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.

The present-day geodynamic state of the lithosphere, hints from loading models of the Martian polar caps

As part of a second project, I am investigating the composition and geodynamic state of the lithosphere beneath the polar caps of Mars (see Broquet et al., LPSC51, Broquet et al., LPSC51, Broquet et al., 2020). Using radar data from Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) and SHAllow RADar (SHARAD) coupled with a lithospheric loading model, we self-consistently estimate the density and real dielectric constant of the polar cap, and the elastic thickness of the lithosphere underneath. We show that the lithosphere below the north polar cap is currently extremely rigid and cold (elastic thickness of 330 km, heat flow of 16 mW m-2). The south polar cap is however supported by a thinner and warmer lithosphere of about 200 km, implying a heat flow of 26 mW m-2. These results suggest that there is a dichotomy in surface heat flow on Mars, which is controlled by the crust that is highly enriched in heat-producing elements. Our inferred compositions suggest that for reasonable dust content, a minimum of 10 vol.% CO2 ice is buried in the north polar deposits, which may have important implications for the climate evolution of Mars.

Among the several tools I developed for these projects, I use the computationally efficient and well documented SHTOOLS software package of Wieczorek et al. (2018).

Education

2017 -                  Observatoire de la Côte d'Azur, Nice, France - Ph. D., Planetary Geophysics (advisor, Mark Wiezcorek)
2016 - 2017         Institut de Physique du Globe, Paris, France - Master 2, Solid-Earth Geophysics
2015 - 2016         Université Pierre et Marie Curie, Paris, France - Master 1, Earth Sciences
2012 - 2015         Université Pierre et Marie Curie, Paris, France - B.ScEarth Sciences

Internship

2017 (5 months)  Institut de Physique du Globe, Paris, France - Modelling the gravitational signature of the small Martian volcanoes (advisor, Mark Wiezcorek)
2016 (4 months)  Institut de Physique du Globe, Paris, France - Cross-spectral study of the Martian gravity & topography (advisor, Mark Wiezcorek)
2015 (2 months)  Geo212, Paris, France - Lead developer of a statistical tool to analyze OpenStreetMap data (advisor, Nicolas Saporiti)

Professional position

2018 -                 InSight collaborator

2017 -  2019       Teaching assistant - Department of Earth, Environment & Space, Université de Nice Côte d'Azur, Nice, France. Classes per level:  1st year - Planetology, Stratigraphy & Paleontology ; 2nd year - Sedimentology, Magmatism, Metamorphism, Mineralogy, Geophysics ; 3rd year - Mass wasting & risks, Geophysics

Conference, talk & poster

2020                      Public outreach - InSight@School, Sophia Antipolis, France (Mars 1st)
                             
Invited speaker: The lithosphere of Mars

2020                      InSight Science Team meeting, Nice, France (Feb. 24-28)
                             
Talk & Poster: Flexure of the lithosphere beneath the polar caps of Mars 

2019                      Public outreach - Club Véga - Ollioules, France (May 18th)
                              Invited speaker: The worlds around us: history and future of space exploration                              

2019                      50th Lunar and Planetary Science Conference, Houston TX, USA (Mar. 18-22)
                             Talk: Geodynamic state of the lithosphere beneath the northern polar cap of Mars. Abstract link

2018                     InSight French Team meeting, Paris, France (November 5th)
                             Talk: Constraints on the Martian lithosphere from gravity and topography

2018                     Workshop in Geology and Geophysics of the Solar System, Petnica, Serbia (June 23-July 1)
                             Poster: The lithosphere of Mars

2018                     NASA InSight Launch, Buellton CA, USA (May. 4-8)
                            
Poster: Gravitational signature of Martian volcanoes

2018                     49th Lunar and Planetary Science Conference, Houston TX, USA (Mar. 19-23)
                            
Talk: Gravitational signature of Martian volcanoes. Abstract link

2018                     Martian crust workshop, Ecole Normale Supérieure de Lyon, France (Feb. 8)
                            
Talk: The lithosphere of Mars as seen by gravity and topography

2018                     DLR Institute, Berlin, Germany (Janv. 17-19)
                            
Invited speaker: Composition, elastic and thermo-mechanical properties of the Martian lithosphere

2017                     Geodesy & Rheology, Université Valrose, Nice, France (Nov. 13-15)
                            
Talk: Gravitational signature of Martian volcanoes

2017                     Accretion and early differentiation of the Earth and terrestrial planets, Saint Paul Hotel, Nice, France (May 29-June 3)

2017                     LOLA/Diviner/SpaceIL team meeting, Tel Aviv, Israel (Apr. 17-22)

2017                     Origin and diversity of planetary systems from the microscope to the telescope, Villefranche, France (Feb. 20-23)

2016                     29th International School of Space Science - Planetary Interiors, L'Aquila, Italy (Sep. 12-16)
                            
Poster: Gravitational signature of small Martian volcanoes

Award, grant & scholarship

2019                     My thesis in 180s, Final in Nice, 3rd place (video link)

2018                     Workshop in Geology and Geophysics of the Solar System scholarship

2016                     29th International School of Space Science scholarship

       

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