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.

The goals of Mars exploration are evolving beyond describing environmental habitability at global and regional scales to targeting specific locations for biosignature detection, sample return, and eventual human exploration. An increase in the specificity of scientific goals—from follow the water to find the biosignatures —requires parallel developments in strategies that translate terrestrial Mars-analog research into confident identification of rover-explorable targets on Mars. Precisely how to integrate terrestrial, ground-based analyses with orbital data sets and transfer those lessons into rover-relevant search strategies for biosignatures on Mars remains an open challenge. Here, leveraging small Unmanned Aerial System (sUAS) technology and state-of-the-art fully convolutional neural networks for pixel-wise classification, we present an end-to-end methodology that applies Deep Learning to map geomorphologic units and quantify feature identification confidence. We used this method to assess the identification confidence of rover-explorable habitats in the Mars-analog Salar de Pajonales over a range of spatial resolutions and found that spatial resolutions two times better than are available from Mars would be necessary to identify habitats in this study at the 1-σ (85%) confidence level. The approach we present could be used to compare the identifiability of habitats across Mars-analog environments and focus Mars exploration from the scale of regional habitability to the scale of specific habitats. Our methods could also be adapted to map dome- and ridge-like features on the surface of Mars to further understand their origin and astrobiological potential.

Understanding when and how evolved rocks formed on terrestrial planets remains a fundamental question in planetary geology. Mars preserves ancient surface rocks, providing unique opportunities to study early crustal formation. Here we report extensive detections of feldspathic, possibly anorthositic, rocks uplifted by the Argyre basin-forming impact using orbital remote sensing. The radial distribution of these feldspathic outcrops indicates they originated from the lower martian crust, positioned stratigraphically between olivine-bearing materials and low-calcium pyroxene-bearing upper crustal rocks. This stratigraphy resembles observations at the Hellas basin, suggesting feldspar-rich rocks are widespread in the lower crust. The spatial association of plagioclase-rich outcrops with pyroxene and olivine-bearing units resembles layered mafic intrusions on Earth, suggesting large-scale melting early in Mars’s history produced differentiated igneous complexes. These observations offer direct evidence to support and refine existing models of early crust formation, addressing key questions regarding crustal properties and formation that have lacked widespread observational constraints.

Bright geological formations called hollows were observed at Mercury and are mostly found in impact craters. While their composition is unknown, subsurface sulfur could be responsible for their formation. Mercury's porous surface favors the subsurface diffusion of returning exospheric atoms. With time, diffusing atoms could form reservoirs in Mercury's subsurface. We use a 3‐D Exospheric Global Model (EGM) including subsurface diffusion to study the evolution of sulfur and sodium in Mercury's subsurface. We identify similar spatial distributions of the sulfur reservoir and of the hollows, but no relationship in the case of sodium. EGM predicts that the longitudinal distribution of hollows correlates with the peak subsurface S column density, ∼60° west of each “hot longitude.” Deeper reservoirs at Mercury's hot poles are predicted, potentially explaining the larger hollows found in these regions. The subsurface diffusion could continuously replenish the sulfur reservoir that could be responsible for the formation of hollows.