" or d-sugars, is a unique property of life that is crucial for molecular recognition, enzymatic function, information storage and structure and is thought to be a prerequisite for the origin or early evolution of life. Therefore, the detection of l- or d-enantiomeric excesses (ee) of chiral amino acids and sugars could be a powerful indicator for extant or extinct life on another world. However, studies of primitive meteorites have revealed they contain extraterrestrial amino acids and sugar acids (aldonic acids) with large enantiomeric excesses of the same chirality as terrestrial biology resulting from nonbiological processes, complicating the use of chiral asymmetry by itself as a definitive biosignature. Here we review our current knowledge of the distributions and enantiomeric and isotopic compositions of amino acids and polyols found in meteorites compared to terrestrial biology and propose a set of criteria for future life detection missions that can be used to help establish the origin of chiral asymmetry.

Development of Solid-State Nanopore Life Detection Technology

Biomarkers collected from past and present life on Earth are a central guide in the development of flight-ready technologies designed to search for life on other planets and moons. Data from numerous laboratory and field studies have revealed the importance of incorporating capabilities that minimize the risk of false negatives or false positives during life detection missions. Biomolecules are the most unambiguous and information-rich of all known biosignatures. Indeed, the identification in a sample of biopolymers akin to DNA, RNA or proteins, would be difficult to refute as a successful life detection experiment. In the case of Mars, the search for biomolecules is inescapable, but few technological solutions exist for in situ identification. Thus, our work has focused on the development of technologies designed to detect biochemical polyelectrolyte molecules, which have been argued to be a universal signature for life due to their ability to store information while reducing their tendency to adopt complex tertiary structures that would prohibit their replication. To that end we have developed a solid-state nanopore device that has the ability to detect electrically charged organic polymers, such as those found in deoxyribonucleic acid. Our nanopore technology, when used as a biosensor, consists of two or three ion-filled chambers separated by voltage-biased thin layer (Figure 1). Nanopores are milled into the thin layer, allowing the chambers to be connected electrically via an ionic solution. Polyelectrolytes are introduced into one of the chambers, and a patch clamp amplifier is connected to the chip via electrodes that serve as a voltage source and an ammeter. When a voltage is applied across the membrane, the ionic current going only through this pore is measured. The polyelectrolytes are captured by the electric field in the pore, then and electrophoretically drawn through the nanopore (translocation) causing the nanopore conductance to decrease. This conductance decrease, or transient current blockage, is evaluated for: 1) mean blockage current (which is caused by the physical passage of the polyelectrolyte through the nanopore, and is measured by a sensitive ammeter in pA); 2) translocation duration (physical time of the translocation, for DNA this is typically 30 bases per second when translocation is controlled by a molecular motor [1]); and 3) integrated area of a blockage, revealing key features of the particle. Solid-state nanopore technology is being aggressively advanced for use as an electrochemistry sensor for a wide variety of biological molecules. Testing of the solid-state nanopore membranes includes the establishment of criteria used to distinguish informational from non-informational output signals. Our work highlights past, current, and future efforts in the development of solid-state nanopore technology for life detection.

Solid Phase Micro Extraction: Potential for Organic Contamination Control for Planetary Protection of Life-Detection Missions to the Icy Moons of the Outer Solar System.

Conclusively detecting, or ruling out the possibility of, life on the icy moons of the outer Solar System will require spacecraft missions to undergo rigorous planetary protection and contamination control procedures to achieve extremely low levels of organic terrestrial contamination. Contamination control is necessary to avoid forward contamination of the body of interest and to avoid the detection of false-positive signals, which could either mask indigenous organic chemistry of interest or cause an astrobiological false alarm. Here we test a new method for rapidly and inexpensively assessing the organic cleanliness of spaceflight hardware surfaces using solid phase micro extraction (SPME) fibers to directly swab surfaces. The results suggest that the method is both time and cost efficient. The SPME-gas chromatography-mass spectrometry (SPME-GC-MS) method is sensitive to common midweight, nonpolar contaminant compounds, for example, aliphatic and aromatic hydrocarbons, which are common contaminants in laboratory settings. While we demonstrate the potential of SPME for surface sampling, the GC-MS instrumentation restricts the SPME-GC-MS technique's sensitivity to larger polar and nonvolatile compounds. Although not used in this study, to increase the potential range of detectable compounds, SPME can also be used in conjunction with high-performance liquid chromatography/liquid chromatography-mass spectrometry systems suitable for polar analytes (Kataoka et al., 2000). Thus, our SPME method presents an opportunity to monitor organic contamination in a relatively rapid and routine way that produces information-rich data sets.

Alteration of Proteomes in First-Generation Cultures of Bacillus pumilus Spores Exposed to Outer Space.

Bacillus pumilus SAFR-032 was originally isolated from the Jet Propulsion Lab Spacecraft Assembly Facility and thoroughly characterized for its enhanced resistance to UV irradiation and oxidative stress. This unusual resistance of SAFR-032 is of particular concern in the context of planetary protection and calls for development of novel disinfection techniques to prevent extraterrestrial contamination. Previously, spores of SAFR-032 were exposed for 18 months to a variety of space conditions on board the International Space Station to investigate their resistance to Mars-like conditions and space travel. Here, proteomic characterization of vegetative SAFR-032 cells from space-surviving spores is presented in comparison to a ground control. Vegetative cells of the first passage were processed and subjected to quantitative proteomics using tandem mass tags. Approximately 60% of all proteins encoded by SAFR-032 were identified, and 301 proteins were differentially expressed among the strains. We found that proteins predicted to be involved in carbohydrate transport/metabolism and energy production/conversion had lower abundance than those of the ground control. For three proteins, we showed that the expected metabolic activities were decreased, as expected with direct enzymatic assays. This was consistent with a decrease of ATP production in the space-surviving strains. The same space-surviving strains showed increased abundance of proteins related to survival, growth advantage, and stress response. Such alterations in the proteomes provide insights into possible molecular mechanisms of B. pumilus SAFR-032 to adapt to and resist extreme extraterrestrial environments.IMPORTANCE Spore-forming bacteria are well known for their resistance to harsh environments and are of concern for spreading contamination to extraterrestrial bodies during future life detection missions. Bacillus pumilus has been regularly isolated from spacecraft-associated surfaces and exhibited unusual resistance to ultraviolet light and other sterilization techniques. A better understanding of the mechanisms of microbial survival and enhanced resistance is essential for developing novel disinfection protocols for the purpose of planetary protection. While genomic analyses did not reveal the unique characteristics that explain elevated UV resistance of space-exposed B. pumilus, the proteomics study presented here provided intriguing insight on key metabolic changes. The observed proteomics aberrations reveal a complex biological phenomenon that plays a role in bacterial survival and adaptation under long-term exposure to outer space. This adaptive ability of microorganisms needs to be considered by those tasked with eliminating forward contamination.

Calculating risk and payoff in planetary exploration and life detection missions

A framework for quantitative assessment of different mission architectures is described using historical data and formal (Bayesian) information value measures. The science value of the result is argued for binary questions (e.g. 'is there life on Europa?) to be proportional to the logarithm of the posterior likelihood ratio of the answers, and can be derived from estimates of the false positive rates of instrumentation and of the presence (PD) of biosignatures at a given site. The expectation payoff is the product of the sought result with Markovian success probabilities of the required steps of launch, landing, sample acquisition etc., and historical planetary mission data are reviewed to derive (sometimes dismaying) estimates of these probabilities, e.g. historical landing successes rates are of the order of 66% and when landing is successful, the conditional rates of individual sample acquisition/analysis/return have similar values. The history of seafloor exploration on Earth is used as an analog, and indicates that in the absence of close reconnaissance data, PD may have rather low values of the order of 1% or less. The data acquisition success framework is demonstrated on the value of single versus multiple landers, on the choice of flyby altitudes for multiple plume fly-through missions, and on the value of surface mobility, which for small values of PD multiplies the science return by the number of sites visited. Bayesian reasoning requires encapsulation of prior information: while such estimates (of biosignature presence, false alarm rates, etc.) are inevitably subjective, the decomposition of that information onto specific factors affords transparency into their contribution to the final result and provides a basis for rational mission evaluation.

Moving Beyond Definitions in the Search for Extraterrestrial Life.

According to the 2015 Astrobiology Strategy, a central goal of astrobiology is to provide a definition of life. A similar claim is made in the 2018 CRC Handbook of Astrobiology. Yet despite efforts, there remains no consensus on a definition of life. This essay explores an alternative strategy for searching for extraterrestrial life: Search for potentially biological anomalies (as opposed to life per se) using tentative (vs. defining) criteria. The function of tentative criteria is not, like that of defining criteria, to provide an estimate (via a decision procedure) of the likelihood that an extraterrestrial phenomenon is the product of life. Instead, it is to identify phenomena that resist classification as living or nonliving as worthy of further investigation for novel life. For as the history of science reveals, anomalies are a driving force behind scientific discovery and yet (when encountered) are rarely recognized for what they represent because they violate core theoretical beliefs about the phenomena concerned. While the proposed strategy resembles that of current life-detection missions, insofar as it advocates the use of a variety of lines of evidence (biosignatures), it differs from these approaches in ways that increase the likelihood of noticing truly novel forms of life, as opposed to dismissing them as just another poorly understood abiological phenomenon. Moreover, the strategy under consideration would be just as effective at detecting forms of life closely resembling our own as a definition of life.

Airborne Bacteria in Earth's Lower Stratosphere Resemble Taxa Detected in the Troposphere: Results From a New NASA Aircraft Bioaerosol Collector (ABC).

Airborne microorganisms in the upper troposphere and lower stratosphere remain elusive due to a lack of reliable sample collection systems. To address this problem, we designed, installed, and flight-validated a novel Aircraft Bioaerosol Collector (ABC) for NASA's C-20A that can make collections for microbiological research investigations up to altitudes of 13.7 km. Herein we report results from the first set of science flights—four consecutive missions flown over the United States (US) from 30 October to 2 November, 2017. To ascertain how the concentration of airborne bacteria changed across the tropopause, we collected air during aircraft Ascent/Descent (0.3 to 11 km), as well as sustained Cruise altitudes in the lower stratosphere (~12 km). Bioaerosols were captured on DNA-treated gelatinous filters inside a cascade air sampler, then analyzed with molecular and culture-based characterization. Several viable bacterial isolates were recovered from flight altitudes, including Bacillus sp., Micrococcus sp., Arthrobacter sp., and Staphylococcus sp. from Cruise samples and Brachybacterium sp. from Ascent/Descent samples. Using 16S V4 sequencing methods for a culture-independent analysis of bacteria, the average number of total OTUs was 305 for Cruise samples and 276 for Ascent/Descent samples. Some taxa were more abundant in the flight samples than the ground samples, including OTUs from families Lachnospiraceae, Ruminococcaceae and Erysipelotrichaceae as well as the following genera: Clostridium, Mogibacterium, Corynebacterium, Bacteroides, Prevotella, Pseudomonas, and Parabacteroides. Surprisingly, our results revealed a homogeneous distribution of bacteria in the atmosphere up to 12 km. The observation could be due to atmospheric conditions producing similar background aerosols across the western US, as suggested by modeled back trajectories and satellite measurements. However, the influence of aircraft-associated bacterial contaminants could not be fully eliminated and that background signal was reported throughout our dataset. Considering the tremendous engineering challenge of collecting biomass at extreme altitudes where contamination from flight hardware remains an ever-present issue, we note the utility of using the stratosphere as a proving ground for planned life detection missions across the solar system.

Protection of cyanobacterial carotenoids' Raman signatures by Martian mineral analogues after high‐dose gamma irradiation

AbstractThe future robotic exploration missions to Mars—European Space Agency/Roscosmos's ExoMars2020 and National Aeronautics and Space Administration's Mars2020 rovers—will search for signs of extant or extinct life using, among other instruments, Raman spectrometers for the first time. The question remains whether organic biosignatures—such as pigments and cellular components—may be detected by this method. Evaluating their detection limit under simulated extraterrestrial conditions is therefore crucial for the success of future life detection missions. Ionizing radiation can be considered as the most deleterious factor for the long‐term preservation of potential biomarkers on Mars. Here, we report on the preservation potential of Raman signatures in the Antarctic strain CCCryo 231‐06 of the cyanobacterium Nostoc sp. after high doses of gamma irradiation. The carotenoids' signals, a well‐established biosignature model, dominate the Raman spectra at 532‐nm excitation wavelength due to resonance effects. But comparing their distribution and quantifying their preservation are still problematic in natural samples. To standardize the analyses, we successfully applied Raman mapping and signal‐to‐noise ratio masks to quantify the effects of irradiation. The typical in vivo Raman signatures of carotenoids could be detected after exposure to up to 56 kGy with significant deterioration in terms of signal coverage and signal‐to‐noise ratio. But they remained stable even after the highest dose of γ rays (117 kGy) tested in this study for colonies embedded in two different Martian mineral analogues. Data gathered during these ground‐based irradiation experiments contribute to interpret results from space experiments and will guide our search for life on Mars and other bodies of interest.

Valuing Life-Detection Missions.

Recent discoveries imply that Early Mars was habitable for life-as-we-know-it; that Enceladus might be habitable; and that many stars have Earth-sized exoplanets whose insolation favors surface liquid water. These exciting discoveries make it more likely that spacecraft now under construction - Mars 2020, ExoMars rover, JWST, Europa Clipper - will find habitable, or formerly habitable, environments. Did these environments see life? Given finite resources (\$10bn/decade for the US ), how could we best test the hypothesis of a second origin of life? Here, we first state the case for and against flying life detection missions soon. Next, we assume that life detection missions will happen soon, and propose a framework for comparing the value of different life detection missions: Scientific value = (Reach x grasp x certainty x payoff) / \$ After discussing each term in this framework, we conclude that scientific value is maximized if life detection missions are flown as hypothesis tests. With hypothesis testing, even a nondetection is scientifically valuable.

Electrochemical Detection of the Molecules of Life

Electrochemical sensor devices are of interest in planetary exploration missions where detection of life is a goal. All forms of life on Earth contain cellular machinery that can transform and regulate chemical energy through metabolic pathways. These processes are oxidation-reduction reactions that are performed by four key classes of molecules: flavins, nicotinamides, porphyrins, and quinones (Crawford, 2001). Electrochemical redox may prove a valuable technique for identifying and differentiating these biomolecules and become a useful tool in future life-detecting missions. In this study, we used cyclic voltammetry (CV) and differential pulse voltammetry (DPV) to measure these molecules in a 3D printed flow cell. Electrodes were fabricated from metal wires and interfaced with the flow cell through compression fittings. Glassy carbon (GC) and boron-doped diamond (BDD) electrodes were chosen as working electrodes and used to detect flavin adenine dinucleotide (FAD), riboflavin, plumbagin, anthraquinone, protoporphyrin IX and nicotinamide adenine dinucleotide (NAD). All procedures were performed in a synthetic sea water solution with a Ag/AgCl reference electrode and a platinum wire counter electrode. Limits of detection down to 10 nM were achieved by the GCE. The wide potential window of BDD proved successful in the detection all four classes of redox-active molecules in complex mixtures. Introduction of electrodes into the 3D-printed flow-cell resulted in reliable CV and DPV data, and present this electrode-integrated system as a robust device suitable for future life-detection missions. Crawford, R. (2001). In Search of the Molecules of Life. Icarus, 154(2), 531–539. https://doi.org/10.1006/icar.2001.6714

Biological Characterization of Microenvironments in a Hypersaline Cold Spring Mars Analog.

While many habitable niches on Earth are characterized by permanently cold conditions, little is known about the spatial structure of seasonal communities and the importance of substrate-cell associations in terrestrial cyroenvironments. Here we use the 16S rRNA gene as a marker for genetic diversity to compare two visually distinct but spatially integrated surface microbial mats on Axel Heiberg Island, Canadian high arctic, proximal to a perennial saline spring. This is the first study to describe the bacterial diversity in microbial mats on Axel Heiberg Island. The hypersaline springs on Axel Heiberg represent a unique analog to putative subsurface aquifers on Mars. The Martian subsurface represents the longest-lived potentially habitable environment on Mars and a better understanding of the microbial communities on Earth that thrive in analog conditions will help direct future life detection missions. The microbial mats sampled on Axel Heiberg are only visible during the summer months in seasonal flood plains formed by melt water and run-off from the proximal spring. Targeted-amplicon sequencing revealed that not only does the bacterial composition of the two mat communities differ substantially from the sediment community of the proximal cold spring, but that the mat communities are distinct from any other microbial community in proximity to the Arctic springs studied to date. All samples are dominated by Gammaproteobacteria: Thiotichales is dominant within the spring samples while Alteromonadales comprises a significant component of the mat communities. The two mat samples differ in their Thiotichales:Alteromonadales ratio and contribution of Bacteroidetes to overall diversity. The red mats have a greater proportion of Alteromonadales and Bacteroidetes reads. The distinct bacterial composition of the mat bacterial communities suggests that the spring communities are not sourced from the surface, and that seasonal melt events create ephemerally habitable niches with distinct microbial communities in the Canadian high arctic. The finding that these surficial complex microbial communities exist in close proximity to perennial springs demonstrates the existence of a transiently habitable niche in an important Mars analog site.

Determination of Geochemical Bio-Signatures in Mars-Like Basaltic Environments.

Bio-signatures play a central role in determining whether life existed on early Mars. Using a terrestrial basalt as a compositional analog for the martian surface, we applied a combination of experimental microbiology and thermochemical modeling techniques to identify potential geochemical bio-signatures for life on early Mars. Laboratory experiments were used to determine the short-term effects of biota on the dissolution of terrestrial basalt, and the formation of secondary alteration minerals. The chemoorganoheterotrophic bacterium, Burkholderia sp. strain B_33, was grown in a minimal growth medium with and without terrestrial basalt as the sole nutrient source. No growth was detected in the absence of the basalt. In the presence of basalt, during exponential growth, the pH decreased rapidly from pH 7.0 to 3.6 and then gradually increased to a steady-state of equilibrium of between 6.8 and 7.1. Microbial growth coincided with an increase in key elements in the growth medium (Si, K, Ca, Mg, and Fe). Experimental results were compared with theoretical thermochemical modeling to predict growth of secondary alteration minerals, which can be used as bio-signatures, over a geological timescale. We thermochemically modeled the dissolution of the basalt (in the absence of biota) in very dilute brine at 25°C, 1 bar; the pH was buffered by the mineral dissolution and precipitation reactions. Preliminary results suggested that at the water to rock ratio of 1 × 107, zeolite, hematite, chlorite, kaolinite, and apatite formed abiotically. The biotic weathering processes were modeled by varying the pH conditions within the model to adjust for biologic influence. The results suggested that, for a basaltic system, the microbially-mediated dissolution of basalt would result in “simpler” secondary alteration, consisting of Fe-hydroxide and kaolinite, under conditions where the abiotic system would also form chlorite. The results from this study demonstrate that, by using laboratory-based experiments and thermochemical modeling, it is possible to identify secondary alteration minerals that could potentially be used to distinguish between abiotic and biotic weathering processes on early Mars. This work will contribute to the interpretation of data from past, present, and future life detection missions to Mars.

Inhabited or Uninhabited? Pitfalls in the Interpretation of Possible Chemical Signatures of Extraterrestrial Life.

The “Rare Earth” hypothesis—put forward by Ward and Brownlee in their 2000 book of the same title—states that prokaryote-type organisms may be common in the universe but animals and higher plants are exceedingly rare. If this idea is correct, the search for extraterrestrial life is essentially the search for microorganisms. Various indicators may be used to detect extant or extinct microbial life beyond Earth. Among them are chemical biosignatures, such as biomolecules and stable isotope ratios. The present minireview focuses on the major problems associated with the identification of chemical biosignatures. Two main types of misinterpretation are distinguished, namely false positive and false negative results. The former can be caused by terrestrial biogenic contaminants or by abiotic products. Terrestrial contamination is a common problem in space missions that search for biosignatures on other planets and moons. Abiotic organics can lead to false positive results if erroneously interpreted as biomolecules, but also to false negatives, for example when an abiotic source obscures a less productive biological one. In principle, all types of putative chemical biosignatures are prone to misinterpretation. Some, however, are more reliable (“stronger”) than others. These include: (i) homochiral polymers of defined length and sequence, comparable to proteins and polynucleotides; (ii) enantiopure compounds; (iii) the existence of only a subset of molecules when abiotic syntheses would produce a continuous range of molecules; the proteinogenic amino acids constitute such a subset. These considerations are particularly important for life detection missions to solar system bodies such as Mars, Europa, and Enceladus.

The nature of organic records in impact excavated rocks on Mars.

Impact ejected rocks are targets for life detection missions to Mars. The Martian subsurface is more favourable to organic preservation than the surface owing to an attenuation of radiation and physical separation from oxidising materials with increasing depth. Impact events bring materials to the surface where they may be accessed without complicated drilling procedures. On Earth, different assemblages of organic matter types are derived from varying depositional environments. Here we assess whether these different types of organic materials can survive impact events without corruption. We subjected four terrestrial organic matter types to elevated pressures and temperatures in piston-cylinder experiments followed by chemical characterisation using whole-rock pyrolysis-gas chromatography-mass spectrometry. Our data reveal that long chain hydrocarbon-dominated organic matter (types I and II; mainly microbial or algal) are unresistant to pressure whereas aromatic hydrocarbon-dominated organic matter types (types III and IV; mainly land plant, metamorphosed or degraded, displaying some superficial chemical similarities to abiotic meteoritic organic matter) are relatively resistant. This suggests that the impact excavated record of potential biology on Mars will be unavoidably biased, with microbial organic matter underrepresented while metamorphosed, degraded or abiotic meteoritic organic matter types will be selectively preserved.