¹A. Khan,²C. Liebske,¹A. Rozel,³A. Rivoldini,⁴F. Nimmo,²J. A. D. Connolly,⁵A.-C. Plesa,¹D. Giardini
Journal of Geophysical Research, Planets (in Press) Link to Article [DOI: 10.1002/2017JE005371]
¹Institute of Geophysics, ETH Zürich, Switzerland
²Institute of Geochemistry and Petrology, ETH Zürich, Switzerland
³Royal Observatory of Belgium, Brussels, Belgium
⁴Department of Earth and Planetary Sciences, UC Santa Cruz, California, USA
⁵German Aerospace Center (DLR), Berlin, Germany
Published by arrangement with John Wiley & Sons

We invert the Martian tidal response and mean mass and moment of inertia for chemical composition, thermal state, and interior structure. The inversion combines phase equilibrium computations with a laboratory-based viscoelastic dissipation model. The rheological model, which is based on measurements of anhydrous and melt-free olivine, is both temperature and grain size sensitive and imposes strong constraints on interior structure. The bottom of the lithosphere, defined as the location where the conductive geotherm meets the mantle adiabat, occurs deep within the upper mantle (∼250–500 km depth) resulting in apparent upper mantle low-velocity zones. Assuming an Fe-FeS core, our results indicate: 1) a Mantle with a Mg# (molar Mg/Mg+Fe) of ∼0.75 in agreement with earlier geochemical estimates based on analysis of Martian meteorites; 2) absence of bridgmanite- and ferropericlase-dominated basal layer; 3) core compositions (13.5–16 wt% S), core radii (1640–1740 km), and core-mantle-boundary temperatures (1560–1660 ∘ C) that, together with the eutectic-like core compositions, suggest the core is liquid; and 4) bulk Martian compositions that are overall chondritic with a Fe/Si (wt ratio) of 1.63–1.68. We show that the inversion results can be used in tandem with geodynamic simulations to identify plausible geodynamic scenarios and parameters. Specifically, we find that the inversion results are reproduced by stagnant lid convection models for a range of initial viscosities (∼1019–1020 Pa·s) and radioactive element partitioning between crust and mantle around 0.001. The geodynamic models predict a mean surface heat flow between 15–25 mW/m2.

¹Jean-Christophe Viennet,¹Benjamin Bultel,²Lucie Riu,¹Stephanie C. Werner
Journal of Geophysical Research, Planets (in Press) Link to Article [DOI: 10.1002/2017JE005343]
¹Centre for Earth Evolution and Dynamics, Department for Geosciences, University of Oslo, Norway
²Institut d’Astrophysique Spatiale, Université Paris-Sud, Orsay, France
Published by arrangement with John Wiley & Sons

Widespread occurrence of Fe,Mg-phyllosilicates have been observed on Noachian Martian terrains. Therefore, the study of Fe,Mg-phyllosilicates formation, in order to characterize early Martian environmental conditions, is of particular interest to the Martian community. Previous studies have shown that the investigation of Fe,Mg-smectite formation alone helps to describe early Mars environmental conditions, but there are still large uncertainties in terms of pH range, oxic/anoxic conditions, etc… Interestingly, carbonates and/or zeolites have also been observed on Noachian surfaces in association with the Fe,Mg-phyllosilicates.

Consequently, the present study focuses on the di/trioctahedral phyllosilicate/carbonate/zeolite formation as a function of various CO2 contents (100% N2, 10% CO2 / 90% N2, 100% CO2), from a combined approach including closed system laboratory experiments for 3 weeks at 120°C and geochemical simulations. The experimental results show that as the CO2 content decreases, the amount of dioctahedral clay minerals decreases in favour of trioctahedral minerals. Carbonates and dioctahedral clay minerals are formed during the experiments with CO2. When Ca-zeolites are formed, no carbonates and dioctahedral minerals are observed. Geochemical simulation aided in establishing pH as a key parameter in determining mineral formation patterns. Indeed, under acidic conditions dioctahedral clay minerals and carbonate minerals are formed, while trioctahedral clay minerals are formed in basic conditions with a neutral pH value of 5.98 at 120°C. Zeolites are favoured from pH >~7.2. The results obtained shed new light on the importance of dioctahedral clay minerals versus zeolites and carbonates versus zeolites competitions, to better define the aqueous alteration processes throughout early Mars history.