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Searching for biosignatures on Mars is changing the focus of exploration from the characterization of habitability to the investigation of a potential coevolution, that is, the spatiotemporal interactions of life with its environment. Yet, the intellectual framework underpinning the preparation of Mars 2020 and ExoMars along with future life-seeking missions is essentially the same as the one that has guided the exploration of Mars for the past 15 years (e.g., Mustard et al., 2013 , Hays et al., 2017 , Cabrol, 2018 ); see also Chapters 11 and 12 . This framework is articulated around the terrestrial analogy principle of habitability. While this principle is helpful in characterizing Mars habitability potential over time, it is limiting and potentially misleading for the exploration of biosignatures as its focus is primarily on the spatiotemporal dynamics of environmental factors.

Processes, environments, and the energy associated with the transport and deposition of sand at Gusev Crater are characterized at the microscopic scale through the comparison of statistical moments for particle size and shape distributions. Bivariate and factor analyses define distinct textural groups at 51 sites along the traverse completed by the Spirit rover as it crossed the plains and went into the Columbia Hills. Fine-to-medium sand is ubiquitous in ripples and wind drifts. Most distributions show excess fine material, consistent with a predominance of wind erosion over the last 3.8 billion years. Negative skewness at West Valley is explained by the removal of fine sand during active erosion, or alternatively, by excess accumulation of coarse sand from a local source. The coarse to very coarse sand particles of ripple armors in the basaltic plains have a unique combination of size and shape. Their distribution display significant changes in their statistical moments within the ~400 m that separate the Columbia Memorial Station from Bonneville Crater. Results are consistent with aeolian and/or impact deposition, while the elongated and rounded shape of the grains forming the ripples, as well as their direction of origin, could point to Ma'adim Vallis as a possible source. For smaller particles on the traverse, our findings confirm that aeolian processes have dominated over impact and other processes to produce sands with the observed size and shape patterns across a spectrum of geologic (e.g., ripples and plains soils) and aerographic settings (e.g., wind shadows).

The Mars Orbital Camera onboard the Mars Global Surveyor spacecraft, which is currently orbiting about Mars, has revealed hundreds of pristine lobate and tongue-shaped flows that closely display the morphological characteristics of terrestrial rock glaciers, both tongue- and lobe-shaped forms. Generally located between 30°S and 47°S latitude on Mars, these terrestrial-like flows have important paleoenvironmental implications, including marking environmental change from current, present cold and dry desert martian conditions to cold wetter climates in the past. Paleoenvironmental conditions, hypothesized to have significantly influenced the dimensions of the terrestrial-like flows, is supported through a simple dynamic model with the power-law rheology. The presence of periglacial landforms on Mars indicates the possible presence of permafrost and potential caches of water for future exobiological exploration.

Lakes on Mars were formed under periglacial to glacial climates. Extreme conditions prevailed including freezing temperatures, low atmospheric pressure, high evaporation/sublimation rates, and liquid water reservoirs locked in aquifers below a thick cryosphere. Although many of the Martian paleolakes display evidence of a short period of activity consistent with these conditions, others display clear evidence of lifetimes ranging from 104 to 105 years. The discovery of young seeping processes in impact craters and pole-facing valley slopes along with young volcanic activity raise questions about the conditions and limitations of liquid water flow and potential lacustrine activity today on Mars. Current climate models show that in today's conditions there exist regions on Mars of sols above the triple point and below boiling point of water that could provide hydrogeological conditions comparable to these of the Antarctic Dry Valley lakes (with the exception of the atmosphere pressure). The locations of the most recent Martian paleolakes are correlated with these regions. Throughout the history of Mars, lakes generated diversified environments, which could have provided potential habitats for life. The recent discovery of young energy sources from volcanism and the potential for liquid water reinforces the possibility of extant life on Mars, and suggests recent ponds and ancient paleolakes as primary targets for rover and sample return missions.

The discovery of gullies and debris aprons raises the question of the existence of aqueous environments on Mars in recent geological times and its astrobiological implications. Three cases of such environments are surveyed at MOC high resolution in the E-Gorgonum chaos and Newton and Hale craters. The regional setting of these craters suggests that the mechanisms of aquifer destabilization, flow discharge, and gully formation in these three cases result from local geological triggers that can include impact cratering, and tectonic processes, rather than climate or insolation factor. We take as a working hypothesis that microbial life appeared on Mars in ancient geological times, probably in a geothermal environment but potentially evolving via infrared detection systems to give photosynthetic communities under the selective pressure of energetic solar radiation. We hypothesize that some microbial communities could have survived underground in either dormant or active state, or that their biomolecules could be preserved either frozen or desiccated in the subsurface beneath the upper oxidized zone. We assess the known environmental constraints for life and what type of potential habitats are provided in these three craters by aquifer discharges using comparison with terrestrial analogues and their associated microbial communities. These environments include: (1) the release of water on a dry crater floor in E-Gorgonum and the possibility for microorganisms and preserved biomolecules to be flushed out and mixed in with the sediment exposed at the surface; (2) the evidence of a recent lacustrine episode in the Newton crater with analogy to Antarctic Dry Valley lakes; (3) the exposure on the floor of Hale crater of material from a regional subsurface that is likely to have retained traces of one of the oldest martian bodies of water recognized to date (Parker et al. 2000) in the Argyre basin. We show how the water in Argyre (∼3.8 billion years ago (Gya) was likely to have been alterated by hydrothermal processes and how the subsequent formation of the 150-km Hale crater on the northern ring of Argyre generated hydrothermal pumping. This accounts for the anomalously high location of the springs on the crater crests today with respect to the rest of the regional subsurface distribution. Finally, we envision current impact cratering as a factor for destabilizing aquifers on Mars today, thus creating new environments. We analyze the implications of impacts for two geological types of rock units that could harbor traces of life. As a result, we compare the potential of astrobiological exploration of crater floors, rims, and ejecta on future missions to Mars.

Among the 2208 martian fluvial features plotted in a previous study [Z. Geomorphol. 37-1 (1993) 57], the hydrological characteristics of 71 pristine fluvial valley networks, located between 18.0°N and 55.0°S, were surveyed using Viking Orbiter photomosaic bases at 1:2 million scale. The surveyed networks display: (1) systems organized mainly as first-order streams, 85% of which have lengths not exceeding 1000 km with a large amount of the statistical sample having lengths between 200 and 500 km (generally shorter than the degraded valleys studied by Baker and Partridge [J. Geophys. Res. 91 (1986) 3561]), (2) drainage basins with average areas of about 5000 km2, (3) a random distribution, and (4) headwater tributary drainage densities (kd) ranging systematically between 0.03 and 0.2 km−1, which are comparable to terrestrial drainage densities in arid climates. The compositions of the surveyed pristine valley networks are comparable to the degraded fluvial systems that dissect older martian surface materials. Thus, it appears that no dramatic changes occurred in martian hydrology since the earlier period of formation of the “degraded valleys”, such as described by Baker and Partridge [J. Geophys. Res. 91 (1986) 3561]. Subsurface aquifer drainage may have been a large contributor to drainage basin formation since early in Mars history.

During the Nomad Rover Field Experiment in the Atacama Desert (Chile), a potential fossil was identified in a boulder by the science team remotely located at NASA Ames Research Center, California. The science team requested the collecting of the boulder that was returned for laboratory analysis. This analysis confirmed the evidence of paleolife. As the first fossil identified and sampled by a remotely located science team using a rover, we use the case of sample 1-250697 to describe the process, both in the field and later in the laboratory during the rock analysis, which led to the identification, characterization, and confirmation of the evidence of paleolife evidence in 1-250697. We point out the lessons that this case provides for future Mars sample return missions.

This study surveys the evolution of martian impact crater paleolakes through time by establishing their age, comparing their locations, and describing the formation of lacustrine sedimentary structures and features over time. We explore their meaning in terms of global climate changes, but also on local and regional scales. We observe a co-location between recent impact crater paleolakes and regions where GCM modeling shows that Mars is currently experiencing days above triple point and below boiling point of water. Some source areas show morphologies that could be consistent with ancient glacier features in the Argyre Basin/Darwin Crater region, in locations that are also consistent with GCM models of past snow precipitation on Mars. We propose that, at least for some paleolakes, our results are best explained by recent, and abundant, aqueous activity on Mars, with some meteorological contribution. These results are supported by other recent findings on martian meteorite ages, by the analysis of new MGS data showing recent volcanic and fluvial activity, and by climate modeling. They point toward the possibility that Mars currently is only hydrologically dormant and that there is a potential for hydrological activity to be generated again in the future following changes in the planet's orbital parameters.

Paleolakes in impact craters on Mars are characterized at global scale using the Viking Orbiter data. We identified 179 paleolakes in impact structures formed by the influx of water and sediment derived from valley networks and channels that can be classified into three different fluviolacustrine systems: closed, open, and lake-chain systems. We show the hydrogeologic implications for each of the three systems and their significance in terms of duration of fluviolacustrine activity. This study provides a catalog of areographic, physical, and physiographic data for each of the studied impact crater lakes, associated with the description of the sedimentary structures observed. It also identifies environments where life and/or precursors to life could have found favorable conditions to evolve.

Ma'adim Vallis and Gusev crater (Aeolis subquadrangle of Mars) formed a complex hydrogeologic system through time. Crater statistics over five geologic units, coupled with morphologic and geologic observations, show that this system was active over a period ranging from 0.7 to about 2 billion years. These results make the floor of Gusev crater one of the most prominent martian depositional features. The last aqueous sedimentary deposition could have occurred during Early Amazonian.

The survey of the hydrogeologic system formed by Gusev crater and Ma'adim Vallis (Aeolis subquadrangle of Mars) points out evidence for the existence of an ice-covered lake in Gusev crater. A first lake was formed by the drainage of the aquifer in the region surrounding Gusev before the entry of Ma'adim Vallis in the crater. The existence of a former lake in Gusev is deduced from the morphology of the Ma'adim delta. Its comparison with terrestrial Antarctic analogs argues for the presence of an ice-covered lake in Gusev at the time that the southern part of the crater's rampart was breached by Ma'adim first release, and for a subice–lacustrine construction of the valley's delta. Our survey shows that Ma'adim Vallis may have entered Gusev crater as late as Late Hesperian/Early Amazonian as part of a second lake episode. The relationship between the variation of the Gusev lake water-level, the volume of the lake, and the surface of the lake bed is established by our bathymetric model. The elevation of the former lake is deduced from the elevation of the mesa-like structures in the delta of Ma'adim Vallis. Furthermore, the correlation of the crater frequency of Gusev rampart with Mars' stratigraphic age shows that lakes may have occupied Gusev crater over a period of time covering 2 Gyrs., from the formation of the crater to the last episode of water release from Ma'adim Vallis. Though it is most likely that the lake was episodical, the recurrence of abundant water in Gusev crater makes this site a high priority for missions, either for martian resource exploration, or for the search of life.

We illustrate the impact of climate variability on lake habitat at high altitude as a proxy to the evolution of declining lakes on Mars. The geophysical environment at the summit lake of the Licancabur volcano (5970 m) in the Central Andes of Chile and Bolivia was monitored between 2002 and 2006. The lake’s environment presents analogies with Mars at a time when the planet was transitioning from a wetter to a drier, colder climate: thin atmosphere, high solar irradiance, low ozone, high daily and yearly temperature fluctuations with low averages, ice, reduced yearly precipitation, and volcanic geology. The Licancabur lake is also located in a region impacted by climate change resulting in enhanced evaporation and strong negative water balance that rapidly modifies lake habitat. Data show that although the decline is not monotical, interannual fluctuations in precipitation, water balance, major ion concentration, and pH are well marked. Microorganisms dwelling near or at the water/atmosphere interface are exposed to a solar irradiance 160% that of sea level. Maximum averaged UVB peaks at 4 W/m2 and shorter (260–270 nm) radiation is also detected at the surface. The thin and cold atmosphere generates sudden and significant inverse relationship between UV radiation and temperatures. In this cold, unstable environment the lake ecosystem is abundant. In addition to adaptation strategies, data suggest that the timing of key cycles is a critical factor in life survival and provide important information on the lake habitability and life survival potential on Mars as the climate changed.

This chapter discusses the morphological, geological, and mineralogical features of lakes on Mars. The existence of lakes on ancient Mars is now widely accepted, but that was not always the case. The history of science shows that knowledge on any scientific question is shaped by the means of exploration and those means are molded by what we think the world is. Prior to MGS of the late 1990s, the relatively low resolution of orbital imagery made it difficult to confirm Martian paleolakes by direct observations, though their existence was inferred because valley networks had already been identified on Viking and Mariner 9 images. Whether the early climate of Mars was much warmer and wetter in the Noachian compared to the later geological epochs is still the subject of ongoing debate. The existence of standing bodies of water on Mars required that at some point in its history, possibly repeatedly, physicochemical and environmental conditions allowing water to circulate and to pond were met. In addition to mineralogy and morphology, another clue about the conditions at the time of lake formation can be revealed by numbers. At Viking resolution, 179 putative impact crater lakes were identified and 210 open lake systems were cataloged from a global survey using the most recent datasets. Combined, the results from morphology, mineralogy, and estimation of lake production support the idea that favorable conditions did exist on Mars to produce a few large bodies of water, and many more, smaller lakes that formed by processes analogous to those occurring on Earth.

[1] The science goals of the Life in the Atacama (LITA) robotic field experiment are to understand habitat and seek out life in the Atacama Desert, Chile, as an analog to future missions to Mars. To those ends, we present a new data analysis tool, the LITA Data Scoring System (DSS), which (1) integrates rover and orbital data relevant to environmental habitability and life detection, and (2) provides a standard metric, or “score” to evaluate (a) the potential habitability, and (b) the strength of evidence for life at all locales along the rover's traverse. Designed and tested during the 2005 field campaign, first results from the DSS indicate that the three selected sites in the Atacama Desert are generally inhospitable. The strength of evidence for life is positively correlated with potential habitability at two of the three sites. Using factor analysis, we find three factors explain 79.9% of the variance in biological observations and five factors explain 96.2% of the variance in potential habitability across all sites. These factors are used to focus a discussion of scoring variable definitions for future robotic missions in the Atacama and of instrument selection and strategy development for future robotic missions on Earth and Mars.

[1] The Life in the Atacama project examined six different sites in the Atacama Desert (Chile) over 3 years in an attempt to remotely detect the presence of life with a rover. The remote science team, using only orbital and rover data sets, identified areas with a high potential for life as targets for further inspection by the rover. Orbital data in the visible/near infrared (VNIR) and in the thermal infrared (TIR) were used to examine the mineralogy, geomorphology, and chlorophyll potential of the field sites. Field instruments included two spectrometers (VNIR reflectance and TIR emission) and a neutron detector: this project represents the first time a neutron detector has been used as part of a “science-blind” rover field test. Rover-based spectroscopy was used to identify the composition of small scale features not visible in the orbital images and to improve interpretations of those data sets. The orbital and ground-based data sets produced consistent results, suggesting that much of the field sites consist of altered volcanic terrains with later deposits of sulfates, quartz, and iron oxides. At one location (Site A), the ground-based spectral data revealed considerably greater compositional diversity than was seen from the orbital view. One neutron detector transect provided insight into subsurface hydrogen concentrations, which correlated with life and surface features. The results presented here have implications for targeting strategies, especially for future Mars rover missions looking for potential habitats/paleohabitats.

The impact of individual extremes on life, such as UV radiation (UVR), temperatures, and salinity is well documented. However, their combined effect in nature is not well-understood while it is a fundamental issue controlling the evolution of habitat sustainability within individual bodies of water. Environmental variables combine in the Bolivian Altiplano to produce some of the highest, least explored and most poorly understood lakes on Earth. Their physical environment of thin atmosphere, high ultraviolet radiation, high daily temperature amplitude, ice, sulfur-rich volcanism, and hydrothermal springs, combined with the changing climate in the Andes and the rapid loss of aqueous habitat provide parallels to ancient Martian lakes at the Noachian/Hesperian transition 3.7-3.5 Ga ago. Documenting this analogy is one of the focuses of the High-Lakes Project (HLP). The geophysical data we collected on three of them located up to 5,916 m elevation suggests that a combination of extreme factors does not necessarily translate into a harsher environment for life. Large and diverse ecosystems adapt to UVR reaching 200%-216% that of sea level in bodies of water sometimes no deeper than 50 cm, massive seasonal freeze-over, and unpredictable daily evolution of UVR and temperature. The HLP project has undertaken the first complete geophysical and biological characterization of these lakes and documents how habitability is sustained and prolonged in declining lakes despite a highly dynamical environment. The same process may have helped life transition through climate crises over time on both Earth and Mars.

[1] The Life in the Atacama project investigated the regional distribution of life and habitats in the Atacama Desert of Chile. We sought to create biogeologic maps through survey traverses across the desert using a rover carrying biologic and geologic instruments. Elements of our science approach were to: Perform ecological transects from the relatively wet coastal range to the arid core of the desert; use converging evidence from science instruments to reach conclusions about microbial abundance; and develop and test exploration strategies adapted to the search of scattered surface and shallow subsurface microbial oases. Understanding the ability of science teams to detect and characterize microbial life signatures remotely using a rover became central to the project. Traverses were accomplished using an autonomous rover in a method that is technologically relevant to Mars exploration. We present an overview of the results of the 2003, 2004, and 2005 field investigations. They include: The confirmed identification of microbial habitats in daylight by detecting fluorescence signals from chlorophyll and dye probes; the characterization of geology by imaging and spectral measurement; the mapping of life along transects; the characterization of environmental conditions; the development of mapping techniques including homogeneous biological scoring and predictive models of habitat location; the development of exploration strategies adapted to the search for life with an autonomous rover capable of up to 10 km of daily traverse; and the autonomous detection of life by the rover as it interprets observations on-the-fly and decides which targets to pursue with further analysis.

[1] The Atacama Desert, one of the most arid landscapes on Earth, serves as an analog for the dry conditions on Mars and as a test bed in the search for life on other planets. During the Life in the Atacama (LITA) 2004 field experiment, satellite imagery and ground-based rover data were used in concert with a ‘follow-the-water’ exploration strategy to target regions of biological interest in two (1 coastal, 1 inland) desert study sites. Within these regions, environments were located, studied and mapped with spectroscopic and fluorescence imaging (FI) for habitats and microbial life. Habitats included aqueous sedimentary deposits (e.g., evaporites), igneous materials (e.g., basalt, ash deposits), rock outcrops, drainage channels and basins, and alluvial fans. Positive biological signatures (chlorophyll, DNA, protein) were detected at 81% of the 21 locales surveyed with the FI during the long-range, autonomous traverses totaling 30 km. FI sensitivity in detecting microbial life in extreme deserts explains the high percentage of positives despite the low actual abundance of heterotrophic soil bacteria in coastal (

Water, vital for life, not only maintains the integrity of structural and metabolic biomolecules, it also transports them in solution or colloidal suspension. Any flow of water through a dormant or fossilized microbial community elutes molecules that are potentially recognizable as biomarkers. We hypothesize that the surface seepage channels emanating from crater walls and cliffs in Mars Orbiter Camera images results from fluvial erosion of the regolith as low-temperature hypersaline brines. We propose that, if such flows passed through extensive subsurface catchments containing buried and fossilized remains of microbial communities from the wet Hesperian period of early Mars (approximately 3.5 Ga ago), they would have eluted and concentrated relict biomolecules and delivered them to the surface. Life-supporting low-temperature hypersaline brines in Antarctic desert habitats provide a terrestrial analog for such a scenario. As in the Antarctic, salts would likely have accumulated in water-filled depressions on Mars by seasonal influx and evaporation. Liquid water in the Antarctic cold desert analogs occurs at -80 degrees C in the interstices of shallow hypersaline soils and at -50 degrees C in salt-saturated ponds. Similarly, hypersaline brines on Mars could have freezing points depressed below -50 degrees C. The presence of hypersaline brines on Mars would have extended the amount of time during which life might have evolved. Phototrophic communities are especially important for the search for life because the distinctive structures and longevity of their pigments make excellent biomarkers. The surface seepage channels are therefore not only of geomorphological significance, but also provide potential repositories for biomolecules that could be accessed by landers.

We report on a field experiment held near Silver Lake playa in the Mojave Desert in February 1999 with the Marsokhod rover. The payload (Descent Imager, PanCam, Mini-TES, and Robotic Arm Camera), data volumes, and data transmission/receipt windows simulated those planned for the Mars Surveyor mission selected for 2001. A central mast with a pan and tilt platform at 150 cm height carried a high-resolution color stereo imager to simulate the PanCam and a visible/near-infrared fiberoptic spectrometer (operating range 0.35–2.5 μm). Monochrome stereo navigation cameras were mounted on the mast and the front and rear of the rover near the wheels. A field portable infrared spectroradiometer (operating range 8–14 μm) simulated the Mini-TES. A Robotic Arm Camera, capable of close-up color imaging at 23 μm/pixel resolution, was used in conjunction with the excavation of a trench into the subsurface. The science team was also provided with simulated images from the Mars Descent Imager and orbital panchromatic and multispectral imaging of the site obtained with the French SPOT, airborne Thermal Infrared Mapping Spectrometer, and Landsat Thematic Mapper instruments. Commands sequences were programmed and sent daily to the rover, and data returned were limited to 40 Mbits per communication cycle. During the simulated mission, 12 commands were uplinked to the rover, it traversed ∼90 m, six sites were analyzed, 11 samples were collected for laboratory analysis, and over 5 Gbits of data were collected. Twenty-two scientists, unfamiliar with the location of the field site, participated in the science mission from a variety of locations, accessing data via the World Wide Web. Remote science interpretations were compared with ground truth from the field and laboratory analysis of collected samples. Using this payload and mission approach, the science team synergistically interpreted orbital imaging and infrared spectroscopy, descent imaging, rover-based imaging, infrared spectroscopy, and microscopic imaging to deduce a consistent and largely correct interpretation of the geology, mineralogy, stratigraphy, and exobiology of the site. Use of imaging combined with infrared spectroscopy allowed source outcrops to be identified for local rocks on an alluvial fan. Different lithologies were distinguished both near the rover and at distances of hundreds of meters or more. Subtle differences such as a contact between dolomite and calcite were identified at a distance of 0.5 km. A biomarker for endolithic microbiota, a plausible life form to be found on Mars, was successfully identified. Microscopic imaging of soils extracted from the surface and subsurface allowed the mineralogy and fluvial history of the trench site to be deduced. The scientific productivity of this simulation shows that this payload and mission approach has high science value and would contribute substantially to achieving the goals of Mars exploration.