Alteration history of Séítah formation rocks inferred by PIXL x-ray fluorescence, x-ray diffraction, and multispectral imaging on Mars.

Collocated crystal sizes and mineral identities are critical for interpreting textural relationships in rocks and testing geological hypotheses, but it has been previously impossible to unambiguously constrain these properties using in situ instruments on Mars rovers. Here, we demonstrate that diffracted and fluoresced x-rays detected by the PIXL instrument (an x-ray fluorescence microscope on the Perseverance rover) provide information about the presence or absence of coherent crystalline domains in various minerals. X-ray analysis and multispectral imaging of rocks from the Séítah formation on the floor of Jezero crater shows that they were emplaced as coarsely crystalline igneous phases. Olivine grains were then partially dissolved and filled by finely crystalline or amorphous secondary silicate, carbonate, sulfate, and chloride/oxychlorine minerals. These results support the hypothesis that Séítah formation rocks represent olivine cumulates altered by fluids far from chemical equilibrium at low water-rock ratios.

Assessing the Sampleability of Bennu's Surface for the OSIRIS-REx Asteroid Sample Return Mission.

NASA's first asteroid sample return mission, OSIRIS-REx, collected a sample from the surface of near-Earth asteroid Bennu in October 2020 and will deliver it to Earth in September 2023. Selecting a sample collection site on Bennu's surface was challenging due to the surprising lack of large ponded deposits of regolith particles exclusively fine enough ( ≤ 2 cm diameter) to be ingested by the spacecraft's Touch-and-Go Sample Acquisition Mechanism (TAGSAM). Here we describe the Sampleability Map of Bennu, which was constructed to aid in the selection of candidate sampling sites and to estimate the probability of collecting sufficient sample. "Sampleability" is a numeric score that expresses the compatibility of a given area's surface properties with the sampling mechanism. The algorithm that determines sampleability is a best fit functional form to an extensive suite of laboratory testing outcomes tracking the TAGSAM performance as a function of four observable properties of the target asteroid. The algorithm and testing were designed to measure and subsequently predict TAGSAM collection amounts as a function of the minimum particle size, maximum particle size, particle size frequency distribution, and the tilt of the TAGSAM head off the surface. The sampleability algorithm operated at two general scales, consistent with the resolution and coverage of data collected during the mission. The first scale was global and evaluated nearly the full surface. Due to Bennu's unexpected boulder coverage and lack of ponded regolith deposits, the global sampleability efforts relied heavily on additional strategies to find and characterize regions of interest based on quantifying and avoiding areas heavily covered by material too large to be collected. The second scale was site-specific and used higher-resolution data to predict collected mass at a given contact location. The rigorous sampleability assessments gave the mission confidence to select the best possible sample collection site and directly enabled successful collection of hundreds of grams of material.

Bacterial Growth in Brines Formed by the Deliquescence of Salts Relevant to Cold Arid Worlds.

Hygroscopic salts at Mars' near-surface (MgSO4, (per)chlorates, NaCl) may form brines by absorbing moisture from the atmosphere at certain times through the process of deliquescence. We have previously shown strong bacterial growth in saturated MgSO4 (∼67% w/v as epsomite) at room temperature, and growth was observed at the MgSO4 eutectic point (43% w/v at -4°C). Here, we have investigated the growth of salinotolerant microbes (Halomonas, Marinococcus, Planococcus) from Hot Lake, Washington; Basque Lake, British Columbia; and Great Salt Plains, Oklahoma under deliquescing conditions. Bacterial cultures were grown to mid-log phase in SP medium supplemented with 50% MgSO4 (as epsomite), 20% NaClO3, or 10% NaCl (w/v), and small aliquots in cups were dried by vacuum desiccation. When the dried culture was rehydrated by the manual addition of water, the culture resumed growth in the reconstituted brine. When desiccated cultures were maintained in a sealed container with a brine reservoir of the matching growth medium controlling the humidity of the headspace, the desiccated microbial culture evaporites formed brine by deliquescence using humidity alone. Bacterial cultures resumed growth in all three salts once rehydrated by deliquescence. Cultures of Halomonas sp. str. HL12 showed robust survival and growth when subjected to several cycles of desiccation and deliquescent or manual rehydration. Our laboratory demonstrations of microbial growth in deliquescent brines are relevant to the surface and near-subsurface of cold arid worlds like Mars. When conditions become wetter, hygroscopic evaporite minerals can deliquesce to produce the earliest habitable brines. Survival after desiccation and growth in deliquescent brines increases the likelihood that microbes from Earth, carried on spacecraft, pose a contamination risk to Mars.

Origin of Life on Mars: Suitability and Opportunities.

Although the habitability of early Mars is now well established, its suitability for conditions favorable to an independent origin of life (OoL) has been less certain. With continued exploration, evidence has mounted for a widespread diversity of physical and chemical conditions on Mars that mimic those variously hypothesized as settings in which life first arose on Earth. Mars has also provided water, energy sources, CHNOPS elements, critical catalytic transition metal elements, as well as B, Mg, Ca, Na and K, all of which are elements associated with life as we know it. With its highly favorable sulfur abundance and land/ocean ratio, early wet Mars remains a prime candidate for its own OoL, in many respects superior to Earth. The relatively well-preserved ancient surface of planet Mars helps inform the range of possible analogous conditions during the now-obliterated history of early Earth. Continued exploration of Mars also contributes to the understanding of the opportunities for settings enabling an OoL on exoplanets. Favoring geochemical sediment samples for eventual return to Earth will enhance assessments of the likelihood of a Martian OoL.

Macrobiont: Cradle for the Origin of Life and Creation of a Biosphere.

Although the cellular microorganism is the fundamental unit of biology, the origin of life (OoL) itself is unlikely to have occurred in a microscale environment. The macrobiont (MB) is the macro-scale setting where life originated. Guided by the methodologies of Systems Analysis, we focus on subaerial ponds of scale 3 to 300 m diameter. Within such ponds, there can be substantial heterogeneity, on the vertical, horizontal, and temporal scales, which enable multi-pot prebiotic chemical evolution. Pond size-sensitivities for several figures of merit are mathematically formulated, leading to the expectation that the optimum pond size for the OoL is intermediate, but biased toward smaller sizes. Sensitivities include relative access to nutrients, energy sources, and catalysts, as sourced from geological, atmospheric, hydrospheric, and astronomical contributors. Foreshores, especially with mudcracks, are identified as a favorable component for the success of the macrobiont. To bridge the gap between inanimate matter and a planetary-scale biosphere, five stages of evolution within the macrobiont are hypothesized: prebiotic chemistry → molecular replicator → protocell → macrobiont cell → colonizer cell. Comparison of ponds with other macrobionts, including hydrothermal and meteorite settings, allows a conclusion that more than one possible macrobiont locale could enable an OoL.

Cultivation and characterization of the bacterial assemblage of epsomic Basque Lake, BC.

Athalassohaline waters that are rich in divalent ions are good analogues for the chemical environments of Mars and the ocean worlds. Sulfate salts, along with chlorides, are important in Mars regolith with Ca, Fe, Mg, and Na counterions. Certain lakes in the Pacific Northwest are saturated with MgSO4 as epsomite. Here we report on the microbial community of Basque Lake, BC, a group of playas that is saturated with MgSO4. More than 60 bacterial isolates were obtained from Basque Lake soils by enrichment culture and repetitive streak-plating using media containing 10% (~ 1.7 M) NaCl or 50% (~ 2 M) MgSO4. Most of the isolates (~ 75%) were Gram-positive, motile, and produced endospores. Isolates related to Marinococcus halophilus and Virgibacillus marismortui dominated the collection. Halomonas and Salinivibrio were Gram-negative genera found at Basque Lake. Nearly all of the Basque Lake isolates grew at 50% MgSO4, with 65% growing at 60% MgSO4. Several isolates could grow in saturated (67%) MgSO4 (aw = 0.90). All of the isolates grew at 10% NaCl with 70% growing at 20% salinity (~ 3.5 M NaCl; aw = 0.82). Basque Lake isolates grew better at basic pH than acidic pH, with 80% growing at pH 9 and 30% growing at pH 10. Only 20% of the isolates grew at pH 5. Numerical taxonomy dendrograms based on 44 phenetic characteristics showed a strong correspondence to phylogenetic trees constructed from 16S rRNA gene sequences. Pyrosequencing of 16S rRNA gene sequences from direct DNA extracts of Basque Lake soils recovered predominantly Proteobacteria (60%), Firmicutes (11%), and unclassified bacteria (27%). Microbes capable of growth under the extreme chemical conditions of Mars are a particular concern for forward planetary protection should they contaminate a spacecraft.

Bacterial Growth in Saturated and Eutectic Solutions of Magnesium Sulphate and Potassium Chlorate with Relevance to Mars and the Ocean Worlds.

Liquid water on Mars might be created by deliquescence of hygroscopic salts or by permafrost melts, both potentially forming saturated brines. Freezing point depression allows these heavy brines to remain liquid in the near-surface environment for extended periods, perhaps as eutectic solutions, at the lowest temperatures and highest salt concentrations where ices and precipitates do not form. Perchlorate and chlorate salts and iron sulfate form brines with low eutectic temperatures and may persist under Mars near-surface conditions, but are chemically harsh at high concentrations and were expected to be incompatible with life, while brines of common sulfate salts on Mars may be more suitable for microbial growth. Microbial growth in saturated brines also may be relevant beyond Mars, to the oceans of Ceres, Enceladus, Europa and Pluto. We have previously shown strong growth of salinotolerant bacteria in media containing 2 M MgSO4 heptahydrate (~50% w/v) at 25 °C. Here we extend those observations to bacterial isolates from Basque Lake, BC and Hot Lake, WA, that grow well in saturated MgSO4 medium (67%) at 25 °C and in 50% MgSO4 medium at 4 °C (56% would be saturated). Psychrotolerant, salinotolerant microbes isolated from Basque Lake soils included Halomonas and Marinococcus, which were identified by 16S rRNA gene sequencing and characterized phenetically. Eutectic liquid medium constituted by 43% MgSO4 at -4 °C supported copious growth of these psychrotolerant Halomonas isolates, among others. Bacterial isolates also grew well at the eutectic for K chlorate (3% at -3 °C). Survival and growth in eutectic solutions increases the possibility that microbes contaminating spacecraft pose a contamination risk to Mars. The cold brines of sulfate and (per)chlorate salts that may form at times on Mars through deliquescence or permafrost melt have now been demonstrated to be suitable microbial habitats, should appropriate nutrients be available and dormant cells become vegetative.

Comet Pond II: Synergistic Intersection of Concentrated Extraterrestrial Materials and Planetary Environments to Form Procreative Darwinian Ponds.

In the “comet pond” model, a rare combination of circumstances enables the entry and landing of pristine organic material onto a planetary surface with the creation of a pond by a soft impact and melting of entrained ices. Formation of the constituents of the comet in the cold interstellar medium and our circumstellar disk results in multiple constituents at disequilibrium which undergo rapid chemical reactions in the warmer, liquid environment. The planetary surface also provides minerals and atmospheric gases which chemically interact with the pond’s organic- and trace-element-rich constituents. Pond physical morphology and the heterogeneities imposed by gravitational forces (bottom sludge; surface scum) and weather result in a highly heterogeneous variety of macro- and microenvironments. Wet/dry, freeze/thaw, and natural chromatography processes further promote certain reaction sequences. Evaporation concentrates organics less volatile than water. Freezing concentrates all soluble organics into a residual liquid phase, including CH3OH, HCN, etc. The pond’s evolutionary processes culminate in the creation of a Macrobiont with the metabolically equivalent capabilities of energy transduction and replication of RNA (or its progenitor informational macromolecule), from which smaller organisms can emerge. Planet-wide dispersal of microorganisms is achieved through wind transport, groundwater, and/or spillover from the pond into surface hydrologic networks.

Prevalence of sucretolerant bacteria in common soils and their isolation and characterization.

Sucretolerant microbes grow in the presence of sugar concentrations high enough to substantially lower water activities. Natural habitats high in sugars are mainly limited to dried fruit, floral nectar, honey, sugarcane, and associated soils. Organisms that tolerate extremes of solute concentration, high enough to lower water activities, might not be expected in common oligoosmotic soils. We report on the isolation of sucretolerant bacteria from common soils using media supplemented with 50% sucrose (a w 0.91) and their physiological characterization and identification by 16S rRNA gene phylogeny. Fifteen of these sucretolerant isolates from common soils were related to four Bacillus spp. A Lysinibacillus and a Microbacterium (actinomycete) also were collected. All grew at 50% sucrose and 13 grew at 60% sucrose. Most probable number counts were used to determine the abundance of sucretolerant microbes in several common soil types, including agricultural, managed turf, and native prairie. Microbial abundance (with fungicides) was about 105 and 103 cells g-1 soil in media containing 50 or 70% sucrose, respectively. The abundances of sucretolerant bacteria in common soils mirror those of halotolerant bacteria that grow at 10 and 20% NaCl. However, there is not a correlation between halotolerance and sucretolerance in our isolates, nor can predictions be made based on taxonomy. Specific solute effects may be at work, rather than biological responses to a single physicochemical parameter such as a w. The occurrence of spore-forming sucretolerant bacteria in common soils has relevance to forward planetary protection and astrobiology. Extraterrestrial habitable regions are defined in part by tolerance to high solute concentrations and osmotolerant soil microbes may contaminate spacecraft.

Automating X-ray Fluorescence Analysis for Rapid Astrobiology Surveys.

UNLABELLED: A new generation of planetary rover instruments, such as PIXL (Planetary Instrument for X-ray Lithochemistry) and SHERLOC (Scanning Habitable Environments with Raman Luminescence for Organics and Chemicals) selected for the Mars 2020 mission rover payload, aim to map mineralogical and elemental composition in situ at microscopic scales. These instruments will produce large spectral cubes with thousands of channels acquired over thousands of spatial locations, a large potential science yield limited mainly by the time required to acquire a measurement after placement. A secondary bottleneck also faces mission planners after downlink; analysts must interpret the complex data products quickly to inform tactical planning for the next command cycle. This study demonstrates operational approaches to overcome these bottlenecks by specialized early-stage science data processing. Onboard, simple real-time systems can perform a basic compositional assessment, recognizing specific features of interest and optimizing sensor integration time to characterize anomalies. On the ground, statistically motivated visualization can make raw uncalibrated data products more interpretable for tactical decision making. Techniques such as manifold dimensionality reduction can help operators comprehend large databases at a glance, identifying trends and anomalies in data. These onboard and ground-side analyses can complement a quantitative interpretation. We evaluate system performance for the case study of PIXL, an X-ray fluorescence spectrometer. Experiments on three representative samples demonstrate improved methods for onboard and ground-side automation and illustrate new astrobiological science capabilities unavailable in previous planetary instruments. KEY WORDS: Dimensionality reduction-Planetary science-Visualization.

A new analysis of Mars "Special Regions": findings of the second MEPAG Special Regions Science Analysis Group (SR-SAG2).

A committee of the Mars Exploration Program Analysis Group (MEPAG) has reviewed and updated the description of Special Regions on Mars as places where terrestrial organisms might replicate (per the COSPAR Planetary Protection Policy). This review and update was conducted by an international team (SR-SAG2) drawn from both the biological science and Mars exploration communities, focused on understanding when and where Special Regions could occur. The study applied recently available data about martian environments and about terrestrial organisms, building on a previous analysis of Mars Special Regions (2006) undertaken by a similar team. Since then, a new body of highly relevant information has been generated from the Mars Reconnaissance Orbiter (launched in 2005) and Phoenix (2007) and data from Mars Express and the twin Mars Exploration Rovers (all 2003). Results have also been gleaned from the Mars Science Laboratory (launched in 2011). In addition to Mars data, there is a considerable body of new data regarding the known environmental limits to life on Earth-including the potential for terrestrial microbial life to survive and replicate under martian environmental conditions. The SR-SAG2 analysis has included an examination of new Mars models relevant to natural environmental variation in water activity and temperature; a review and reconsideration of the current parameters used to define Special Regions; and updated maps and descriptions of the martian environments recommended for treatment as "Uncertain" or "Special" as natural features or those potentially formed by the influence of future landed spacecraft. Significant changes in our knowledge of the capabilities of terrestrial organisms and the existence of possibly habitable martian environments have led to a new appreciation of where Mars Special Regions may be identified and protected. The SR-SAG also considered the impact of Special Regions on potential future human missions to Mars, both as locations of potential resources and as places that should not be inadvertently contaminated by human activity.

Molecular and Phenetic Characterization of the Bacterial Assemblage of Hot Lake, WA, an Environment with High Concentrations of Magnesium Sulfate, and Its Relevance to Mars.

Hot Lake (Oroville, WA) is an athalassohaline epsomite lake that can have precipitating concentrations of MgSO4 salts, mainly epsomite. Little biotic study has been done on epsomite lakes and it was unclear whether microbes isolated from epsomite lakes and their margins would fall within recognized halotolerant genera, common soil genera, or novel phyla. Our initial study cultivated and characterized epsotolerant bacteria from the lake and its margins. Approximately 100 aerobic heterotrophic microbial isolates were obtained by repetitive streak-plating in high-salt media including either 10% NaCl or 2 M MgSO4. The collected isolates were all bacteria, nearly evenly divided between Gram-positive and Gram-negative clades, the most abundant genera being Halomonas, Idiomarina, Marinobacter, Marinococcus, Nesterenkonia, Nocardiopsis, and Planococcus. Bacillus, Corynebacterium, Exiguobacterium, Kocuria, and Staphylococcus also were cultured. This initial study included culture-independent community analysis of direct DNA extracts of lake margin soil using PCR-based clone libraries and 16S rRNA gene phylogeny. Clones assigned Gram-positive bacterial clades (70% of total clones) were dominated by sequences related to uncultured actinobacteria. There were abundant Deltaproteobacteria clones related to bacterial sulfur metabolisms and clones of Legionella and Coxiella. These epsomite lake microbial communities seem to be divided between bacteria primarily associated with hyperhaline environments rich in NaCl and salinotolerant relatives of common soil organisms. Archaea appear to be in low abundance and none were isolated, despite near-saturated salinities. Growth of microbes at very high concentrations of magnesium and other sulfates has relevance to planetary protection and life-detection missions to Mars, where scant liquid water may form as deliquescent brines and appear as eutectic liquids.

Identification of carbonate-rich outcrops on Mars by the Spirit rover.

Decades of speculation about a warmer, wetter Mars climate in the planet's first billion years postulate a denser CO2-rich atmosphere than at present. Such an atmosphere should have led to the formation of outcrops rich in carbonate minerals, for which evidence has been sparse. Using the Mars Exploration Rover Spirit, we have now identified outcrops rich in magnesium-iron carbonate (16 to 34 weight percent) in the Columbia Hills of Gusev crater. Its composition approximates the average composition of the carbonate globules in martian meteorite ALH 84001. The Gusev carbonate probably precipitated from carbonate-bearing solutions under hydrothermal conditions at near-neutral pH in association with volcanic activity during the Noachian era.

The Urey instrument: an advanced in situ organic and oxidant detector for Mars exploration.

The Urey organic and oxidant detector consists of a suite of instruments designed to search for several classes of organic molecules in the martian regolith and ascertain whether these compounds were produced by biotic or abiotic processes using chirality measurements. These experiments will also determine the chemical stability of organic molecules within the host regolith based on the presence and chemical reactivity of surface and atmospheric oxidants. Urey has been selected for the Pasteur payload on the European Space Agency's (ESA's) upcoming 2013 ExoMars rover mission. The diverse and effective capabilities of Urey make it an integral part of the payload and will help to achieve a large portion of the mission's primary scientific objective: "to search for signs of past and present life on Mars." This instrument is named in honor of Harold Urey for his seminal contributions to the fields of cosmochemistry and the origin of life.

Elemental compositions of comet 81P/Wild 2 samples collected by Stardust.

We measured the elemental compositions of material from 23 particles in aerogel and from residue in seven craters in aluminum foil that was collected during passage of the Stardust spacecraft through the coma of comet 81P/Wild 2. These particles are chemically heterogeneous at the largest size scale analyzed ( approximately 180 ng). The mean elemental composition of this Wild 2 material is consistent with the CI meteorite composition, which is thought to represent the bulk composition of the solar system, for the elements Mg, Si, Mn, Fe, and Ni to 35%, and for Ca and Ti to 60%. The elements Cu, Zn, and Ga appear enriched in this Wild 2 material, which suggests that the CI meteorites may not represent the solar system composition for these moderately volatile minor elements.

A soft solid surface on Titan as revealed by the Huygens Surface Science Package.

The surface of Saturn's largest satellite--Titan--is largely obscured by an optically thick atmospheric haze, and so its nature has been the subject of considerable speculation and discussion. The Huygens probe entered Titan's atmosphere on 14 January 2005 and descended to the surface using a parachute system. Here we report measurements made just above and on the surface of Titan by the Huygens Surface Science Package. Acoustic sounding over the last 90 m above the surface reveals a relatively smooth, but not completely flat, surface surrounding the landing site. Penetrometry and accelerometry measurements during the probe impact event reveal that the surface was neither hard (like solid ice) nor very compressible (like a blanket of fluffy aerosol); rather, the Huygens probe landed on a relatively soft solid surface whose properties are analogous to wet clay, lightly packed snow and wet or dry sand. The probe settled gradually by a few millimetres after landing.

An integrated view of the chemistry and mineralogy of martian soils.

The mineralogical and elemental compositions of the martian soil are indicators of chemical and physical weathering processes. Using data from the Mars Exploration Rovers, we show that bright dust deposits on opposite sides of the planet are part of a global unit and not dominated by the composition of local rocks. Dark soil deposits at both sites have similar basaltic mineralogies, and could reflect either a global component or the general similarity in the compositions of the rocks from which they were derived. Increased levels of bromine are consistent with mobilization of soluble salts by thin films of liquid water, but the presence of olivine in analysed soil samples indicates that the extent of aqueous alteration of soils has been limited. Nickel abundances are enhanced at the immediate surface and indicate that the upper few millimetres of soil could contain up to one per cent meteoritic material.

Water alteration of rocks and soils on Mars at the Spirit rover site in Gusev crater.

Gusev crater was selected as the landing site for the Spirit rover because of the possibility that it once held a lake. Thus one of the rover's tasks was to search for evidence of lake sediments. However, the plains at the landing site were found to be covered by a regolith composed of olivine-rich basaltic rock and windblown 'global' dust. The analyses of three rock interiors exposed by the rock abrasion tool showed that they are similar to one another, consistent with having originated from a common lava flow. Here we report the investigation of soils, rock coatings and rock interiors by the Spirit rover from sol (martian day) 1 to sol 156, from its landing site to the base of the Columbia hills. The physical and chemical characteristics of the materials analysed provide evidence for limited but unequivocal interaction between water and the volcanic rocks of the Gusev plains. This evidence includes the softness of rock interiors that contain anomalously high concentrations of sulphur, chlorine and bromine relative to terrestrial basalts and martian meteorites; sulphur, chlorine and ferric iron enrichments in multilayer coatings on the light-toned rock Mazatzal; high bromine concentration in filled vugs and veins within the plains basalts; positive correlations between magnesium, sulphur and other salt components in trench soils; and decoupling of sulphur, chlorine and bromine concentrations in trench soils compared to Gusev surface soils, indicating chemical mobility and separation.

Surface of young Jupiter family comet 81P/Wild 2: view from the Stardust Spacecraft.

Images taken by the Stardust mission during its flyby of 81P/Wild 2 show the comet to be a 5-kilometer oblate body covered with remarkable topographic features, including unusual circular features that appear to be impact craters. The presence of high-angle slopes shows that the surface is cohesive and self-supporting. The comet does not appear to be a rubble pile, and its rounded shape is not directly consistent with the comet being a fragment of a larger body. The surface is active and yet it retains ancient terrain. Wild 2 appears to be in the early stages of its degradation phase as a small volatile-rich body in the inner solar system.