Venus’ steep-sided domes are circular volcanoes ~10s of km wide and ~1 km tall, which are known for their characteristic flat tops and steep sides. However, their composition remains mysterious. These “pancake” domes are likely formed by a high-viscosity lava, and other studies have predicted a range of compositions, from rhyolite to basalt. In this study, we build on previous work modeling pancake domes as spreading viscous gravity currents. However, previous models of dome formation assumed that they form over a rigid lithosphere. We confirmed that lithospheric flexure occurs at pancake domes, and therefore build a new model of dome formation over a bending elastic lithosphere. We find that flexure during formation can influence the shape of the resulting pancake dome. Our results also support the idea that pancake domes continue to spread for a long time after their emplacement. In comparing our model to the topography of a real pancake dome, we find a range of high, though variable, lava viscosities. Our range of lava viscosities is related to the size of the observed dome, and our results for a large dome agree with those of other studies. We test different lava densities and find that a lava density of ~2400 – 2700 kg/m3 best reproduces the flexural signatures observed at pancake domes. Low-density lava (~1500 kg/m3) does not reproduce the flexural signatures, implying that dome-forming lava is not highly vesiculated.

The Martian surface composition appears mainly mafic but recent observations have revealed the presence of differentiated rocks, only in the Highlands. Here, we demonstrate that differentiated melts can form during the construction of thick crustal regions on Mars by fractional crystallisation of a mafic protolith, without plate tectonics. On a stagnant-lid planet, regions of thicker crusts contain more heat-producing elements and are associated to thinner lithospheres and to higher mantle melt fractions. This induces larger crustal extraction rates where the crust is thicker. This positive feedback mechanism is favoured at large wavelengths and can explain the formation of the Martian dichotomy. We further develop an asymmetric parameterised thermal evolution model accounting for crustal extraction, where the well-mixed convective mantle is topped by two lithospheres (North/South) characterised by specific thermal and crustal structures. We use this model in a Bayesian inversion to investigate the conditions that allow crustal temperatures to be maintained above the basalt solidus during crustal growth, resulting in the formation of evolved melts. Among the thermal evolution models matching constraints on the structure of the Martian crust and mantle provided by the InSight NASA mission, a non-negligible fraction allows partial melting and differentiation of the crust in the south, which can occur very early (<100 Myr) as well as during the Hesperian ; partial melting in the north appears unlikely. Although crustal differentiation may occur on a hemispheric scale on Mars, its vertical extent is limited to less than a third of the crustal thickness.

We present inversions for the structure of Mars using the first Martian seismic record collected by the InSight lander. We identified and used arrival times of direct, multiples, and depth phases of body waves, for seventeen marsquakes to constrain the quake locations and the one-dimensional average interior structure of Mars. We found the marsquake hypocenters to be shallower than 40 km depth, most of them being located in the Cerberus Fossae graben system, which could be a source of marsquakes. Our results show a significant velocity jump between the upper and the lower part of the crust, interpreted as the transition between intrusive and extrusive rocks. The lower crust makes up a significant fraction of the crust, with seismic velocities compatible with those of mafic to ultramafic rocks. Additional constraints on the crustal thickness from previous seismic analyses, combined with modeling relying on gravity and topography measurements, yield constraints on the present-day thermochemical state of Mars and on its long-term history. Our most constrained inversion results indicate a present-day surface heat flux of 22±1 mW/m2, a relatively hot mantle (potential temperature: 1740±90 K) and a thick lithosphere (540±120 km), associated with a lithospheric thermal gradient of 1.9±0.3 K/km. These results are compatible with recent seismic studies using a reduced data set and different inversions approaches, confirming that Mars’ mantle was initially relatively cold (1780±50 K) compared to its present-day state, and that its crust contains 10-12 times more heat-producing elements than the primitive mantle.