The below list represents only planned manuscripts. Some of these
manuscripts have not been received by the Editorial Office yet. Papers
submitted to MDPI journals are subject to peer-review.
Type of Paper: Article
Title: The Vast Subsurface: The Role of Caves and Mines as Terrestrial Analogs for Planetary Environments
Author: Penny Boston
Affiliation: Earth & Environmental Sciences Dept., New Mexico Institute of Mining & Technology, Socorro, New Mexico 87801, USA; E-Mail: email@example.com
Abstract: The role of caves, vugs, and mines in providing terrestrial analogs for near subsurface environments on other planets is a major new direction for analog studies. The use of such underground terrain has been slow to develop, perhaps because most investigators are not experienced in entering natural and artificial cavities, or because they are unacquainted with the tremendous varieties of geochemistries, mineralogies, physical parameters, and microbial communities that are found in such environments.
There is a cave or mine environment that provides an example of almost all major environmental challenges, with the obvious exceptions of ultraviolet wavelengths and ionizing radiation. Cave temperatures range from extremely high (e.g., ~ 40–60 °C) to subfreezing, depending upon the location and altitude of the cavity and its proximity to geothermal sources. The cave air ranges from ordinary ambient Earth gas constituents to extremely exotic mixtures of CO2, CO, H2S, SO2, CH4, aldehydes, and other compounds. Caves can be found in almost every lithology expressed in the Earth’s crust, including carbonates (e.g., limestone, dolomite, and marble), evaporates (e.g., gypsum, anhydrite, and halite), silicates (e.g., quartzite and sandstones), volcanic basalts and tuffs, igneous rocks (e.g., granite), water ice, and even unconsolidated sediments. Dominant pH values for caves and mines depend upon the interaction of the geo- and atmospheric chemistries of a cavity with the life that it contains; pH levels range from hyperacidic (pH = 0–3) to alkaline (pH ~ 8–9.5). Some caves have extensive hydrological input: sometimes, even major rivers flow through them. In contrast, some caves occur in hyperarid deserts, including the Atacama. Some caves are at high altitudes, and some are at low altitudes. Some caves are so immense that they even house their own rain clouds, while some are so tiny that direct human entrance is not possible. Nevertheless, all of these caves provide a comprehensive menu of conditions that can be germane to various planetary environments.
The microbial communities that inhabit this vast array of subterranean real estate are extremely unusual, and in many instances, unique. The number of novel strains that appear in virtually every assessment of caves and mines far outstrips the number of known strains in our databases (as determined by both non-culture dependent and culturing techniques). The stringent partitioning of subsurface habitats (because of limited opportunities for transport), the very slow intrinsic “pace of life” of many subsurface organisms, and the often high degree of heterogeneity of relevant geochemical and physical parameters within the subsurface habitats all appear to contribute to the apparent high degree of endemism.
A suite of similarities unifies subsurface microbial communities, even though the lithologies, geochemistries, and identities of individual organisms may be radically different from one cave to another. The ecological, energetic, and evolutionary consequences of inhabiting the subsurface also contribute to a unifying set of principles that can be applied broadly in the subsurface.
The physical depth to which microbial inhabitants can exist in either the continental or marine crust has not yet been established. However, the deep continental subsurface (to depths of 4 km) has yielded not only microorganisms, but also a multicellular eukaryote. The cavities through which we as humans may go are only the most accessible part of a much greater, biologically rich environment: the rock fracture habitat. This habitat is found throughout the continental and island land masses, and increasingly in ocean drilling samples.
In summary, this “hidden” part of our planet’s biosphere may rival, in terms of both diversity and biomass, other biomes on the Earth’s surface.
Type of Paper: Article
Title: Photosynthesis in a Hydrogen-rich Atmosphere
Author: William Bains
Affiliation: Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA; E-Mail: firstname.lastname@example.org
Abstract: The diversity of extrasolar planets discovered in the last decade shows that we should not be constrained to look for life in environments similar to early or present-day Earth. Super-earth exoplanets are being discovered with increasing frequency, and some will be able to retain a stable, hydrogen-dominated atmosphere. I explore the possible chemistry of photosynthesis in such an atmosphere. Life needs diverse chemicals to carry out its functions, and must derive these from its environment using energy-consuming redox reactions. The nature of the reactions and the amount of energy needed depends on the environment. Based on these constraints and the thermodynamics relevant to the possible chemistry of photosynthesis, I evaluate the potential chemical routes for photosynthesis in an environment with 10% and 90% hydrogen in an otherwise inert atmosphere, and the possible biosignature gases that the photosynthetic chemistry could generate. I comment on the energetics of photosynthesis on a hydrogen-dominated world, and speculate on whether photosynthesis, and the biomass that it can support, is more or less likely to evolve on a hydrogen-rich world.
Type of Paper: Article
Title: A passenger blimp experiment measuring the survival of bacterial endospores in the atmosphere
Author: David J. Smith 1, Finlay Maguire 2, Megan A. Morford 1, Christina L. Khodadad 3, Parag A. Vaishampayan 4, Philip R. Maloney 5, James B. McQuaid 6 and Kasthuri J. Venkateswaran 4
1 NASA John F. Kennedy Space Center, Surface Systems Office, Kennedy Space Center, Florida, USA; E-Mails: email@example.com (D.J.S.); firstname.lastname@example.org (M.A.M.)
2 University College London & Natural History Museum, London, United Kingdom; Email: email@example.com
3 Sierra Lobo, Inc., Kennedy Space Center, Florida, USA; E-Mail: firstname.lastname@example.org
4 California Institute of Technology, Jet Propulsion Laboratory, Biotechnology and Planetary Protection Group, Pasadena, California, USA; E-Mails: email@example.com (P.A.V.); firstname.lastname@example.org (K.J.V.)
5 NASA Postdoctoral Fellow, Kennedy Space Center, Florida, USA; E-Mail: email@example.com
6 School of Earth and Environment, University of Leeds, United Kingdom; E-Mail: firstname.lastname@example.org
Abstract: Airborne microorganisms experience a combination of stressors in the upper atmosphere similar to extraterrestrial conditions (i.e., low temperature and pressure; high desiccation and irradiation). We conducted flight experiments outside a passenger blimp with Bacillus pumilus SAFR-032 to develop a methodology for measuring the survival and response of bacterial spores in the atmosphere. Ground storage tests over 4 months confirmed that SAFR-032 coupons remained stable when frozen and desiccated. A cloud simulation test indicated that spores would be vulnerable to washing off the coupons after condensation, so blimp flights were restricted to clear skies. For ribonucleic acid (RNA) analysis requirements, we determined the minimum amount of surviving spores on test coupons was about 1 x 107 total cells, but this concentration created some layering (potentially protecting cells underneath from sunlight). Subsequent flights would benefit from larger coupons with a higher starting concentration of spores and ultraviolet light sensors located adjacent to the experimental arrays. More studies of this type may help formulate inactivation models for predicting the viability of microbes traveling through Earth’s harsh upper atmosphere. In addition, documenting which taxa survive (and how) in the atmosphere could inform our understanding of the consequences of terrestrial microbes dispersed by spacecraft on other planets.
Type of Paper: Article
Title: The Exobiological Relevance of the Hypanis and Sabrina Valles, Mars
Author: Alexis Rodríguez 1 and Alberto Fairén 2
Affiliation: 1. NASA Ames Research Center & Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ 85719-2395, USA; E-Mail: email@example.com
2. Department of Astronomy, Cornell University, 426 Space Science Bldg, Ithaca, NY 14853, USA; E-Mail: firstname.lastname@example.org
Abstract: The Hypanis and Sabrina Valles cut across a portion of Xanthe Terra, which is located west of Shalbatana Valles. These channels are likely to have been formed by flowing groundwater; the groundwater was likely sourced from regional aquifer systems that induced catastrophic floods in the Chryse outflow channels. We have identified layered fluvial deposits that retain primary depositional morphologies and perhaps hydrated materials that have not been subject to dehydration cycles. Sampling these materials would represent an opportunity to examine the composition and possible biosignatures of ancient water brines that may have remained confined in the subsurface for eons.
Title: The Photochemical Origin of Desert Varnish
Author: Henry Sun
Affiliation: Desert Research Institute
Abstract: Desert varnish, which forms on rocks in semiarid regions on Earth and possibly on Mars, is widely considered to be a product of bacterial biomineralization. In this paper, I provide several lines of evidence that argue against the conventional wisdom, and suggest instead that varnish is precipitated by light-stimulated metal transformations. First, the colonization of rock surfaces in deserts by epilithic or endolithic microbial communities and the formation of varnish are mutually exclusive. Second, biogenesis is an implausible argument from the standpoint of energetics. A one-millimeter thick varnish veneer takes several thousand years to form. No bacteria can survive by oxidizing such small amounts of iron and manganese. Third, metals can be reduced and oxidized photochemically. A photochemical theory, not the biogenic hypothesis, explains why varnish forms only on sun-exposed rock surfaces.
Title: Model Environment for Early Life on Earth and Mars Harbors Unusual Microbial Diversity
Author: Marina Resendes de Sousa Walther-António 1,2, David B. Finkelstein 3,4, Lisa M.Pratt 3 and Carl E. Bauer 1
Affiliation: 1 Department of Biology, Indiana University, Bloomington, IN 47405, USA
2 Department of Surgery, Mayo Clinic, Rochester, MN 55901,USA
3 Department of Geology, Indiana University, Bloomington, IN 47405, USA
4 Current address: Geoscience Department, Hobart and William Smith Colleges, Geneva, NY 14456, USA
Abstract: Warner Valley, Oregon is an alluvial system containing numerous geothermal springs and evaporative alkaline lakes underlain and hosted by basaltic flows and deposits from Pleistocene Lake Warner. One of its lakes, Anderson, is an alkaline (pH 8.3–10.5), NaCl dominated system (brackish/saline) that is enriched in arsenic (0.4–16.6 μM). During a single day of monitoring, it was revealed that pH, temperature, and conductivity were sensitive to diurnal changes in evaporation.
Despite the extreme environment, a 16S rDNA library (>1000 sequences) constructed from a microbial mat recovered at the lake showed a redundancy level of only 61%, with the diversity spreading throughout 3 domains and 27 bacterial phyla.
Title: Extreme Deserts on Earth: Analogs to Present and Past Mars
Author: Alfonso Davila 1, Christopher P. McKay 2, Henry Sun 3
Abstract: Mars has been a desert planet for most of its history. However, we now know that early in the planet’s history, conditions were suitable for life to evolve. By studying extreme deserts on Earth, we can learn valuable lessons concerning how life could have evolved and survived during the Martian surface’s increasing desertification. Such extreme deserts include the Atacama Desert in Chile and the Antarctic Dry Valleys. While both deserts are extremely dry, and in the case of the Dry Valleys, also extremely cold, both are capable of sustaining life inside rock niches. This is a survival strategy that is exclusive to extreme deserts, and could have been realized on Mars. Extreme deserts on Earth are also informative of non-biological processes on Mars. Examples include the formation of atmospheric oxidants in the Atacama Desert, and of vapor-deposited ground ice in the Dry Valleys (the only place other than Mars where this type of ice is known to occur). Finally, the study of extreme deserts can help advance and fine-tune the search for life on Mars, both from the point of view of basic science strategies, and from the perspective of technology development and testing.
Title: Volcanogenic Fluvial and Lacustrine Environments and Their Implications for the Habitability of Mars
Author: Claire R Cousins
Affiliation: UK Centre for Astrobiology, School of Physics and Astronomy, University of Edinburgh, Edinburgh EH9 3JZ, UK
Abstract: The search for once-habitable environments on Mars has become increasingly focused on environments dominated by fluvial and lacustrine processes, such as those recently identified by the MSL Curiosity rover. The abundance of liquid water, coupled with the potential longevity of such systems, has rendered these localities prime targets for the future exploration of Martian biosignatures. Fluvial-lacustrine environments associated with basaltic volcanism and volcanic terrains are highly relevant to Mars, but their terrestrial counterparts have been overlooked as useful analogs for both the robotic identification of sedimentary deposits and mineral assemblages, and for the assessment of past habitability. Such environments are common in Iceland, where rift- and hotspot-dominated basaltic volcanism interacts with surface ice to produce large volumes of liquid water within an otherwise cold and dry environment. Large volumes of meltwater can be stored to create subglacial and proglacial lakes, be released as catastrophic floods, or steadily feed proglacial braided channels and rivers. Sedimentary deposits and terrains produced by the resulting fluvial-lacustrine activity are extensive, with lithologies typically dominated by basaltic minerals, low-temperature alteration phases (including smectite clays and sulfates), and amorphous material (such as basaltic glass, palagonite, nanophase iron oxides, and amorphous silica). Finally, the volcanogenic lacustrine environments investigated so far have been found to support active microbial communities driven by chemolithotrophic energy production. This paper reviews these environments, their sedimentary deposits, and their microbiology, within the context of identifying similar processes on Mars where volcanism and cryospheric processes have dominated surface activity for much of its history.