Glacial Isostatic Adjustment Reveals Mars’ Interior Viscosity Structure

Abstract

Investigating glacial isostatic adjustment has been the standard method to decipher our Earth’s interior viscosity structure [1], but such approach has been rarely applied to other planets because of lack of observational data [2].

The north polar cap of Mars is the only million years old surface feature that can induce measurable surface deformation on this planet, thereby holding clues to its present-day internal viscosity structure. Orbital radar sounders have mapped the Martian ice caps, and the lack of measurable lithospheric flexure beneath the north polar deposits (Fig. 1) was used to infer the planet’s interior to be cold with a thick elastic lithosphere [3,4]. While the results of these studies are used as constraints on the present-day thermal state of Mars’ interior [5], geodynamic evolution models [6] struggle to explain the cold and thick lithosphere inferred at the north pole and, at the same time, the planet’s young volcanism [7] and ongoing plume activity [8].

Geologic observations and global climate models suggest the north polar cap is only a few Myr old, but the exact age remains uncertain [9]. Due to this young age, calculations suggested that an ongoing viscoelastic relaxation could result in estimating thinner elastic lithospheres beneath the north polar cap [3,4]. However, in those models a constant mantle viscosity was assumed, the ice loading history was neglected, and the load was represented by one single wavelength. As a result, limited insights into the possible effects of viscoelastic relaxation were provided. Here, we investigate the status of the north polar cap by combining thermal evolution and viscoelastic deformation models with radar observations. We show that downward motion of the north polar region is ongoing and can be further constrained by analyses of the time-variable gravity field and InSight seismicity.