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Environmental ethics and policy have been largely developed around the concept of nature as a dynamic collection of living beings in direct interaction with humans. However, when we consider the Moon, Mars, and other worlds we encounter profoundly static and lifeless nature with essentially no history of human interaction. On what basis do we make decisions on the preservation or utilization of such lifeless landscapes? Here I suggest that static lifeless landscapes on other worlds have some parallels on Earth in the upper elevations of the Antarctic Dry Valleys and the central depression of the Atacama Desert in Chile and Peru. In the soil of both locations life is absent or cannot grow and the timescales of landscape evolution vastly exceed human timescales. For all intents and purposes, these are static and lifeless landscapes. Today, human activities in Antarctica are carefully regulated with a view toward preservation while in the Atacama Desert resource mining has extensively altered the landscape. Thus, these two sites provide alternate possible futures for the Moon and Mars. Preservation of static lifeless landscapes could be based on two attributes. First, they provide a way to further define and understand the Earth and the limits of life—which is the current scientific motivation for biological and geological research activities there. Second, both on Earth and particularly on other worlds, lifeless landscapes may express states that are new to human experience akin to new forms of art or new branches of science. We don't understand these environments, their nature, or their relationship to us. In the case of Mars there may come to be a tension between preserving a lifeless landscape and introducing or enhancing life on that planet. [ABSTRACT FROM AUTHOR]

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The Atacama Desert is one of the driest places on Earth, with an arid core highly adverse to the development of hypolithic cyanobacteria. Previous work has shown that when rain levels fall below ~1 mm per year, colonization of suitable quartz stones falls to virtually zero. Here, we report that along the coast in these arid regions, complex associations of cyanobacteria, archaea, and heterotrophic bacteria inhabit the undersides of translucent quartz stones. Colonization rates in these areas, which receive virtually no rain but mainly fog, are significantly higher than those reported inland in the hyperarid zone at the same latitude. Here, hypolithic colonization rates can be up to 80%, with all quartz rocks over 20 g being colonized. This finding strongly suggests that hypolithic microbial communities thriving in the seaward face of the Coastal Range can survive with fog as the main regular source of moisture. A model is advanced where the development of the hypolithic communities under quartz stones relies on a positive feedback between fog availability and the higher thermal conductivity of the quartz rocks, which results in lower daytime temperatures at the quartz-soil interface microenvironment. [ABSTRACT FROM AUTHOR]

Abstract: In desert environments with low water and salt contents, rapid thermal variations may be an important source of rock weathering. We have obtained temperature measurements of the surface of rocks in hyper-arid hot and cold desert environments at a rate of 1/s over several days. The values of temperature change over 1-second intervals were similar in hot and cold deserts despite a 30 °C difference in absolute rock surface temperature. The average percentage of the time dT/dt>2 °C/min was ~8±3%, >4 °C/min was 1±0.9%, and >8 °C/min was 0.02±0.03%. The maximum change over a 1-second interval was ~10 °C/min. When sampled to simulate data taken over intervals longer than 1 s, we found a reduction in time spent above the 2 °C/min temperature gradient threshold. For 1-minute samples, the time spent above any given threshold was about two orders of magnitude lower than the corresponding value for 1-second sampling. We suggest that a rough measure of efficacy of weathering as a function of frequency is the product of the percentage of time spent above a given threshold value multiplied by the damping depth for the corresponding frequency. This product has a broad maximum for periods between 3 and 10 s. [Copyright &y& Elsevier]

In most climates on Earth, biological processes control soil N. In the Atacama Desert of Chile, aridity severely limits biology, and soils accumulate atmospheric NO3-. We examined this apparent transformation of the soil N cycle using a series of ancient Atacama Desert soils (>2 My) that vary in rainfall (21 to <2 mm yr-1). With decreasing rainfall, soil organic C decreases to 0.3 kg C m-2 and biological activity becomes minimal, while soil NO3- and organic N increase to 4 kg N m-2 and 1.4 kg N m-2, respectively. Atmospheric NO3- (Δ17O = 23.0%o) increases from 39% to 80% of total soil NO3- as rainfall decreases. These soils capture the transition from a steady state, biologically mediated soil N cycle to a dominantly abiotic, transient state of slowly accumulating atmospheric N. This transition suggests that oxidized soil N may be present in an even more arid and abiotic environment: Mars. [ABSTRACT FROM AUTHOR]

The failure of Viking Lander thermal volatilization (TV) (without or with thermal degradation)-gas chromatography (GC)-MS experiments to detect organics suggests chemical rather than biological interpretations for the reactivity of the martian soil. Here, we report that TV-GC-MS may be blind to low levels of organics on Mars. A comparison between TV-GC-MS and total organics has been conducted for a variety of Mars analog soils. In the Antarctic Dry Valleys and the Atacama and Libyan Deserts we find 10–90 μg of refractory or graphitic carbon per gram of soil, which would have been undetectable by the Viking TV-GC-MS. In iron-containing soils (jarosites from Rio Tinto and Panoche Valley) and the Mars simulant (palogonite), oxidation of the organic material to carbon dioxide (CO2) by iron oxides and/or their salts drastically attenuates the detection of organics. The release of 50–700 ppm of CO2 by TV-GC-MS in the Viking analysis may indicate that an oxidation of organic material took place. Therefore, the martian surface could have several orders of magnitude more organics than the stated Viking detection limit. Because of the simplicity of sample handling, TV-GC-MS is still considered the standard method for organic detection on future Mars missions. We suggest that the design of future organic instruments for Mars should include other methods to be able to detect extinct and/or extant life. [ABSTRACT FROM AUTHOR]

The occurrence of hypolithic cyanobacteria colonizing translucent stones was quantified along the aridity gradient in the Atacama Desert in Chile, from less arid areas to the hyperarid core where photosynthetic life and thus primary production reach their limits. As mean rainfall declines from 21 to ≤2 mm year−1, the abundance of hypolithic cyanobacteria drops from 28 to <0.1%, molecular diversity declines threefold, and organic carbon residence times increase by three orders of magnitude. Communities contained a single Chroococcidiopsis morphospecies with heterotrophic associates, yet molecular analysis revealed that each stone supported a number of unique 16S rRNA gene-defined genotypes. A fivefold increase in steady-state residence times for organic carbon within communities in the hyperarid core (3200 years turnover time) indicates a significant decline in biological carbon cycling. Six years of microclimate data suggest that the dry limit corresponds to ≤5 mm year−1 rainfall and/or decadal periods of no rain, with <75 h year−1 of liquid water available to cyanobacteria under light conditions suitable for photosynthesis. In the hyperarid core, hypolithic cyanobacteria are rare and exist in small spatially isolated islands amidst a microbially depauperate bare soil. These findings suggest that photosynthetic life is extremely unlikely on the present-day surface of Mars, but may have existed in the past. If so, such microhabitats would probably be widely dispersed, difficult to detect, and millimeters away from virtually lifeless surroundings. [ABSTRACT FROM AUTHOR]

Preferential uptake of either levorotatory (L) or dextrorotatory (D) enantiomer of a chiral molecule is a potential planetary life-detection method. On Earth, bacteria, as a rule, metabolize D-sugars and L-amino acids. Here, we use growth experiments to identify exceptions to the rule and their potential impact on the method's reliability. Our experiments involve six strains of Bacillus and collective uptake of the sugars glucose and arabinose, and the amino acids alanine, glutamic acid, leucine, cysteine, and serine—all of which are highly soluble. We find that selective uptake is not evident unless (1) each sugar is tested individually and (2) multiple amino acids are tested together in a mixture. Combining sugars should be avoided because, as we show in Bacillus bacteria, the same organisms may catabolize one sugar, glucose, in D-form and another sugar, arabinose, in L-form. Single amino acids should be avoided because bacteria can access certain proteinogenically incompatible enantiomers using specific racemases. Specifically, bacteria contain an alanine acid racemase and can catabolize D-alanine if no other D-amino acids are present. The proposed improvements would reliably separate nonselective chemical reactions from biological reactions and, if life is indicated, inform whether the selective patterns for amino acids and sugars are the same as on Earth. [ABSTRACT FROM AUTHOR]

Abstract: A key question in understanding life on Mars under dry(ing) conditions is how arid soils respond to small levels of liquid water. We have conducted a series of simulated rain experiments in the hyperarid core region of the Atacama Desert. Rain amounts from 0.24 to 3.55mm were applied in the early evening to the soil. We conclude that rain events of less than 1mm do not saturate the surface, and the soil humidity at the surface remains below 100%. Rain events of 2mm or more generate free water in the pore space of the soil surface, which may be necessary to support biological activity in the soil. The crust on the surface of the soil is a strong barrier to the diffusion of subsurface moisture and subsequent evaporation. Our results show that once the relative humidity in hyperarid soils begins to fall below 100% the rate of decrease is quite rapid. Thus, the precise value assumed for the limits of life or water activity, do not appreciably change the time of water availability resulting from small desert rains. The Atacama Desert results may be applied to models of (H2O) wetting in the upper soils of Mars due to light rains, melting snow and heavy precipitating fog. [Copyright &y& Elsevier]

Abstract: TEGA, one of several instruments on board of the Phoenix Lander, performed differential scanning calorimetry and evolved gas analysis of soil samples and ice, collected from the surface and subsurface at a northern landing site on Mars. TEGA is a combination of a high temperature furnace and a mass spectrometer (MS) that was used to analyze samples delivered to the instrument via a robotic arm. The samples were heated at a programmed ramp rate up to 1000°C. The power required for heating can be carefully and continuously monitored (scanning calorimetry). The evolved gases generated during the process can be analyzed with the evolved gas analyzer (a magnetic sector mass spectrometer) in order to determine the composition of gases released as a function of temperature. Our laboratory has developed a sample characterization method using a pyrolyzer integrated to a quadrupole mass spectrometer to support the interpretations of TEGA data. Here we examine the evolved gas properties of six types of hyperarid soils from the Pampas de La Joya in southern Peru (a possible analog to Mars), to which we have added with microorganisms (Salmonella typhimurium, Micrococcus luteus, and Candida albicans) in order to investigate the effect of the soil matrix on the TEGA response. Between 20 and 40mg of soil, with or without ∼5mg of lyophilized microorganism biomass (dry weight), were placed in the pyrolyzer and heated from room temperature to 1200°C in 1h at a heating rate of 20°C/min. The volatiles released were transferred to a MS using helium as a carrier gas. The quadrupole MS was ran in scan mode from 10 to 200m/z. In addition, ∼20mg of each microorganism without a soil matrix were analyzed. As expected, there were significant differences in the gases released from microorganism samples with or without a soil matrix, under similar heating conditions. Furthermore, samples from the most arid environments had significant differences compared with less arid soils. Organic carbon released in the form of CO2 (ion 44m/z) from microorganisms evolved at temperatures of ∼326.0±19.5°C, showing characteristic patterns for each one. Others ions such as 41, 78 and 91m/z were also found. Interestingly, during the thermal process, the release of CO2 increased and ions previously found disappeared, demonstrating a high-oxidant activity in the soil matrix when it was subjected to high temperature. Finally, samples of soil show CO2 evolved up to 650°C consistent with thermal decomposition of carbonates. These results indicate that organics mixed with these hyperarid soils are oxidized to CO2. Our results suggest the existence of at least two types of oxidants in these soils, a thermolabile oxidant which is highly oxidative and other thermostable oxidant which has a minor oxidative activity and that survives the heat-treatment. Furthermore, we find that the interaction of biomass added to soil samples gives a different set of breakdown gases than organics resident in the soil. The nature of oxidant(s) present in the soils from Pampas de La Joya is still unknown. [Copyright &y& Elsevier]

The article presents a report on low levels of culturable organisms and no recoverable DNA in the surface soils of the extreme arid core of the Atacama Desert, which is near the abandoned town of Yungay. This upper limit is orders of magnitude less than the concentrations of bacteria found in soils south of this Mars-like region of the Atacama. In more recent published work, the researchers have reported that below the surface, there are discrete layers with higher numbers of culturable bacteria. The researchers could not claim that there was no life in these soils on the basis of their results, and therefore they presented their data as indicating an upper limit of 100 culturable heterotrophic bacteria per gram of soil.

Abstract: The soil carbon content and its relation to site characteristics are important in evaluating current local, regional, and global soil C storage and projecting future variations in response to climate change. In this study we analyzed the concentration of organic and inorganic carbon and their relationship with in situ climatic and geological characteristics in 485 samples of surface soil and 17 pits from the hyper-arid area and 51 samples with 2 pits from the arid–semiarid region from the Atacama Desert located in Peru and Chile. The soil organic carbon (SOC) in hyperarid soils ranged from 1.8 to 50.9μgC per g of soil for the 0–0.1m profile and from 1.8 to 125.2μgC per g of soil for the 0–1m profile. The analysis of climatic (temperature and precipitation), elevation, and some geologic characteristics (landforms) associated with hyper-arid soils explained partially the SOC variability. On the other hand, soil inorganic carbon (SIC) contents, in the form of carbonates, ranged from 200 to 1500μgC per g of soil for the 0–0.1m profile and from 200 to 3000μgC per g of soil for the 0–1.0m profile in the driest area. The largest accumulations of organic and inorganic carbon were found near to arid–semiarid areas. In addition, the elemental carbon concentrations show that the presence of other forms of inorganic carbon (e.g. graphite, etc.) was negligible in these hyperarid soils. Overall, the top 1m soil layer of hyperarid lands contains ∼11.6Tg of organic carbon and 344.6Tg of carbonate carbon. The total stored carbon was 30.8-fold the organic carbon alone. To our knowledge, this is the first study evaluating the total budget carbon on the surface and shallow subsurface on ∼160,000km2 of hyperarid soils. [Copyright &y& Elsevier]

Abstract: The objective of this work was to investigate the variability of surface organic carbon within the hyper-arid Yungay region of the Atacama Desert. The fraction of Labile Organic Carbon (LOC) in these samples varied from 2 to 73μg per gram of soil with a bi-modal distribution with average content of 17±9μg LOC and 69±3μg LOC for “low” and “high” samples, respectively. Interestingly, there was no relation between organic levels and geomorphologic shapes. While organics are deposited and distributed in these soils via eolic processes, it is suggested that fog is the dynamic mechanism that is responsible for the variability and peaks in organic carbon throughout the area, where a “high” LOC content sample could be indicative of a biological process. It was determined that there was no significant difference between topological feature or geographical position within the hyper-arid samples and LOC. This very curious result has implications for the investigation of run-off gullies on the planet Mars as our work suggests a need for careful consideration of the expectation of increases in concentrations of organic materials associated with following aqueous altered topology. [Copyright &y& Elsevier]

Molecular biology and microscopy techniques were used to characterize the microbial communities inside halite evaporites from different parts of the Atacama Desert. Denaturing gradient gel electrophoresis (DGGE) analysis revealed that the evaporite rocks harbor communities predominantly made up of cyanobacteria, along with heterotrophic bacteria and archaea. Different DGGE profiles were obtained for the different sites, with the exception of the cyanobacterial profile, in which only one phylotype was detected across the three sites examined. Chroococcidiopsis-like cells were the only cyanobacterial components of the rock samples, although the phylogenetic study revealed their closer genetic affinity to Halothece genera. Gene sequences of the heterotrophic bacteria and archaea indicated their proximity to microorganisms found in other hypersaline environments. Microorganisms colonizing these halites formed microbial aggregates in the pore spaces between halite crystals, where microbial interactions occur. In this exceptional, salty, porous halite rock habitat, microbial consortia with a community structure probably conditioned by the environmental conditions occupy special microhabitats with physical and chemical properties that promote their survival. [ABSTRACT FROM AUTHOR]

Abstract: The soils of the Atacama Desert in northern Chile have long been known to contain large quantities of unusual salts, yet the processes that form these soils are not yet fully understood. We examined the morphology and geochemistry of soils on post-Miocene fans and stream terraces along a south-to-north (27° to 24° S) rainfall transect that spans the arid to hyperarid transition (21 to ∼2mm rain y−1). Landform ages are ⩾2 My based on cosmogenic radionuclide concentrations in surface boulders, and Ar isotopes in interbedded volcanic ash deposits near the driest site indicate a maximum age of 2.1My. A chemical mass balance analysis that explicitly accounts for atmospheric additions was used to quantify net changes in mass and volume as a function of rainfall. In the arid (21mmrainy−1) soil, total mass loss to weathering of silicate alluvium and dust (−1030kgm−2) is offset by net addition of salts (+170kgm−2). The most hyperarid soil has accumulated 830kgm−2 of atmospheric salts (including 260kg sulfate m−2 and 90kgchloridem−2), resulting in unusually high volumetric expansion (120%) for a soil of this age. The composition of both airborne particles and atmospheric deposition in passive traps indicates that the geochemistry of the driest soil reflects accumulated atmospheric influxes coupled with limited in-soil chemical transformation and loss. Long-term rates of atmospheric solute addition were derived from the ion inventories in the driest soil, divided by the landform age, and compared to measured contemporary rates. With decreasing rainfall, the soil salt inventories increase, and the retained salts are both more soluble and present at shallower depths. All soils generally exhibit vertical variation in their chemistry, suggesting slow and stochastic downward water movement, and greater climate variability over the past 2My than is reflected in recent (∼100y) rainfall averages. The geochemistry of these soils shows that the transition from arid to hyperarid rainfall levels marks a fundamental geochemical threshold: in wetter soils, the rate and character of chemical weathering results in net mass loss and associated volumetric collapse after 105 to 106 years, while continuous accumulation of atmospheric solutes in hyperarid soils over similar timescales results in dramatic volumetric expansion. The specific geochemistry of hyperarid soils is a function of atmospheric sources, and is expected to vary accordingly at other hyperarid sites. This work identifies key processes in hyperarid soil formation that are likely to be independent of location, and suggests that analogous processes may occur on Mars. [Copyright &y& Elsevier]

The Viking missions showed the martian soil to be lifeless and depleted in organic material and indicated the presence of one or more reactive oxidants. Here we report the presence of Mars-like soils in the extreme arid region of the Atacama Desert. Samples from this region had organic species only at trace levels and extremely low levels of culturable bacteria. Two samples from the extreme arid region were tested for DNA and none was recovered. Incubation experiments, patterned after the Viking labeled-release experiment but with separate biological and nonbiological isomers, show active decomposition of organic species in these soils by nonbiological processes. [ABSTRACT FROM AUTHOR]