Composition, elastic and thermo-mechanical properties of the Martian lithosphere
The strong and cold outermost portion of a planet is conventionally referred to as the lithosphere. When this layer is loaded by volcanoes, magmatic intrusions, or even ice caps, it deviates from its original state and bends. This compensation mechanism gives rise to two principal observables that are detectable from orbit: gravity anomalies, generally measured by applying Doppler tracking methods on orbiting spacecraft, and topographic expressions unveiled by laser altimeters or radar sounders.
By considering that, on geologic timescales, the Martian lithosphere behaves as a thin (compared to the wavelength of the deformation) and isotropic elastic shell overlaying an inviscid mantle, it is straightforward to model the deflection of the lithosphere under imposed loads and match the theoretical gravitational or topographic signatures to observations. On the basis of these comparisons, it is possible to estimates both the elastic thickness of the lithosphere (linked to its thermal state) that is required to support the investigated edifice, at the time it was emplaced, and the internal and surface densities (hence composition).
The gravitational signature of Martian volcanoes and the thermal evolution of Mars
As a first project, I conducted an analysis (Broquet & Wieczorek, 2019) of the gravitational signature of kilometer-sized Martian volcanoes. In this study, we model the deformation of the lithosphere under volcanic loads and compare the resulting theoretical gravitation signal to observations. The bulk density of the volcanic structures is found to have a mean value of 3200 ± 200 kg m-3, which is representative of iron-rich basalts as sampled by the Martian basaltic meteorites. The elastic part of the lithosphere, that maintains loads over geologic timescale, is constrained to have been weak when the oldest volcanoes ( > 3.2 Ga) formed, which implies that the lithosphere was hot and thin early in geologic history. Conversely, we obtain that younger volcanoes ( < 3 Ga) were emplaced on cold and strong elastic lithospheres. This chronology of formation tailors a geodynamic history in which the lithosphere strengthens with time as the planet cools and controls the surface expression of magmatism.