The magmatic crust of Vesta

Les astéroïdes Cérès et Vesta ont motivé la mission spatiale Dawn parce qu'ils représentent deux embryons planétaires différents restés relativement intacts depuis leur formation. Vesta est large- ment considéré comme le corps parent des météorites HED témoins d'une activité magmatique probablement due à la présence de l'isotope radioactif 26Al qui était suffisamment abondant pour permettre la fusion interne des corps rocheux primitifs. La composition d'une surface planétaire peut être mesurée grâce à l'analyse des rayons gammas qu'elle produit. Pour la sonde Dawn cela est rendu possible par l'instrument GRaND et la scintillation d'un cristal de BGO. Cette thèse présente l'analyse des spectres gammas de Vesta par deux outils de séparation aveugle de source: l'analyse en composantes indépendantes (ICA) et la factorisation en matrice non-négative (NMF). Ces méthodes sont aussi appliquées à un jeu de données lunaire comparable et déjà bien interprété. Des spectres synthétiques lunaires permettent de tester ICA et NMF. La séparation de spectres élémentaires s'avère délicate même si on peut distinguer les éléments K, Th et Fe en raison des propriétés statistiques de leur signaux sources plus favorables. On mesure la sensibilité d'ICA-NMF à la variabilité chimique de la surface pour des Lunes artificielles, ce qui permet d'expliquer l'absence de séparation d'un signal élémentaire clair dans le cas de Vesta. Malgré les observations de la sonde Dawn et le nombre important d'informations fournies par les HED, il n'y a pas de consensus sur la formation des HED. On met souvent en avant l'existence d'un océan magmatique global sur Vesta, alors que la migration de la principale source chaleur, contenue dans le premier minéral fondu, le plagioclase, ne permet pas la fusion totale. On met en oeuvre un modèle de migration des magmas, basé sur les équations de la compaction. On adapte ce modèle en utilisant un diagramme d'équilibre de phase olivine-anorthite-quartz. Cela permet de calculer l'évolution de la minéralogie en fonction du temps et de la profondeur. Les résultats montrent que les eucrites et les diogénites pourraient être une caractéristique commune des gros corps accrétés tôt dans l'histoire du système solaire.Asteroids Vesta and Ceres motivated the space mission Dawn because they represent two different planetary embryos that remained relatively intact since their formation. Vesta is broadly considered as the parent body of the HED meteorites suite that are witnesses of a magmatic activity probably due to the presence of the radioactive isotope 26 Al which was present in significant amount to cause internal melting of primitive rocky bodies. The composition of a planetary surface can be quantified through the analysis of the gamma rays it produces. This is made possible for the Dawn spacecraft by the instrument GRaND and the scintillation of a BGO crystal. This thesis presents the analysis of gamma ray spectra from Vesta by two blind source separation methods: the independent component analysis and the non negative matrix factorization. These methods are also applied to an equivalent lunar dataset already well interpreted. Lunar synthetic spectra are used to test ICA and NMF. The separation of elementary spectra is delicate although K, Th and Fe can be discriminated due to the more favorable statistical properties of their source signals. The sensitivity of separation to the chemical variability is assessed based on artificial lunar spectra, which allows to explain the lack of separation of a clear elemental signal in the case of Vesta. Despite the observations of Dawn and the important collection of HED data, there is no consensus on the conditions of the vestan magmatism. A global magma ocean is often put forward, whereas the migration of the heat source, contained in the easiest mineral to melt, plagioclase, does not allow it. A model of melt migration is implemented, based on two-phase flow equations. This model is combined with the olivine-anorthite-quartz equilibrium phase diagram. This allows to predict the mineralogy as a function of depth and time. Results obtained show that eucrites and diogenites may be a common feature of large bodies accreted early in solar system history

Implication of the lopsided growth for the viscosity of Earth's inner core

Two main seismic features characterize the Earth's inner core: a North-South polar anisotropy and an East-West asymmetry of P-wave velocity and attenuation. Anisotropy is expected if shear deformation is induced by convective motions. Translation has recently been put forward as an important mode of convection of the inner core. Combined with a simple diffusive grain growth model, this mechanism is able to explain the observed seismic asymmetry, but not the bulk anisotropy. The source of anisotropy has therefore to be sought in the shear motions caused by higher modes of convection. Using a hybrid finite-difference spherical harmonics Navier-Stokes solver, we investigate the interplay between translation and convection in a 3D spherical model with permeable boundary conditions at the inner core boundary. Three parameters act independently: viscosity, internal heating and convection velocity in the outer core. Our numerical simulations show the dominance of pure translation for viscosities of the inner core higher than $10^{20}$ Pas. Translation is almost completely hampered by convective motions for viscosities lower than $10^{18}$ Pas. Between these values, translation and convection develop, but convective downwellings are restricted to the coldest hemisphere where crystallization occurs. On the opposite side, shear is almost absent, thereby allowing grain growth. We propose that the coexistence of translation and convection observed in our numerical models leads to a seismic asymmetry but localizes deformation only in one hemisphere.Comment: Submitted to PNAS on Februray 10th 2012, rejected on March 19th 2012. Submitted to Earth Planet. Sci. Lett. on March 20th, 2012. (10 pages, 8 figures

Chemical Mapping of Vesta and Ceres

Following successful science operations at Vesta, the Dawn spacecraft is headed for an encounter with Ceres in 2015. What have we learned at Vesta? And, what do we expect to learn by comparing Vesta and Ceres? We will address these questions from the standpoint of geochemistry. Dawn's Gamma Ray and Neutron Detector (GRaND) is sensitive to the elemental composition of surface materials to depths of a few decimeters [1]. Gamma rays and neutrons, produced by the steady bombardment of galactic cosmic rays and by the decay of naturally ]occurring radioisotopes (K, Th, U), provide a chemical fingerprint of the regolith. Analysis of planetary radiation emissions enables mapping of specific elements (such as Fe, Mg, Si, Cl, and H) and compositional parameters (such as average atomic mass), which provide information about processes that shaped the planet1s surface and interior. Dawn has exceeded operational goals for GRaND at Vesta, accumulating an abundance of nadir-pointed data during five months in a 210 km, low altitude mapping orbit around Vesta (265-km mean radius). Chemical information from gamma ray and neutron measurements was used to test the connection between Vesta and the howardite, eucrite, and diogenite (HED) meteorites [2]. Additionally, GRaND searched for evolved, igneous lithologies [3], mantle and upper crustal materials exposed in large impact basins, mesosiderite compositions, and hydrogen in Vesta1s bulk regolith. Results of our analyses and their implications for thermal evolution and regolith-processes will be presented. The possibility of a subcrustal ocean [4, 5] and lack of cerean meteorites makes water-rich Ceres a compelling target of exploration [6]. If Ceres underwent aqueous differentiation, then crustal overturn or gas driven volcanism may have significantly modified its primitive surface; and products of aqueous alteration (e.g. [7]) would detectable by GRaND [1]. For example, the presence of Cl in salts, associated with liquid-water-processes, would have a profound effect on the thermal neutron leakage flux. GRaND is sensitive to H and H-layering, which may be in the form of endogenic water ice or hydrous minerals on Ceres. Ammonia ice (e.g., from recent cryovolcanism) would produce a distinctly different neutron signature than water ice [1]. Prospective results for GRaND at Ceres will be presented in the context of what we have learned about Vesta

La croûte magmatique de Vesta

Asteroids Vesta and Ceres motivated the space mission Dawn because they represent two different planetary embryos that remained relatively intact since their formation. Vesta is broadly considered as the parent body of the HED meteorites suite that are witnesses of a magmatic activity probably due to the presence of the radioactive isotope 26 Al which was present in significant amount to cause internal melting of primitive rocky bodies. The composition of a planetary surface can be quantified through the analysis of the gamma rays it produces. This is made possible for the Dawn spacecraft by the instrument GRaND and the scintillation of a BGO crystal. This thesis presents the analysis of gamma ray spectra from Vesta by two blind source separation methods: the independent component analysis and the non negative matrix factorization. These methods are also applied to an equivalent lunar dataset already well interpreted. Lunar synthetic spectra are used to test ICA and NMF. The separation of elementary spectra is delicate although K, Th and Fe can be discriminated due to the more favorable statistical properties of their source signals. The sensitivity of separation to the chemical variability is assessed based on artificial lunar spectra, which allows to explain the lack of separation of a clear elemental signal in the case of Vesta. Despite the observations of Dawn and the important collection of HED data, there is no consensus on the conditions of the vestan magmatism. A global magma ocean is often put forward, whereas the migration of the heat source, contained in the easiest mineral to melt, plagioclase, does not allow it. A model of melt migration is implemented, based on two-phase flow equations. This model is combined with the olivine-anorthite-quartz equilibrium phase diagram. This allows to predict the mineralogy as a function of depth and time. Results obtained show that eucrites and diogenites may be a common feature of large bodies accreted early in solar system history.Les astéroïdes Cérès et Vesta ont motivé la mission spatiale Dawn parce qu’ils représentent deux embryons planétaires différents restés relativement intacts depuis leur formation. Vesta est large- ment considéré comme le corps parent des météorites HED témoins d’une activité magmatique probablement due à la présence de l’isotope radioactif 26Al qui était suffisamment abondant pour permettre la fusion interne des corps rocheux primitifs. La composition d’une surface planétaire peut être mesurée grâce à l’analyse des rayons gammas qu’elle produit. Pour la sonde Dawn cela est rendu possible par l’instrument GRaND et la scintillation d’un cristal de BGO. Cette thèse présente l’analyse des spectres gammas de Vesta par deux outils de séparation aveugle de source: l’analyse en composantes indépendantes (ICA) et la factorisation en matrice non-négative (NMF). Ces méthodes sont aussi appliquées à un jeu de données lunaire comparable et déjà bien interprété. Des spectres synthétiques lunaires permettent de tester ICA et NMF. La séparation de spectres élémentaires s’avère délicate même si on peut distinguer les éléments K, Th et Fe en raison des propriétés statistiques de leur signaux sources plus favorables. On mesure la sensibilité d’ICA-NMF à la variabilité chimique de la surface pour des Lunes artificielles, ce qui permet d’expliquer l’absence de séparation d’un signal élémentaire clair dans le cas de Vesta. Malgré les observations de la sonde Dawn et le nombre important d’informations fournies par les HED, il n’y a pas de consensus sur la formation des HED. On met souvent en avant l’existence d’un océan magmatique global sur Vesta, alors que la migration de la principale source chaleur, contenue dans le premier minéral fondu, le plagioclase, ne permet pas la fusion totale. On met en oeuvre un modèle de migration des magmas, basé sur les équations de la compaction. On adapte ce modèle en utilisant un diagramme d’équilibre de phase olivine-anorthite-quartz. Cela permet de calculer l’évolution de la minéralogie en fonction du temps et de la profondeur. Les résultats montrent que les eucrites et les diogénites pourraient être une caractéristique commune des gros corps accrétés tôt dans l’histoire du système solaire