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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]

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]