The Composition of the South Polar Cap of Mars Derived From Orbital Data

다국어 초록 (Multilingual Abstract)

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.
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.



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 patterns

The 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%


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 patterns
The 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%