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Abstract Liquid water is a critical component of habitability. However, the production and stability of surficial liquid water can be challenging on planets outside the Habitable Zone and devoid of adequate greenhouse warming. On such cold, icy exo-Earths, basal melting of regional/global ice sheets by geothermal heat provides an alternative means of forming liquid water. Here, we model the thermophysical evolution of ice sheets to ascertain the geophysical conditions that allow liquid water to be produced and maintained at temperatures above the pressure-controlled freezing point of water ice on exo-Earths. We show that even with a modest, Moon-like geothermal heat flow, subglacial oceans of liquid water can form at the base of and within the ice sheets on exo-Earths. Furthermore, subglacial oceans may persist on exo-Earths for a prolonged period due to the billion-year half-lives of heat-producing elements responsible for geothermal heat. These subglacial oceans, often in contact with the planet’s crust and shielded from the high energy radiation of their parent star by thick ice layers, may provide habitable conditions for an extended period.

Based on the analysis of chemical maps of Thorium and Potassium derived in the Eridania region on Mars, the authors show how radiogenic heat driven hydrothermal systems may have persisted on Mars.

Constraining the onset of dynamic recrystallization (DRX) and its effects on the mechanical properties of firn is crucial for firn densification modeling. To that end, samples from a depth of 13 m in a Summit, Greenland (72°35′N, 38°25′W) firn core were subjected to creep tests at −14°C and 0.21 MPa compressive stress to strains of 7%, 12%, 18%, and 29%. Microstructural analyses using thin‐section imaging and microcomputed x‐ray tomography (micro‐CT) revealed smaller grain sizes, reduced specific surface area and connectivity, and increased density in relation to reduced porosity as the strain increases. These results show that DRX occurs in firn under creep, with strain‐induced boundary migration (SIBM) and nucleation and growth starting at ∼7%. DRX leads to elongated grains, reduced grain size, and the development of a preferred crystallographic orientation, indicating that DRX occurs by both SIBM and nucleation and growth. Plain Language Summary: Firn is multi‐year snow that undergoes densification due to the load from the snow overburden and from sintering. Understanding firn densification is important for interpreting ice core records, predicting ice sheet mass balance and elevation changes, and studying climate change effects. Previous densification models focused on accumulation rate and temperature, overlooking the role of recrystallization. To address this gap, compression tests were performed on Greenland firn samples from a depth of 13 m. The deformation resulted in reduced median grain size, preferred crystallographic orientation, and increased density. Our findings indicate that dynamic recrystallization starts when the firn is subjected to a strain of about 7% through boundary migration of old grains, around which new stress‐free grains also start to form. Key Points: Dynamic Recrystallization occurs in firn through strain‐induced boundary migration, and nucleation and growthAverage grain size in firn decreases under constant temperature and compressive stress in firn [ABSTRACT FROM AUTHOR]

Modern and ancient hypersaline brines have been identified across the solar system, but the habitability of these environments remains unknown. Here, we evaluate organic matter (OM) production in MgSO4 and Na2CO3 rich hypersaline lakes whose chemistries resemble deposits on Mars such as those identified in Jezero crater. We focus our analysis on lipid biomarkers including fatty acids, alkanes, and ether‐bound lipids in modern brines, salt deposits, and surface sediments. We also report total organic carbon (TOC), carbon/nitrogen (C/N) ratios, and bulk OC (δ13C and δ15N) isotopes to contextualize the lipid biomarker data. In all lakes, the predominant biosignatures include midchain (12 < C < 23) fatty acids and alkanes suggesting microbial origin. Sediments also incorporate a greater diversity of lipids. Ether‐bound lipids derived from archaea and bacteria constitute a minor but measurable fraction of the lipids. This result contrasts with typical findings from other studies with NaCl brines which contain significant archeal biomass. TOC concentrations in all sediments are high, ranging from 0.7% to 12% with sulfate‐rich sediments having the highest concentrations. The isotopic and elemental compositions of TOC corroborate the biomarker results, showing δ13C values and C/N values indicative of aquatic microbial origin. This richness of organic matter and in situ microbial biosignatures differ from previously studied Cl‐dominated Mars‐analog sites which have shown limited OM production and preservation and acidic SO4‐rich hypersaline environments which were dominated by terrestrial inputs. Overall, our results suggest that MgSO4‐rich hypersaline environments are conducive for life and harbor a rich microbial biomarker landscape. Plain Language Summary: The minerals found on Mars suggest that it had abundant hypersaline waters early in its history. However, liquid water no longer persists on its surface. As such, we must use environments on Earth that are similar to Mars and other astrobiological targets to better understand the potential for life elsewhere. Here, we examine fatty molecules and organic carbon from hypersaline lakes on Earth that resemble water compositions thought to have been present on early Mars. We also use bulk carbon isotopic data to help inform the results from our biomarker analysis. Overall, data from these sites shows that circumneutral SO4‐rich environments are conducive for life and harbor a suite of microbial biosignatures. In comparison to other Martian‐analog environments on Earth, this study shows that circumneutral SO4‐dominated lakes produce in situ biological signatures in abundance. Key Points: SO4‐dominated hypersaline lakes produce more abundant organic matter (OM) than a Na2CO3‐dominated site of comparable salinityThe circumneutral MgSO4‐dominated hypersaline lakes studied here harbor abundant microbial biosignatures compared to other Martian analogsLipid biosignatures produced in situ predominate within sedimentary OM rather than biosignatures produced by surrounding vegetation [ABSTRACT FROM AUTHOR]

Non‐ice impurities within the ice shells of ocean worlds (e.g., Europa, Enceladus, Titan, Ganymede) are believed to play a fundamental role in their geophysics and habitability and may become a surface expression of subsurface ocean properties. Heterogeneous entrainment and distribution of impurities within planetary ice shells have been proposed as mechanisms that can drive ice shell overturns, generate diverse geological features, and facilitate ocean‐surface material transport critical for maintaining a habitable subsurface ocean. However, current models of ice shell composition suggest that impurity rejection at the ice‐ocean interface of thick contemporary ice shells will be exceptionally efficient, resulting in relatively pure, homogeneous ice. As such, additional mechanisms capable of facilitating enhanced and heterogeneous impurity entrainment are needed to reconcile the observed physicochemical diversity of planetary ice shells. Here we investigate the potential for hydrologic features within planetary ice shells (sills and basal fractures), and the unique freezing geometries they promote, to provide such a mechanism. By simulating the two‐dimensional thermal and physicochemical evolution of these hydrological features as they solidify, we demonstrate that bottom‐up solidification at sill floors and horizontal solidification at fracture walls generate distinct ice compositions and provide mechanisms for both enhanced and heterogeneous impurity entrainment. We compare our results with magmatic and metallurgic analogs that exhibit similar micro‐ and macroscale chemical zonation patterns during solidification. Our results suggest variations in ice‐ocean/brine interface geometry could play a fundamental role in introducing compositional heterogeneities into planetary ice shells and cryoconcentrating impurities in (re)frozen hydrologic features. Plain Language Summary: The ice shells of ocean worlds are not pure water ice but contain significant amounts of salts and other ocean‐derived impurities. These impurities are believed to play an important role in the dynamics, evolution, and habitability of ice‐ocean worlds. Additionally, observations of salts on the surfaces of these moons can inform us of their underlying ocean properties. That said, salts are not easily entrained into ice under the thermal and geometric (vertically downward freezing) conditions present at the ice‐ocean interfaces of these ice shells, leading to relatively pure ice, and necessitating an explanation for how the compositional variations observed in ice shells are introduced. Here we explore a possible solution: the freezing of water bodies within ice shells that have surfaces that freeze vertically upward (floors) and horizontally (walls). We find that these freezing geometries facilitate enhanced and spatially variable salt entrainment. Our results suggest that solidifying water bodies within ice shells could play a key role in explaining the compositional diversity observed on ice‐ocean world surfaces and that constraining the dynamics that govern these ice‐brine systems will be critical in linking spacecraft measurements of planetary ice compositions to the properties of subsurface water reservoirs. Key Points: When a brine freezes, the direction of solidification affects the structure and composition of the resulting iceIce formation at sill floors and fracture walls provides a mechanism for heterogeneous and amplified impurity entrainment in ice shellsCryoconcentration of impurities in freezing hydrological features can impact ice shell material properties, geophysics, and habitability [ABSTRACT FROM AUTHOR]

Liquid water is a critical component of habitability. However, the production and stability of surficial liquid water can be challenging on planets outside the Habitable Zone and devoid of adequate greenhouse warming. On such cold, icy exo-Earths, basal melting of regional/global ice sheets by geothermal heat provides an alternative means of forming liquid water. Here, we model the thermophysical evolution of ice sheets to ascertain the geophysical conditions that allow liquid water to be produced and maintained at temperatures above the pressure-controlled freezing point of water ice on exo-Earths. We show that even with a modest, Moon-like geothermal heat flow, subglacial oceans of liquid water can form at the base of and within the ice sheets on exo-Earths. Furthermore, subglacial oceans may persist on exo-Earths for a prolonged period due to the billion-year half-lives of heat-producing elements responsible for geothermal heat. These subglacial oceans, often in contact with the planet's crust and shielded from the high energy radiation of their parent star by thick ice layers, may provide habitable conditions for an extended period. Liquid water is key for life as we know it. Here, the authors show even with a modest geothermal heat flow, subglacial oceans of liquid water can form at the base of and within the ice sheets on exo-Earths, which may provide habitable conditions for an extended period. [ABSTRACT FROM AUTHOR]

In this study, we report the results of an active-source seismology and ground-penetrating radar survey performed in northwestern Greenland at a site where the presence of a subglacial lake beneath the accumulation area has previously been proposed. Both seismic and radar results show a flat reflector approximately 830–845 m below the surface, with a seismic reflection coefficient of -0.43 ± 0.17, which is consistent with the acoustic impedance contrast between a layer of water and glacial ice. Additionally, in the seismic data we observe an intermittent lake bottom reflection arriving between 14–20 ms after the lake top reflection, corresponding to a lake depth of approximately 10–15 m. A strong coda following the lake top and lake bottom reflections is consistent with a package of lake bottom sediments although its thickness and material properties are uncertain. Finally, we use these results to conduct a first-order assessment of the lake origins using a one-dimensional thermal model and hydropotential modeling based on published surface and bed topography. Using these analyses, we narrow the lake origin hypotheses to either anomalously high geothermal flux or hypersalinity due to local ancient evaporite. Because the origins are still unclear, this site provides an intriguing opportunity for the first in situ sampling of a subglacial lake in Greenland, which could better constrain mechanisms of subglacial lake formation, evolution, and relative importance to glacial hydrology. [ABSTRACT FROM AUTHOR]

Heat flow measurements are important for our understanding of planetary interior composition, structure, and evolution. In the absence of direct measurement, a first‐order estimate of a planet's interior heat flow can be made by modeling the lithosphere's viscoelastic response to stress exerted by large surface loads. Here, we model the Martian lithosphere's viscoelastic response to the south polar layered deposits and estimate the local mantle heat flow to be less than ∼10 mW/m2. Combined with our previous estimate of the low mantle heat flow from the north polar region (∼7 mW/m2), our results suggest that the Martian mantle may be globally depleted in heat‐producing elements. The relatively low mantle heat flow has significant implications for Mars' long‐term thermal evolution and on the possibility of basal melting in the south polar region. Plain Language Summary: The outermost layer of a planet is called the lithosphere. The lithosphere can bend due to the weight of large mountains or thick glaciers. The total amount of the bending depends on the stress imparted by topographic loads and strength or thickness of the lithosphere, which depends on the planet's interior heat. Despite the huge amount of stress exerted by the south polar cap of mars, the lithosphere underneath is remarkably flat. In this work, we model the lithospheric bending using numerical methods and provide a first‐order estimate of the heat being generated in the Martian mantle. Our results suggest that the lithosphere in the southern polar region is thick and that Mars' interior is relatively cool. Key Points: We estimate the mantle heat flow in Mars's south polar region to not exceed 10 mW/m2The low mantle heat flow estimate suggests the Martian mantle to be depleted in heat‐producing elementsBased on our heat flow constraints, present‐day basal melting of the south polar layered deposits is highly unlikely [ABSTRACT FROM AUTHOR]

Exploration of the furthest reaches of our planet, as well as other planetary bodies, typically requires the use of robotic platforms due to the extreme environments encountered. Some of the harshest conditions on earth are found in Antarctica and require the use of autonomous underwater vehicles (AUVs) to explore remote and hazardous areas beneath the ice. The custom-built Icefin under-ice unmanned underwater vehicle (UUV) has been developed for deployment in permanently ice-covered oceans, such as those found in Antarctica, with the intent of furthering relevant technology for future missions to Europa, a moon of Jupiter with an icecovered ocean. The design of the vehicle flows from program requirements that provide a road map for maximizing scientific data collection with the low-risk and low-logistical impact needed for polar science. Lessons learned from Antarctic deployment of the Icefin vehicle can be extrapolated for future polar AUV design. [ABSTRACT FROM AUTHOR]