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Detecting evidence of life on other planetary bodies requires a certain understanding of known biomarkers and their chemical nature, preservation potential, or biological specificity. In a planetary search for life, carbonates are of special interest due to their known association with life as we know it. On Earth, carbonates serve as an invaluable paleogeochemical archive of fossils of up to billions of years old. Here, we investigated biomarker profiles on three Chilean Triassic–Jurassic sedimentary records regarding our search for signs of past and present life over ∼200 Ma. A multianalytical platform that combines lipid-derived biomarkers, metaproteomics, and a life detector chip (LDChip) is considered in the detection of biomolecules with different perdurability and source-diagnosis potential. The combined identification of proteins with positive LDChip inmunodetections provides metabolic information and taxonomic affiliation of modern/subrecent biosignatures. Molecular and isotopic analysis of more perdurable hydrocarbon cores allows for the identification of general biosources and dominant autotrophic pathways over time, as well as recreation of prevailing redox conditions over ∼200 Ma. We demonstrate how extraterrestrial life detection can benefit from the use of different biomarkers to overcome diagnosis limitations due to a lack of specificity and/or alteration over time. Our findings have implications for future astrobiological missions to Mars. This work has been funded by the Spanish Ministry of Science and Innovation and Fondo Europeo de Desarrollo Regional (MICINN/FEDER) through the projects Nr. RYC2018-023943-I (L.S.-G.), RYC-2014-19446 (D.C.), RTI2018-094368-B-I00 (V.P.), and ESP2017-89053-C2-1-P (O.P.-B.). The Project Nr. MDM-2017-0737 Unidad de Excelencia “María de Maeztu”- Centro de Astrobiología (CSIC-INTA) was funded by the Spanish State Research Agency (AEI). M.A.L. was supported by a Juan de la Cierva postdoctoral fellowship (FJC2018-037246-I). Peerreview

Mineralogy is the key to understanding the origin of Phobos and its position in the evolution of the Solar System. In situ Raman spectroscopy on Phobos is an important tool to achieve the scientific objectives of the Martian Moons eXploration (MMX) mission, and maximize the scientific merit of the sample return by characterizing the mineral composition and heterogeneity of the surface of Phobos. Conducting in situ Raman spectroscopy in the harsh environment of Phobos requires a very sensitive, compact, lightweight, and robust instrument that can be carried by the compact MMX rover. In this context, the Raman spectrometer for MMX (i.e., RAX) is currently under development via international collaboration between teams from Japan, Germany, and Spain. To demonstrate the capability of a compact Raman system such as RAX, we built an instrument that reproduces the optical performance of the flight model using commercial off-the-shelf parts. Using this performance model, we measured mineral samples relevant to Phobos and Mars, such as anhydrous silicates, carbonates, and hydrous minerals. Our measurements indicate that such minerals can be accurately identified using a RAX-like Raman spectrometer. We demonstrated a spectral resolution of approximately 10 cm−1, high enough to resolve the strongest olivine Raman bands at ~ 820 and ~ 850 cm−1, with highly sensitive Raman peak measurements (e.g., signal-to-noise ratios up to 100). These results strongly suggest that the RAX instrument will be capable of determining the minerals expected on the surface of Phobos, adding valuable information to address the question of the moon’s origin, heterogeneity, and circum-Mars material transport. Graphical Abstract

The study area (148.155 km2, Figure 1) is located in the southern equatorial region of Mars, approximately centered at 26.0° S and 6.5° E. Although subjected to extensive surface erosion, this heavily cratered region, selected as a representative section of the densely cratered highlands [1], still shows evident signs of past water erosion in the form of valley networks that regularly dissect the surface. As such, is an ideal area to study the role of water in modifying the surface of the Martian highlands. The top elevation of the mapped region is located in the central and eastern areas, reaching a maximum elevation of 2,606 meters above the Martian datum (a.m.d.). The points of lower elevation in the area are locally inside impact craters (-678 m a.m.d.). We found two unusually large impact craters. One is at the northeast, about 180 km in diameter, and only its southern half is inside the area. The other is at the southeast, irregularly shaped, and 116 km in diameter. The topography in the mapped region is also defined by two large valley network systems which carve the surface flowing from south to north, and surround the positive relief at each flank. We have produced a 1:500.000 scale geomorphological map with unprecedented detail, using ArcGIS 10.3 Desktop Software (ESRI) to draw and compiled a combination of a mosaic of hi-resolution CTX images, complemented by available Context Camera (CTX) images for cover gaps [2], a mosaic composed by MOLA and HRSC topography [3], and THEMIS-IR day imagery [4]. We mapped the main morphological units to contribute to the understanding of the hydrology ofthis understudied region of Mars. We mainly focus on describing eleven different morphological unitsand four geomorphic features (Figure 2), related to the past presence of water, as both ice and liquid,to allow us to characterize the past environment and eventually to identify their presence andpersistence. Among these units we would like to highlight the watershed unit, formed by incisions on thesurface that we interpret as evidence of aqueous activity, in which water came from channels thatflowed by the runoff wall unit. We differentiate a few types of impact craters, highlighting the invertedcrater unit and the sediment-filled impact crater unit, both filled up with sedimentary materials maybesourced by paleo-lakes. We differentiate two ejecta units related to liquid and ice water reservoirs;and a polygonal ridged unit and a knobby terrain unit associated with permafrost environments. Acknowledgments: This research is a contribution of the Project ”MarsFirstWater”, EuropeanResearch Council, Consolidator Grant no. 818602. Authors also thank the Agencia Estatal deInvestigación (AEI) project no. MDM-2017-0737 Unidad de Excelencia ”María de Maeztu”, and ReyJuan Carlos University. References: [1] Forsberg-Taylor, et al., (2004). Journal of Geophysical Research: Planets, 109(E5). [2] Fergason R.L. et al. (2018) Astrogeology PDS Annex, USGS. [3] Dickson J.L. et al. (2018)49th LPSC, Abstract#2083. [4] Christensen, P. R et al. (2004). Space Science Reviews, 110 (1), 85–130.

The ExoMars Rosalind Franklin rover is scheduled to land in Oxia Planum, at the outlet of Coogoon Valles, in 2023. The site is located in the western Arabia Terra region, the most gradual transition of the planetary topographic dichotomy, also considered to be one of the oldest terrains of Mars (Davis et al., 2016). Despite the high potential of the area as a promising location to search for ancient life, some aspects of its regional geologic context remain elusive. The extremely complex hydrologic history of Coogoon Valles system (Molina et al., 2017) is one example of the difficulty in addressing its geologic record. The Arabia Terra surface is extensively modified, and the paucity of valley networks (Figure 1A) and the presence of many inverted structures (Hynek et al., 2010; Davis et al., 2016; Davis et al., 2019) seem to indicate a widespread denudation. Besides, some of the phyllosilicate-bearing light-toned layered materials found in the area could have been formed by weathered airborne transported volcanic tuffs or ashes, that may cover older features. The absence of obvious volcanic edifices near the study area, that could be a source for these materials (Kerber et al., 2012), does not exclude the presence of yet undiscovered volcanic sources in the vicinity. Some crater-like structures, which lack typical features of impact origin, could have been formed by tectonics, groundwater, thermokarst, or volcanic collapses (or a combination of all; Figure 2). Their morphology favors a volcanic origin (Molina et al., 2019), perhaps similar to the plain-style calderas described to the NE of Arabia Terra (Figure 2; Michalski and Bleacher, 2013). The distribution of relatively small water- and volcanic-related features in the region provides additional evidence for processes that are difficult to assess on a regional scale. We have performed a grid-based mapping (Ramsdale et al., 2017; Voelker and Ramsdale, 2019) in two perpendicular study areas that overlap in the landing site area (Figure 1). The northern study area was separated into 2,171 grids (each 20x20 km, yellow in Figure 1B). It follows the dichotomy scarp from the landing site until reaching the plain-style calderas to the northeast (Michalski and Bleacher, 2013). Here we looked for features related to volcanic and thermokarstic activity as depressions pits, flows, and ridges. The other study area (2,563 grids, white in Figure 1B) transverses the dichotomy from the landing site southbound to Sinus Sabaeus in the Noachian highlands, where records of fluvial activity are much more prevalent (Robas et al., 2019). In this case, we looked for modified craters and various water-related features. In addition to that, we mapped further landforms found in both study areas, as knob fields, light-toned deposits, and channels. To identify the different features we used the global mosaic composed of CTX images (Malin et al., 2007; Dickson et al., 2018), a topographic mosaic from MOLA and HRSC data (Smith et al. 2001; Jaumann, R. et al. 2007; Fergason et al., 2018), and the quantitative THEMIS-derived global thermal inertia mosaic (Christensen et al., 2004, 2013). Acknowledgments: This research is a contribution of the Project ”MarsFirstWater”, European Research Council Consolidator Grant no. 818602. The authors also thank the Agencia Estatal de Investigación (AEI) project no. MDM-2017-0737 Unidad de Excelencia ”María de Maeztu”, and Rey Juan Carlos University. References: Christensen P. R. et al., Space Sci Rev. 110, 85–130 (2004). Christensen P. R., R. L. Fergason, C. S. Edwards, J. Hill, in LPSC, 44, No. 1719, id. 2822 (2013). Davis J. M., M. Balme, P. M. Grindrod, R. M. E. Williams, S. Gupta, Geology. 44, 847–850 (2016). Davis J.M. et al, J Geophys Res Planets. 124(7), 1913–1934. (2019). Dickson J. L. , L. A. Kerber, C. I. Fassett, B. L. Ehlmann, in LPSC, 49, No. 2083, id. 2480 (2018). Fergason R. , T. Hare, J. Laura, Astrogeology PDS Annex, US Geological Survey (2018). Hynek B. M., M. Beach, M. R. T. Hoke, J Geophys Res Planets. 115 (2010). Jaumann R. et al., Planet Space Sci. 55, 928–952 (2007). Malin M. C. et al., Journal of Geophysical Research: Planets, 112, E5 (2018). Michalski J. R. , J. E. Bleacher, Nature. 502, 47–52 (2013). Molina A. et al., Icarus. 293, 27–44 (2017). Molina A., O. Prieto-Ballesteros, I. López, C. Robas, A. G. Fairén, in EANA Conf., 282577 (2019). Ramsdale J. D. et al., Planet Space Sci. 140, 49–61 (2017). Robas C., A. Molina, I. López, O. Prieto-Ballesteros, A. G. Fairén, in EANA Conf., 280888 (2019). Voelker M., J. D. Ramsdale, in Planetary Cartography and GIS, H. Hargitai, Ed. (Springer International Publishing, Cham, 2019), pp. 293–302.

The JUpiter ICy moons Explorer (JUICE) (ESA) and Europa Clipper (NASA) missions will be launched in the next decade in order to spend a number of years making detailed observations of the giant gaseous planet Jupiter and three of its largest moons, Ganymede, Callisto, and Europa. The focus of both missions is to scrutinize the nature of these icy moons and characterize the conditions that may have led to the emergence of habitable environment for life. A particular aspect of both JUICE and Europa Clipper missions is the exploration of the icy crusts, which comprises a new and very focused objective for outer solar system missions. A good example however of such investigations has been presented by Cassini’s RADAR instrument. One of JUICE’s tasks is to characterize the structure and properties of the ice shell by probing the subsurface of Ganymede down to a depth of a few kilometers. An additional task of JUICE’s ice penetrating RIME and Clipper’s radar REASON is to obtain profiles of subsurface thermal, compositional, and structural horizons down to a maximum depth of 1 to 9 km depending on the crust’s properties. However, the interpretation of radar sounding experiments relies on the ability to decipher the backscattering patterns. The icy moons of Jupiter have been reported to be efficient backscatters of cm-wavelength radiations, showing polarization effects that must be due to factors other than the only single scattering process at the space-body surface interface. The identification of the characteristics of the backscattering radiation results from the superposition of backscattering layers with different electrical properties as a function of depth. Amongst the properties likely to modify the backscattering efficiency are: the amount of silicate (partial absorbers), the ice salinity, and the distribution of cracks present in the layer (inhomogeneity of the medium at size scales comparable to the radar wavelengths). Consequently, direct measurements of the dielectric values (electrical conductivity and permittivity) of an icy layer with properties controlled in the lab could unveil the backscattering efficiency of the sample. We provide here the first version of a database of measurements, following a number of laboratory experiments made at CAB-CSIC-INTA, in collaboration with UCM and CSIC-ITEFI (Please see abstracts EPSC Muñoz-Iglesias et al. and EPSC Gonzalez-Díaz et al.), collected and classified based on the parameters and properties of the different ice samples, which are manufactured to resemble the Jovian moon ice compositions. During the experiments, different conditions of T and P were applied and the recorded measurements of electrical properties were collected in this database of ‘ice behavior’. Since the next remote sensing data are not expected anytime before the arrival of JUICE and Europa Clipper (around 2030), the database provides us with the opportunity to compare with the current literature and the data available for the Jovian icy moons from previous missions, but also add information that is currently missing and relate that information to relative surface processes. The ice database would facilitate the interpretation of future radar sounding measurements, providing a better constrain on the true nature of ices of the surfaces of JUICE’s and Europa Clipper’s future targets.

Coogoon Valles is an intricate fluvial system, and its main channel was formed during the Noachian period through the erosion of the clay-bearing basement of the Western Arabia Terra. This region is characterized by a thinner crust compared to the rest of the highlands and by the occurrence of massive phyllosilicate-bearing materials. The origin of this region is still under discussion. Its surface has been exposed to a large-scale volcanism, and several episodes of extensive denudation were primarily controlled by fluvial activity. In this regard, the study of the oldest channels in Arabia Terra is crucial for understanding the global geological evolution of early Mars. The reactivation of the hydrological system by sapping followed by aeolian erosion had reshaped the channel, as well as exposed ancient materials and landforms. The examination of the bed deposits suggests an old episode of detrital sedimentation covering the Noachian basement followed by an erosive event that formed the current Coogoon Valles configuration. A complex system of deltas and alluvial fans is situated at the termination of this channel, which has been proposed as a landing site for the upcoming ExoMars and Mars 2020 missions.

The Jovian moon Europa is a prime target for astrobiology. A global subsurface water ocean and a geologically young surface provide evidence of an active planetary body with a potential deep habitable environment. Tectonism and cryomagmatism are both agents of resurfacing, with structures on the surface spatially related to reddish non-icy materials that could represent crystallized volatile and salt-rich fluids from the interior, possibly from the ocean or shallower aqueous bodies. Cryomagmatism could therefore be a mechanism for exposing the underlying liquid layers to the surface and could hold paramount importance for understanding the physical and chemical evolution of fluids during their ascent and emplacement and their connection with geological features at the surface. With these premises, we perform a set of laboratory experiments simulating the evolution of different fluids under the conditions in Europa's crust. These experiments allow us to constrain the physico-chemical and textural changes experienced by the different fluids and solids that are potentially emplaced within the icy crust and determine how they are affected by such secondary processes as reheating, melting, and ultimate recrystallization (e.g., in response to the emplacement of a second diapir close to the first one or tidal reheating). Based on these experimental results, we explore the connection of cryomagmas and their evolution near the surface to geologic features present on Europa's surface, such as pits, uplifts/domes, and microchaos regions, as well as the link with explosive cryovolcanism responsible for putative plumes at Europa., With funding from the Spanish government through the "María de Maeztu Unit of Excellence" accreditation (MDM-2017-0737)

a ESCET-Area de Geologia, Universidad Rey Juan Carlos, 28933 Mostoles, Madrid, Spain b Centro de Astrobiologia (INTA-CSIC), INTA Campus, Ctra Ajalvir km 4, Torrejon de Ardoz, 28850 Madrid, Spain c International Geophysical Technology SL, Fuerteventura 4, 28703 San Sebastian de los Reyes, Madrid, Spain d Instituto Geologico y Minero de Espana (IGME), Rios Rosas 23, 28003 Madrid, Spain e Centro de Biologia Molecular Severo Ochoa, C/Nicolas Cabrera 1, Universidad Autonoma de Madrid, 28048 Madrid, Spain

The acidic waters of the Rio Tinto rise from several acidic springs that emerge in the area surrounding Pena de Hierro ( Fernandez-Remolar et al., 2005 ). These springs are located above minor normal faults that act as natural conduits for the water from the underlying deep aquifer. Although it has been suggested that the acidity of the river originates from the biooxidation of massive and stockwork sulfides ( Fernandez-Remolar et al., 2008a ), the location of the source for these acidic solutions has not previously been established. This lack of evidence has been used to suggest that the acidity of the Rio Tinto may be the product of the most conspicuous of the possible source, the extensive mining of the area over approximately the last 5000 years ( Davis et al., 2000 ). In this paper, we report resistivity and time-domain electromagnetic sounding data from the Rio Tinto aquifer to a depth of ∼600 m, revealing the locations for the acidic sources. Both types of data support the presence of two distinct geological units that we interpret as thrust sheets emplaced onto each other during the Variscan orogeny of the Carboniferous. These units, both of which contain massive and stockwork sulfides, act as the aquifer for the acidic waters of the Rio Tinto. Under this scenario, which is in agreement with the geological record of the Rio Tinto fluvial system for the past 6 Ma ( Moreno et al., 2003 ), our results imply that mining activity had little influence on the generation of the acidic river waters.

Recent observations of carbonate minerals in ancient Martian rocks have been interpreted as evidence for the former presence of circumneutral solutions optimal for carbonate precipitation. Sampling from surface and subsurface regions of the low-pH system of Río Tinto has shown, unexpectedly, that carbonates can form under diverse macroscopic physicochemical conditions ranging from very low to neutral pH (1.5-7.0). A multi-technique approach demonstrates that carbonate minerals are closely associated with microbial activity. Carbonates occur in the form of micron-size carbonate precipitates under bacterial biofilms, mineralization of subsurface colonies, and possible biogenic microstructures including globules, platelets and dumbbell morphologies. We propose that carbonate precipitation in the low-pH environment of Río Tinto is a process enabled by microbially-mediated neutralization driven by the reduction of ferric iron coupled to the oxidation of biomolecules in microbially-maintained circumneutral oases, where the local pH (at the scale of cells or cell colonies) can be much different than in the macroscopic environment. Acidic conditions were likely predominant in vast regions of Mars over the last four billion years of planetary evolution. Ancient Martian microbial life inhabiting low-pH environments could have precipitated carbonates similar to those observed at Río Tinto. Preservation of carbonates at Río Tinto over geologically significant timescales suggests that similarly-formed carbonate minerals could also be preserved on Mars. Such carbonates could soon be observed by the Mars Science Laboratory, and by future missions to the red planet.

– A model is presented in which the aqueous conditions needed to generate phyllosilicate minerals in the absence of carbonates found in the ancient Noachian crust are maintained by an early CO2-rich atmosphere, that, together with iron (II) oxidation, would prevent carbonate formation at the surface. After cessation of the internal magnetic dynamo, a CO2-rich primordial atmosphere was stripped by interactions with the solar wind and surface conditions evolved from humid to arid, with ground waters partially dissolving subsurface carbonate and sulfide minerals to produce acid-sulfate evaporitic deposits in areas with upwelling ground water. In a subsequent geochemical state (Late Noachian to Hesperian), surface and subsurface acidic solutions were neutralized in the subsurface through interaction with basaltic crust, allowing the precipitation of secondary carbonates. This model suggests that, in the early Noachian, the surface waters of Mars maintained acidity because of a drop in temperature. This would have favored increased dissolution of CO2 and a reduction in atmospheric pressure. In this scenario, physicochemical conditions precluded the formation of surface carbonates, but induced the precipitation of carbonates in the subsurface.

We present the scientific case for inclusion of penetrators into the Europan surface, and the candidate instruments which could significantly enhance the scientific return of the joint ESA/NASA Europa-Jupiter System Mission (EJSM). Moreover, a surface element would provide an exciting and inspirational mission highlight which would encourage public and political support for the mission. Whilst many of the EJSM science goals can be achieved from the proposed orbital platform, only surface elements can provide key exploration capabilities including direct chemical sampling and associated astrobiological material detection, and sensitive habitability determination. A targeted landing site of upwelled material could provide access to potential biological material originating from deep beneath the ice. Penetrators can also enable more capable geophysical investigations of Europa (and Ganymede) interior body structures, mineralogy, mechanical, magnetic, electrical and thermal properties. They would provide ground truth, not just for the orbital observations of Europa, but could also improve confidence of interpretation of observations of the other Jovian moons. Additionally, penetrators on both Europa and Ganymede, would allow valuable comparison of these worlds, and gather significant information relevant to future landed missions. The advocated low mass penetrators also offer a comparatively low cost method of achieving these important science goals. A payload of two penetrators is proposed to provide redundancy, and improve scientific return, including enhanced networked seismometer performance and diversity of sampled regions. We also describe the associated candidate instruments, penetrator system architecture, and technical challenges for such penetrators, and include their current status and future development plans.

The presence of extensive phyllosilicate deposits from the early Noachian of Mars are often interpreted as having formed from neutral to subalkaline solutions. In this paper we examine the Rio Tinto fluvial basin, an early Mars analog, that hosts clay production and sedimentation along the entire course of the river. At Rio Tinto, phyllosilicate minerals including clays and micas are sourced by volcanosedimentary bedrock of rhyolitic and andesitic composition affected by Carboniferous hydrothermal alteration. Pleistocene to modern acidic weathering of those materials chemically altered the volcanic and sedimentary materials to K/Na–clay–(montmorillonite/smectites)–kaolinite assemblages in paleosoils and fractures while physical weathering degrades phyllosilicates more resistant to acidic attack. During the wet season, phyllosilicates are eroded, transported and deposited from both acidic headwaters and neutral tributaries. During the dry season, sulfates and nanophase oxyhydroxides co-precipitate. Late summer storms that cause fast flooding events mix illite, quartz, feldspars, iron oxides and other minerals in fluvial deposits where these minerals are stabilized and aggrade until the following wet season. As a result, chemical precipitates, primary phyllosilicates and secondary clays form mineral admixtures that explain the compositional diversity of the fluvial deposits. These deposits reveal the persistence of smectites, whose occurrence is explained given that the reaction kinetics under acidic conditions of degradation is lowered by seasonal discharges of the river. The longevity of phyllosilicate minerals within fluvial deposits depends on climatic and geochemical conditions and processes which are in turn are correlated to temperature, persistence of water, hydrological cycling, hydrogeochemistry and composition of the source materials in the basement. These parameters are universal and have to be characterized in order to understand the distribution of mineral composition on any planetary surface, including Mars.

9 páginas, 4 figuras., The 2005 Mars Astrobiology Research and Technology Experiment (MARTE) project conducted a simulated 1-month Mars drilling mission in the Río Tinto district, Spain. Dry robotic drilling, core sampling, and biological and geological analytical technologies were collectively tested for the first time for potential use on Mars. Drilling and subsurface sampling and analytical technologies are being explored for Mars because the subsurface is the most likely place to find life on Mars. The objectives of this work are to describe drilling, sampling, and analytical procedures; present the geological analysis of core and borehole material; and examine lessons learned from the drilling simulation. Drilling occurred at an undisclosed location, causing the science team to rely only on mission data for geological and biological interpretations. Core and borehole imaging was used for micromorphological analysis of rock, targeting rock for biological analysis, and making decisions regarding the next day's drilling operations. Drilling reached 606 cm depth into poorly consolidated gossan that allowed only 35% of core recovery and contributed to borehole wall failure during drilling. Core material containing any indication of biology was sampled and analyzed in more detail for its confirmation. Despite the poorly consolidated nature of the subsurface gossan, dry drilling was able to retrieve useful core material for geological and biological analysis. Lessons learned from this drilling simulation can guide the development of dry drilling and subsurface geological and biological analytical technologies for future Mars drilling missions.

Chemolithotrophic communities that colonize subsurface habitats have great relevance for the astrobiological exploration of our Solar System. We hypothesize that the chemical and thermal stabilization of an environment through microbial activity could make a given planetary region habitable. The MARTE project ground-truth drilling campaigns that sampled cryptic subsurface microbial communities in the basement of the Río Tinto headwaters have shown that acidic surficial habitats are the result of the microbial oxidation of pyritic ores. The oxidation process is exothermic and releases heat under both aerobic and anaerobic conditions. These microbial communities can maintain the subsurface habitat temperature through storage heat if the subsurface temperature does not exceed their maximum growth temperature. In the acidic solutions of the Río Tinto, ferric iron acts as an effective buffer for controlling water pH. Under anaerobic conditions, ferric iron is the oxidant used by microbes to decompose pyrite through the production of sulfate, ferrous iron, and protons. The integration between the physical and chemical processes mediated by microorganisms with those driven by the local geology and hydrology have led us to hypothesize that thermal and chemical regulation mechanisms exist in this environment and that these homeostatic mechanisms could play an essential role in creating habitable areas for other types of microorganisms. Therefore, searching for the physicochemical expression of extinct and extant homeostatic mechanisms through physical and chemical anomalies in the Mars crust (i.e., local thermal gradient or high concentration of unusual products such as ferric sulfates precipitated out from acidic solutions produced by hypothetical microbial communities) could be a first step in the search for biological traces of a putative extant or extinct Mars biosphere.

Several substances besides water ice have been detected on the surface of Europa by spectroscopic sensors, including CO 2 , SO 2 , and H 2 S. These substances might occur as pure crystalline ices, as vitreous mixtures, or as clathrate hydrate phases, depending on the system conditions and the history of the material. Clathrate hydrates are crystalline compounds in which an expanded water ice lattice forms cages that contain gas molecules. The molecular gases that may constitute Europan clathrate hydrates may have two possible ultimate origins: they might be primordial condensates from the interstellar medium, solar nebula, or jovian subnebula, or they might be secondary products generated as a consequence of the geological evolution and complex chemical processing of the satellite. Primordial ices and volatile-bearing compounds would be difficult to preserve in pristine form in Europa without further processing because of its active geological history. But dissociated volatiles derived from differentiation of a chondritic rock or cometary precursor may have produced secondary clathrates that may be present now. We have evaluated the current stability of several types of clathrate hydrates in the crust and the ocean of Europa. The depth at which the clathrates of SO 2 , CO 2 , H 2 S, and CH 4 are stable have been obtained using both the temperatures observed in the surface [Spencer, J.R., Tamppari, L.K., Martin, T.Z., Travis, L.D., 1999. Temperatures on Europa from Galileo photopolarimeter–radiometer: Nighttime thermal anomalies. Science 284, 1514–1516] and thermal models for the crust. In addition, their densities have been calculated in order to determine their buoyancy in the ocean, obtaining different results depending upon the salinity of the ocean and type of clathrate. For instance, assuming a eutectic composition of the system MgSO 4 H 2 O for the ocean, CO 2 , H 2 S, and CH 4 clathrates would float but SO 2 clathrate would sink to the seafloor; an ocean of much lower salinity would allow all these clathrates to sink, except that CH 4 clathrate would still float. Many geological processes may be driven or affected by the formation, presence, and destruction of clathrates in Europa such as explosive cryomagmatic activity [Stevenson, D.J., 1982. Volcanism and igneous processes in small icy satellites. Nature 298, 142–144], partial differentiation of the crust driven by its clathration, or the local retention of heat within or beneath clathrate-rich layers because of the low thermal conductivity of clathrate hydrates [Ross, R.G., Kargel, J.S., 1998. Thermal conductivity of Solar System ices, with special reference to martian polar caps. In: Schmitt, B., De Berg, C., Festou, M. (Eds.), Solar System Ices. Kluwer Academic, Dordrecht, pp. 33–62]. On the surface, destabilization of these minerals and compounds, triggered by fracture decompression or heating could result in formation of chaotic terrain morphologies, a mechanism that also has been proposed for some martian chaotic terrains [Tanaka, K.L., Kargel, J.S., MacKinnon, D.J., Hare, T.M., Hoffman, N., 2002. Catastrophic erosion of Hellas basin rim on Mars induced by magmatic intrusion into volatile-rich rocks. Geophys. Res. Lett. 29 (8); Kargel, J.S., Prieto-Ballesteros, O., Tanaka K.L., 2003. Is clathrate hydrate dissociation responsible for chaotic terrains on Earth, Mars, Europa, and Triton? Geophys. Res. 5. Abstract 14252]. Models of the evolution of the ice shell of Europa might take into account the presence of clathrate hydrates because if gases are vented from the silicate interior to the water ocean, they first would dissolve in the ocean and then, if the gas concentrations are sufficient, may crystallize. If any methane releases occur in Europa by hydrothermal or biological activity, they also might form clathrates. Then, from both geological and astrobiological perspectives, future missions to Europa should carry instrumentation capable of clathrate hydrate detection.

The complex geology of Europa is evidenced by many tectonic and cryomagmatic resurfacing structures, some of which are “painted” into a more visible expression by exogenic alteration processes acting on the principal endogenic cryopetrology. The surface materials emplaced and affected by this activity are mainly composed of water ice in some areas, but in other places there are other minerals involved. Non-ice minerals are visually recognized by their low albedo and reddish color either when first emplaced or, more likely, after alteration by Europan weathering processes, especially sublimation and alteration by ionizing radiation. While red chromophoric material could be due to endogenic production of solid sulfur allotropes or other compounds, most likely the red substance is an impurity produced by radiation alteration of hydrated sulfate salts or sulphuric acid of mainly internal origin. If the non-ice red materials or their precursors have a source in the satellite interior, and if they are not merely trace contaminants, then they can play an important role in the evolution of the icy crust, including structural differentiation and the internal dynamics. Here we assume that these substances are major components of Europa's cryo/hydrosphere, as some models have predicted they should be. If this is an accurate assumption, then these substances should not be neglected in physical, chemical, and biological models of Europa, even if major uncertainties remain as to the exact identity, abundance, and distribution of the non-ice materials. The physical chemical properties of the ice-associated materials will contribute to the physical state of the crust today and in the geological past. In order to model the influence of them on the thermal state and the geology, we have determined the thermal properties of the hydrated salts. Our new lab data reveal very low thermal conductivities for hydrated salts compared to water ice. Lower conductivities of salty ice would produce steeper thermal gradients than in pure ice. If there are salt-rich layers inside the crust, forming salt beds over the seafloor or a briny eutectic crust, for instance, the high thermal gradients may promote endogenic geological activity. On the seafloor, bedded salt accumulations may exhibit high thermochemical gradients. Metamorphic and magmatic processes and possible niches for thermophilic life at shallow suboceanic depths result from the calculated thermal profiles, even if the ocean is very cold.

We have considered a wide array of scenarios for Europa's chemical evolution in an attempt to explain the presence of ice and hydrated materials on its surface and to understand the physical and chemical nature of any ocean that may lie below. We postulate that, following formation of the jovian system, the europan evolutionary sequence has as its major links: (a) initial carbonaceous chondrite rock, (b) global primordial aqueous differentiation and formation of an impure primordial hydrous crust, (c) brine evolution and intracrustal differentiation, (d) degassing of Europa's mantle and gas venting, (e) hydrothermal processes, and (f) chemical surface alteration. Our models were developed in the context of constraints provided by Galileo imaging, near infrared reflectance spectroscopy, and gravity and magnetometer data. Low-temperature aqueous differentiation from a carbonaceous CI or CM chondrite precursor, without further chemical processing, would result in a crust/ocean enriched in magnesium sulfate and sodium sulfate, consistent with Galileo spectroscopy. Within the bounds of this simple model, a wide range of possible layered structures may result; the final state depends on the details of intracrustal differentiation. Devolatilization of the rocky mantle and hydrothermal brine reactions could have produced very different ocean/crust compositions, e.g., an ocean/crust of sodium carbonate or sulfuric acid, or a crust containing abundant clathrate hydrates. Realistic chemical–physical evolution scenarios differ greatly in detailed predictions, but they generally call for a highly impure and chemically layered crust. Some of these models could lead also to lateral chemical heterogeneities by diapiric upwellings and/or cryovolcanism. We describe some plausible geological consequences of the physical–chemical structures predicted from these scenarios. These predicted consequences and observed aspects of Europa's geology may serve as a basis for further analys is and discrimination among several alternative scenarios. Most chemical pathways could support viable ecosystems based on analogy with the metabolic and physiological versatility of terrestrial microorganisms.

In this paper, we provide a detailed review of Ganymede's characteristics that are germane to any consideration of its planetary protection requirements. Ganymede is the largest moon in our solar system and is the subject of one of the main science objectives of the JUICE mission to the jovian system. We explore the probability of the occurrence of potentially habitable zones within Ganymede at present, including those both within the deep liquid ocean and those in shallow liquid reservoirs. We consider the possible exchange processes between the surface and any putative habitats to set some constraints on the planetary protection approach for this moon. As a conclusion, the “remote” versus “significant” chance of contamination will be discussed, according to our current understanding of this giant icy moon. Based on the different estimates we investigate here, it appears extremely unlikely that material would be exchanged downward through the upper icy layer of Ganymede and, thus, bring material into the ocean over timescales consistent with the survival of microorganisms. Key Words: Planetary science—Planetary protection—Ice.

International audience; Past exploration of Jupiter's diverse satellite system has forever changed our understanding of the unique environments to be found around gas giants, both in our solar system and beyond. The detailed investigation of three of Jupiter's Galilean satellites (Ganymede, Europa, and Callisto), which are believed to harbour subsurface water oceans, is central to elucidating the conditions for habitability of icy worlds in planetary systems in general. The study of the Jupiter system and the possible existence of habitable environments offer the best opportunity for understanding the origins and formation of the gas giants and their satellite systems. The JUpiter ICy moons Explorer (JUICE) mission, selected by ESA in May 2012 to be the first large mission within the Cosmic Vision Program 2015-2025, will perform detailed investigations of Jupiter and its system in all their inter-relations and complexity with particular emphasis on Ganymede as a planetary body and potential habitat. The investigations of the neighbouring moons, Europa and Callisto, will complete a comparative picture of the Galilean moons and their potential habitability. Here we describe the scientific motivation for this exciting new European-led exploration of the Jupiter system in the context of our current knowledge and future aspirations for exploration, and the paradigm it will bring in the study of giant (exo) planets in general. [All rights reserved Elsevier].

In the acidic environment of Rio Tinto, there is a close relationship between microbial mat growth and channel configuration. This extreme environment is characterized by solutions with a low pH (1–3) and high Eh (300–450 mV), and high concentration in ferric iron (0.5–30 g l−1) and sulfates (1–120 g l−1). Under these conditions, biofilms have several morphologies that adapt to the local parameters controlled by the water flow, light, and hydrochemistry. Seasonal water availability induces different rates in the oxidation degree and concentration of water chemicals. Therefore, seasonal changes favor cycles between high increment in microbial mass and late preservation under coating by inorganic polymers mainly composed by ferric iron and sulfate. Under these circumstances, massive coating of biofilms during the dry season induce clear changes in the river bed in the form of pool and riffle corrugated pattern that has some impact on the river flow. Moreover, sediment accretion and lateral progradation is also found in those areas where the river has a meandering patter or floods on the lower bank. In this last case, lateral sedimentation is favored by the mineralization on biofilms.

[1] Perchlorate salts (mostly magnesium and sodium perchlorate) have been detected on Mars' arctic soil by the Phoenix lander, furthermore chloride salts have been found on the Meridiani and Gusev sites and on widespread deposits on the southern Martian hemisphere. The presence of these salts on the surface is not only relevant because of their ability to lower the freezing point of water, but also because they can absorb water vapor and form a liquid solution (deliquesce). We show experimentally that small amounts of sodium perchlorate (∼ 1 mg), at Mars atmospheric conditions, spontaneously absorb moisture and melt into a liquid solution growing into ∼ 1 mm liquid spheroids at temperatures as low as 225 K. Also mixtures of water ice and sodium perchlorate melt into a liquid at this temperature. Our results indicate that salty environments make liquid water to be locally and sporadically stable on present day Mars.

Dr. David C. Fernandez-Remolar is currently researcher in the Centro de Astrobiologia (INTA-CSIC). He obtained his Ph.D. from the Complutense University at Madrid in 1999 studying Lower Cambrian phosphatized skeletons of Sierra de Cordoba. At the Centro de Astrobiologia he is currently researching geobiology and biogeochemistry of extreme environments and geohistorical Mars analogs such as Rio Tinto focused in the surface and subsurface astrobiological exploration of Mars. Other areas of interest are the astrobiological exploration of Europa and the geobiology of the Proterozoic and Lower Cambrian deposits of Spain.

Geometrical properties of landscapes result from the geological processes that have acted through time. The quantitative analysis of natural relief represents an objective form of aiding in the visual interpretation of landscapes, as studies on coastlines, river networks, and global topography, have shown. Still, an open question is whether a clear relationship between the quantitative properties of landscapes and the dominant geomorphologic processes that originate them can be established. In this contribution, we show that the geometry of topographic isolines is an appropriate observable to help disentangle such a relationship. A fractal analysis of terrestrial isolines yields a clear identification of trenches and abyssal plains, differentiates oceanic ridges from continental slopes and platforms, localizes coastlines and river systems, and isolates areas at high elevation (or latitude) subjected to the erosive action of ice. The study of the geometrical properties of the lunar landscape supports the existence of a correspondence between principal geomorphic processes and landforms. Our analysis can be easily applied to other planetary bodies., Comment: 21 pages, 7 figures

Terrestrial subsurface geomicrobiology is a matter of growing interest. On a fundamental level, it seeks to determine whether life can be sustained in the absence of radiation, whereas it also aims to develop practical applications in environmental biotechnology. Subsurface ecosystems are also intriguing exobiological models , useful for the re-creation of life on early Earth (Widdel et al. 1993) or the representation of life as it would occur in other planetary bodies (Boston et al. 1992). Subsurface ecosystems were originally reported in basalt aquifers (Stevens and McKinley 1995; Chapelle et al. 2002) and later in sedimentary aquifers, petroleum reservoirs, and alkaline and saline goldmine groundwater (Lin et al. 2006). Results obtained by deep-sea subsurface exploration initiatives are widening the scope of our knowledge in this field (D’Hondt et al. 2004). In this field there is a serious debate on whether the source of electron donors and/or acceptors is dependent on radiation-mediated reactions and also on contamination problems associated with drilling technologies, their mitigation, and control. In spite of the interest of subsurface ecosystems, information concerning microbial abundance, diversity, and sustainability is still scarce, mainly due to methodological limitations. Among the different minerals, the metallic sulfides have the potential to be a good source of energy for subsurface chemolithotrophs . The micro-organisms that aerobically oxidize iron sulfides are well characterized (Gonzalez-Toril et al. 2003), however, little is known about the possibility of subsurface chemolithoautotrophic metabolism in anoxic conditions. The Mars Analog Research and Technology Experiment (MARTE) project (Stoker et al. 2004; Fernandez-Remolar et al. 2005a) outlined in this chapter was designed to search for this type of life in the nonporous volcanically hosted massive sulfide deposits (VHMS) of the Rio Tinto basement at Pena de Hierro (Iberian Pyritic Belt).

25 páginas, 13 figuras, 7 tablas.-- et al., The Mars Astrobiology Research and Technology Experiment (MARTE) simulated a robotic drilling mission to search for subsurface life on Mars. The drill site was on Peña de Hierro near the headwaters of the Río Tinto river (southwest Spain), on a deposit that includes massive sulfides and their gossanized remains that resemble some iron and sulfur minerals found on Mars. The mission used a fluidless, 10-axis, autonomous coring drill mounted on a simulated lander. Cores were faced; then instruments collected color wide-angle context images, color microscopic images, visible–near infrared point spectra, and (lower resolution) visible–near infrared hyperspectral images. Cores were then stored for further processing or ejected. A borehole inspection system collected panoramic imaging and Raman spectra of borehole walls. Life detection was performed on full cores with an adenosine triphosphate luciferin-luciferase bioluminescence assay and on crushed core sections with SOLID2, an antibody array-based instrument. Two remotely located science teams analyzed the remote sensing data and chose subsample locations. In 30 days of operation, the drill penetrated to 6 m and collected 21 cores. Biosignatures were detected in 12 of 15 samples analyzed by SOLID2. Science teams correctly interpreted the nature of the deposits drilled as compared to the ground truth. This experiment shows that drilling to search for subsurface life on Mars is technically feasible and scientifically rewarding., MARTE was funded by the NASA Astrobiology Science and Technology for Exploring Planets (ASTEP) program through NRA 02-OSS-01. Spanish participation in MARTE was funded by the Centro de Astrobiología.

4 pages, 3 figures.-- ISI Article Identifier: 000250543900004., Full-text version available Open Access at: http://duff.geology.washington.edu/grg/publications/pdfs/Mars_Kargel.pdf, Numerical simulations and geologic studies suggest that high thermal anomalies beneath, within, and above thermally insulating layers of buried hydrated salts and gas hydrates could have triggered and sustained hydrologic processes on Mars, producing or modifying chaotic terrains, debris flows, gullies, and ice-creep features. These simulations and geologic examples suggest that thick hydrate deposits may sustain such geothermal anomalies, shallow ground-water tables, and hydrogeologic activity for eons. The proposed mechanism may operate and be self-reinforcing even in today's cold Martian climate without elevated heat flux., This work was partially supported by a grant to Kargel from the National Aeronautics and Space Administration Mars Fundamental Research Program.

Recent data from space missions reveal that there are ongoing climatic changes and erosive processes that continuously modify surface features of Mars. We have investigated the seasonal dynamics of a number of morphological features located at Inca City, a representative area at high southern latitude that has undergone seasonal processes. By integrating visual information from the Mars Orbiter Camera on board the Mars Global Surveyor and climatic cycles from a Mars' General Circulation Model, and considering the recently reported evidence for the presence of water-ice and aqueous precipitates on Mars, we propose that a number of the erosive features identified in Inca City, among them spiders, result from the seasonal melting of aqueous salty solutions.

Tirez Lake (La Mancha, central Spain) is proposed as a terrestrial analogue of Europa's ocean. The proposal is based on the comparison of the hydrogeochemistry of Tirez Lake with the geochemical features of the alteration mineralogy of meteoritic precursors and with Galileo's Near Infrared Mapping Spectrometer data on Europa's surface. To validate the astrobiological potential of Tirez Lake as an analog of Europa, different hydrogeochemical, mineral, and microbial analyses were performed. Experimental and theoretical modeling helped to understand the crystallization pathways that may occur in Europa's crust. Calculations about the oxidation state of the hypothetical Europan ocean were estimated to support the sulfate-rich neutral liquid model as the origin of Europa's observed hydrated minerals and to facilitate their comparison with Tirez's hydrogeochemistry. Hydrogeochemical and mineralogical analyses showed that Tirez waters corresponded to Mg-Na-SO(4)-Cl brines with epsomite, hexahydrite, and halite as end members. A preliminary microbial ecology characterization identified two different microbial domains: a photosynthetically sustained community represented by planktonic/benthonic forms and microbial mat communities, and a subsurficial anaerobic realm in which chemolithotrophy predominates. Fluorescence in situ hybridization has been used to characterize the prokaryotic diversity of the system. The subsurficial community seemed to be dominated by sulfate-reducing bacteria and methanogens. Frozen Tirez brines were analyzed by Fourier-transform infrared techniques providing spectra similar to those reported previously using pure components and to the Galileo spectral data. Calorimetric measurements of Tirez brines showed pathways and phase metastability for magnesium sulfate and sodium chloride crystallization that may aid in understanding the processes involved in the formation of Europa's icy crust. The use of fluorescence hybridization techniques for microbial detection and characterization in hypersaline environments makes this methodology strongly advisable for future Europa astrobiological missions.

The presence of methane has been recently detected in the martian atmosphere, suggesting a contemporary source such as volcanism or microbial activity. Here we show that methane may be released by the destabilization of methane clathrate hydrates, triggered by the interglacial climate change starting 0.4 Ma. Clathrate hydrates are nonstoichiometric crystalline compounds in which a water ice lattice forms cages that contain apolar gas molecules, such as methane [CH4· n H2O] and carbon dioxide [CO2· n H2O]. The loss of shallow ground ice eliminates confining pressure, initiating the destabilization of clathrate hydrates and the release of methane to the atmosphere. This alternative process does not restrict the methane's age to 430 yr (maximum residence time of methane gas in martian atmosphere), because clathrate hydrates can preserve (encage) methane of ancient origin for long time periods.

We have produced a 1:650,000 scale geomorphological map of the southwest Sinus Sabaeus, a region of Mars approximately centered at 25.0°S and 6.5°E and located in the topographic transition between Arabia Terra and Noachis Terra, in the Martian highlands. This heavily cratered region, subjected to extensive surface erosion, shows a complex valley network system known as Marikh Vallis. In this work, we study the history and role of water in and around Marikh Vallis, focusing on the modification and evolution of this area during the earliest Martian times, the Noachian period. The map described in this paper was produced through the analysis of a combination of available imagery data, topography, and thermal inertia, which together allow defining different geomorphological units in this area. This new map provides a basis for identifying the ancient presence of water in the region, both in the liquid state and in the ice phase. This work was supported by the European Research Council Consolidator Grant number 818602; the Agencia Estatal de Investigación (AEI) under Grant PID2019-107442RB-C32; and the AEI under Grant MDM-2017-0737 Unidad de Excelencia ‘María de Maeztu’. The authors also thank the Rey Juan Carlos University.This work was supported by Agencia Estatal de Investigación: [grant number MDM-2017-0737,PID2019-107442RB-C32]. European Research Council: [grant num ber 818602]. Peerreview