Measuring N2 Pressure Using Cyanobacteria Discipline: Geomicrobiology

The evolution of Earth's atmosphere has been governed by biological evolution. Dinitrogen (N2) has been a major constituent of Earth's atmosphere throughout the planet's history, yet only a few constraints exist for the partial pressure of N2 (pN2). In this study we evaluate two new potential proxies for pN2: the physical spacing between heterocysts and the isotopic signature of nitrogen fixation in filamentous cyanobacteria. Heterocyst-forming filamentous cyanobacteria are some of the oldest photosynthetic microorganisms on Earth, and debated fossilized specimens have been found in sedimentary rocks as old as 2 Ga. These organisms overcome nitrogen limitation in their aqueous environment through cellular differentiation along their filaments. The specialized cells that develop, known as heterocysts, fix the nitrogen and laterally distribute it to neighboring cells along the filaments. Because the concentration of the dissolved N2 available to the filaments correlates directly with pN2, any preservable physiological response of the organism to the changed N2 availability constitutes a potential proxy for pN2. In the laboratory, we have examined how pN2 is reflected in the heterocyst spacing pattern and in the isotopic signature of nitrogen fixation by subjecting the representative species Anabaena cylindrica and Anabaena variabilis to different N2 partial pressures during growth at constant temperature and lighting (in media free of combined nitrogen). We show experimentally that the distance between heterocysts and the nitrogen isotope fractionation measured in bulk biomass reflect the pN2 experienced by Anabaena cylindrica. Current work is investigating these responses in Anabaena variabilis. When heterocystous cyanobacteria fossilize, these morphological and isotopic signatures should preserve information about pN2 at that time. Application of this relationship to the rock record may provide a paleoproxy to complement the two existing geobarometers

Thermodynamic Predictions of Hydrogen Generation during the Serpentinization of Harzburgite with Seawater-derived Brines

Salty aqueous solutions (brines) occur on Earth and may be prevalent elsewhere. Serpentinization represents a family of geochemical reactions where the hydration of olivine-rich rocks can release aqueous hydrogen, H2(aq), as a byproduct, and hydrogen is a known basal electron donor for terrestrial biology. While the effects of lithological differences on serpentinization products have been thoroughly investigated, effects focusing on compositional differences of the reacting fluid have received less attention. In this contribution, we investigate how the chemistry of seawater-derived brines affects the generation of biologically available hydrogen resulting from the serpentinization of harzburgite. We numerically investigate the serpentinization of ultramafic rocks at equilibrium with an array of brines at different water activities (a proxy for salt concentration in aqueous fluids and a determinant for habitability) derived from seawater evaporation. Because the existing supersaturation of aqueous calcium carbonate, a contributor to dissolved inorganic carbon (DIC) in natural seawater, cannot be captured in equilibrium calculations, we bookend our calculations by enabling and suppressing carbonate minerals when simulating serpentinization. We find that the extent of DIC supersaturation can provide an important control of hydrogen availability. Increased DIC becomes a major sink for hydrogen by producing formate and associated complexes when the reacting fluids are acidic enough to allow for CO2. Indeed, H2(aq) reduces CO2(aq) to formate, leading to a hydrogen deficit. These conclusions provide additional insights into the habitability of brine systems, given their potential for serpentinization across diverse planetary bodies such as on Mars and ocean worlds

Perspective: science policy through public engagement

While tensions may lie between science and policy, we argue that dissemination and public engagement are key in alleviating those perceived tensions. Science being valued by society results in fact-based policy-making being demanded by constituents. Constituents’ demands will yield representatives who are familiar with the scientific process and research to inform policy decisions.https://academic.oup.com/spp/article/47/6/890/590080

The Benefits and Harms of Transmitting Into Space

Deliberate and unintentional radio transmissions from Earth propagate into space. These transmissions could be detected by extraterrestrial watchers over interstellar distances. Here, we analyze the harms and benefits of deliberate and unintentional transmissions relevant to Earth and humanity. Comparing the magnitude of deliberate radio broadcasts intended for messaging to extraterrestrial intelligence (METI) with the background radio spectrum of Earth, we find that METI attempts to date have much lower detectability than emissions from current radio communication technologies on Earth. METI broadcasts are usually transient and several orders of magnitude less powerful than other terrestrial sources such as astronomical and military radars, which provide the strongest detectable signals. The benefits of radio communication on Earth likely outweigh the potential harms of detection by extraterrestrial watchers; however, the uncertainty regarding the outcome of contact with extraterrestrial beings creates difficulty in assessing whether or not to engage in long-term and large-scale METI.Comment: Published in Space Polic

Ladakh: Diverse, high-altitude extreme environments for off-earth analogue and astrobiology research

This paper highlights unique sites in Ladakh, India, investigated during our 2016 multidisciplinary pathfinding expedition to the region. We summarize our scientific findings and the site's potential to support science exploration, testing of new technologies and science protocols within the framework of astrobiology research. Ladakh has several accessible, diverse, pristine and extreme environments at very high altitudes (3000-5700 m above sea level). These sites include glacial passes, sand dunes, hot springs and saline lake shorelines with periglacial features. We report geological observations and environmental characteristics (of astrobiological significance) along with the development of regolith-landform maps for cold high passes. The effects of the diurnal water cycle on salt deliquescence were studied using the ExoMars Mission instrument mockup: HabitAbility: Brines, Irradiance and Temperature (HABIT). It recorded the existence of an interaction between the diurnal water cycle in the atmosphere and salts in the soil (which can serve as habitable liquid water reservoirs). Life detection assays were also tested to establish the best protocols for biomass measurements in brines, periglacial ice-mud and permafrost melt water environments in the Tso-Kar region. This campaign helped confirm the relevance of clays and brines as interest targets of research on Mars for biomarker preservation and life detection.The team would like to express its gratitude to BirbalSahni Institute of Palaeosciences, Department of Science and Technology,Office of Chief Wildlife Warden of Ladakh, Government of India for helpingarrange the requisite clearances and permits for the conducted work. Projectmentoring and guidance provided by Spaceward Bound members at NASAAmes Research Center. Financial and logistics support provided by TataMotors Ltd, Inspired Journeys Co, Pearl Travels Ltd and NationalGeographic Traveller India. Website and IT support provided by the BlueMarble Space Institute of Science. Audio-video documentation support pro-vided by Astroproject India and The H

Stability and heat transfer characteristics of unsteady condensing and evaporating films

The stability and heat transfer characteristics of an unsteady condensing and evaporating n-pentane film on the underside of a cooled, flat, horizontal plate was studied experimentally. Unsteady conditions were produced by varying the system pressure in a cyclic fashion. The film was imaged using a double-pass shadowgraph system, and an embedded heat flux sensor measured the spatially averaged heat flux. Surface conditions were obtained using an inverse method. Images and data point collection were synchronized to permit direct correlation between thermal data and film behavior. The heat flux was affected by the Rayleigh-Taylor instability after an initial rise due to condensate formation. Hysteresis was observed in the heat flux over each pressure variation cycle, where the heat flux during condensation varied differently with the degree of subcooling than during evaporation. An additional study examined the film stability of non-condensing, growing films with mass addition but without thermal effects. Experiments showed that the film thickness at the point of first droplet break-off increased with increased pumping rate. © 2006 Elsevier Ltd. All rights reserved

Eolianite grain size distributions as a proxy for large changes in planetary atmospheric density

Atmospheres are dynamic over geologic timescales, making large changes in planetary air density possible. For the Earth, geological proxies suggest that air density in the Neoarchean was similar to or lower than today. This air density variation possibly affected eolian dune grain sizes by controlling the trajectories of grains though the air. Balancing the fall velocity and threshold friction velocity, a metric separating saltation and suspension transport, suggests that a lower air density could increase the mean grain size of dunes because decreased air drag extends the size range of grains in modified saltation and incipient suspension regimes. Consequently, the dune-forming sand left behind in pure saltation, the dominant dune-forming transport mode, could have coarser grains. We analyzed size distributions of two eolianites from 2.64 and 1.5 Ga (billion years ago) for deviations from modern sand dunes emplaced at sea level, which globally exhibit similar mean grain sizes. Both aeolianites have mean grain sizes within one standard deviation of the modern mean and are not statistically separable at 95% confidence. Overall, this suggests that while air density is important in eolian physics, a factor of 2 to 4 change in density is insufficient to produce an unambiguous grain size signal. This suggests that while eolian dune grain sizes have not significantly changed over the range of Earth’s atmospheric conditions, they could be useful when investigating the order of magnitude changes thought to have occurred on Mars.A digitized version of the Ahlbrandt (1979) global dune dataset and data for distributions in Figure are available at https://github. com/erikgoosmann/AeolianData.The Simons Collaboration on the Origin of Life Award 511570 to D. C. C. and by the NASA Astrobiology Institute’s Virtual Planetary Laboratory, grant NNA13AA93A.https://agupubs.onlinelibrary.wiley.com/journal/21699100Geolog

Microbial diversity and activity in Southern California salterns and bitterns: analogues for remnant ocean worlds

Concurrent osmotic and chaotropic stress make MgCl -rich brines extremely inhospitable environments. Understanding the limits of life in these brines is essential to the search for extraterrestrial life on contemporary and relict ocean worlds, like Mars, which could host similar environments. We sequenced environmental 16S rRNA genes and quantified microbial activity across a broad range of salinity and chaotropicity at a Mars-analogue salt harvesting facility in Southern California, where seawater is evaporated in a series of ponds ranging from kosmotropic NaCl brines to highly chaotropic MgCl brines. Within NaCl brines, we observed a proliferation of specialized halophilic Euryarchaeota, which corresponded closely with the dominant taxa found in salterns around the world. These communities were characterized by very slow growth rates and high biomass accumulation. As salinity and chaotropicity increased, we found that the MgCl -rich brines eventually exceeded the limits of microbial activity. We found evidence that exogenous genetic material is preserved in these chaotropic brines, producing an unexpected increase in diversity in the presumably sterile MgCl -saturated brines. Because of their high potential for biomarker preservation, chaotropic brines could therefore serve as repositories of genetic biomarkers from nearby environments (both on Earth and beyond) making them prime targets for future life-detection missions