Special Issue "Planetary Exploration: Habitats and Terrestrial Analogs"

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A special issue of Life (ISSN 2075-1729). This special issue belongs to the section "Physics".

Deadline for manuscript submissions: closed (30 May 2014)

Special Issue Editors

Guest Editor
Prof. Dr. Dirk Schulze-Makuch
School of the Environment, Webster Hall 1148, Washington State University, Pullman, WA 99164, USA
E-Mail: dirksm@wsu.edu
Phone: +1 509 335 1180
Interests: planetary habitability; astrobiology; evolutionary biology; extreme environments; geobiology; space missions

Guest Editor
Dr. Alberto G. Fairen
Department of Astronomy, Cornell University, 426 Space Science Bldg, Ithaca, NY 14853, USA
Website: http://www.astro.cornell.edu/members/alberto-fairen.html
E-Mail: agfairen@cornell.edu
Phone: +1 607 255-5907
Interests: mars evolution; exploration; hydrogeology; geochemistry; mineralogy; astrobiology

Special Issue Information

Dear Colleagues,

Planetary exploration is moving at a fast pace as we learn about environmental conditions on various planetary bodies in our solar system and beyond. Habitable conditions at some time during the history of the solar system have been proposed to have existed on Mars, Venus, and a number of icy moons of the outer solar system, some of which may still exist today. For this “LIFE” Special Issue, we particularly encourage submissions describing habitable conditions on planetary bodies and of how life could have interacted with them; also a description of analog environments on Earth from which we can learn about possible adaptations and life strategies on other planets and moons.

Prof. Dr. Dirk Schulze-Makuch
Dr. Alberto G. Fairen
Guest Editors

Submission

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. Papers will be published continuously (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

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Keywords

  • planets
  • moons
  • habitat
  • analog environment
  • exploration
  • astrobiology

Published Papers

No papers have been published in this special issue yet, see below for planned papers.

Planned Papers

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: Review
Title: Río Tinto as a Geochemical Terrestrial Analogue of Mars
Author:
Ricardo Amils
Affiliation:
Centro de Astrobiología, INTA-CSIC, Torrenjón de Ardoz, Madrid 28850, Spain;
E-Mail: ramils@cbm.uam.es
Abstract:
The geomicrobiological characterization of Río Tinto (Huelva, Southwestern Spain) has proven the importance of the iron and the sulfur cycles, not only in generating the extreme conditions of the habitat (low pH, high concentration of toxic heavy metals), but also maintaining the high level of microbial diversity detected in the basin. It has been proved that the extreme acidic conditions of the Tinto basin are not the product of 5000 years of mining activity in the area but the consequence of an active underground bioreactor that obtains its energy from the massive sulfidic minerals existing in the Iberian Pyrite Belt. Two drilling projects, MARTE (2003–2006) and IPBSL (2011–2015), have been developed to provide evidence of subsurface microbial activities and the potential resources to support these activities. The oxidants that drive the system appear to come from the rock matrix. These resources need only groundwater to launch different microbial metabolisms. There are several similarities between the vast deposits of sulfates and iron oxides on Mars and the main sulfide bioleaching products found in the Tinto basin. These similarities have given to Río Tinto the status of geochemical Mars terrestrial analogue.

Type of Paper: Article
Title:
Mud Volcanoes of Trinidad as Astrobiological Analogs for Martian Environments
Authors:
Riad Hosein 1, Shirin Haque 2,* and Denise Beckles 1
Affiliation:
1 Department of Chemistry, University of the West Indies, St. Augustine, Trinidad, West, Indies; Email: riad.hosein@sta.uwi.edu (R.H.); denise.beckles@sta.uwi.edu (D.B.)
2 Department of Physics, University of the West Indies, St. Augustine, Trinidad, West Indies; Email:shirin.haque@sta.uwi.edu
Abstract:
Mud volcanoes in Trinidad emit methane. If this methane is biogenic in origin (i.e., is generated from subsurface microbes), the mud volcanoes can act as analogs for Martian environments and provide insight into the possibility of Mars harboring microbial life. The chemical profiles of eleven mud volcanoes (located in the southern region of Trinidad) were investigated in terms of their chemical, mineralogical, and soil properties; such factors were analyzed to determine whether mud volcanic soil microbes could exist. Methane Gas Analysis, pH analysis, Cation Exchange Capacity (CEC), and Percentage Water Content Analysis were performed on soil samples from the mud volcanoes. Chromium, strontium, and silicon were found in the mud volcanic soil; these elements all play a role in the proliferation of microbial life. Samples from three of the volcanoes were used to successfully culture bacterial colonies under anaerobic conditions. A general comparison of the mud volcanic soil in Trinidad with Martian soil indicates that similar processes may enable Mars to harbor microbial life.

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: pboston@nmt.edu
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: bains@mit.edu
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: david.j.smith-3@nasa.gov (D.J.S.); megan.a.morford@nasa.gov (M.A.M.)
2
University College London & Natural History Museum, London, United Kingdom; Email: finlay.maguire.11@ucl.ac.uk
3
Sierra Lobo, Inc., Kennedy Space Center, Florida, USA; E-Mail: christina.l.khodadad-1@nasa.gov
4
California Institute of Technology, Jet Propulsion Laboratory, Biotechnology and Planetary Protection Group, Pasadena, California, USA; E-Mails: parag.a.vaishampayan@jpl.nasa.gov (P.A.V.); kjvenkat@jpl.nasa.gov (K.J.V.)
5
NASA Postdoctoral Fellow, Kennedy Space Center, Florida, USA; E-Mail: phillip.r.maloney@nasa.gov
6
School of Earth and Environment, University of Leeds, United Kingdom; E-Mail: j.b.mcquaid@leeds.ac.uk
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: alexis@psi.edu
2.  Department of Astronomy, Cornell University, 426 Space Science Bldg, Ithaca, NY 14853, USA; E-Mail: agfairen@cornell.edu
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.

Type of Paper: Article
Title
: Models of Formation and Activity of Spring Mounds in the Mechertate-Chrita-Sidi El Hani System, Eastern Tunisia: Implications for the Habitability of Mars
Author
: Elhoucine Essefi 1,2,*, Goro Komatsu 3, Alberto G. Fairén 4, Marjorie A. Chan 5, Hayet Ben Jmaa 6 and Chokri Yaich 1,2
1
National Engineering School of Sfax, Sfax 3038, Tunisia; Email: hocinsefi@yahoo.fr
2
RU: Sedimentary Dynamics and Environment (DSE) (Code 03/UR/10-03), National Engineering School of Sfax, University of Sfax
3
International Research School of Planetary Sciences, Università d’Annunzio, Viale Pindaro 42, 65127 Pescara, Italy
4
Department of Astronomy, Cornell University, Ithaca 14853 New York, USA; E-Mail: agfairen@cornell.edu
5
Department of Geology & Geophysics, University of Utah, 115 S. 1460 E. Rm. 383 FASB, Salt Lake City, UT 84112, USA
6
Faculty of Arts and Humanities of Sfax, University of Sfax, Road of the Airport 5, Sfax 3023, Tunisia
Abstract
: Spring mounds on Earth and on Mars could represent optimal niches for life to develop in. If life ever evolved on Mars, ancient spring deposits would be excellent localities to search in for morphological or chemical remnants of past life. In this work, we investigate models of formation and activity in the well-exposed spring mounds of the Mechertate-Chrita-Sidi El Hani (MCSH) system (eastern Tunisia). We then use these models to explore possible spring mound formation on Mars. In the MCSH system, the genesis of the spring mounds is a direct consequence of groundwater upwelling, which is triggered by tectonics and/or hydraulics. These mounds are often considered to be fault spring mounds; their preferential orientation indicates tectonic involvement in spring mound formation. On the other hand, the hydraulic pressure generated by the convergence of aquifers toward the surface of the system also suggests that some mounds may have formed as artesian spring mounds. In the case of the MCSH system, the geologic data we present here show that both models are valid, and we propose a combined hydro-tectonic model as the likely formation mechanism of artesian-fault spring mounds. During their evolution from the embryonic (early) to the islet (“island”) stages, spring mounds are also shaped by eolian accumulations, which are enhanced by the induration process. Similarly, spring mounds may be relatively common in certain areas on the Martian surface, but their mode of formation is still a matter of debate. We propose that the tectonic, hydraulic, and combined hydro-tectonic models describing the spring mounds at MCSH are relevant Martian analogs because: (i) the Martian subsurface may be over-pressured, which could expel mineral rich waters as spring mounds on the surface, (ii) the subsurface may be fractured to cause alignment of the spring mounds in preferential orientations, and (iii) indurated eolian sedimentation and erosional remnants are common features on Mars. The spring mounds further bear diagnostic mineralogic and magnetic properties (in comparison with their immediate surroundings). Consequently, remote sensing techniques can be very useful for identifying similar spring mounds on Mars.

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: Fluorine-rich and Supercritical CO2 Planetary Environments as Possible Habitats for Life
Author:
N. Budisa 1 and D. Schulze-Makuch 2
Affiliation:
1 Technical University of Berlin
2
School of the Environment, Webster Hall 1148, Washington State University, Pullman, WA 99164, USA
Abstract:
Fluorine might be an element of choice for life in apolar, aprotic organic solvents. Fluorinated building blocks may be used as monomers of choice for sell-assembling life polymers. In the earth’s crust, fluorine comes in the form of fluoride minerals and is one of the most abundant halogen elements. However, fluorine-based chemistry has not evolved in living cells on Earth; this may possibly be due to its low bioavailability, since fluoride is almost insoluble (e.g., sea water contains 1.3 ppm fluorine). On the other hand, the replacement of hydrogen with fluorine in organic compounds is often accompanied by profound and unexpected changes in their properties (e.g., those of the perfluorinated polymer, Teflon). Such H®F replacements in the context of proteins in cells usually produce unexpected changes in biological activity, structural stability, and folding cooperativity. Perfluorocarbons are known as fluorophilic, and not as hydrophobic or hydrophilic: i.e., such molecules are expected not to interact with either hydrophilic or hydrophobic molecules. Therefore, the hydrophobic effect (i.e., the exclusion of apolar residues from the aqueous phase), as the main driving force of protein folding processes for life on earth, should also hold for fluorous proteins in both aqueous and organic phases. Such polymers or self-sorting assemblies are capable of resisting denaturation by organic solvents by excluding fluorocarbon side chains from the organic phase and forming a compact interior (or core) that is shielded from the surrounding solvent. Thus, we can anticipate that fluorine-containing “Teflon”-like or “non-sticking” building blocks would be substances of choice for the synthesis of organized polymeric structures in fluorine-rich planetary environments. No fluorine-rich planetary environment is known, but it can be theoretically hypothesized. For example, all molecular oxygen may be used up by oxidation reactions on a planetary surface and fluorine gas could be released from F-rich magma later in the history of a planetary body.
Another exotic possibility of life is that supercritical fluids are used as solvents for life: in particular, supercritical carbon dioxide. Feasibly, under these seemingly extreme environments, enzymes or even whole cells can catalyze reactions that are difficult or impossible in water. Supercritical fluids are fluids held at temperatures and pressures above the critical point. They possess gas-like low viscosities and high diffusivities, and their liquid-like solubilizing features and physicochemical properties depend significantly on external pressure. Several bacterial species that tolerate these conditions have been recently identified. In such habitats, enzymes can even become more stable (as they are conformationally rigid in the dehydrated state). Enzyme function is dependent upon solvent features; these enzymes can catalyze reactions that do not occur in water and which have novel substrate selectivities, as well as unique stereo-, regio- and chemoselectivities, etc. Certainly, one of the most intriguing properties of enzymes in organic solvents is the ‘molecular memory’ effect, where an enzyme, upon transfer from one solvent to another, is capable of “remembering” a conformation or pH state. For example, the catalytic activity of an enzyme in a particular solvent reflects the pH of the last solution to which it was previously exposed. In this way, enzyme properties in a particular solvent are dependent on their history. Planetary environments with supercritical fluids (particularly supercritical carbon dioxide) exist, even on Earth (below the ocean floor), and on Venus.

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.

Last update: 28 March 2014

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