Ana-Catalina Plesa

Thermal evolution and Urey ratio of Mars

The upcoming InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) mission, to be launched in 2016, will carry out the first in situ Martian heat flux measurement, thereby providing an important baseline to constrain the present-day heat budget of the planet and, in turn, the thermal and chemical evolution of its interior. The surface heat flux can be used to constrain the amount of heat-producing elements present in the interior if the Urey ratio (Ur)—the planets heat production rate divided by heat loss—is known. We used numerical simulations of mantle convection to model the thermal evolution of Mars and determine the present-day Urey ratio for a variety of models and parameters. We found that Ur is mainly sensitive to the efficiency of mantle cooling, which is associated with the temperature dependence of the viscosity (thermostat effect), and to the abundance of long-lived radiogenic isotopes. If the thermostat effect is efficient, as expected for the Martian mantle, assuming typical solar system values for the thorium-uranium ratio and a bulk thorium concentration, simulations show that the present-day Urey ratio is approximately constant, independent of model parameters. Together with an estimate of the average surface heat flux as determined by InSight, models of the amount of heat-producing elements present in the primitive mantle can be constrained.

On the habitability of a stagnant lid Earth

Context. Plate tectonics is considered a fundamental component for the habitability of the Earth. Yet whether it is a recurrent feature of terrestrial bodies orbiting other stars or unique to the Earth is unknown. The stagnant lid may rather be the most common tectonic expression on such bodies. Aims. To understand whether a stagnant-lid planet can be habitable, i.e. host liquid water at its surface, we model the thermal evolution of the mantle, volcanic outgassing of H 2 O and CO 2 , and resulting climate of an Earth-like planet lacking plate tectonics. Methods. We used a 1D model of parameterized convection to simulate the evolution of melt generation and the build-up of an atmosphere of H 2 O and CO 2 over 4.5 Gyr. We then employed a 1D radiative-convective atmosphere model to calculate the global mean atmospheric temperature and the boundaries of the habitable zone (HZ). Results. The evolution of the interior is characterized by the initial production of a large amount of partial melt accompanied by a rapid outgassing of H 2 O and CO 2 . The maximal partial pressure of H 2 O is limited to a few tens of bars by the high solubility of water in basaltic melts. The low solubility of CO 2 instead causes most of the carbon to be outgassed, with partial pressures that vary from 1 bar or less if reducing conditions are assumed for the mantle to 100–200 bar for oxidizing conditions. At 1 au, the obtained temperatures generally allow for liquid water on the surface nearly over the entire evolution. While the outer edge of the HZ is mostly influenced by the amount of outgassed CO 2 , the inner edge presents a more complex behaviour that is dependent on the partial pressures of both gases. Conclusions. At 1 au, the stagnant-lid planet considered would be regarded as habitable. The width of the HZ at the end of the evolution, albeit influenced by the amount of outgassed CO 2 , can vary in a non-monotonic way depending on the extent of the outgassed H 2 O reservoir. Our results suggest that stagnant-lid planets can be habitable over geological timescales and that joint modelling of interior evolution, volcanic outgassing, and accompanying climate is necessary to robustly characterize planetary habitability.

A geophysical perspective on the bulk composition of Mars

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.

Impact-induced changes in source depth and volume of magmatism on Mercury and their observational signatures

Mercury’s crust is mostly the result of partial melting in the mantle associated with solid-state convection. Large impacts induce additional melting by generating subsurface thermal anomalies. By numerically investigating the geodynamical effects of impacts, here we show that impact-generated thermal anomalies interact with the underlying convection modifying the source depth of melt and inducing volcanism that can significantly postdate the impact depending on the impact time and location with respect to the underlying convection pattern. We can reproduce the volume and time of emplacement of the melt sheets in the interior of Caloris and Rembrandt if at about 3.7–3.8 Ga convection in the mantle of Mercury was weak, an inference corroborated by the dating of the youngest large volcanic provinces. The source depth of the melt sheets is located in the stagnant lid, a volume of the mantle that never participated in convection and may contain pristine mantle material.Mantle partial melting produced the volcanic crust of Mercury. Here, the authors numerically model the formation of post-impact melt sheets and find that mantle convection was weak at around 3.7–3.8 Ga and that the melt sheets of Caloris and Rembrandt may contain partial melting of pristine mantle material.

Present‐Day Mars' Seismicity Predicted From 3‐D Thermal Evolution Models of Interior Dynamics

The Interior Exploration using Seismic Investigations, Geodesy and Heat Transport mission, to be launched in 2018, will perform a comprehensive geophysical investigation of Mars in situ. The Seismic Experiment for Interior Structure package aims to detect global and regional seismic events and in turn offer constraints on core size, crustal thickness, and core, mantle, and crustal composition. In this study, we estimate the present-day amount and distribution of seismicity using 3-D numerical thermal evolution models of Mars, taking into account contributions from convective stresses as well as from stresses associated with cooling and planetary contraction. Defining the seismogenic lithosphere by an isotherm and assuming two end-member cases of 573 K and the 1073 K, we determine the seismogenic lithosphere thickness. Assuming a seismic efficiency between 0.025 and 1, this thickness is used to estimate the total annual seismic moment budget, and our models show values between 5.7 × 1016 and 3.9 × 1019 Nm.

Large Scale Numerical Simulations of Planetary Interiors

The massive increase of computational power over the past decades has established numerical models of planetary interiors to one of the principal tools to investigate the thermo-chemical evolution of terrestrial bodies. Large scale computational models have become state of the art to investigate the interior heat transport, surface tectonics and chemical differentiation of planetary bodies across the Solar System and beyond. In the present work we present large scale numerical simulations performed using the mantle convection code Gaia in spherical and Cartesian geometry. The results have been obtained on the HLRS system Hornet running on 54 × 103 computational cores. The strong scaling results show an optimal speedup for a grid with 55 million computational points corresponding to 275 million unknowns.

Mercury's low-degree geoid and topography controlled by insolation-driven elastic deformation

Mercury experiences an uneven insolation that leads to significant latitudinal and longitudinal variations of its surface temperature. These variations, which are predominantly of spherical harmonic degrees 2 and 4, propagate to depth, imposing a long-wavelength thermal perturbation throughout the mantle. We computed the accompanying density distribution and used it to calculate the mechanical and gravitational response of a spherical elastic shell overlying a quasi-hydrostatic mantle. We then compared the resulting geoid and surface deformation at degrees 2 and 4 with Mercurys geoid and topography derived from the MErcury, Surface, Space ENvironment, GEochemistry, and Ranging spacecraft. More than 95% of the data can be accounted for if the thickness of the elastic lithosphere were between 110 and 180 km when the thermal anomaly was imposed. The obtained elastic thickness implies that Mercury became locked into its present 3:2 spin orbit resonance later than about 1 Gyr after planetary formation.

Magma Ocean Cumulate Overturn and Its Implications for the Thermo-chemical Evolution of Mars

Early in the history of terrestrial planets, the fractional crystallization of primordial magma oceans may have led to the formation of large scale chemical heterogeneities. These may have been preserved over the entire planetary evolution as suggested for Mars by the isotopic analysis of the so-called SNC meteorites. The fractional crystallization of a magma ocean leads to a chemical stratification characterized by a progressive enrichment in heavy elements from the core-mantle boundary to the surface. This results in an unstable configuration that causes the overturn of the mantle and the subsequent formation of a stable chemical layering. Assuming scaling parameters appropriate for Mars, we first performed simulations of 2D thermo-chemical convection in Cartesian geometry with the numerical code YACC. We ran a large set of simulations spanning a wide parameter space, by varying systematically the buoyancy ratio B, which measures the relative importance of chemical to thermal buoyancy, in order to understand the basic physics governing the magma ocean cumulate overturn and its consequence on mantle dynamics. Moreover, we derived scaling laws that relate the time over which chemical heterogeneities can be preserved (mixing time) and the critical yield stress (maximal yield stress that allows the lithosphere to undergo brittle failure) to the buoyancy ratio. We have found that the mixing time increases exponentially with B, while the critical yield stress shows a linear dependence. We investigated then Mars early thermo-chemical evolution using the code GAIA in a 2D cylindrical geometry and assuming a detailed magma ocean crystallization sequence as obtained from geochemical modeling. A stagnant lid forms rapidly because of the strong temperature dependence of the viscosity. This immobile layer at the top of the mantle prevents the uppermost dense cumulates to sink, even when allowing for a plastic yielding mechanism. The convection pattern below this dense stagnant lid is dominated by small-scale structures caused by perturbations in the chemical component. Therefore, large-scale volcanic features observed over Mars surface cannot be reproduced. Assuming that the stagnant lid will break, the inefficient heat transport due to the stable density gradient and the entire amount of heat sources above the core-mantle-boundary (CMB) lead to a strong increase of the temperature to values that exceed the liquidus. We conclude that a fractionated global and deep magma ocean is difficult to reconcile with observations. Other scenarios like shallow or hemispherical magma ocean or even another freezing mechanism, which would reduce the strength of chemical gradient need to be considered.

A particle-in-cell Method to model the Influence of Partial Melt on Mantle Convection

Solid-state convection is the principal mechanism that controls the global dynamics and thermal evolution of the terrestrial planets. Observations such as seismology and mission data from geological structures at the planetary surfaces offer important constraints for the interior dynamics. However, our main knowledge stems from laboratory experiments and in particular from computer models. In the last years, due to the significant improve of high performance computing, computer simulations have became the most powerful access to this fluid-dynamical problem by solving partial differential equations in a discrete formulation to describe the flow in space and time. In the present work we will present results obtained using the cylindrical/spherical code GAIA with a particle-in-cell method to account for compositional changes due to partial melting of the mantle.

Modeling the Interior Dynamics of Terrestrial Planets

Over the past years, large scale numerical simulations of planetary interiors have become an important tool to understand physical processes responsible for the surface features observed by various space missions visiting the terrestrial planets of our Solar System. Such large scale applications need to show good scalability on thousands of computational cores while handling a considerable amount of data that needs to be read from and stored to a file system. To this end, we analyzed numerous approaches to write files on the Cray XC40 Hazel Hen supercomputer. Our study shows that HPC applications parallelized using MPI highly benefit from utilizing the MPI I/O facilities. By implementing MPI I/O in Gaia, we improved the I/O performance up to a factor of 100. Additionally, in this study we present applications of the fluid flow solver Gaia using high resolution regional spherical shell grids to study the interior dynamics and thermal evolution of terrestrial bodies of our Solar System.