Salar de Pajonales, a Ca-sulfate salt flat in the Chilean High Andes, showcases the type of polyextreme environment recognized as one of the best terrestrial analogs for early Mars because of its aridity, high solar irradiance, salinity, and oxidation. The surface of the salar represents a natural climate-transition experiment where contemporary lagoons transition into infrequently inundated areas, salt crusts, and lastly dry exposed paleoterraces. These surface features represent different evolutionary stages in the transition from previously wetter climatic conditions to much drier conditions today. These same stages closely mirror the climate transition on Mars from a wetter early Noachian to the Noachian/Hesperian. Salar de Pajonales thus provides a unique window into what the last near-surface oases for microbial life on Mars could have been like in hypersaline environments as the climate changed and water disappeared from the surface. Here we open that climatological window by evaluating the narrative recorded in the salar surface morphology and microenvironments and extrapolating to similar paleosettings on Mars. Our observations suggest a strong inter-dependence between small and large scale features that we interpret to be controlled by extrabasinal changes in environmental conditions, such as precipitation-evaporation-balance changes and thermal cycles, and most importantly, by internal processes, such as hydration/dehydration, efflorescence/deliquescence, and recrystallization brought about by physical and chemical processes related to changes in groundwater recharge and volcanic processes. Surface structures and textures record a history of hydrological changes that impact the mineralogy and volume of Ca-sulfate layers comprising most of the salar surface. Similar surface features on Mars, interpreted as products of freeze-thaw cycles, could, instead, be products of water-driven, volume changes in salt deposits. On Mars, surface manifestations of such salt-related processes would point to potential water sources. Because hygroscopic salts have been invoked as sources of localized, transient water sufficient to support terrestrial life, such structures might be good targets for biosignature exploration on Mars.

In extreme environments, microbial organisms reside in pockets with locally habitable conditions. Micro-climates conducive to the persistence of life in an otherwise inhospitable environment—“refugia”—are spatially restricted and can be micro- to centimeters in extent. If martian microbes are preserved in fossil refugia, this presents a double-edged sword for biosignature exploration: these locations will be specific and targetable but small and difficult to find. To better understand what types of features could be refugia in martian salt-encrusted basins, we explore a case study of two terrestrial habitats in salt-encrusted paleo-lake basins (salars): Salar Grande (SG) in the Atacama Desert and Salar de Pajonales (SdP) in the Altiplano Puna plateau of Chile. We review the formation of salt constructs within SG and SdP, which are the features that serve as refugia in those salars, and we explore the connection between the formation of salt constructs at the local scale with the larger-scale geologic phenomena that enable their formation. Our evaluation of terrestrial salars informs an assessment of which chloride basins on Mars might have had a high potential to form life-hosting salt constructs and may preserve biosignatures, or even host extant life. Our survey of martian salars identifies 102 salars in regions with a geographic context conducive to the formation of salt constructs, of which 17 have HiRISE coverage. We investigate these 17 martian salars with HiRISE coverage and locate the presence of possible salt constructs in 16 of them. Salt constructs are features that have may have been continuously habitable for the past ~3.8 Byr, have exceptional preservation potential, and are accessible by robotic exploration. Future work could explore in detail the mechanisms involved in the formation of the topographic features we identified in salt-encrusted basins on Mars to test the hypothesis that they are salt constructs.

<p>Introduction</p> <p>In environments where it is difficult for life to function, microbial organisms tend to inhabit pockets of locally favorable climatic conditions. Micro-climates conducive to the persistence of life in an otherwise inhospitable environment – “refugia” – are spatially restricted and can be < centimeters in extent [1], [2]. Refugia may have been (and perhaps still are) perennially prevalent on Mars where conditions were likely never globally favorable to life for sustained periods of time [3]. The tendency for refugia to be small means that it may be difficult to locate features that could have served (or perhaps still do serve) as refugia for microorganisms on Mars. The spatial distribution of refugia in extreme environments across larger geographic extents is often non-random and may depend on many factors, biotic and abiotic [4]–[6]. Understanding patterns that refugia follow across larger geographic contexts as well as particular geologic phenomena (e.g., volcanic vents, dikes, stress fields) that are commonly associated with refugia may provide a way to infer regions of astrobiological interest, even if the specific, small, habitable patches (refugia) are below the resolving power of orbital instruments [6]. Here, we explore a case study of two terrestrial habitats in salt-encrusted paleo-lake basins (salars) in the Atacama and Altiplano of Chile to assess their characteristics and what factors are common between them. The Neogene salars of the Atacama and Altiplano are perhaps the best analogs on Earth for the Noachian/Hesperian salt-encrusted paleo-lakes of Mars [4], [7]–[12].</p> <p>Evaporite habitats at Salar Grande and Salar de Pajonales</p> <p>Salar Grande hosts decimeter scale nodules made of halite that serve as refugia for endolithic microbes [9]. [1] proposed a model to describe the evolution of nodules in halite-encrusted salars at the edges of polygonal features. To briefly summarize their model, halite nodules initiate at polygon edges in a salar with active ground water. Growth continues after ground water activity ceases as winds drive a moisture gradient, along which brines travel, toward the apex of the relatively higher relief nodules where more halite is deposited (Fig 1). The action of brines in halite nodules generates porosity at multiple spatial scales (nanometers to millimeters), contributing positively to their habitability [1].</p> <p><img src="" alt="" /></p> <p><strong><em>Fig. 1 Halite nodules at Salar Grande. </em></strong><em>A) Drone-view of nodules and nodule clusters. Humans for scale. B) Close up of halite nodule showing endolithic community. C) Halite nodule evolution from </em><em>[1]</em><em>.</em></p> <p>Like Salar Grande, Salar de Pajonales hosts endoliths in refugia habitats. In a gypsum-covered region of Salar de Pajonales, alabaster (a high-porosity polymorph of gypsum) is the most reliable indicator for the presence of life [4]. Alabaster refugia are most commonly found associated with decimeter- to meter-tall ridges and domes [6], [10]. The domes and ridges form via water-related processes: hydration/dehydration cycles, volume changes associated with mineral precipitation from brines, and/or efflorescence deliquescence [10]. The formation of alabaster is likely predicated on the action of the near-surface water that drives the formation of ridges and domes (Fig. 2), though microbial activity may play a role as well [4]. Therefore, at Salar de Pajonales water activity generates positive topographic salt constructs and physiochemical changes to gypsum (formation of high-porosity alabaster) that foster an environment favorable to life.</p> <p><img src="" alt="" /></p> <p><strong><em>Fig. 2 Models of ridge and dome formation at SdP.</em></strong><em> A) Drone-perspective view of ridges at SdP. B) Model for ridge formation from </em><em>[10]</em><em> involving volume change at the phreatic-vadose zone interface. C) Image of domes at SdP in different stages of development. D) Possible model of gyspsum dome formation from </em><em>[16]</em><em>. </em></p> <p>Discussion</p> <p>            Across two salt-encrusted environments, one in the Atacama and the other in the Altiplano, with distinct evaporite mineralogy (halite versus gypsum), the activity of water resulted in decimeter- to meter-tall topographic constructs with nanometer- to millimeter-scale porosity conducive to the persistence of endoliths. We hypothesize that decimeter- to meter-tall topographic constructs (as opposed to erosional remnants or boulders) may be general indicators for relatively enhanced habitability in salt-encrusted paleo-lake basins because they require water to form. Although refugia – such as the precise location of endoliths in halite nodules or alabaster in gypsum domes and ridges – may not be observable from orbit, decimeter- to meter-scale salt constructs may be possible to identify with HiRISE or future orbital imagers with higher resolving power [6]. Chloride basins should be the targets of high-resolution imaging campaigns and efforts should be made to distinguish salt constructs from erosional remnants, boulders, and other relative topographic highs with which they could be confused. Salt constructs may be one of the few features, other than (fossil) hydrothermal vents, that have a high potential to both host and preserve microbial organisms, and that are specific targets, possibly identifiable from orbit, to which a rover could be driven. These characteristics make them attractive targets for future missions to Mars.</p> <p> </p> <p>Reference:</p> <p>[1]       O. Artieda <em>et al.</em>, 2015, doi: 10.1002/esp.3771.</p> <p>[2]       L. Hays, “NASA Astrobiology Strategy.” 2015.</p> <p>[3]       R. Wordsworth <em>et al.</em>, 2021, doi: 10.1038/s41561-021-00701-8.</p> <p>[4]       K. Warren-Rhodes <em>et al.</em>, <em>Nature Astronomy</em>. in review.</p> <p>[5]       M. S. Phillips <em>et al.</em>, <em>Astrobiology</em>, in review.</p> <p>[6]       K. A. Warren-Rhodes <em>et al.</em>, 2019. doi: 10.3389/fmicb.2019.00069.</p> <p>[7]       M. M. Osterloo, et al., 2010, doi: 10.1029/2010JE003613.</p> <p>[8]       T. D. Glotch, et al., 2016, doi: 10.1002/2015JE004921.</p> <p>[9]       A. F. Davila <em>et al.</em>, 2008, doi: 10.1029/2007JG000561.</p> <p>[10]     N. W. Hinman <em>et al.</em>, 2022, https://www.frontiersin.org/article/10.3389/fspas.2021.797591</p> <p>[11]     N. A. Cabrol <em>et al.</em>, 2007, doi: 10.1029/2006JG000298.</p> <p>[12]     E. K. Leask and B. L. Ehlmann, 2022, doi: 10.1029/2021AV000534.</p> <p>[13]     A. Szynkiewicz, et al., JGR, vol. 115, 2010.</p>