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The flexure of the lithosphere under stresses imposed by the geologically young south polar cap is one of the few clues we have regarding the south polar cap composition and the present‐day thermal state of Mars. Here, we combine radar, gravity, and topography data with a flexural loading model to estimate the bulk density (ρ) and average real dielectric constant (ε′) of the south polar cap, and the elastic thickness of the lithosphere (Te). Given the uncertainties of the data, our results constrain ρ to be 1,100–1,300 kg m−3 (best fit of 1,220 kg m−3), ε′ to be 2.5–3.4 (best fit of 3.3), and Te to be greater than 150 km (best fit of 360 km). Based on these results, the maximum lithospheric flexure is 770 m, and the polar cap volume could be up to 26% larger than previous estimates that did not account for lithospheric flexure. Our inferred compositions imply that the dust concentration would be at least 9 vol% if the CO2 ice content were negligible, and that the CO2 ice concentration would be more than the known 1 vol% CO2 if the dust concentration were less than 9 vol%. The 1‐σ lower limit on Te implies a surface heat flow that is less than 23.5 mW m−2. This lower limit is significantly less than the range of acceptable values at the north pole (330–450 km, heat flow of 11–16 mW m−2), and helps satisfy global thermal evolution simulations that predict hemispheric differences in surface heat flow. Plain Language Summary: The south polar cap of Mars is a tremendous reservoir of ices. It is transparent to radar wavelengths allowing us to probe its structure and composition. Here, we combine radar, gravity, and topography data with a flexural loading model to invert for the south polar cap composition and elastic thickness of the underlying lithosphere. We find that the lithosphere below the south pole is cold and strong, but with a rigidity that is potentially lower than at the north pole. The allowed range of lithospheric rigidities for the northern and southern lithospheres is compatible with hemispheric differences in heat flow of the planet predicted by thermal evolution models. The polar cap volume could be up to 26% higher than previous estimates that did not account for lithospheric flexure, which represents a significant quantity of trapped volatiles. Such as for the north pole, the south polar deposits are made mostly of water ice, and may have sequestered more CO2 ice than has currently been detected depending on the assumed dust content. An independent estimate on the dust content would allow placing crucial constraints on the amount of CO2 trapped in the south polar deposits. Key Points: Radar and gravity data are used to invert for the south polar cap composition and elastic thickness of the underlying lithosphere (Te)Te lower than at the north pole are allowed, which helps satisfy thermal evolution models that predict hemispheric heat flow patternsThe polar cap is made mostly of H2O and has more than the known 1 vol% CO2 if the dust concentration is less than 9 vol% [ABSTRACT FROM AUTHOR]

The geodynamical response of the lithosphere under stresses imposed by the geologically young north polar cap is one of the few clues we have to constrain both the polar cap composition and the present‐day thermal state of Mars. Here we combine radar data with a flexural loading model to self‐consistently estimate the density (ρ) and real dielectric constant (ε′) of the polar cap, and the elastic thickness of the lithosphere underneath (Te). Our results show that ρ ranges from 920 to 1,520 kg m −3, ε′ is 2.75 (+0.40, −0.35), and Te is between 330 and 450 km. We determine a polar cap volume that is up to 30% larger than current estimates that all neglect lithospheric flexure. Our inferred compositions suggest that, for dust content larger than about 6 vol%, 10 vol% CO 2 ice is mixed within the polar deposits, which has important implications for the climate evolution of Mars. Plain Language Summary: The north polar cap of Mars is a tremendous reservoir of ices and dust of unknown concentration and composition. It is transparent to radar giving us a unique insight into its structure and composition. Here we use a novel technique that combines radar and elevation data along with a flexure model, to invert for the polar cap composition and the strength of the underlying lithosphere. Similar to previous studies, we find that the lithosphere below the north pole is extremely rigid and does not deform much under the load of the polar cap. This implies that the north polar region is currently colder than the rest of the planet, which has profound implications for our understanding of the structure and evolution of the Martian interior. Our inferred compositions suggest that for reasonable dust contents, about 10 vol% CO 2 ice is mixed within the polar deposits. This is the first time a large quantity of CO 2 ice is constrained to exist in the north polar cap. Like on Earth, where the composition of buried ices gives hints on the climatic evolution, having CO 2 ice at the north pole of Mars will help improve scenarios for the climate evolution of the planet. Key Points: Radar data and a flexural loading model are used to probe the composition of the north polar cap of Mars and the strength of the lithosphereThe elastic thickness beneath the north polar cap is found to be between 330 and 450 km, implying regional heat flows of 11 to 16 mW m −2For reasonable dust contents, large quantities of CO 2 ice must be mixed within the polar cap to be consistent with our inferred compositions [ABSTRACT FROM AUTHOR]

The surface of Europa is heavily irradiated by high-energy particles with the energy ranges of 1–100 MeV for primary electrons and ∼10 keV for secondary electrons. These irradiations may cause oxidation of surface materials, such as chlorides and sulfur-bearing species, through reactions with oxidative radicals formed by dissociation of H 2 O ice. Similar irradiation-induced oxidation could have occurred in near-surface ice on Mars. However, most of previous experiments simulated ∼10 keV electron irradiation onto ice materials on Europa and Mars, and few studies have investigated the oxidation of chlorides and sulfur by MeV electron irradiation. Here we performed laboratory experiments on MeV electron irradiation, as well as 10 keV electron and UV irradiations, onto mixtures of H 2 O ice and chlorides, or elemental sulfur, at low temperatures. Our experimental results indicate selective oxidation of sulfur against chlorides, with negligible formation of oxychlorides on irradiation of H 2 O ice and chloride mixtures, regardless of experimental conditions (temperature, electron energy, or presence of oxidants in ice). Sulfate is effectively formed by the electron irradiation. This selective oxidation may be caused by the different stabilities of atoms within irradiated H 2 O ice. If sulfate on Europa's surface is transported into the subsurface ocean, this could provide energy sources for possible chemoautotrophic life in the ocean. On Europa, sulfur would be one of key elements in redox chemistry on the surface and possibly in the subsurface ocean. • Laboratory experiments on irradiation-induced oxidation on planetary surface. • MeV and 10 keV electron irradiation onto mixtures of H 2 O ice and chlorides or sulfur. • Selective oxidation of sulfur against chlorides, with negligible formation of oxychlorides. • Sulfur would be one of key elements in redox chemistry on Europa. [ABSTRACT FROM AUTHOR]