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Co-auteur étranger; International audience; While major impacts during late accretion of a Mars type planet occur on a differentiated body, the characteristics of the shockwave propagation are poorly knownwithin these layered objects. Here, we use iSALE-2D hydrocode simulations to calculate shock pressure in a differentiated Mars type body for impact velocitiesranging from 5 to 20 km/s, impactor radii ranging from 50 to 200 km, and different rheologies. To better represent the distribution of shock pressure as a function ofdistance from the impact site at the surface, we propose two distinct regions in the mantle: a near field region that extends to 7–15 times the projectile radius into thetarget, where the peak shock pressure decays exponentially with increasing the distance from the impact site, and a far field region where the pressure decaysstrongly with the distance following a power law. At the core-mantle boundary, the peak shock pressure increases from the mantle side to the core side. The refractedshockwave travels within the core where the shock pressure decreases following a second power law. In this study, we fit the output obtained from iSALE hydrocodesimulations to determine scaling laws that illustrate the influence of the distance from the impact site, the ray angle, the target rheology, the impactor size and theimpact velocity. Finally we combine these shock-pressure scaling laws with the formalism proposed by Watters et al. [2009] to determine the impact heating inducedby large impacts within a differentiated Mars

The core dynamos of Mars and the Moon have distinctly different histories. Mars had no core dynamo at the end of accretion. It took ∼100 Myr for the core to create a strong dynamo that magnetized the martian crust. Giant impacts during 4.2–4.0 Ga crippled the core dynamo intermittently until a thick stagnant lithosphere developed on the surface and reduced the heat flux at the core–mantle boundary, killing the dynamo at ∼3.8 Ga. On the other hand, the Moon had a strong core dynamo at the end of accretion that lasted ∼100 Myr and magnetized its primordial crust. Either precession of the core or thermochemical convection in the mantle or chemical convection in the core created a strong core dynamo that magnetized the sources of the isolated magnetic anomalies in later times. Mars and the Moon indicate dynamo reversals and true polar wander. The polar wander of the Moon is easier to explain compared to that of Mars. It was initiated by the mass deficiency at South Pole Aitken basin, which moved the basin southward by ∼68° relative to the dipole axis of the core field. The formation of mascon maria at later times introduced positive mass anomalies at the surface, forcing the Moon to make an additional ∼52° degree polar wander. Interaction of multiple impact shock waves with the dynamo, the abrupt angular momentum transfer to the mantle by the impactors, and the global overturn of the core after each impact were probably the factors causing the dynamo reversal.

The youngest of the giant impact basins on Mars are either weakly magnetized or completely demagnetized, indicating that a global magnetic field was not present at the time those basins formed. Eight basins are sufficiently large that the impact heating associated with their formation could have penetrated below the core–mantle boundary (CMB). Here we investigate the thermal evolution of the martian interior and the fate of the global magnetic field using 3D mantle convection models coupled to a parameterized 1D core thermal evolution model. We find that the survival of the impact-induced temperature anomalies in the upper mantle is strongly controlled by the mantle viscosity. Impact heating from subsequent impacts can accumulate in stiffer mantles faster than it can be advected away, resulting in a thermal blanket that insulates an entire hemisphere. The impact heating in the core will halt dynamo activity, at least temporarily. If the mantle is initially cold, and the core initially superheated, dynamo activity may resume as quickly as a few Myr after each impact. However unless the lower mantle has either a low viscosity or a high thermal conductivity, this restored dynamo will last for only a few hundred Myr after the end of the sequence of impacts. Thus, we find that the longevity of the magnetic field is more strongly controlled by the lower mantle properties and relatively insensitive to the impact-induced temperature anomalies in the upper mantle.

The analysis of the Lunar Prospector magnetic data over South Pole Aitken basin reveals two categories of magnetic anomalies with distinctly different polarities. Modeling the anomalies in terms of vertical prism source bodies shows two separate true polar wander paths of the Moon. One is driven by the giant surface mass deficiency resulted from the formation of the South Pole Aitken basin and the other by the giant surface mass concentrations (mascons) formed inside the large impact basins, such as Serenitatis and Imbrium, and a vast volcanic deposit on Procellarum Terrane. A total of ~100o true polar wander path suggests that the source bodies are formed and magnetized during a long period. Moreover, the distinctly different polarities of the two categories of the anomalies supports the idea that the core dynamo underwent a reversal, or shutdown and started up with a different polarity, caused by the latter impacts. The impact-induced shock pressures of 10-20 GPa traveling inside the lunar core, the appreciable angular momentum transfer to the lunar mantle by the impacting bodies, and the impact-induced stratification of the core probably reversed the polarity of the core dynamo. We also model the north Crisium magnetic anomaly, extracted from the Lunar Prospector magnetic data. The paleomagnetic pole position obtained for the source body reveals that the north Crisium anomaly belongs to the first category of the anomalies.

The formation of an inner core during the accretion of Earth is investigated using self-gravitating and compressible Earth models formed by accreting a total of 25 or 50 Moon to Mars size planetary embryos. The impact of an embryo heats the proto-Earth's interior differentially, more below the impact site than elsewhere. The rotating core dynamically overturns and stratifies shortly after each impact, creating a spherically symmetric and radially increasing temperature distribution relative to an adiabatic profile. Merging of an embryo to the proto-Earth increases the lithostatic pressure that results in compressional temperature increase, while further enhances the melting temperature of the core causing solidification. A total of 36 thermal evolution models of the growing proto-Earth's core are calculated to investigate effects of major physical parameters. No solidification is considered in the first 21 models where modified two-body escape velocities are used as the impact velocities of the embryos. At the end of accretion, temperatures in the upper part of the core are significantly different among these models, whereas temperatures in the deeper parts are similar. The core solidification considered in the remaining 15 models, where impact velocities higher than the modified two-body escape velocities are adopted, drastically changes the temperature distribution in the deeper parts of the core. All of the models produce partially solidified stiff inner cores, 1000-2100 km in radius, at the end of accretion, where the solid fraction is larger than 50%. The innermost of the stiff inner cores are completely solidified to radii 250-1500 km.

The Rayleigh number-Nusselt number, and the Rayleigh number-thermal boundary layer thickness relationships are determined for the three-dimensional convection in a spherical shell of constant physical parameters. Several models are considered with Rayleigh numbers ranging from 1.1 x 102 to 2.1 x 105 times the critical Rayleigh number. At lower Rayleigh numbers the Nusselt number of the three-dimensional convection is greater than that predicted from the boundary layer theory of a horizontal layer but agrees well with the results of an axisymmetric convection in a spherical shell. At high Rayleigh numbers of about 105 times the critical value, which are the characteristics of the mantle convection in terrestrial planets, the Nusselt number of the three-dimensional convection is in good agreement with that of the boundary layer theory. At even higher Rayleigh numbers, the Nusselt number of the three-dimensional convection becomes less than those obtained from the boundary layer theory. The thicknesses of the thermal boundary layers of the spherical shell are not identical, unlike those of the horizontal layer. The inner thermal boundary is thinner than the outer one, by about 30- 40%. Also, the temperature drop across the inner boundary layer is greater than that across the outer boundary layer.

International audience; While major impacting bodies during accretion of a Mars type planet have very low velocities (