We developed radiative transfer models to reproduce Titan’s visible and near infrared spectra, to determine the effects of the haze, and retrieve the methane abundances during Titan’s current southern summer. With ground-based high resolution spectra of CH3D absorption at 1.6 μm, we measured the global CH₃D abundance. Combined with observations of 8.6 μm emission of CH₃D and CH₄ that indicate their relative abundances, we thus determined the global CH₄ abundance. We expanded on these ground-based measurements, with improved radiative transfer models based on the Huygens DISR models, and spectra which resolve the spatial variation of the CH₃D lines. The profiles of CH3D thus obtained revealed that the methane abundance on the lowest 10 km of Titan’s atmosphere does not vary by more than 20% over 32°S-32°N. With the extensive coverage of Cassini VIMS observations at 0.35-1.6 μm, we determined the latitudinal variation of the methane at 20-50 km and of the haze. We find an ambiguity between the methane and haze abundances, so their gradients become coupled. At the lower limit of the methane gradient, the spectral variation observed can be reproduced with no methane change, and a haze density increase of 60% between 20°S and 10°S. The largest methane variation allowed by the data, derived assuming no haze variation with latitude, is a drop of 60% over latitudes 27°S to 19°N. Our analysis indicates that the latitudinal variations in Titan’s visible to near-IR albedo, the North/South Asymmetry, result primarily from variations in the thickness of the haze above 80 km altitude. The range of methane latitudinal variations allowed between 27°S to 19°N indicates temperature variations of no more than 1.5 K at 20-30 km, altitudes where the Huygens profile is saturated.

Titan is an important planetary body for aeolian research because of the vast equatorial sand seas that span 20% of its surface. Previous studies have determined the general margins of sand and sand seas on Titan, and have speculated about the source of Titan's sand. Little research has been done concerning where sand collects in the sand seas. Additionally, the relationships be-tween material distributions as observed by the Cassini Visual and Infrared Mapping Spectrometer (VIMS) and the history of erosion and transportation of sediments across equatorial latitudes is not fully understood. This work focuses on an in depth evaluation of sand distribution and abundance across the sand seas, and presents evidence for an alternative sand source. This work also addresses a potential stratigraphy for the equatorial regions based on the excavation of materials from impact craters. We mapped the extent of relative sand abundances by comparing different Cassini image datasets, largely by mapping where the Imaging Science Subsystems (ISS) regions were darkest, in-dicating the presence of more sand. Our results revealed that greater abundances of sand accumu-late near the eastern margins of sand seas. This is in agreement with previous studies that demon-strated general W to E transport, and fits a general model of sand transport across the sand seas to collect at the downwind margins, perhaps ahead of topographic obstacles that mark the eastern ends of the sand seas. Additionally, we found that the largest continuous expanse of abundant sand de-posits lie across Belet, a large sand sea that occupies a broad equatorial lowland. Another sand sea of interest is Shangri-La, which has a recessed SE margin adjacent to the broad, albedo-bright de-pression Xanadu. We also found abundant sand deposits within Shangri-La across a corridor be-tween highlands and along the SE boundary of the sand sea. Dune crest orientations across eastern Shangri-La indicate WNW to ESE transport in the region. We propose that the low topography of Xanadu, coupled with the strong gradient in albedo between Shangri-La and Xanadu would gener-ate atmospheric disturbances similar to those responsible for transporting sand across positive changes in elevation on Mars, and may be responsible for the distinct boundary. VIMS-blue materials are generally associated with water ice mixed with organic com-pounds. We found that VIMS-blue surfaces across equatorial latitudes tend to be directly adjacent to and upwind of sand seas. This, coupled with geomorphological observations of erosional charac-teristics and examination of material properties, suggests that sand could at least in part be derived from VIMS-blue materials. We propose 3 environments (alluvial fans, dry lakebeds, and ejecta from impact craters) for sand production using this interpretation and making comparisons with SAR, ISS, and VIMS imagery. Modeling suggests that Titan's lithosphere significantly thickened 500 m.y. ago. We inter-pret an elongate exposure of VIMS-blue materials adjacent to Aztlan to be a rift caused by a thick-ening of the lithosphere, similar to many of the other icy bodies of the solar system. Our interpreta-tion is further supported by the distribution of cryovolcanic features alongside the proposed rift. Anomalous VIMS-blue and bright regions within eastern Xanadu are distributed in a pattern that resembles a multi-ringed impact basin. Additionally, when a value threshold was applied to ISS imagery, a bright circular feature was revealed within western Xanadu. These observations suggest two large impacts may have been significantly responsible for creating Xanadu. Comparisons of impact crater models with VIMS imagery of Paxsi, Menrva, Sinlap, Selk, and other craters suggest alternating layers of VIMS-bright and VIMS-blue cover much of the equatorial latitudes of Titan. We completed ground penetrating radar (GPR) and global positioning system (GPS) surveys across margins of the Kelso Dunes to evaluate the effects of fluvial interaction on sand depth. Our terres-trial model was compared to sand seas on Titan that appear to also have interactions with fluvial channels. Distributions of sand suggest that in both the Kelso Dunes and on Titan, fluvial obstruc-tion is temporary and on Titan isolated across small regions. This work leads to a better understanding of sand production, accumulation and transport on Titan and in sand seas in general, and reveals a basic stratigraphy of the equatorial regions of Titan. This region is of particular interest because it is the landing site of the Dragonfly mission, now in design.

L'observation d'occultations du Soleil et d'étoiles par Titan permet d'étudier l'atmosphère épaisse de ce satellite de Saturne du point de vue de sa composition en gaz et en aérosols. Le principe de ces observations, réalisées par le spectro-imageur visible/infrarouge VIMS de la sonde Cassini, est de mesurer la transmission du flux solaire ou stellaire à travers l'atmosphère de Titan. Les données sont constituées de courbes de lumière à différentes longueurs d'onde et de spectres infrarouges pour différentes altitudes de visée. L'étude des courbes de lumière montre qu'il s'agit d'occultations par absorption et non par réfraction différentielle comme c'est le cas pour les occultations observées depuis la Terre. La baisse de signal observée est donc due à l'absorption du flux lumineux par le gaz et les aérosols de l'atmosphère.<br>Les spectres en transmission présentent des bandes d'absorption du méthane à 1,2, 1,4, 1,7, 2,3 et 3,3 µm et du monoxyde de carbone à 4,7 µm. Un code de transfert radiatif en géométrie sphérique et utilisant la méthode de calcul raie par raie a été développé afin de modéliser les bandes observées. L'étude du méthane est centrée principalement sur la bande à 2,3 µm. Au-dessus de 200 km, nos données sont compatibles avec une abondance uniforme de 1,4 - 1,6% telle que mesurée par d'autres instruments. En dessous de 200 km, un effet systématique mal compris empêche une mesure fiable. La molécule de CO est détectée en dessous de 180 km. Une abondance de 33±10 ppm est mesurée entre 70 et 130 km d'altitude. En dessous de 500 km environ, une absorption supplémentaire, centrée sur 3,4 µm se superpose à la bande du méthane à 3,3 µm. Cette bande caractérise la vibration des liaisons C - H au sein de longues chaînes aliphatiques rattachées à de larges molécules organiques qui composent les aérosols. <br>L'absorption des aérosols fixe le niveau de continu des spectres étudiés. Celle-ci est plus forte aux courtes longueurs d'onde et augmente lorsque l'altitude décroît. Un code d'inversion du continu a été développé afin de déterminer les profils de densité des aérosols et de modéliser leur transmission. L'hypothèse de départ est que les aérosols sont des agrégats fractals composés de sphères de 0,05 µm de rayon dont les propriétés optiques sont celles des tholins de Khare et al. (1984). Les modèles de transmission obtenus révèlent que seuls les agrégats comportant plus de 1 000 sphères sont compatibles avec les observations. De plus, l'absence des deux absorptions caractéristiques à 3 et 4,6 µm dans nos données soulignent les différences significatives entre les tholins et les aérosols réels. Les profils de densité des aérosols indiquent une augmentation exponentielle en dessous de 450 km, caractérisée par une échelle de hauteur de l'ordre de 60 km pour les données de l'occultation solaire (71°S) et de l'ordre de 50 km pour celle de l'occultation de Gamma Crucis (24°N). L'écart constaté est peut-être attribuable à la différence de latitude entre ces deux observations. Enfin, les données de l'occultation rasante d'Antarès comportent de<br>nombreuses variations de flux rapides et intenses (« spikes ») : elles sont attribuées à des ondes de gravité se propageant dans l'atmosphère de Titan.