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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]

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]