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I -- Importance of phase changes and nucleation in Titan's lower Atmosphere -- We have pointed out the interactions between phase changes in the lower atmosphere and other processes which occur on Titan. Phase change processes can affect photo-chemistry, the nature of Titan's exposed surface and hence its spectral properties, and the atmospheric thermal profile. We have critically reviewed clues that suggest that nucleation is difficult in Titan's atmosphere and we have tried to make clear why the description of nucleation is a priority in modeling phase changes. The results of the analysis of IRIS spectra and the low temperatures that imply solid rather than liquid phases are the main arguments for nucleation modeling. In the frame of the classical theory of nucleation, we have proposed a definition and a value of the critical nucleation rate which are adapted to the study of Titan's atmosphere. From this critical rate, we have computed the contact angle of methane on aerosols that is needed if the inhibition of methane nucleation is the reason for the super-saturation suggested by IRIS observations. -- II -- Estimation of contact angles and surface free enthalpies of solids -- Those parameters are essential for the description of nucleation. As experimental data for the solid phases of the species we are interested in are unavailable, we have selected and pieced together estimation methods for contact angles and surface free enthalpies of solids, from the existing literature. The surface free enthalpy of a solid may be linked to the surface tension of the liquid and the latent heats of vaporization and sublimation. However, a few variations on this idea have been proposed. In order to choose the adequate correlation for solid hydrocarbons and nitriles, the solid surface free enthalpy for a few selected reference species may also be estimated from the Hamaker constant. The Hamaker constant is itself linked to the dielectric permittivity, hence to the absorption spectra of the solid phase. Thus, we have opened up a way between ongoing laboratory investigations on the spectra of solid hydrocarbons and nitriles, and the computations of surface free enthalpies and contact angles. -- III -- Building models for the description of phase changes in Titan's lower atmosphere -- Essential phenomena to be described in a model of phase changes, beside condensation and evaporation, are nucleation, aerosol settling and gas transport by eddy diffusion. Nucleation is described with the classical theory. The contact angle is not assumed to be zero, it is a free parameter. We have managed to bring together these phenomena in the equations, addressing such problems as the volume of the aerosols just after nucleation and the treatment of aerosols with size smaller than the Kelvin equilibrium size. As we expect non-stationary evolution for some values of the contact angle, we have made the model time-dependent. Therefore, the model (numbered 3) consists of a set of partial differential equations. In order to check the validity of the model itself (the way nucleation is built into the equations) and the validity of the numerical resolution to come, we have considered a simpler model (numbered 2) with instantaneous nucleation, and we have demonstrated that the solutions of model 3 should tend toward a solution of model 2 when the contact angle and the surface free enthalpy of the condensed phase tend toward zero. -- IV -- We have studied two more simple models. One model (numbered 1) assumes negligible super-saturation of all species. This very easy-to-use model provides insight into the influence of such parameters as the eddy diffusion coefficient, the mass flux of tholins falling from the upper atmosphere, the radius of tholin particles, the gas mole fractions at the surface. In particular, for plausible values of those parameters (as found in the literature), by studying the self-consistency of the model, we have shown that methane super-saturation would rather be due to nucleation inhibition or coagulation than the dynamics of gas transport and condensation (after nucleation). Thus, small super-saturation is really not a hypothesis, and model 1 gives correct results in the event of easy nucleation and unimportant coagulation. Then the evaporation-condensation cycle of methane in the troposphere involves the evaporation of typically a few centimeters, up to fifty centimeters, of methane per year, and the concomitant exchange of latent heat may be on the order of 0.1 W m-2, up to 3 W m-2. Condensation of acetylene starts at about 61 km, and condensation of ethane at 51 km. At the tropopause, the radius of aerosols reaches approximately 3 times the radius of tholin nuclei. At 20 km, the radius has increased to typically 200 times the radius of tholin nuclei due to methane condensation. Finally, a model is specifically set up to demonstrate the potential periodic behavior associated to some values of the contact angle. As opposed to the other models mentioned above, it does not aim at providing values for properties of Titan's atmosphere, and gives a qualitative information. Periodicity is expected when the abundance from the instantaneous nucleation model is distinctly lower than the critical abundance (obtained from the critical nucleation rate), and the critical abundance itself is distinctly lower than the abundance without condensation. The period then diminishes with the critical abundance., I -- Importance des changements de phase et de la nucléation dans la basse atmosphère de Titan -- Nous soulignons les interactions entre les changements de phase dans la basse atmosphère et d'autres phénomènes qui se produisent sur Titan. Les processus de changements de phase peuvent affecter la photochimie, la nature de la surface exposée de Titan et donc ses propriétés optiques, et le profil de température de l'atmosphère. Nous passons en revue les indices d'une nucléation difficile dans l'atmosphère de Titan et nous essayons de montrer que la description de la nucléation est une priorité dans la modélisation des changements de phase. Les résultats de l'analyse des spectres d'IRIS et les basses températures qui impliquent des phases solides et non liquides fournissent les principaux arguments en faveur de la modélisation de la nucléation. Dans le cadre de la théorie classique de la nucléation, nous proposons une définition et une valeur du taux de nucléation critique adaptées à l'étude de l'atmosphère de Titan. A partir de ce taux critique, nous calculons l'angle de contact du méthane sur les aérosols nécessaire pour que l'inhibition de la nucléation du méthane puisse expliquer la sursaturation suggérée par les observations d'IRIS. -- II -- Estimation des angles de contact et des enthalpies libres surfaciques des phases solides -- Ces paramètres sont essentiels pour la description de la nucléation. Comme les données expérimentales pour les phases solides des espèces qui nous intéressent ne sont pas disponibles, nous relevons dans la littérature et mettons bout à bout des méthodes pour l'estimation des angles de contact et des enthalpies libres surfaciques des solides. L'enthalpie libre surfacique s d'un solide peut être reliée à la tension superficielle du liquide et aux chaleurs latentes de vaporisation et de sublimation. Cependant, quelques variations sur ce thème ont été proposées. Afin de choisir la corrélation adéquate pour les hydrocarbures et les nitriles solides, s peut être estimé pour quelques espèces de référence à partir de la constante d'Hamaker. La constante d'Hamaker est elle-même liée à la permittivité diélectrique, donc au spectre d'absorption de la phase solide. Ainsi, nous établissons un pont entre les études de laboratoire en cours sur les spectres des hydrocarbures et des nitriles solides, et le calcul des enthalpies libres surfaciques et des angles de contact. -- III -- Élaboration de modèles pour la description des changements de phase dans la basse atmosphère de Titan -- Les phénomènes essentiels qui doivent être décrits dans un modèle de changements de phase, outre la condensation et l'évaporation, sont la nucléation, la sédimentation des aérosols et le transport de gaz par diffusion turbulente. La nucléation est décrite à l'aide de la théorie classique. L'angle de contact n'est pas supposé nul, c'est un paramètre libre. Nous parvenons à mettre tous ces phénomènes ensemble en équation, en gérant des problèmes tels que le volume des aérosols juste après la nucléation et le traitement des aérosols de taille inférieure au rayon d'équilibre associé à l'effet Kelvin. Comme nous nous attendons à une évolution non stationnaire pour certaines valeurs de l'angle de contact, nous laissons le modèle dépendre du temps. Par conséquent, le modèle (numéroté 3) est représenté par un système d'équation aux dérivées partielles. Afin de vérifier la validité du modèle lui-même (la manière dont la nucléation est mise en équations) et la validité de la résolution numérique à venir, nous considérons un modèle (numéroté 2) plus simple avec nucléation instantanée, et nous démontrons que les solutions du modèle 3 doivent tendre vers une solution du modèle 2 lorsque l'angle de contact et l'enthalpie libre surfacique de la phase condensée tendent vers zéro. -- IV -- Nous étudions deux autres modèles simples. L'un des modèles (numéroté 1) suppose une sursaturation négligeable pour toutes les espèces. Ce modèle très facile à mettre en oeuvre permet de comprendre l'influence de certains paramètres comme le coefficient de diffusion turbulente, le flux de masse de tholins arrivant de la haute atmosphère, le rayon des particules de tholins et les fractions molaires en phase gazeuse à la surface. En particulier, pour des valeurs plausibles de ces paramètres (trouvées dans la littérature), en étudiant l'auto-cohérence du modèle, nous montrons que la sursaturation éventuelle du méthane serait due à l'inhibition de la nucléation ou à la coagulation plutôt qu'à la dynamique du transport gazeux et de la condensation (après la nucléation). Ainsi, la faible sursaturation n'est pas réellement une hypothèse et le modèle 1 donne des résultats corrects si la nucléation est facile et la coagulation peu importante. Dans ce cas, le cycle d'évaporation et de condensation du méthane dans la troposphère entraîne l'évaporation de typiquement quelques centimètres, et jusqu'à 50 cm, de méthane par an. L'échange de chaleur latente concomitant peut être de l'ordre de 0,1 W.m-2, et jusqu'à 3 W.m-2. La condensation de l'acétylène commence à environ 61 km, celle de l'éthane à 51 km. A la tropopause, le rayon des aérosols atteint approximativement 3 fois le rayon des noyaux de tholins. A 20 km, suite à la condensation du méthane, le rayon a augmenté jusqu'à une valeur qui est typiquement 200 fois supérieure au rayon des noyaux de tholins. Enfin, nous construisons un modèle spécialement pour illustrer le comportement périodique potentiel associé à certaines valeurs de l'angle de contact. Par opposition aux autres modèles, celui-ci ne prétend pas fournir des valeurs pour les propriétés de l'atmosphère de Titan, mais seulement une information qualitative. La périodicité est attendue quand l'abondance donnée par le modèle avec nucléation instantanée est nettement plus faible que l'abondance critique (déduite du taux de nucléation critique), elle-même nettement plus faible que l'abondance en l'absence de condensation. La période diminue alors avec l'abondance critique.

39 pages; The uncertainty about possible supersaturation of methane, condensation of volatile species and the existence of clouds in Titan's lower atmosphere affects our understanding of photochemistry, the nature of the surface and the atmospheric thermal structure. Indeed, photochemistry depends on the depth of penetration of energetic photons, affected by methane abundance. Radar and infrared observations of bright surface regions have been explained by rain washing of highlands. As for the thermal profile, it is sensitive to CH4-N2 gas opacity, cloud opacity and could be influenced by latent heat exchange. A rudimentary model with no methane supersaturation and gas transport by eddy diffusion indicates a methane latent heat release of 0.2 W m-2 between 20 and 30 km altitude for a surface mole fraction of 4.4 % and an eddy diffusion coefficient of 0.2 m2 s-1. Description of nucleation seems to be one of the first improvements which should be included in a model of phase changes. The suspicion of difficult methane nucleation comes from analysis of Voyager IRIS spectra. Moreover, species are expected to condense to the solid phase, which excludes very efficient nucleation and condensation processes associated with the presence of a liquid phase, such as deliquescence. The classical theory of heterogeneous nucleation, despite its deficiencies, is employed in atmospheric models, owing to its general nature and relative simplicity. Yet, it requires physical quantities for which experimental values do not exist. We show how surface free enthalpies of solids and contact angles may be linked to other material properties which are within reach of laboratory experiments, mainly ultraviolet absorption spectra of solid phases. We find that a value of 10^{-9} s-1 - 10^{-7} s-1 for the 'critical nucleation rate' (per nucleus) is adapted to the case of Titan, though we question the ability of the critical rate concept to make predictions for the condensation altitudes. A possible consequence of difficult methane nucleation is periodic evolution of the lower atmosphere, on a time scale of the order of 10^2 years.

Observations from the Gravity Recovery and Interior Laboratory (GRAIL) mission indicate a marked change in the gravitational signature of lunar impact structures at the morphological transition, with increasing diameter, from complex craters to peak-ring basins. At crater diameters larger than ~200 km, a central positive Bouguer anomaly is seen within the innermost peak ring, and an annular negative Bouguer anomaly extends outward from this ring to the outer topographic rim crest. These observations demonstrate that basin-forming impacts remove crustal materials from within the peak ring and thicken the crust between the peak ring and the outer rim crest. A correlation between the diameter of the central Bouguer gravity high and the outer topographic ring diameter for well-preserved basins enables the identification and characterization of basins for which topographic signatures have been obscured by superposed cratering and volcanism. The GRAIL inventory of lunar basins improves upon earlier lists that differed in their totals by more than a factor of 2. The size-frequency distributions of basins on the nearside and farside hemispheres of the Moon differ substantially; the nearside hosts more basins larger than 350 km in diameter, whereas the farside has more smaller basins. Hemispherical differences in target properties, including temperature and porosity, are likely to have contributed to these different distributions. Better understanding of the factors that control basin size will help to constrain models of the original impactor population.

A technique for converting gamma-ray count rates measured by the Gamma-Ray Spectrometer on the MESSENGER spacecraft to spatially resolved maps of the gamma-ray emission from the surface of Mercury is utilized to map the surface distributions of the elements Si, O, and K over the planet's northern hemisphere. Conversion of the K gamma-ray count rates to elemental abundances on the surface reveals variations from 300 to 2400 ppm. A comparison of these abundances with models for the maximum surface temperature suggests the possibility that a temperature-related process is controlling the K abundances on the surface as well as providing K to the exosphere. The abundances of K and Th have been determined for several geologically distinct regions, including Mercury's northern smooth plains and the plains interior to the Caloris basin. The lack of a significant variation in the measured Th abundances suggests that there may be considerable variability in the K/Th abundance ratio over the mapped regions.

The volcanic plains that fill the Caloris basin, the largest recognized impact basin on Mercury, are deformed by many graben and wrinkle ridges, among which the multitude of radial graben of Pantheon Fossae allow us to resolve variations in the depth extent of associated faulting. Displacement profiles and displacement-to-length scaling both indicate that faults near the basin center are confined to a ~ 4-km-thick mechanical layer, whereas faults far from the center penetrate more deeply. The fault scaling also indicates that the graben formed in mechanically strong material, which we identify with dry basalt-like plains. These plains were also affected by changes in long-wavelength topography, including undulations with wavelengths of up to 1300 km and amplitudes of 2.5 to 3 km. Geographic correlation of the depth extent of faulting with topographic variations allows a first-order interpretation of the subsurface structure and mechanical stratigraphy in the basin. Further, crosscutting and superposition relationships among plains, faults, craters, and topography indicate that development of long-wavelength topographic variations followed plains emplacement, faulting, and much of the cratering within the Caloris basin. As several examples of these topographic undulations are also found outside the basin, our results on the scale, structural style, and relative timing of the topographic changes have regional applicability and may be the surface expression of global-scale interior processes on Mercury.