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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]

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]