Glacial isostatic adjustment on Mars
Authors: Broquet Adrien , Plesa Ana-Catalina , Breuer Doris , Wieczorek Mark A. , Root Bart C. , Klemann Volker , Genova Antonio , Andrews-Hanna Jeffrey C. 
Published: July 2024

Introduction:&#160; The response of a planet to loading is intimately linked to its interior structure. On Earth, studying the time-variable response of the lithosphere to the growth and decay of large ice sheets, or glacial isostatic adjustment (GIA), has been the standard approach to constrain the mantle structure [1]. However, such models have rarely been applied to the other terrestrial planets because of a lack of observational data [2].Mars harbors two geologically young (60&#176;N) is discretized into 68 constant-thickness layers with given viscosity, density, and rigidity, from the core to the surface (Fig. 2). The time evolution of the deformation for these models is predicted using ALMA and the polar cap load potential [5,7]. Our model assumes that the ice cap formed instantly at a given time in Martian history. We also estimate the rate of deformation and the associated time-variable degree-2 zonal gravity coefficient (J2). A model is deemed acceptable if the predicted viscoelastic deformation matches radar observations [5].Results and conclusions: &#160;The present-day interior structure beneath the northern cap shows mantle viscosities ranging from 1021 to 1024 Pa s (Fig. 2). Given the large interior viscosity, the equilibrium response of the interior to loading is achieved over long-timescales (>100 Myr). The north polar cap is young (150 km), and large mantle viscosity (>1022 Pa s). A thick crust is consistent with the independent reassessment of InSight seismic data [16]. Models with a higher crustal heat production cool faster, have higher mantle viscosities, slower interior response to loading and thus better fit radar observations. For accepted models, GIA in the polar regions show rates >10-3 mm/year and strain rates >10-20 s-1.Our analysis further constrains the bulk of the northern cap to have formed no more than 7 Myr ago, which is in remarkable agreement with independent Global Climate Model [15]. Models with a linear ice cap formation would imply even younger ages. If the mantle viscosity were precisely known, our framework could be used to place constraints on the formation history of the ice cap.GIA affects the long-wavelength gravity field. Over the last 20 years, J2 may have varied by up to 10-11 m s-2. Together, our results implies that GIA could be measured from precise laser ranging or with a dedicated GRAIL-like gravity mission to Mars [17,18]. Alternatively, reanalyses and extended collection of radio tracking data could enable discerning a trend in the gravity field.References: [1] Peltier W. R. (1974). [2] Greve R., et al. (2003) [3] Byrne S. (2009). [4] Phillips R. et al. (2008). [5] Broquet A. et al. (2020). [6] Broquet A. (2020) AB-Ares/Te_HF_Conversion [7] Broquet A. et al. (2021) JGR:Planets. [8] Khan A. et al. (2021). [9] Plesa A.-C. et al. (2022). [10] Voigt J. R. C. et al. (2023). [11] Broquet A. &amp; Andrews-Hanna J. C. (2023). [12] Wieczorek M. A. et al. (2022). [13] Hahn B. C. et al. (2011). [14] Spada G. (2008). [15] Levrard B. et al. (2007). [16] Kim D. et al. (2023). [17] Genova A. (2020). [18] W&#246;rner L. et al. (2023).