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This master's thesis is devoted to assess whether a small impactor can lead to destabilization of clathrates and degassing of methane. Therefore, the increase of temperature as a result of impact energy is theoretically and numerically investigated. First, a discussion on the theoretical model, which is used to calculate the temperature increase following the impact event as a function of the impactor radius, velocity and impact angle, is presented. A comparison study of the two theoretical models considered in this study, Gault-Heitowit and Murnaghan equation of state, is then proposed. The Martian subsurface temperatures are also calculated from the heat equation using the Crank-Nicholson method. The sensitivity of temperature predictions on the thermal conductivity is investigated. The impact-induced temperatures are then added to the initial subsurface temperatures for different case studies. The final step consists of verifying if the temperature at a certain depth allows the destabilization of methane clathrates, and thus degassing of methane, both for equatorial and polar regions, for different thermal conductivities of the subsurface (ranging from 0.039 W/mK to 2.5 W/mK), radii (0.06 m, 0.12 m and 0.20 m) and velocities (8 km/s, 10 km/s and 12 km/s) of the impactor.

Mars --- Impacts --- Methane` --- Physique, chimie, mathématiques & sciences de la terre > Aérospatiale, astronomie & astrophysique

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The objective of this work is to establish the Martian regions whose surface conditions are favourable to the deliquescence of salt. This process allows the formation of salt water (a brine) in which life can, under certain conditions, develop. A global scale modelling (MarsWRF Global Circulation Model) is performed to acquire the surface conditions. First, diurnal and seasonal variations for the entire planet are analysed. This shows that the areas meeting the conditions are mainly in the Northern Hemisphere in summer, between 50°N and 50°S in spring and autumn and mainly around the equator in winter. Then, some interesting locations were investigated in more detail. On the one hand, landing sites and on the other, sites where the absorption spectrum revealed the presence of salts. What emerges primarily from this study is that calcium perchlorate is the most likely salt to deliquesce under Martian conditions. For sites located at high latitudes, they can host brines in the first half of the year. Mid-latitude sites in the north allow salt deliquescence throughout the year but the maximum number of hours for which conditions are satisfied is about ten hours around the winter solstice. The same is true for locations near the equator but a hollow period is visible during the autumn and the maximum number of consecutive hours encountering the conditions is lower. In the Southern Hemisphere, only Hale Crater showed surface conditions favourable for the deliquescence of calcium perchlorate. To conclude, many locations have surface conditions allowing salts deliquescence for a determined interval of consecutive hours, but this study does not establish whether during this time brines form and remain stable.

brine --- deliquescence --- Mars --- water --- Physique, chimie, mathématiques & sciences de la terre > Aérospatiale, astronomie & astrophysique

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It is only recently that the periods during which the radars MARSIS and SHARAD are incapacitated, have been correlated to coronal mass ejections (CME) and solar energetic particles (SEP) events. MAVEN measurements of enhancements of the SEP electrons flux happen to be consistent with these radar blackouts. This phenomenon was attributed to an extra-ionization of the neutral atmosphere of Mars by the increased flux of SEP electrons, inducing an increase of the electron density which attenuates the radar signal. However, the effect of this phenomenon on the physical properties of the atmosphere was never observed as the instruments generally used, experienced blackouts. This study proposes the use of radio occultations to monitor the physical properties of the neutral Martian atmosphere during CME and SEP events as the radio signals used in this technique are not attenuated by the extra-ionization. The two case studies of this work suggest that CME/SEP events are correlated to a decrease of the upper atmosphere refractivity due to an increase of the electron density induced by an extra-ionization caused by the SEP electrons. An atmospheric disturbance is also observed during CME/SEP events further suggesting an extra-ionization of this same region. Our results are coherent with the MAVEN and MEX measurements of the SEP electrons as well as with MARSIS and SHARAD blackouts. This study presents the first global analysis of such modifications of the physical properties of the neutral atmosphere of Mars. Ce n'est que récemment que les périodes durant lesquelles les incapacités des radars MARSIS et SHARAD ont été corrélées avec les événements d'éjections de masses coronales (EMC) et de particules énergiques solaires (PES). Les mesures de MAVEN des augmentations du flux d'électrons de PES sont consitantes avec les "blackouts" des radars. Ce phénomène a été attribué à une ionisation supplémentaire de l'atmosphère neutre de Mars par les flux accrus d'électrons de PES, causant une augmentation de la densité d'électrons, ce qui attenue le signal radar. Cependant, l'effet de ce phénomène sur les propriétes physiques de l'atmosphère n'a jamais été observé car les instruments utilisés habituellement expérimentaient des "blackouts". Cette étude propose l'utilisation d'occultations radio afin de mesurer les propriétés physiques de l'atmosphère neutre martienne lors des événements EMC et PES car les signaux radio utilisés par cette méthode ne sont pas atténués par l'ionisation supplémentaire. Les deux études de cas de cette recherche suggèrent que les événements ECM/PES sont corrélés avec une diminution de la réfractivité dans la haute atmosphère de Mars causée par une augmentation de la densité d'électrons, elle-même induite par une ionisation supplémentaire due aux électrons PES. Une perturbation atmosphérique est aussi observée durant les événements EMC/PES suggérant d'autant plus une ionisation supplémentaire de cette région. Nos résultats sont cohérents avec les mesures des électrons PES de MAVEN et MEX ainsi qu'avec les "blackouts" de MARSIS et SHARAD. Ceci présente la première analyse globale de tels effets sur les propriétés physiques de l'atmosphère neutre de Mars.

Mars --- Radio occultation --- Mars Express --- Atmosphere --- Ionosphere --- Remote Sensing --- Mars --- Occultation radio --- Mars Express --- Atmosphère --- Ionosphère --- Télédétection --- Physique, chimie, mathématiques & sciences de la terre > Aérospatiale, astronomie & astrophysique