Theses defended

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Ludivine HAREL - 29th November

"Modelling of heat transfer in external ice shell of ocean-moons: Scaling laws and thermal evolution applied to Ganymede"

In the outer solar system, many planetary bodies are covered with a layer of ice and a salty liquid ocean under their icy crust have been detected for a few of them. The existence of these oceans depends on the efficiency of the internal cooling, controlling the crystallization of the hydrosphere. The aim of this thesis is to understand and to describe the heat transfer through the outer ice layer of these satellites by performing numerical thermal convection simulations using the CHEOPS-2D code in spherical annulus geometry for a viscosity law taking into account the different creep mechanisms of the ice. The performed numerical simulations show that the grain size of ice plays a key role in the convective dynamics of the outer ice layer. New scaling laws taking into account the specificities of the ice-I rheology have been derived and applied to the thermal evolution of Ganymede. We show that taking into account a more realistic rheology decreases the efficiency of heat transfer, thus allowing the internal ocean to persist longer. These new scaling laws will provide a better description of the thermal history of Ganymede and more realistic evolution scenarios, essential for the interpretation of future data collected by the ESA JUICE mission.

Chloé LARRE - 25th November

"H2O and S behaviors in martin magmatic system: An experimental and spectroscopic approach"

Volatiles (H,C,N,O,Cl,F,S,P) are basis compounds for development of life since they are the origin of everything and serve as nutrient sources. Those elements are detected on Mars surface through sedimentary analysis thanks to former spatial missions (such as Curiosity) and thanks to multiple analysis of martian meteorites. Theses volatiles come from, in the major part, magmas degassing which compositions are still not well constrain. This project aims to detect martian volatiles contents through identification and quantification of igneous rocks. Basalts are the main volcanic rocks representative of Mars surface. They can be in a glassy of amorphous form. Basalts come from an oxygen-poor but iron-rich mantle (FeOtot=18wt%). Thus, they have been synthesized under reduced conditions. Precise analysis of mineralogical and chemical compositions of these igneous rocks collected at the surface, can constrain mantle composition. Especially, it can determine volatile contents in basaltic magmas which, that we know, are important for the partial melt of the martian mantle. To determine each volatile content and which species are present in martian melts, we have to produce some calibrations for the SuperCam apparatus. With the help of a piston-cylinder we can reproduce martian conditions which create martian basalts. We also will be able to make several analysis on our experiments with a Raman and InfraRed spectrometers, available in our laboratory, in order to elaborate a calibration of those devices. These calibrations will be directly used by the SuperCam instrument, in particular when it will identified volatile contents in igneous rocks. Thus, degassing of this kind of magmatism can be explain and the budget of volatiles on Mars can be eluded, included unanswered questions about the presence of liquid water on the martian primitive surface.

Briz PARENT - 11th September

"Development of a biotic index based on benthic foraminifera; : application in the French Mediterranean coastal waters "

Coastal environments suffer from anthropogenic activities. Various types of pressure can have large impacts on benthic ecosystems. In Europe, international regulations, such as the Water Framework Directive (WFD), have been implemented. The WFD aims to attain and sustain a good ecological status for all coastal waters, up to one mile from the coast. To verify whether this good status has been reached, adapted and efficient measuring tools are needed. The use of biotic indices based on foraminiferal faunas are a mean to assess the ecosystem status. The use of indices based on foraminiferal faunas rapidly increases, especially in the Mediterranean. In terms of methodology, we were able to improve a method to concentrate benthic foraminifera by density separation, which accelerates the sample treatment. The TSI-Med index includes a correction to take into account the natural trophic state, i.e., the natural enrichment rate in organic matter, on the basis of sediment grain size. However, our data suggest that the applied correction is still insufficient. We also compared several biotic indices based on benthic foraminiferal faunas along the French Mediterranean coast. Indices based on diversity, such as the exp(H’bc), are not suited for our oligotrophic study area. The results of three tested indices based on ecological groups (TSI-Med, Foram-AMBI, FSI) are well correlated. The main factor inducing differences between the indices is the list of ecological assignments for the various species. In general, too many species have not yet been assigned to ecological categories and the assignment of several major species is problematic. Nevertheless, we show that the indices are already efficient in detecting the impact of diffusive organic matter enrichment or enrichment from point sources, such as sewage outlets (Israelian coast). To further perfection the foraminiferal indices, the efforts of ecological assignments of species have to be continued.