Au-delà de Neptune, les petits corps du système solaire se trouvent dans un environnement préservant le matériel primordial dont ils sont formés. Aussi, la caractérisation de la structure, l'aspect et la composition d'objets tels que les comètes révèle des informations vitales sur les processus de formation et évolutions qu'ils ont subis. Les petits corps du système solaire externe n'ont connus qu'un léger retraitement thermique et collisionnel, préservant ainsi des indices vitaux de l'histoire du système solaire primitif et permettant de contraindre ses propriétés. En conséquence, l'objectif de la mission ROSETTA de l'ESA était d'effectuer la première étude approfondie d'une comète (67P/Churyumov-Gerasimenko), en la suivant, l'observant et mesurant l'évolution de son activité sur les trois quarts de son orbite. La mission constituait la pierre d'angle de l'étude des petits corps du système solaire de l'ESA.L'objectif de cette thèse fut de déterminer les propriétés photométriques et spectrales, dans le visible, de la surface de la comète en utilisant les images de l'instrument OSIRIS. À cet effet, j'ai développé uneapproche pour préparer et analyser les données OSIRIS: j'ai utilisé etdéveloppé des méthodes existantes pour projeter les images calibrées surdes modèles 3D de la comète, j'ai crée et utilisé des codes pour calculerles géométries d'observations et simulé les images à partir d'éphémérides de la comète et de Rosetta, j'ai implémenté des modèles photométrique afin de déterminer les paramètres donnant le meilleur ajustement aux données. À l'aide de ces outils, j'ai analysé des jeux d'images d'OSIRIS lors de trois manoeuvres de survols effectuées en Août 2014, Février 2015 et en Avril 2016. Durant ces trois manoeuvres, la surface fut cartographiée avec une résolution métrique et centrimétrique, ainsi que sous de nombreuses angles de vue. J'ai aussi analysé des images prises au cours de la mission afin d'étudier certaines particularités de la surface et d'observer leur évolution temporelle. L'ajustement des jeux de données avec le modèle photométrique de Hapke indique que le noyau a une surface très sombre (un albédo de 4.2% à 650 nm), qu'elle diffuse la lumière plus vers la source plus que l'observateur, qu'elle est extrêmement poreuse (à plus de 80%), et que sa réflectivité augmente légèrementde manière non-linéaire, de manière explicable par la disparition des ombres. Outre la nature bi-lobale du noyau cométaire, les analyses de ces images ont montrée la présence d'hétérogénéités de morphologie, de couleurs et d'albédo sur des échelles hectométrique et décimétrique, confirmant ainsiles tendances globales mesurées par ROSETTA/OSIRIS et PHILAE/CIVA. Entre250 nm et 1000 nm, le spectre du noyau ne présente pas de signatures spectrale. La pente du spectre en fonction de la longueur d'onde est strictement positive comme pour certains Centaures et des astéroïdes de type D. Trois types de surface ont été identifiés à l'aide de la pente spectrale. Les terrains et particularités avec les pentes les plus grandes semblent poussiéreuses et desséchées, alors que ceux avec les pentes les plus faibles sont associées avec la présence de matériel riche en glace d'eau. Les images OSIRIS ont également permis de mesurer pour la première fois le rougissement de phase d'un noyau cométaire: la variation de la pente spectral avec la géométrie d'observation. Les deux années de données ont également permis de déterminer que le rougissement de phase varie avec la distance héliocentrique, atteignant sa valeur la plus faible quand la comète est proche du périhélie. Tout comme l'observation de variations diurnes et de falaises fraîchement fracturées, ce résultat indique que sous la surface du noyau, se trouve du matériel riche en glace d'eau. / Small bodies of the outer solar system, beyond Neptune, are in an environment that can preserve the base material from which they are formed. Hence the determination of the structure, aspect, and composition of objects such as comets yield vital informations about the formation and evolution processes they went through. Small bodies from the outer solar system have undergone weak thermal and collisionnal reprocessing, thus preserving vital clues on the history of the early solar system, which can constrain its properties. Hence the objective of the European Space Agency/ROSETTA mission was to perform the first in-depth study of a comet (67P/Churyumov-Gerasimenko), following it inbound to and outbound from perihelion, observing it and monitoring the evolution of its activity during most of its orbit. This mission constituted the cornerstone of the study of small bodies of the solar system by ESA.The aim of this thesis has been to determine the photometric characteristics and the spectral properties, from the near-ultraviolet to the near-infrared, of the comet' surface using the images taken by the OSIRIS instrument. For this purpose, I developped an approach to prepare and analyse OSIRIS' datasets: I used and build on existing methods to register calibrated images to a 3D model of the comet, I created and used codes to compute the observational geometries and simulate OSIRIS images using the comet's and Rosetta's ephemerids, I implemented photometric models to determine the parameters required to fit the datasets.Using those tools, I analysed sets of images acquired by OSIRIS during three flyby maneuvers executed in August 2014, in February 2015 and in April 2016. On those three events, the surface was mapped at a meter and sub-meter resolution and also under multiple observing conditions. I also further analysed images taken throughout the mission to investigate particular surface features and signs of temporal evolution. In the description of the Hapke photometric model, the fitting of those dataset point to a nucleus with a very dark surface (4.2% albedo at 650 nm), scattering more light towards the source than the observer, with a high upper-surface porosity (over than 80%), and displaying a limited non-linear increase in reflectivity when source and observer are aligned over the comet' surface, most probably associated with the shadow-hiding phenomemon. Beyond the obvious bilobate nature of the cometary nucleus, the analyses of those images have shown that it present heterogeneities in morphology, colours and albedo of the comet' surface from the hundred of meters to the decimeter scale, confirming the trend noticed from other ROSETTA/OSIRIS and PHILAE/CIVA observations. In the wavelength domain between 250 nm and 1000 nm, the spectrum of the nucleus does not present any band features. The slope of the spectrum increases monotonously with the wavelength in a similar way to certain Centaurs and D-type asteroids. Three categories of surface were identified based on this spectral slope. Terrains and features with the largest slopes appear dusty and dessicated while those with small or flat slopes have associated with the presence of water-ice-rich material. The OSIRIS images have also allowed to measure for the first time the phase reddening effect on a cometary nucleus, that is the variation of the spectral slope with the viewing geometry. The two years of monitoring have also allowed to further determine that the phase reddening of the nucleus varies with the heliocentric distance, reaching its lowest value while the comet was close to perihelion. This result along with observations of diurnal colour variations and of freshly fractured cliffs point to the presence of a higher proportion of water-ice material at a mere distance under the nucleus surface.

Context. Studying the coma environment of comet 67P/Churyumov-Gerasimenko (67P) is one of the primary scientific goals of the VIRTIS experiment on the ESA Rosetta mission. Aims. The distribution and variability of water vapour and carbon dioxide in the comet's coma are needed to estimate their production rate, abundances in the nucleus, and the spatial distribution of the active regions. Methods. Infrared emission lines from vibrational bands of water and carbon dioxide at 2.67 and 4.27 mu m, respectively, were observed by the VIRTIS-M imaging channel and mapped from close to the nucleus up to similar to 10 km altitude with a resolution of similar to 40 m/px. A dataset consisting of 74 observations in the 1 5 mu m spectral range acquired from 8 to 14 April 2015 when 67P was at a heliocentric distance of 1.9 AU is analysed in this work. A statistical correlation between the gas distribution and the surface's active regions was performed. Results. The maximum H2O emission is observed within 3 km from the nucleus and is mainly concentrated above two active regions, Aten-Babi and Seth-Hapi, while the CO2 distribution appears more uniform with significant emissions coming from both the "head" and southern latitude regions. In the equatorial region, the column densities of both species decrease with altitude, although CO2 decreases more rapidly than H2O. The calculated CO2/H2O column density ratios above Aten-Babi and Seth-Hapi are 2.4 +/- 0.6% and 3.0 +/- 0.7%, respectively. A value equal to 3.9 +/- 1.0% is observed at equatorial latitudes in the region encompassing Imothep. Conclusions. VIRTIS-M has mapped the distribution of water vapour and carbon dioxide around the nucleus of 67P with unprecedented spatial resolution. The different water and carbon dioxide outgassing above the surface, seen in the VIRTIS-M data, might be indicative of a different thermal history of the northern and southern hemispheres of 67P.

This work is a part of a more global effort aimed at understanding and interpreting in situ and remote sensing data acquired by instruments on board Rosetta. This study aims at deriving the dust mass source rate and the location of the dust jet source observed by Rosetta VIRTIS-M on 2015 April 12. The analysis is performed by means of the coupled kinetic modelling of gas and dust in the coma of comet 67P/Churyumov-Gerasimenko, which were used for calculating the coma brightness as it would be seen from the Rosetta spacecraft. The dust mass production rate and a possible location of the jet origin needed to explain the Rosetta VIRTIS-M dust brightness image were inferred by comparing the calculated brightness with VIRTIS-M data. Our analysis suggests that the dust mass production rate needed to maintain the observed jet is about 1.9 kg s(-1). According to our analysis, the location of the observed jet surface footprint is outside of the nucleus area characterized by the highest gas production rate, which suggests that gas and dust source rates are not necessarily proportional to each other across the entire nucleus surface. The inferred location of the possible jet origin is consistent with that of the observed active pits. In this paper, we show that the jet intensity is variable in time, and has a lifetime of at least 10 h.

Cette thèse porte sur l’étude expérimentale et théorique de quatre instabilités associées à l’émergence de motifs réguliers sur des substrats érodables ou fortement déformables,instabilités liées à l’hydrodynamique sur un relief modulé.La première partie porte sur l’étude de l’instabilité d’une plaque élastique fixée aux deux bouts et soumise à un écoulement fluide permanent. La solution plane est instable vis-à-vis d’ondes propagatives, lorsque l’écoulement est suffisamment fort. La sélection de fréquence et de longueur d’onde est caractérisée expérimentalement en fonction de la vitesse de l’écoulement. Ces quantités suivent remarquablement les lois d’échelle obtenues par l’analyse de stabilité linéaire du problème. Le principe de l’expérience pourrait être appliqué à la récupération d’énergie.La deuxième partie porte sur une analyse théorique de la formation de rides géantes sur la comète 67P, récemment observées par la sonde Rosetta. Nous montrons comment le dégazage de vapeur se produit au travers d’une couche poreuse granulaire superficielle et comment l’alternance jour/nuit conduit à des gradients de pression gigantesques qui engendrent des vents thermiques de surface. Ces motifs apparaissent comme étant les analogues de rides qui se forment à la surface de lit sableux dans un écoulement visqueux.L’analyse de stabilité linéaire du problème permet de prédire quantitativement l’émergence de ces rides à la longueur d’onde et à la vitesse de propagation observées. Cette description fournit un outil robuste et fiable pour décrire les processus d’érosion et d’accrétion dans l’évolution des petits corps.Dans la troisième partie, nous proposons un modèle pour l’apparition de motifs de sublimation sur Pluton, tels que ceux observés sur Sputnik Planum. La formation et l’évolution de ces motifs proviennent de la sublimation/condensation différentielle de la glace d’azote.Nous montrons que l’atmosphère de Pluton possède des propriétés (température et pression)peu variables en espace et en temps. Nous analysons les différents mécanismes d’instabilité en compétition et concluons à un mécanisme original, basé sur le mélange et e transport de chaleur dans l’atmosphère, plutôt qu’au mécanisme des pénitents, basé sur l’auto-éclairement de la surface de glace.Enfin, nous avons étudié théoriquement l’instabilité de formation des rides éoliennes en considérant les trajectoires des grains résonantes avec le relief. Cette modélisation prend en compte de manière simple et effective les effets collectifs du transport de sédiments. Le modèle est validé à partir de simulations numériques existantes, elles mêmes calées sur des expériences contrôlées. / This thesis is devoted to the experimental and theoretical investigations of four instabilitiesassociated with the emergence of regular patterns over erodible/flexible substrates, andrelated to hydrodynamics over a modulated relief.First, the instability of a flexible sheet clamped at both ends and submitted to a permanentwind is investigated. The flat sheet solution is unstable towards propagative waves, forstrong enough wind. We experimentally study the selection of frequency and wavenumberas a function of the wind velocity. These quantities obey simple scaling laws derived froma linear stability analysis of the problem. This phenomenon may be applied for energyharvesting.Second, an explanation is proposed for the giant ripples observed by spacecraft Rosettaat the surface of the comet 67P. We show that the outgassing flow across a porous surfacegranular layer and the strong pressure gradient associated with the day-night alternanceare responsible for thermal superficial winds. We show that these unexpected patterns areanalogous to ripples emerging on granular beds submitted to viscous shear flows. Linearstability analysis of the problem quantitatively predicts the emergence of bedforms at theobserved wavelength and their propagation. This description provides a reliable tool topredict the erosion and accretion processes controlling the evolution of small solar systembodies.Third, we propose a model for rhythmic, dune-like patterns observed on SputnikPlanum of Pluto. Their emergence and evolution are related to the differential condensation/sublimation of nitrogen ice. We show that the temperature and pressure in Pluto’satmosphere are almost homogeneous and steady, and that heat flux from the atmospheredue to convection and turbulent mixing is responsible for the emergence of these sublimationpatterns, in contrast to the penitentes instability due to solar radiation.Last, we report an analytical model for the aeolian ripple instability by considering theresonant grain trajectories over a modulated sand bed, taking the collective effect in thetransport layer into account. The model is tested against existing numerical simulationsthat match experimental observations.

No description available.

Infrared observations of the coma of 67P/Churyumov-Gerasimenko were carried out from 2015 July to September, i.e. around perihelion (2015 August 13), with the high-resolution channel of the Visible and Infrared Thermal Imaging Spectrometer instrument onboard Rosetta. We present the analysis of fluorescence emission lines of H2O, CO2, (CO2)-C-13, OCS, and CH4 detected in limb sounding with the field of view at 2.7-5 km from the comet centre. Measurements are sampling outgassing from the illuminated Southern hemisphere, as revealed by H2O and CO2 raster maps, which show anisotropic distributions, aligned along the projected rotation axis. An abrupt increase of water production is observed 6 d after perihelion. In the meantime, CO2, CH4, and OCS abundances relative to water increased by a factor of 2 to reach mean values of 32, 0.47, and 0.18 per cent, respectively, averaging post-perihelion data. We interpret these changes as resulting from the erosion of volatile-poor surface layers. Sustained dust ablation due to the sublimation of water ice maintained volatile-rich layers near the surface until at least the end of the considered period, as expected for low thermal inertia surface layers. The large abundance measured for CO2 should be representative of the 67P nucleus original composition, and indicates that 67P is a CO2-rich comet. Comparison with abundance ratios measured in the Northern hemisphere shows that seasons play an important role in comet outgassing. The low CO2/H2O values measured above the illuminated Northern hemisphere are not original, but the result of the devolatilization of the uppermost layers.

Context. Since its rendezvous with comet 67P/Churyumov-Gerasimenko (67P), the Rosetta spacecraft has provided invaluable information contributing to our understanding of the cometary environment. On board, the VIRTIS and ROSINA instruments can both measure gas parameters in the rarefied cometary atmosphere, the so-called coma, and provide complementary results with remote sensing and in situ measurement techniques, respectively. The data from both ROSINA and VIRTIS instruments suggest that the source regions of H2O and CO2 are not uniformly distributed over the surface of the nucleus even after accounting for the changing solar illumination of the irregularly shaped rotating nucleus. The source regions of H2O and CO2 are also relatively different from one another. Aims. The use of a combination of a formal numerical data inversion method with a fully kinetic coma model is a way to correlate and interpret the information provided by these two instruments to fully understand the volatile environment and activity of comet 67P. Methods. In this work, the nonuniformity of the outgassing activity at the surface of the nucleus is described by spherical harmonics and constrained by ROSINA-DFMS data. This activity distribution is coupled with the local illumination to describe the inner boundary conditions of a 3D direct simulation Monte-Carlo (DSMC) approach using the Adaptive Mesh Particle Simulator (AMPS) code applied to the H2O and CO2 coma of comet 67P. Results. We obtain activity distribution of H2O and CO2 showing a dominant source of H2O in the Hapi region, while more CO2 is produced in the southern hemisphere. The resulting model outputs are analyzed and compared with VIRTIS-M/-H and ROSINADFMS measurements, showing much better agreement between model and data than a simpler model assuming a uniform surface activity. The evolution of the H2O and CO2 production rates with heliocentric distance are derived accurately from the coma model showing agreement between the observations from the different instruments and ground-based observations. Conclusions. We derive the activity distributions for H2O and CO2 at the surface of the nucleus described in spherical harmonics, which we couple to the local solar illumination to constitute the boundary conditions of our coma model. The model presented reproduces the coma observations made by the ROSINA and VIRTIS instruments on board the Rosetta spacecraft showing our understanding of the physics of 67P’s coma. This model can be used for further data analyses, such as dust modeling, in a future work.

La majeure partie de mon travail de thèse a porté sur l’étude de la composition chimique de la surface d’Europe. Afin d’apporter une plus-value par rapport aux résultats de l’instrument NIMS à bord de la sonde Galileo qui orbita dans le système jovien de 1995 à 2003, une campagne d’observations depuis le sol a été menée avec le spectromètre imageur infra-rouge SINFONI au VLT. Cinq observations en optique adaptative possédant une résolution spatiale d’environ 160 km et une résolution spectrale R=1500 dans le proche infra-rouge furent acquises. Une procédure complexe de réduction des données a été mise en place afin de construire un cube hyperspectral global en réflectance de la surface. La modélisation linéaire de chacun des spectres du cube a permis d’aboutir aux premières cartes d’abondances absolues jamais obtenues pour la surface d’Europe. Ces cartes confirment la présence des deux espèces majoritaires, la glace d’eau et l’acide sulfurique hydraté. La distribution de l’acide sulfurique est centrée sur l’hémisphère orbital arrière qui est préférentiellement impacté par un flux d’ions de soufre originaires du tore de plasma produit par l’activité volcanique d’Io. Cependant, deux résultats inattendus ont été obtenus. Le premier concerne la glace d’eau, dont la forme cristalline est près de deux fois plus abondante que la forme amorphe selon les résultats de la modélisation. Ce résultat, surprenant compte tenu du taux d’irradiation très élevé auquel la surface est soumise, pourrait s’expliquer par l’existence d’un fort gradient de cristallinité au sein de la couche de glace, mais aussi par l’existence d’une activité endogène relativement soutenue qui se traduit visuellement par une surface peu cratérisée, donc jeune. La corrélation entre la distribution des grains cristallins et la géomorphologie semble accréditer la seconde hypothèse. Le second résultat concerne la détection de sels chlorés à partir de la modélisation des spectres hautement résolus de SINFONI, et non de sulfates, remettant en cause les détections marginales annoncées par les observations de l’instrument NIMS/Galileo. La distribution des sels chlorés, tout comme celle de la glace d’eau cristalline, est corrélée à la géomorphologie, ce qui confirme le rôle important des apports endogènes. Des processus tectoniques et cryovolcaniques mis en évidence récemment pourraient être à l’origine de cette distribution. L’autre versant de ma thèse a été consacré aux caractéristiques physiques des grains cométaires de 67P/C-G. L’instrument COSIMA embarqué sur l’orbiter de la sonde Rosetta a permis la collecte, l'imagerie et l'analyse chimique élémentaire des grains présents dans l’environnement proche de 67P/C-G. Une approche automatisée de la détection des grains à partir des images prises par la caméra « COSISCOPE » a été mise en place et a permis de détecter environ 35000 grains ayant une surface de plus de 100 µm2 entre août 2014 et mai 2016. La résolution de 13.7 µm/pixel a rendu possible la caractérisation en détail de la forme et de la structure des grains, et le nombre important de détections a permis d’obtenir des statistiques robustes concernant la distribution en taille et l’évolution de celle-ci au cours du temps. Deux grandes familles de grains ont été identifiées : les grains compacts, qui ne représentent qu’une faible minorité des grains et qui ont été majoritairement collectés en début de mission, et les agrégats, qui ont une structure très poreuse similaire à celle des IDPs et des micrométéorites collectées en Antarctique. La distribution en taille obtenue suit une loi de puissance intégrale en r-2.66. La comparaison avec des lois obtenues à des échelles différentes par d’autres instruments met en évidence des différences qui peuvent être interprétées par des mécanismes d’éjection dépendant de la taille associés à un biais du processus de collecte en orbite. / The major part of my work focused on the study of the chemical composition of Europa’s surface. In order to provide additional insights in comparison to the results of the NIMS instrument onboard the Galileo spacecraft that orbited in the Jovian system from 1995 to 2003, a ground-based observations campaign was conducted with the infrared imaging spectrometer SINFONI on the VLT. Several observations using adaptive optics with a spatial resolution of about 160 km and a spectral resolution R = 1500 in the near-infrared were acquired and then combined. A specific data reduction pipeline was developed to build a global hyperspectral cube in surface reflectance. The linear modeling of each spectra of this cube leads to the first global abundance maps ever obtained for the surface of Europa. These maps confirm the presence of the two major species, namely water ice and hydrated sulfuric acid. The distribution of the hydrated sulfuric acid is centered on the trailing orbital hemisphere preferentially affected by a sulfur ion flux coming from the plasma torus produced by Io volcanic activity. Two surprising results were obtained. The first one concerns water ice, which crystalline form is about twice more abundant than the amorphous form according to the modeling results. This result, unexpected given the very high radiation rate on the surface, could be explained by a strong crystallinity gradient through the ice slab. However, it could also point out an endogenous activity possibly strong as first suggested by its poorly well-known cratered surface. The correlation between the crystalline grains distribution and the geomorphology seems in favor to the second hypothesis. The second result is related to the detection of chlorinated salts from the modeling of highly resolved spectra from SINFONI. Sulfates, first reported by several analyses of NIMS observations are marginal in the modeled composition, hence challenging their presence on the surface of Europa. The chloride distribution, as well as the one of the crystalline water ice, is correlated to geomorphology, potentially confirming significant endogenous contributions as the result of tectonic and cryovolcanic processes recently highlighted. The second part of my PhD was dedicated to the physical characteristics of the 67P/C-G’s cometary grains. The COSIMA instrument onboard the Rosetta orbiter allowed the collection, imaging and elemental chemical analysis of grains present in the immediate environment of 67P/C-G. An automated approach of the grains detection based on images taken by the camera "COSISCOPE" was set up and able to detect about 35.000 grains having an area of more than 100 µm² between August 2014 and May 2016. The resolution of 13.7 µm/pixel allowed to perform a detailed characterization of the shape and the structure of the grains, and the large number of detections permitted to obtain significant statistics on the size distribution and its evolution over the time. Two large families of grains have been identified: the compact grains, which represent only a small minority of grains mostly collected at the beginning of the mission, and aggregates, which have a very porous structure similar to those of IDPs and micrometeorites collected in Antarctica. The size distribution obtained follows an integrated power law in r-2.66. The comparison with the laws obtained at different scales by other instruments highlights differences that can be interpreted by ejection mechanisms depending on the size associated with a selection bias during the collection process in orbit.

The plasma environment of an active comet provides a unique setting for plasma physics research. The complex interaction of newly created cometary ions with the flowing plasma of the solar wind gives rise to a plethora of plasma physics phenomena, that can be studied over a large range of activity levels as the distance to the sun, and hence the influx of solar energy, varies. In this thesis, we have used measurements of the spacecraft potential by the Rosetta Langmuir probe instrument (LAP) to study the evolution of activity of comet 67P/Churyumov-Gerasimenko as it approached the sun from 3.6 AU in August 2014 to 2.1 AU in March 2015. The measurements are validated by cross-calibration to a fully independent measurement by an electrostatic analyzer, the Ion Composition Analyzer (ICA), also on board Rosetta. The spacecraft was found to be predominantly negatively charged during the time covered by our investigation, driven so by a rather high electron temperature of ~5 eV resulting from the low collision rate between electrons and the tenuous neutral gas. The spacecraft potential exhibited a clear covariation with the neutral density as measured by the ROSINA Comet Pressure Sensor (COPS) on board Rosetta. As the spacecraft potential depends on plasma density and electron temperature, this shows that the neutral gas and the plasma are closely coupled. The neutral density and negative spacecraft potential were higher in the northern hemisphere, which experienced summer conditions during the investigated period due to the nucleus spin axis being tilted toward the sun. In this hemisphere, we found a clear variation of spacecraft potential with comet longitude, exactly as seen for the neutral gas, with coincident peaks in neutral density and spacecraft potential magnitude roughly every 6 h, when sunlit parts of the neck region of the bi- lobed nucleus were in view of the spacecraft. The plasma density was estimated to have increased during the investigated time period by a factor of 8-12 in the northern hemisphere and possibly as much as a factor of 20-44 in the southern hemisphere, due to the combined effects of seasonal changes and decreasing heliocentric distance. The spacecraft potential measurements obtained by LAP generally exhibited good correlation with the estimates from ICA, confirming the accuracy of both of these instruments for measurements of the spacecraft potential.

No description available.