Dynamique et composition des atmosphères supérieures de Mars et Vénus observées par les spectrographes ultraviolets à bord de Mars Express et Vénus Express

Venus has a dense CO2-dominated atmosphere. Above 50 km, the atmospheric circulation is composed of two dominants patterns: the Retrograde Superrotating Zonal (RSZ) circulation up to 65 km and the subsolar to antisolar (SSAS) circulation beyond 120 km. The SSAS is caused by the temperature gradient between the day and the night hemispheres and it generates a global flow from the dayside to the nightside. Between 65 and 120 km, the circulation is complex as it is the superposition of the two main components. I have used the nightglow emitted by the NO* molecule (180-300 nm) produced by recombination of N(4S) and O(3P) atoms. These atoms are created by photodissociation of N2 and CO2 molecules on the dayside and carried to the nightside by the SSAS circulation. I analyze the full set of measurements of the NO δ and γ bands measured by the SPICAV instrument on board the ESA Venus Express spacecraft. I discuss the altitude and brightness of the emission peak (60 kR at 115 km) and show that the emission profile exhibits a secondary peak in ~10% of the cases. This additional peak is probably caused by upward-moving gravity waves propagating from below. I have developed an inversion method of the observed limb profiles to compare the results with a chemical-diffusive model of the atmosphere of Venus and constrain the vertical fluxes of N and O. On the basis of an extended statistical analysis, I characterize the presence of a bright spot of the nightglow of NO shifted from the antisolar point and located around 0230LT, 10°S. I use the vast SPICAV airglow database to statistically demonstrate that the NO nightglow is highly variable, both temporally and spatially.
Mars experiences seasons as its rotation axis is inclined from its spin axis. Its upper atmospheric dynamics is dominated by summer-to-winter global transport. I study the upper atmosphere of Mars using nightglow observations performed by the SPICAM instrument on board ESA’s Mars Express spacecraft. I analyze the δ and γ bands of molecule NO in the nightside mesosphere to confirm, on statistical basis, the peak brightness and altitude of the NO UV emission (5 kR at 72 km). I show that the NO nightglow is located following the relation Latitude = -80xsin(Solar Longitude), in agreement with results based on stellar occultations and from the LMD model, which simulates the photochemistry and dynamics of the Mars atmosphere.
I have performed a detailed study of the CO Cameron band (170 to 270 nm) and CO2+ doublet (290 nm) dayglow. I have developed a method to deduce the temperature profile around 150 km, an important study as the distribution of the temperature in the Mars atmosphere is poorly known. I showed that the temperature at high altitude is only weakly correlated with the solar
EUV flux. This result suggests that the variability of the upper atmosphere may include internal processes. Comparisons with the Mars Global Ionosphere Thermosphere Model show that the model can reproduce the observation for high solar conditions but predicts lower temperature for low to moderate solar conditions. Further study will therefore be necessary to identify and understand the mechanisms that govern the temperature variability.