Chloé Michaut

Petrological constraints on the density of the Martian crust

New insights into the chemistry of the Martian crust have been made available since the derivation of crustal thickness maps from Mars Global Surveyor gravity and topography data that used a conservative range of density values (2700–3100 kg/m3). A new range of crustal density values is calculated from the major element chemistry of Martian meteorites (3100–3700 kg/m3), igneous rocks at Gusev crater (3100–3600 kg/m3) and from the surface concentration of Fe, Al, Ca, Si, and K measured by the Gamma-Ray Spectrometer on board Mars Odyssey (3250–3450 kg/m3). In addition, the density of mineral assemblages resulting from low-pressure crystallization of primary melts of the primitive mantle are estimated for plausible conditions of partial melting corresponding to the Noachian to Amazonian periods (3100–3300 kg/m3). Despite the differences between these approaches, the results are all consistent with an average density above 3100 kg/m3 for those materials that are close to the surface. The density may be compatible with the measured mass of Mars and the moment of inertia factor, but only if the average crustal thickness is thicker than previously thought (approaching 100 km). A thicker crust implies that crustal delamination and recycling could be possible and may even control its thickness, globally or locally. Alternatively, and considering that geoid-to-topography ratios argue against such a thick crust for the highlands, our results suggest the existence of a buried felsic or anorthositic component in the southern hemisphere of Mars.

Domes, pits, and small chaos on Europa produced by water sills

Pits, domes, and small chaos on Europas surface are quasi-circular features a few to a few tens of kilometers in diameter. We examine if injection of water sills into Europas ice shell and their subsequent evolution can induce successive surface deformations similar to the morphologies of these features. We study the dynamics of water spreading within the elastic part of the ice shell and show that the mechanical properties of ice exert a strong control on the lateral extent of the sill. At shallow depths, water makes room for itself by lifting the overlying ice layer and water weight promotes lateral spreading of the sill. In contrast, a deep sill bends the underlying elastic layer and its weight does not affect its spreading. In that case, the sill lateral extent is limited by the fracture toughness of ice and the sill can thicken substantially. After emplacement, cooling of the sill warms the surrounding ice and thins the overlying elastic ice layer. As a result, preexisting stresses in the elastic part of the ice shell increase locally to the point that they may disrupt the ice above the sill (small chaos). Disruption of the surface also allows for partial isostatic compensation of water weight, leading to a topographic depression at the surface (pit), of the order of ∼10 2 m. Complete water solidification finally causes expansion of the initial sill volume and results in an uplifted topography (dome) of ∼10 2 m.

A model for the dynamics of crater‐centered intrusion: Application to lunar floor‐fractured craters

Lunar floor-fractured craters are a class of craters modified by post-impact mechanisms. They are defined by distinctive shallow floors that are convex or plate-like, sometimes with a wide floor moat bordering the wall region. Radial, concentric, and polygonal floor fractures suggest an endogenous process of modification. Two mechanisms have been proposed to account for such deformations: viscous relaxation and spreading of a magma intrusion at depth below the crater. To test the second assumption and bring more constraints on the intrusion process, we develop a model for the dynamics of magma spreading below an elastic overlying layer with a crater-like topography. As predicted in earlier more qualitative studies, the increase in lithostatic pressure at the crater wall zone prevents the intrusion from spreading laterally, leading to the thickening of the intrusion. Additionally, our model shows that the final crater floor appearance after the uplift, which can be convex or flat, with or without a circular moat bordering the wall zone, depends on the elastic thickness of the layer overlying the intrusion and on the crater size. Our model provides a simple formula to derive the elastic thickness of the overlying layer hence a minimum estimate for the intrusion depth. Finally, our model suggests that crust redistribution by cratering must have controlled magma ascent below most of these craters.