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Uranus and Neptune, the Ice Giants, are the unique planets in the Solar System that have not received a dedicated mission. However, studying these planets is crucial for understanding the formation and evolution of our planetary system and the outer systems, for which the ice planet systems are very common.Our current knowledge comes from Earth and space telescope limited observations and from the brief encounter with the Voyager 2 spacecraft almost three decades ago. The recent decadal survey [1] has established a flag mission to Uranus as the following strategic priority for the Nasa exploration program (apart from the ongoing missions to Mars and Europa). From ESA’s perspective, the outcomes from the Voyage 2050 [2] are also in alignment, recommending the agency’s participation in a future mission in a collaboration framework, as established in previous successful partnerships like Cassini-Huygens.Several reference missions have been proposed during the last decade [3-4], most of them suggesting an orbiter plus a descent probe configuration. For the orbiter, the scientific priorities should be to study the planet's bulk composition and internal structure, magnetic field, atmosphere circulation, rings, and satellite system. In the case of the descent probe, its primary mission should be to obtain the atmospheric noble gas abundances, noble gas isotope ratios, and the thermal structure of the atmosphere using a mass spectrometer and a meteorological package.Understanding the thermal structure and dynamics of Uranus’ atmosphere requires studying the vertically distributed aerosols (hazes and clouds) and their microphysical and scattering properties. Indeed, aerosols affect the absorption and reflection of solar radiation, directly affecting the energy balance that drives the planet. In this work we present a lightweight radiometer, as a part of the descending probe, dedicated to studying Uranus’s aerosols. The principle of measurement is based on the vertical variation of the solar radiance at different wavelengths and geometries of observations as the probe falls using photodetectors, field-of-view masks, and interferential filters. From these observations, information on the vertical structure of clouds and hazes, particle size, or scattering properties could be derived.The radiometer takes its heritage from previous missions for Mars exploration [7-9] where its technology has demonstrated its endurance for extreme environments of operation, using limited resources in terms of power consumption, mass and volume footprints, and data budget. These characteristics make this instrument a valuable complementary probe’s payload for studying Uranus’ atmosphere with a high scientific return. [1] Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. [2] Linda J. Tacconi, Christopher S. Arridge, et al, Voyage2050 Final recommendations from the Voyage 2050 Senior Committee. [3] Christopher S. Arridge, et al.. 2012. [4] Sushil K.AtreyaaMark, et al.,2019 [5] Ian J. Cohen et al 2022 P [6] Athul Pradeepkumar Girija. 2023 [7] I. Arruego et al. 2017. [8] Apestigue, V. et al 2022 [9] Pérez-Izquierdo, J., Sebastián et al, 2016.

Introduction With 13 years of observations, the Visual And Infrared Mapping Spectrometer (VIMS) onboard the \textit{Cassini} spacecraft has observed the surface and atmosphere of Titan through two seasons: winter and spring. In VIMS-IR spectra the surface is only seen in seven atmospheric windows due to the strong methane absorption. To retrieve the surface albedo we use radiative transfer (RT) models to compensate for the signal due to the atmosphere. Thanks to the lander Huygens, we have information about the optical properties of the aerosols above the equator that can be used in RT models. However, using the same aerosols vertical profile at high latitude doesn't work. With the help of the results of and Global Circulation Models and the Composite InfraRed Spectrometer onboard Cassini, we changed the aerosol vertical profile and optical properties in our RT model to better fit VIMS data at high latitude. While this model is not well constrained due to a lack of data, we manage to adjust the optical properties so our RT model based on Coutelier et al., (2021) retrieve surface albedo mostly between 0 and 1 instead of values crossing these boundaries like we had previously. It allow us to study with a RT model the shores and polar seas of Titan. We applied this new model on the same area of Kraken Mare in three consecutive VIMS cubes of the same flyby (subsequently named C1, C2 and C3). It allow us to validate our model on terrains with different albedo, and to notice an interesting feature in Kraken Mare that could be interpreted as sediment transport into the sea. Adaptation of the aerosols optical properties and vertical profile We decided to keep a 2 layers-based aerosol model, separated into haze and mist. We first changed the haze opacity vertical profile, using an exponential law : with the altitude of transition between mist and haze Ztr=70 km, and the scale height Hh=40 km. τh1µm is the opacity calculated by Doose et al., 2016 at 1 µm. We then changed the spectral slope of the optical depth of the mist with a simple power law as a first approximation : with Δτmnorm the normalized optical depth of the atmospheric layer of mean altitude z, τhλ0 the total optical depth at λ0 = 1 µm calculated by Doose et al 2016., λ the wavelength, and the parameter b = 2.2 +/- 0.2 The most influent parameter is the mist single scattering albedo ωm. we decided to change it depending on that of the haze ωh and a factor α = 0.4 +/- 0.1. Application and results on Kraken Mare We tuned and applyed this model on three successive VIMS cubes (full names in Fig. 1) subsequently called C1, C2 and C3. We retrieved the albedo on a zone crossing Kraken Mare, containing pixels from land, shore and methane sea. They are circled in red in Fig.1. The top part shows the VIMS cubes, and the bottom part their footprint on the geomorphologic map of Titan. That way we can have an expectation on the retrieved albedo: dark in the sea, and bright on land. Figure 1 : (top) Successive VIMS cubes from flyby 292TI (colors : R : 5.01, V : 1.28, B : 2.79 μm).(bottom) : Footprint of the VIMS pixels on the geomorphologic map from Lopez et al., 2020. The pixels in our study are circled in red. The pixels circled in blue have mixed signatures. The retrieved albedo are on Fig. 2. We still have remaining problems with negative albedo on dark pixels, mostly in the first atmospheric window. We can still differentiate very well the signatures from different terrains. Those coming from Kraken Mare are in blue, and those coming from the land are in green in Fig. 2. In C2, we have a pixel localized on the shore containing part of land and sea, circled in pink. Its signature is mixed, as we expected. However, we notice that on C1 and C3, two pixels localized in Kraken Mare (also circled in pink) also have a mixed signature. We did check that it was not a mistake in the cube geolocalization, or a difference due to a cloud. Figure 2 : Retrieved albedo of the selected pixels in Fig. 1. from the cubes C1, C2 and C3. The errors bar are calculated from the error on VIMS, and not from the error on the model. They are underestimated as a consequence. Discussion Infrared can penetrate deeply into liquid methane and ethane. The mixed signature we noticed can come from sediment transport carried by rivers flowing into Kraken Mare, issued from the erosion of the bedrock. While this aerosol model for the poles is not exact, nor well constrained, the RT model is working and gives reasonable results on different cubes from the same flyby. We can compare the different surface albedos instead of the absolute values, because the atmospheric model is the same for all of the studied pixels. The combination of the RT analysis with the geomorphologic map is a powerful tool that leads to notice unexpected signatures. With the seasons changes, we can expect that the improved polar aerosol model is not constant, so further studies should be made on other cubes through different seasons. We could that way follow through an other method the seasonal variation of the polar haze and mist layers. References Doose et al. (2016) Vertical structure and optical properties of Titan’s aerosols from radiance measurements made inside and outside the atmosphere. Icarus 270 : 355-375. Coutelier et al. (2021) Distribution and intensity of water ice signature in South Xanadu and Tui Regio. Icarus 364 : 114464. Lopes et al. (2020) A global geomorphologic map of Saturn’s moon Titan." Nature astronomy 4.3 : 228-233.

Introduction Titan is the only moon in the solar system with a thick atmosphere, dominated by nitrogen and organic compounds and methane- and ethane-based climatic cycles similar to the hydrological cycle on Earth. Hence, Titan is a prime target for planetary and astrobiological researches. Heaviest organic materials resulting from atmospheric chemistry (including high atomic number aerosols) precipitate onto the surface and are subject to geological processes (e.g., eolian and fluvial erosion) that lead to the formation of a variety of landscapes, including dune fields, river networks, mountains, labyrinth terrains, canyons, lakes and seas analogous to their terrestrial counterparts but in an exotic context. Its optically thick atmosphere, however, prevents the surface from being probed in the entirety of the near-infrared (NIR) range, and its composition is still largely unknown, or largely debated at the least, preventing to fully understand and quantify the geological processes at play. Incident and reflected solar radiations are indeed strongly affected by gaseous absorption and aerosol scattering in the NIR. Only where the methane absorption is the weakest, a few transmission windows allow the detection of radiation coming from the low atmosphere and the surface, making possible to retrieve the surface albedo. In the 0.88-5.11 μm range (VIMS-IR channel), the Visual and Infrared Mapping Spectrometer (VIMS) instrument on board the Cassini spacecraft has shown that the surface can be observed in eight narrow transmission windows centered at 0.93, 1.08, 1.27, 1.59, 2.03, 2.69, and 2.78 μm, and in the 5.0-5.11 μm interval. Even in these transmission windows, residual gaseous absorption and increasing scattering from aerosols with decreasing wavelength make the analysis of the surface signal and the retrieving of surface albedo complex and delicate. In order to retrieve the surface albedo in the atmospheric windows in the most possible rigorous way, we have developed a radiative transfer (RT) model with up-to-date gaseous abundances profiles and absorption coefficients and improved photochemical aerosol optical properties. We validated our model using in situ observations of Huygens-DISR (Descent Imager / Spectral Radiometer) acquired during descent and once landed. We then applied our RT model to the Selk crater area (the Dragonfly mission landing area) in order to map the surface albedo and discuss the surface properties of the different geomorphological units of the region. Radiative transfer Our RT model is based on the SHDOM solver to solve the RT equations using the plan-parallel approximation. Vertical abundance profiles and absorption lines of CH4 and isotopes, CO, C2H2 and HCN are implemented using the most recent studies. Correlated-k coefficients are used to calculate gases absorption coefficients at VIMS-IR spectral sampling and resolution. Aerosols extinction profile and single scattering albedo are described using a fractal code developed by [1], allowing the aerosol fractal dimension to be varied. Aerosols phase function is modified using a multi-angular VIMS sequence (Sébastien Rodriguez, personal communication). Our model is validated using the in situ observations of Huygens-DISR acquired during the complete descent sequence and once landed. Application We applied our RT model to the Selk crater region by inverting aerosol opacity and surface albedo over 4 VIMS cubes (1578266417_1, 1575509158_1, 1578263500_1, 1578263152_1) acquired over the area. We built local maps of aerosol opacities and surface albedos of the Selk region by combining the 4 VIMS cubes on a geographically projected mosaic (see the mosaic of the 4 raw VIMS observations in Fig. 1). A few longitudinal profiles of the retrieved atmospheric properties are shown in Fig. 2. Slopes and seams between cubes of the aerosol opacities, originally due to varying observation geometries between flybys, have been entirely corrected, confirming the robustness of our RT model and making the retrieved surface albedo more reliable. Retrieved surface albedo have been then corrected for the photometry using in-situ observations ([3]). The resulting albedo maps of the regions are highly contrasted and homogeneous, most of the seams between cubes (due to residual surface photometry) being corrected (Fig. 3). Conclusion We developed and validated a new RT model for Cassini-VIMS observations of Titan with up-to-date atmospheric optical description. Coupled with an efficient inversion scheme, our model can be apply to the complete VIMS dataset for the retrieval of Titan’s atmospheric opacities and surface albedos at regional and global scales. References [1] Rannou, P., McKay, C., & Lorenz, R. 2003, Planetary and Space Science, 51, 963 [2] Karkoschka, E., Schröder, S. E., Tomasko, M. G., & Keller, H. U. 2012, Planetary and Space Science, 60, 342

Titan's atmosphere has one of the most complex chemistry of the Solar System. On the one hand, its two main compounds, nitrogen and methane, are dissociated at high altitude by energetic solar photons and charged particles coming from the Saturnian magnetosphere. They produce a set of complex molecules that generates a layer of photochemical haze which completely covers Titan. On the other hand, there is a methane cycle similar to the hydrological cycle established on Earth (evaporation, condensation, precipitations and presence of rivers and lakes on the surface). Haze, methane and clouds are subject to coupled cycles that are not completely understood. They also contribute to the thermal equilibrium and long-term evolution of Titan. It is therefore important to better characterize these cycles and their interactions. 1. Principle of the work The temperature profile of the satellite allows methane to condense, when it is transported upwards, on the aerosols which serve as a support for nucleation. Thus, after evaporation from the surface reservoirs, methane clouds are produce, precipitate, and return to the surface [Stofan et al., 2007]. The minor species created at high altitude also condense during their transport to the lower layers. These clouds have long been observed thanks to telescopes, and then to Cassini's instruments [Rodriguez et al., 2011]. Cloud activity has also been modeled from physical principles in the 2-D GCM (altitude-latitude) of IPSL [Rannou et al., 2006] and by simple models with other 3-D GCMs (e.g. [Mitchell et al., 2006] and [Lora et al., 2019]). The Global Climate Model of Titan developed at the Institut-Pierre-Simon-Laplace is the ideal tool to understand how these cycles work and how clouds form on Titan. The transition of the model in 3 dimension has significantly improved our knowledge of the mechanisms of Titan's middle atmosphere [Lebonnois et al., 2012]. The microphysical processes associated with haze have been implemented long time ago. We now have added the clouds microphysics (methane, ethane, hydrocyanic acid, etc.) and we further plan to add convective aspects. Aside from that, the morphological structure (fractal dimension) of aerosols has also been questioned recently and is likely to modify the thermal equilibrium of the atmosphere, haze-dominated in the stratosphere. These are the two axes that we will explore in our work. 2. Results & Discussion In this presentation, we will first discuss the impact of changing aerosol structure on the haze cycle and the consequences for the thermal equilibrium of the atmosphere. Recent works show that aerosols appear more compact than previously thought (with a fractal dimension Df ∼ 2.3 instead of Df ∼ 2.0). We studied the effects induced by such a change (change of vertical profiles, annual cycles, aerosol size, etc.). For example, Figure 1 shows the haze extinction in Titan's atmosphere depending on the pressure and latitude for two values of the aerosol fractal dimensions. Fig. 1 Haze extinction at 4000 nm for Df = 2.0 (left) and Df = 2.3 (right) in the Titan Global Climate Model of IPSL. We will detail the consequences on the structure of the haze and on the thermal equilibrium in the stratosphere (figure 2). Fig. 2 Vertical temperature profile of Titan's atmosphere. The bold line corresponds to the profile measured by Huygens. The dotted lines correspond to the temperature profiles predicted with the IPSL model with Df = 2.0 (in red) and Df = 2.3 (in blue). Next, we will describe the general principle of the cloud model used in the GCM and show the first results obtained. We will display the comparisons, on the one hand with observations and on the other hand with other models. This first step aims at recovering, in the 3-D GCM, the level of coupling previously reached in the 2-D model. We will also discuss the missing processes in the model and the strategy to account for. The main missing process is the convective cloud process, which have been observed. This project is naturally in line with the upcoming observations of the JWST, the future Dragonfly mission and all other future missions. This model will allow the best possible characterisation of the climate expected in the Dragonfly landing region. Of course, it will also be a question of providing all possible elements necessary for the understanding of future observation of Titan. References E. R. Stofan, C. Elachi, J. I. Lunine, R. D. Lorenz, B. Stiles, K. L. Mitchell, S. Ostro, L. Soderblom, C. Wood, H. Zebker, S. Wall, M. Janssen, R. Kirk, R. Lopes, F. Paganelli, J. Radebaugh, L. Wye, Y. Anderson, M. Allison, R. Boehmer, P. Callahan, P. Encrenaz, E. Flamini, G. Francescetti, Y. Gim, G. Hamilton, S. Hensley, W. T. K. Johnson, K. Kelleher, D. Muhleman, P. Paillou, G. Picardi, F. Posa, L. Roth, R. Seu, S. Shaffer, S. Vetrella, and R. West, “The lakes of Titan,” Nature, vol. 445, no. 7123, pp. 61–64, Jan. 2007. S. Rodriguez, S. Le Mouélic, P. Rannou, C. Sotin, R. H. Brown, J. W. Barnes, C. A. Griffith, J. Burgalat, K. H. Baines, B. J. Buratti, R. N. Clark, and P. D. Nicholson, “Titan’s cloud seasonal activity from winter to spring with Cassini/VIMS,” Icarus, vol. 216, no. 1, pp. 89–110, Nov. 2011. P. Rannou, F. Montmessin, F. Hourdin, and S. Lebonnois, “The Latitudinal Distribution of Clouds on Titan,” Science, vol. 311, no. 5758, pp. 201–205, Jan. 2006. J. L. Mitchell, R. T. Pierrehumbert, D. M. W. Frierson, and R. Caballero, “The dynamics behind Titan’s methane clouds,” Proceedings of the National Academy of Science, vol. 103, no. 49, pp. 18421–18426, Nov. 2006. Juan M. Lora, Tetsuya Tokano, Jan Vatant d’Ollone, Sébastien Lebonnois, and Ralph D. Lorenz, “A model intercomparison of Titan’s climate and low-latitude environment,” , vol. 333, pp. 113–126, Nov. 2019. Sébastien Lebonnois, Jérémie Burgalat, Pascal Rannou, and Benjamin Charnay, “Titan global climate model: A new 3-dimensional version of the IPSL Titan GCM,” , vol. 218, no. 1, pp. 707–722, Mar. 2012.

We have determined the real and imaginary refractive indices (n and k, respectively), from the visible to the near infrared (0.4 to 1.6 µm), of five laboratory-generated organic refractory materials produced from gas-phase chemistry with the NASA Ames COSmIC facility. The solid samples were produced using a plasma discharge in the stream of a 200-K supersonic jet-cooled expansion of different gas mixtures to study the impact of the molecular precursors on the solid sample optical properties. Three samples were produced from N2:CH4 (95:5) gas mixtures using three different high voltages (700V, 800V and 1000V) to vary the energy in the plasma discharge. One sample was produced from a N2:CH4:C2H2 (94:5:5:0.5) gas mixture, with a high voltage of 1000 V. The fifth sample was produced in an Ar:CH4 (95:5) gas mixture with a high voltage of 1000 V to produce a nitrogen-free hydrocarbon sample. The optical constants, n and k, of these five samples were determined using spectral reflectance measurements. They appear to be positively correlated with the nitrogen content in the solid sample, i.e., a sample with larger nitrogen content exhibits higher n and k values. We have used these refractive indices as input parameters in a radiative transfer model to analyze Cassini Visible Infrared Mapping Spectrometer (VIMS) observations of Titan’s atmosphere. The results show that using the tholin samples with higher n and k values (higher nitrogen content) provides a better fit to the observational data than using the samples with lower n and k values (lower nitrogen content). The Titan tholins with higher nitrogen content therefore appear to be more representative of the Titan aerosols observed by VIMS.

Titan, the largest satellite of Saturn, has a dense atmosphere mainly composed of nitrogen and methane at a percent level. These two molecules generate a complex prebiotic chemistry, a global haze, most of the cloud cover and the rainfalls which model the landscape. Methane sources are located in liquid reservoirs at and below the surface and it sink is the photodissociation at high altitude. Titan’s present and past climates strongly depend on the connection between the surface sources and the atmosphere upper layers. Despite its importance, very little information is available on this topic. In the last two decades, the observations made by the Cassini orbiter and the Huygens probe have greatly improved our knowledge of Titan’s system. The surface, haze, clouds, and chemical species can be studied and characterised with several instruments simultaneously. On the other hand, some compounds of its climatic cycle remain poorly known. This is clearly the case of the methane cycle, which is, however, a critical component of Titan’s climate and of its evolution. We reanalysed four solar occultations by Titan’s atmosphere observed with the infrared part of the Visual Infrared Mapping Spectrometer (VIMS) instrument. These observations were already analysed (Bellucci et al., 2009, Maltagliati et al., 2015), but here we used significantly improved methane spectroscopic data. We retrieved the haze properties (not treated previously) (Figure 1) and the mixing ratios of methane (Figure 2), deuterated methane, and CO in the stratosphere and in the low mesosphere. Figure 1 : Haze extinction as a function of altitude, retrieved for the four observations, at wavelengths 0.884 μm (channel #97), 1.540 μm (channel #137) and 2.199 μm (channel #177). The extinction profiles retrieved by Seignovert et al. (2021) with Cassini/ISS, at wavelength 338 nm (CL-UV3 filters), are shown with green lines (labelled "S2020"). Those from Vinatier et al. (2010) or Vinatier et al. (2015), scaled at the wavelength 1μm, are shown with black lines ("V2010" or "V2015"). The profiles in cyan ("RP83"), are the extinctions retrieved by Rages & Pollack (1983) at 30◦N in August 1981 (wavelength 0.5 μm). The differences in the detached haze altitudes between VIMS-IR (Ls = 26°), Cassini/ISS (Ls = 14.8°) and Voyager 2/ISS (Ls = 18°) are their dates while the detached is falling down (West et al. (2018); Seignovert et al. (2021)). The grey line shows the haze profile by Doose et al. (2016) with DISR in 2005 at 10°S (labelled "D2016"). We find that the methane mixing ratio in the stratosphere is much lower (about 1.1%) than expected from Huygens measurements (about 1.4 to 1.5%). However, this is consistent with previous results obtained with CIRS. Features in the methane vertical profiles clearly demonstrate that there are interactions between the methane distribution and the atmosphere circulation. We find a layer rich in methane at 165 km and at 70°S (mixing ratio 1.45 ± 0.1%) and a dryer background stratosphere (1.1 − 1.2%). In absence of local production, this reveals an intrusion of methane transported into the stratosphere, probably by convective circulation. On the other hand, methane transport through the tropopause at global scale appears quite inhibited. Leaking through the tropopause is an important bottleneck of Titan’s methane cycle at all timescales. As such, it affects the long term evolution of Titan atmosphere and the exchange fluxes with the surface and subsurface reservoirs in a complex way. Figure 2 : Methane mixing ratio retrieved with the four observation sets, with data between 0.88 and 2 μm (top) and between 2 and 2.8 μm (bottom). We also plot the methane mole fraction retrieved with the GCMS onboard Huygens (Niemann et al. (2010)) and with DIRS (Bézard (2014)) and CISR (Lellouch et al. (2014)). The green dashed profile, in the upper left graph, shows the evaluation made by Rannou et al. (2021). We also retrieved the haze extinction profiles and the haze spectral behaviour. We find that aerosols are aggregates with a fractal dimension of Df ≃2.3±0.1, rather than Df ≃2 as previously thought. Our analysis also reveals noticeable changes in their size distribution and their morphology with altitude and time. These changes are also clearly connected to the atmosphere circulation and concerns the whole stratosphere and the transition between the main and the detached haze layers. We conclude that, to fully understand these results, Global Climate Models accounting for haze and cloud physics, thermodynamical feedbacks and convection are needed. Especially, the humidificaton of the stratosphere, at the present time and its evolution under changing conditions at geological timescale appears as a key process, and our work provide strong constraints to guide studies. References Bellucci, A., Sicardy, B., Drossart, P., et al. 2009, Icarus, 201, 198 Bézard, B. 2014, Icarus, 242, 64 Doose, L. R., Karkoschka, E., Tomasko, M. G., & Anderson, C. M. 2016, Icarus, 270, 355 Lellouch, E., Bézard, B., Flasar, F. M., et al. 2014, Icarus, 231, 323 Maltagliati, L., Bézard, B., Vinatier, S., et al. 2015, Icarus, 248, 1 Niemann, H. B., Atreya, S. K., Demick, J. E., et al. 2010, Journal of Geophysical Research (Planets), 115, E12006 Rages, K. & Pollack, J. B. 1983, Icarus, 55, 50 Rannou, P., Coutelier, M., Riviere, E., et al. 2021, Astrophysical Journal, 922Rey, M., Nikitin, A., Bézard, B., et al. 2018, Icarus, 303, 114 Seignovert, B., Rannou, P., West, R. A., & Vinatier, S. 2021, The Astrophysical Journal, 907, 36 Vinatier, S., Bézard, B., de Kok, R., et al. 2010, Icarus, 210, 852 Vinatier, S., Bézard, B., Lebonnois, S., et al. 2015, Icarus, 250, 95 West, R. A., Balloch, J., Dumont, P., et al. 2018, Geophysical Research Letters, 38

Remote sensing observations suffer significant limitations when used to study the bulk atmospheric composition of the giant planets of our solar system. This impacts our knowledge of the formation of these planets and the physics of their atmospheres. A remarkable example of the superiority of in situ probe measurements was illustrated by the exploration of Jupiter, where key measurements such as the determination of the noble gases' abundances and the precise measurement of the helium mixing ratio were only made available through in situ measurements by the Galileo probe. Here we describe the main scientific goals to be addressed by the future in situ exploration of Saturn, Uranus, and Neptune, placing the Galileo probe exploration of Jupiter in a broader context. An atmospheric entry probe targeting the 10-bar level would yield insight into two broad themes: i) the formation history of the giant planets and that of the Solar System, and ii) the processes at play in planetary atmospheres. The probe would descend under parachute to measure composition, structure, and dynamics, with data returned to Earth using a Carrier Relay Spacecraft as a relay station. An atmospheric probe could represent a significant ESA contribution to a future NASA New Frontiers or flagship mission to be launched toward Saturn, Uranus, and/or Neptune., 27 pages, 9 figures, White Paper submitted in response to ESA's Call for Voyage 2050 Science Themes. arXiv admin note: substantial text overlap with arXiv:1708.00235

International audience; This study presents a 13 years survey of haze UV extinction profiles, monitoring the temporal evolution of the detached haze layer (DHL) in Titan's upper atmosphere (350-600 km). As reported by West et al. 2011 (GRL vol.38, L06204) at the equator, we show that the DHL is present at all latitudes below 55 • N during the northern winter (2004-2009). Then, it globally sunk and disappeared in 2012. No permanent DHL was observed between 2012 and 2015. It's only in late-2015, that a new structure emerged from the Northern hemisphere and propagated to the equator. This new DHL is not as pronounced as in 2004 and is much more complex than the one observed earlier. In one specific sequence, in 2005, we were able to investigate the short time scale variability of the DHL and no major changes was observed. When both side of the limb were visible (dawn/dusk), we notice that the extinction of the DHL is slightly higher on the dawn side. Additionally, during a polar flyby in 2009, we observed the longitudinal variability of the DHL and spotted some local inhomogeneities. Finally, comparisons with UVIS stellar occultations and General Climate Models (GCMs) are both consistent with our findings. However, we noticed that the timing of the DHL main pattern predicted by the GMCs can be off by up to 30 • in solar longitude. All these observations bring new perspectives on the seasonal cycle of Titan's upper atmosphere, the evolution of the DHL and its interaction with the dynamics.

International audience; Photochemical aerosols were detected as high as 350 km of altitude in Pluto's atmosphere during the New Horizons fly-by. These aerosols are thought to affect Pluto's climate, by acting as cooling agents, and the colours of Pluto's surface, in particular in the dark regions named Cthulhu and Krun and at the North Pole. Pluto atmospheric and surface models have so far used the optical constants of Titan aerosol analogues (tholins), whereas their chemical composition is known to differ from that of Pluto aerosol analogues.In order to provide a new set of optical constants for Pluto tholins, we synthesized analogues of Pluto's aerosols and determined with spectroscopic ellipsometry their optical constants from 270 to 2100 nm. Three types of samples were produced from N2:CH4:CO gas mixtures differing in their CH4:N2 mixing ratio, representative of different altitudes in Pluto's current atmosphere or different seasons or epochs of Pluto.Our analysis shows a strong absorption by Pluto tholins in the UV and visible spectral ranges, with k index of a few 10−1 at 270 nm, in agreement with N- and O-bearing organic molecules. Pluto tholins are less absorbent in the near-IR than in the UV–Vis wavelength range, with k of a few 10−3 between 600 and 2100 nm. Our comparative study highlights the dependency of n and k indices to the CH4:N2 mixing ratio. Aerosols formed at different altitudes in Pluto's atmosphere or during different seasons or epochs of Pluto will therefore affect the budget of Pluto radiative transfer differently.The optical constants presented in this study were tested with a Pluto surface model and with a model of light scattering. The surface modelling results highlight the suitability of these optical constants to reproduce Pluto compositional observations in the visible spectral range by MVIC and LEISA. The atmospheric modelling results conclude that Pluto tholins absorb 5 to 10 times less than Titan tholins at 500 nm, and this lower absorption is consistent with Alice observations of Pluto's haze.

In response to ESA Voyage 2050 announcement of opportunity, we propose an ambitious L-class mission to explore one of the most exciting bodies in the Solar System, Saturn largest moon Titan. Titan, a "world with two oceans", is an organic-rich body with interior-surface-atmosphere interactions that are comparable in complexity to the Earth. Titan is also one of the few places in the Solar System with habitability potential. Titan remarkable nature was only partly revealed by the Cassini-Huygens mission and still holds mysteries requiring a complete exploration using a variety of vehicles and instruments. The proposed mission concept POSEIDON (Titan POlar Scout/orbitEr and In situ lake lander DrONe explorer) would perform joint orbital and in situ investigations of Titan. It is designed to build on and exceed the scope and scientific/technological accomplishments of Cassini-Huygens, exploring Titan in ways that were not previously possible, in particular through full close-up and in situ coverage over long periods of time. In the proposed mission architecture, POSEIDON consists of two major elements: a spacecraft with a large set of instruments that would orbit Titan, preferably in a low-eccentricity polar orbit, and a suite of in situ investigation components, i.e. a lake lander, a "heavy" drone (possibly amphibious) and/or a fleet of mini-drones, dedicated to the exploration of the polar regions. The ideal arrival time at Titan would be slightly before the next northern Spring equinox (2039), as equinoxes are the most active periods to monitor still largely unknown atmospheric and surface seasonal changes. The exploration of Titan northern latitudes with an orbiter and in situ element(s) would be highly complementary with the upcoming NASA New Frontiers Dragonfly mission that will provide in situ exploration of Titan equatorial regions in the mid-2030s., Comment: arXiv admin note: substantial text overlap with arXiv:1908.01374

A number of images and analyses have demonstrated the presence of hazes and clouds in the atmosphere of the ice giants. While the formation of hazes is attributed to the methane dissociation in the high stratosphere by solar UV and energetic particles that leads to a number of chemical reactions (e.g. Moses et al., Icarus, 307, 2018), the observed clouds are the result of the condensation of CH4 and H2S in the troposphere (e.g. Irwin et al., Nature Astronomy, 2018). However, the lack of current limb observations taken at different tangent heights limits our knowledge about the vertical structure and optical properties of these aerosols. In this work, we will present different results obtained with a coupled cloud-haze microphysical model (Toledo et at., Icarus, 333, 2019; Toledo et at., Icarus, 350, 2020) used to constrain the particle size, density, vertical structure and time scale of aerosols in the ice giants. Our simulations show, among other results, high precipitation rates at pressures greater than 0.5 bar and timescales ranging from years (for the haze) to a few hours (CH4 clouds).

Introduction The thermal profile of Pluto’s atmosphere has been measured from ground-based observations and from the REX instrument on-board New Horizons [1,2]. From the surface to the top of the atmosphere, the profiles show a 3-km deep ~37 K cold layer above the N2 ice-covered surface, a strong positive gradient below 50 km, warming the stratosphere up to 110 K, and a 40 K negative gradient cooling the upper atmosphere to 70 K. Our objective is to study the radiative balance of Pluto’s atmosphere in 1D with a radiative-convective model, and in 3D with the full Pluto Global Climate Model (GCM), taking into account the radiative impact of haze particles. We focus on three aspects of the thermal profile with the GCM: (1) It has been suggested that the organic haze has a significant radiative impact and is responsible for the cooling of the upper atmosphere [3]. However, this has not been explored yet in details in a GCM and remains to be tested against JWST observations of Pluto’s atmosphere thermal emission. (2) The depth of the near-surface cold layer in the GCM is currently only 1 km, versus 3 km in the observations [4,5]. (3) The significant temperature gradient between the equator and the north pole tentatively indicated by recent ALMA observations (PI Lellouch) is not predicted by current models, because the long radiative timescale of Pluto’s atmosphere should prevent horizontal temperature gradients. However, by radiating in the infrared, haze particles could shorten the radiative timescale and trigger significant temperature gradients. The Pluto GCM The GCM is described in detail in [4,5]. It takes into account the sublimation and condensation cycles of N2, CH4, and CO [4], their thermal and dynamical effects, the cloud formation, the vertical turbulent mixing, molecular thermal conduction, and a detailed surface thermal model with different thermal inertia for various timescales (diurnal, seasonal). It also includes a parameterization of the formation of organic haze [6]. We use the 1D radiative-convection version of the GCM to explore the radiative impact of haze depending on the haze properties. We then use the 3D GCM to explore the effect in a climatic context, with consistent 3D predictions for haze formation and methane abundance, which are used as an input for our radiative transfer calculation. At the conference we will present the results obtained with our model for different types of haze particles (spheres, fractal aggregates) and different datasets of laboratory-generated optical constants. We will reveal if the radiative haze can solve the three mysteries mentioned in Section 1, and how it impacts Pluto’s atmosphere dynamics. Acknowledgements T.B. was supported for this research by an appointment to the National Aeronautics and Space Administration (NASA) Post-doctoral Program at the Ames Research Center administered by Universities Space Research Association (USRA) through a contract with NASA. References [1] Hinson, D. P., et al., Radio occultation measurements of Pluto’s neutral atmosphere with New Horizons, Icarus, 290, 96–111, 2017. [2] A. Dias-Oliveira et al., Pluto's atmosphere from stellar occultations in 2012 and 2013, ApJ 811, 53, 2015. [3] Zhang, X., et al, Haze heats Pluto's atmosphere yet explains its cold temperature, Nature, 2017. [4] Forget, F., et al., A post-new horizons global climate model of Pluto including the N2, CH4 and CO cycles, Icarus, 287, 54–71, 2017. [5] Bertrand, T., et al., Pluto’s Beating Heart Regulates the Atmospheric Circulation: Results From High-Resolution and Multiyear Numerical Climate Simulations. JGR: Planets, 125(2), 1–24, 2020. [6] Bertrand, T. and Forget, F.: 3D modeling of organic haze in Pluto’s atmosphere, Icarus, 287:72, 2017.

Titan is a complexe moon of Saturn, with dense atmosphere, hydrocarbon cycle, and water-ice crust. Titan was targeted by two major space missions: Voyager and Cassini, and has been the subject of many studies. Lakes and seas of liquid hydrocarbons were discovered by Cassini's RADAR in Titan's polar regions [15, 14]. This instrument also detected on equatorial regions geomorphological structures related to the presence of liquid, like fluvial valleys incised in the bedrock, and alluvial fans [8, 3]. The existence of evaporitic terrains where also suggested [1, 4, 9], often in place of paleo-sea [12]. Water ice signal is not present everywhere on Titan, contrary to other icy moon of Saturn and Jupiter. It was detected with Cassini's VIMS instrument in Titan's dark region, often at the transition between a dark and bright unit, and mixed with a darkening material [11, 13]. Ref. [6] also highlighted an equatorial corridor of exposed water-ice using a principal component analysis (PCA) with VIMS, showing on a large scale terrains with low and high water ice content. We accentuate our study on Titan's river bed, where the erosion due to liquid hydrocarbon can reveals the bedrock, and where we can have clues on the sediment transport. Here we propose a study of the water ice signal distribution in Xanadu and Tui regio, where we have channels [8] and paleo-seas of hydrocarbures [12]. We focused on 2 radar bright channels [8], and the surrounding areas. The channels are located at the south of Xanadu, directly exiting the mountains. They are at the north of the evaporitic region Tui Regio [10], and form dry dendritic fluvial valleys [7], with a width up to 8~km. We apply a radiative transfer analysis to retrieve the surface albedo of 2 VIMS cubes located in our area of study : 1590648776_1, and 1809727868_1. We establish a criteria δ representing the intensity of the water-ice signature, based on wavelength ratios and means of the surface albedo significant for the identification of the water-ice spectrum. They are detailed in Tab. 1. Those ratio and mean are normalized with respect to themselves. Then, they are added if the expected value indicating a high water ice signal is high or >1, and subtracted otherwise. We then have a range of values going from δ = -4 for no water ice signature, and δ = 1 for a high water ice signal. The results of this multicriteria analysis are in Fig. 1 and 2. An example of the different spectra for a low and high δ are given is Fig.3. Table 1 : Water-ice criteria Wavelength ratioand mean Expected value ifwater-ice signature 1.58/1.28 < 1 2.03/1.28 < 1 2.03/1.58 < 1 2.7/2.8 high 4.85 - 5.1µm low The radiative transfer analysis highlighted areas of high water-ice signal on Xanadu mountains, and also at the end of the dry channels, corresponding to potential deltas or alluvial fans, supporting [2] and [7] hypothesis of sediment transport from the mountains of Xanadu to downstream of the channels. The ice signal is lower inside the channels, perhaps due to their sizes, smaller than the spatial resolution of a pixel. The Fig. 3 shows that water ice lower the surface albedo. This combined analysis also shows a gradient of low water-ice signal perfectly superposed to Tui Regio in the RADAR map. This gradient could correspond to the decreasing thickness from the center to the border of the evaporite layer of Tui Regio, as suggested by [5]. Detailed analysis of the retrieved albedo could bring further information on the composition of those terrains if the error bars are low enough. A map of the inversion of the surface signal allows us to have a finer vision than just an analysis of the I/F. With the parameter δ, we have better contrasts, and a direct appreciation of the strength of the water-ice signature. Acknowledgements The present research was supported by the Programme National de Planétologie (PNP) of CNRS-INSU co-funded by CNES, and also partially supported by the French HPC Center ROMEO, the Région Grand-Est and the University of Reims Champagne-Ardenne. References [1] J. W. Barnes et al., Organic sedimentary deposits in Titan’s dry lakebeds: Probable evaporite. Icarus, 216:136–140, November 2011. [2] Jason W. Barnes et al., Near-infrared spectral mapping of Titan’s mountains and channels. Journal of Geophysical Research (Planets), 112(E11):E11006, Nov 2007. [3] S.P.D. Birch et al., Alluvial fan morphology, distribution and formation on titan. Icarus, 270:238 – 247, 2016. Titan’s Surface and Atmosphere. [4] D. Cordier et al., On the chemical composition of Titan’s dry lakebed evaporites. Icarus, 226(2):1431–1437, 2013. [5] D. Cordier et al., Structure of titan’s evaporites. Icarus, 270:41–56, may 2016. [6] Caitlin A. Griffith et al., A corridor of exposed ice-rich bedrock across Titan’s tropical region. Nat. Astron., page 343, Apr 2019. [7] M. H. Langhans et al., Titan’s fluvial valleys: Morphology, distribution, and spectral properties. Planet. Space Sci., 60(1):34–51, Jan 2012. [8] A. Le Gall et al., Radar-bright channels on Titan. Icarus, 207:948–958, June 2010. [9] S. M. MacKenzie et al., Evidence of Titan’s climate history from evaporite distribution. Icarus, 243:191–207, November 2014. [10] Shannon MacKenzie and Jason Barnes. Compositional Similarities and Distinctions between Titan’s Evaporitic Terrains. The Astrophysical Journal, 821(1):0, 2016. [11] T. B. McCord et al., Composition of Titan’s surface from Cassini VIMS. Planet. Space Sci., 54(15):1524–1539, Dec 2006. [12] Jeffrey M. Moore and Alan D. Howard. Are the basins of titan’s hotei regio and tui regio sites of former low latitude seas ? Geophysical Research Letters, 37(22), 2010. [13] Laurence A. Soderblom et al., Correlations between cassini vims spectra and radar sar images: Implications for titan’s surface composition and the character of the huygens probe landing site. Planetary and Space Science, 55(13):2025 – 2036, 2007. Titan as seen from Huygens. [14] K. Stephan et al., Specular reflection on Titan: Liquids in Kraken Mare. Geophysical Research Letters, 37:L07104, April 2010. [15] E. R. Stofan et al.,The lakes of Titan. Nature, 445:61–64, January 2007.

A 1-dimensional microphysics model has been used to constrain the structure and formation of haze in Neptune's atmosphere. These simulations were coupled to a radiative-transfer and retrieval code (NEMESIS) to model spectral observations of Neptune in the H-band performed by the SINFONI Integral Field Unit Spectrometer on the Very Large Telescope (VLT) in 2013. It was found that observations in the H-band and with emission angles ≤60° are largely unaffected by the imaginary refractive index of haze particles, allowing a notable reduction of the free parameters required to fit the observations. Our analysis shows a total haze production rate of (2.61 ± 0.18) × 10−14 kg m−2 s−1, about 10 times larger than that found in Uranus's atmosphere, and a particle electric charge of q = 8.6 ± 1.1 electrons per μm radius at latitudes between 5 and 15° S. This haze production rate in Neptune results in haze optical depths about 10 times greater than those in Uranus. The effective radius reff was found to be 0.22 ± 0.01 and 0.26 ± 0.02 μm at the 0.1 and 1-bar levels, respectively, with haze number densities of 8.48+1.78−1.31 and 9.31+2.52−1.91 particles per cm3. The fit at weak methane-absorbing wavelengths reveals also the presence of a tropospheric cloud with a total optical depth >10 at 1.46 μm. The tropospheric cloud base altitude was found near the 2.5-bar level, although this estimation may be only representative of the top of a thicker and deeper cloud. Our analysis leads to haze opacities about 3.5 times larger than that derived from Voyager-2 observations ( Moses et al., 1995 ). This larger opacity indicates a haze production rate 2 times larger at least. To study this difference haze opacity or production rate, we performed a timescale analysis with our microphysical model to estimate the time required for haze particles to grow and settle out. Although this analysis shows haze timescales (~15 years) shorter than the time lapsed between Voyager-2 observations and 2013, the solar illumination at the top of the atmosphere has not varied significantly during this period (at the studied latitudes) to explain the increase in haze production. This difference in haze production rate derived for these two periods may arise from: a) the fact that in our analysis we employed spectral observations in the infrared (H-band), while Moses et al. (1995) used photometric images taken at 5 different filters in the visible. While high-phase-angle Voyager observations are more sensitive to small haze particles and at altitudes above the 0.1-bar level, the haze constraints derived from VLT spectra in H-band are limited to pressures greater than 0.1 bar. As a result of the different phase angles of the two set of observations, differences in the estimation of M0 may arise from the use of Mie phase functions as well. b) our 1-dimensional model does not account for latitudinal redistributions of the haze by dynamics. A possible meridional transport of haze with wind velocities greater than ~0.03 m s−1 would result in dynamics timescales shorter than 15 years and thus might explain the observed variations in the haze production rate during this period. Compared with our estimations, photochemical models point to even larger production rates on Neptune (by a factor of 2.4). Assuming that the photochemical simulations are correct, we found that this discrepancy can be explained if haze particles evaporate before reaching the tropospheric-cloud levels. This scenario would decrease the cumulative haze opacity above the 1-bar level, and thus a larger haze production rate would be required to fit our observations. However, to validate this haze vertical structure future microphysical simulations that include the evaporation rates of haze particles are required.

Liquid and solid particles are present in the atmosphere of many Solar System objects. Measuring aerosol properties can provide major constraints about both atmospheric composition and dynamics. While some bulk aerosol properties can be estimated using remote measurements, the size distributions and the typologies of the aerosols, which are related to their formation process, their origin and their evolution, are often poorly known. We propose a new concept of optical instrument dedicated to in situ measurements of aerosols as part of the science payload of an atmospheric entry probe or of a surface module. It relies the Earth atmospheric light aerosol counter LOAC used since 2013 under various types of balloons. This instrument measures the aerosol concentrations in 19 size classes between 0.2 and 50 micrometers, and estimates their typology The LONSCAPE (Light Optical Nephelometer Counter Sizer and Counter for Aerosols in Planetary Environments) concept combines counter measurements and nephelometric measurements at several phase angles, particle by particle, to retrieve for all size classes the concentrations and the scattering functions. This approach is the novelty of the instrument concept. The scattering functions can be compared to results of theoretical calculations but also to laboratory databases obtained for levitating particles and from the microwave analogy technics, to retrieve the refractive indices of the liquid and solid particles to better identify their nature and origin. Up to 10 angles measurements for the scattering function and one angle measurement for the counting provide an optimal configuration to distinguish between liquid, icy and possibly solid particles in the Uranus or Neptune atmosphere. Such an instrument must be able to detect up to tens of particles greater than 0.1 micrometer within 1 cm3. For ice giants, the instrument must work for pressures up to 10 bars. No pumping system should be needed since the aerosols will be injected in the optical chamber by an inlet parallel to the descent motion of the probe under parachutes. Considering the relative velocity between the atmosphere and the probe, the electronics must be able to detect particles crossing the laser beam up to several tens of m/s, which can be done by “conventional” electronics. Preliminary studies show that the instrument could have a total mass of about 2 kg and an electric consumption of about 2W.

International audience; Measurements of light scattered by particles give insight into their physical properties (solid or liquid, size, shape, complex refractive index). We propose a novel instrument that provides in situ optical light-scattering measurements of aerosols in giant planet atmospheres, particularly in the ice giants Uranus and Neptune. Known as LONSCAPE (Light Optical Nephelometer Sizer and Counter for Aerosols for Planetary Environments), the instrument combines the particle counting technique of the LOAC balloon-borne aerosol counter with the well-known nephelometer technique to retrieve both the concentrations and phase functions of aerosols over 20 size classes in the 0.2-50 mm diameter size range. Such measurements allow us to distinguish between liquid, ice and solid (potentially carbonaceous) particles for all size classes, and thus to constrain the aerosols composition and their formation process. Given its low mass, size, and power requirements, LONSCAPE could become one of the key instruments selected to be part of the science payload of an atmospheric entry probe sent to the ice giants in the 2030s.

The spectrum of methane is very important for the analysis and modeling of Titan’s atmosphere but its insufficient knowledge in the near infrared, with the absence of reliable absorption coefficients, is an important limitation. In order to help the astronomer community for analyzing high-quality spectra, we report in the present work the first accurate theoretical methane line lists (T = 50–350 K) of 12 CH 4 and 13 CH 4 up to 13400 cm − 1 ( > 0.75 µm). These lists are built from extensive variational calculations using our recent ab initio potential and dipole moment surfaces and will be freely accessible via the TheoReTS information system ( http://theorets.univ-reims.fr , http://theorets.tsu.ru ). Validation of these lists is presented throughout the present paper. For the sample of lines where upper energies were available from published analyses of experimental laboratory 12 CH 4 spectra, small empirical corrections in positions were introduced that could be useful for future high-resolution applications. We finally apply the TheoRetS line list to model Titan spectra as observed by VIMS and by DISR, respectively onboard Cassini and Huygens. These data are used to check that the TheoReTS line lists are able to model observations. We also make comparisons with other experimental or theoretical line lists. It appears that TheoRetS gives very reliable results better than ExoMol and even than HITRAN2012, except around 1.6 µm where it gives very similar results. We conclude that TheoReTS is suitable to be used for the modeling of planetary radiative transfer and photometry. A re-analysis of spectra recorded by the DISR instrument during the descent of the Huygens probe suggests that the CH 4 mixing ratio decreases with altitude in Titan’s stratosphere, reaching a value of ∼ 10 − 2 above the 110 km altitude.

Titan's surface was revealed by Cassini's instruments, showing the presence of liquid hydrocarbons in lakes, and features like dry riverbed. In order to study the sediment transport in Titan's channels and to map distribution of the water-ice signature in these terrains, we use a radiative transfer model to retrieve the surface albedo, after we estimated VIMS error via an original method. We also establish a criteria related to the intensity of the water ice signature. The tuning of the radiative transfer model shows that the fractal dimension of Titan's aerosols is higher than previously thought, around 2.3 - 2.4. We find spots of increasing signal of water ice downstream, at the margins of Tui Regio, that could correspond to alluvial fans, deltas or crater rims. We also observe a very low water ice signal on Tui Regio, with a positive gradient between the central region and the boundary of the area, possibly due to the thickness variation of an evaporitic layer. The riverbeds show within the error bars a decreasing grain size from the top to the bottom of the channels., 22 pages, 17 figures, submitted to Icarus

Microphysical simulations have been performed to constrain the formation and structure of haze in Uranus's atmosphere. These simulations were coupled to a radiative-transfer code to fit observations performed by the SINFONI Integral Field Unit Spectrometer on the Very Large Telescope (VLT)and by the Wide Field Camera 3 (WFC3)of the Hubble Space Telescope (HST)in 2014. Our simulations yield an effective radius of ∼0.2 μm for the haze particles in the tropopause and a density of ∼2.9 particles per cm 3. Our simulations also provide an estimate for the haze production rate in the stratosphere of between ∼3.10 −16 and 3.10 −15 kg m −2 s −1, about 100 times smaller than that found in Titan's atmosphere (e.g. Rannou et al., 2004). This range of values is very similar to that derived by Pollack et al. (1987)from Voyager-2 observations in 1986, suggesting microphysical timescales greater than the elapsed time between these observations (28 years, or 1/3 of a Uranian year). This result is in agreement with analyses performed with our microphysical model that show timescales for haze particles to grow and settle out to be >∼30 years at pressure levels >0.1 bar. However, these timescales are too big to explain the observed variations in the haze structure over Uranus's northern hemisphere after 2007 equinox (e.g. de Pater et al., 2015). This indicates that dynamics may be the main factor controlling the spatial and temporal distribution of the haze over the poles. A meridional stratospheric transport of haze particles with winds velocities >∼0.025 m s −1 would result in dynamics timescales shorter than 30 years and thus may explain the observed variations in the haze structure.

Saturn’s moon Titan has a dense nitrogen-rich atmosphere, with methane as its primary volatile. Titan’s atmosphere experiences an active chemistry that produces a haze of organic aerosols that settle to the surface and a dynamic climate in which hydrocarbons are cycled between clouds, rain and seas. Titan displays particularly energetic meteorology at equinox in equatorial regions, including sporadic and large methane storms. In 2009 and 2010, near Titan’s northern spring equinox, the Cassini spacecraft observed three distinctive and short-lived spectral brightenings close to the equator. Here, we show from analyses of Cassini spectral data, radiative transfer modelling and atmospheric simulations that the brightenings originate in the atmosphere and are consistent with formation from dust storms composed of micrometre-sized solid organic particles mobilized from underlying dune fields. Although the Huygens lander found evidence that dust can be kicked up locally from Titan’s surface, our findings suggest that dust can be suspended in Titan’s atmosphere at much larger spatial scale. Mobilization of dust and injection into the atmosphere would require dry conditions and unusually strong near-surface winds (about five times more than estimated ambient winds). Such strong winds are expected to occur in downbursts during rare equinoctial methane storms—consistent with the timing of the observed brightenings. Our findings imply that Titan—like Earth and Mars—has an active dust cycle, which suggests that Titan’s dune fields are actively evolving by aeolian processes. Saturn’s moon Titan may have an active dust cycle in equatorial regions driven by storm winds, Cassini observations consistent with dust suspension in Titan’s atmosphere suggest.

International audience; The first of the two missions foreseen in the ExoMars program was successfully launched on 14th March 2016. It included the Trace Gas Orbiter and the Schiaparelli Entry descent and landing Demonstrator Module. Schiaparelli hosted the DREAMS instrument suite that was the only scientific payload designed to operate after the touchdown. DREAMS is a meteorological station with the capability of measuring the electric properties of the Martian atmosphere. It was a completely autonomous instrument, relying on its internal battery for the power supply. Even with low resources (mass, energy), DREAMS would be able to perform novel measurements on Mars (atmospheric electric field) and further our understanding of the Martian environment, including the dust cycle. DREAMS sensors were designed to operate in a very dusty environment, because the experiment was designed to operate on Mars during the dust storm season (October 2016 in Meridiani Planum). Unfortunately, the Schiaparelli module failed part of the descent and the landing and crashed onto the surface of Mars. Nevertheless, several seconds before the crash, the module central computer switched the DREAMS instrument on, and sent back housekeeping data indicating that the DREAMS sensors were performing nominally. This article describes the instrument in terms of scientific goals, design, working principle and performances, as well as the results of calibration and field tests. The spare model is mature and available to fly in a future mission.

The atmosphere of Pluto contains a global thin haze layer, possibly clouds and a variety of gaseous species which may be supersaturated under some conditions. Studies of Pluto climate necessitate a fairly good knowledge of the interactions between gases and aerosols since it has an impact on the vertical profiles of these species and on the fluxes of matter at the planetary scale. In this paper we use the laws of cloud nucleation to evaluate the supersaturation which is needed to trigger condensation. HCN and H2O can supersaturate by factor from several thousand to several billions depending on the type of nucleation which is used. Other species can supersaturate, but with smaller values of saturation. Gaseous species also found as ices at the surface (e.g., CO, N2, CH4) are of special interest. At a surface temperature of 37 K, they can be supersaturated with S between 1.5 to 2 even if condensation nuclei are available in large number. Such supersaturation factors have an impact on fluxes of these species from the surface to the atmosphere.

We have analyzed the complete archive of the Visual and Infrared Mapping Spectrometer (VIMS) data in order to monitor and analyze the evolution of the clouds and haze coverage at both poles of Titan during the entire Cassini mission. Our objective is to give a cartographic synopsis from a VIMS perspective, to provide a global view of the seasonal evolution of Titan's atmosphere over the poles. We leave the detailed comparison with the Imaging Science Subsystem (ISS) and the Composite Infrared Spectrometer (CIRS) data sets to further studies. We have computed global hyperspectral mosaics for each of the 127 targeted flybys of Titan to produce synthetic color maps emphasizing the main atmospheric features. The north pole appears fully covered by a huge cloud as soon as the first observations in 2004 and up to the equinox in 2009 (Le Mou\'elic et al. 2012). The northern skies then became progressively clearer, after the circulation turnover in 2009, revealing the underlying lakes and seas to the optical instruments up to 2017. The reverse situation is observed over the south pole, which was mostly clear of such a high obscuring cloud during the first years of the mission, but started to develop a polar cloud in 2012. This feature grew up month after month until the end of the mission in 2017, with a poleward latitudinal extent of 75$^\circ$S in 2013 up to 58$^\circ$S in April 2017. Thanks to the spectral capabilities of VIMS, we have detected HCN spectral signatures over the north pole in almost all flybys between 2004 and 2008. These HCN signatures started then to show up over the south pole in almost all flybys between 2012 and 2017, so perfectly matching the timing and spatial extent of the northern and southern polar atmospheric features.

Titan's 'detached' haze, seen in Voyager images in 1980 and 1981 and monitored by the Cassini Imaging Science Subsystem (ISS), during the period 2004-2017, provides a measure of seasonal activity in Titan's mesosphere with observations over almost one half of Saturn's seasonal cycle. Here we report on retrieved haze extinction profiles that reveal a depleted layer that visually manifests as a thin layer detached from the main haze below. Our new measurements show the disappearance of the feature in 2012 and its reappearance in 2016, as well as details after the reappearance. These observations highlight the dynamical nature of the detached haze. The reappearance appears congruent but more complex than previously described by climate models. It occurs in two steps, first as haze reappearing at $450\pm20$ km and one year later at $510\pm20$ km. These new observations provide additional tight and valuable constraints about the underlying mechanisms, especially for Titan's mesosphere, that control Titan's haze cycle., Comment: Nature Astronomy, 2018

Titan's atmosphere is optically thick and hides the surface and the lower layers from the view at almost all wavelengths. However, because gaseous absorptions are spectrally selective, some narrow spectral intervals are relatively transparent and allow to probe the surface. To use these intervals (called windows) a good knowledge of atmospheric absorption is necessary. Once gas spectroscopic linelists are well established, the absorption inside windows depends on the way the far wings of the methane absorption lines are cut-off. We know that the intensity in all the windows can be explained with the same cut-off parameters, except for the window at 2 μ m. This discrepancy is generally treated with a workaround which consists in using a different cut-off description for this specific window. This window is relatively transparent and surface may have specific spectral signatures that could be detected. Thus, a good knowledge of atmosphere opacities is essential and our scope is to better understand what causes the difference between the 2 μ m window and the other windows. In this work, we used scattered light at the limb and transmissions in occultation observed with VIMS (Visual Infrared Mapping Spectrometer) onboard Cassini, around the 2 μ m window. Data shows an absorption feature that participates to the shape of this window. Our atmospheric model fits well the VIMS data at 2 μm with the same cut-off than for the other windows, provided an additional absorption is introduced in the middle of the window around ≃ 2.065 μ m. It explains well the discrepency between the cut-off used at 2 μm, and we show that a gas with a fairly constant mixing ratio, possibly ethane, may be the cause of this absorption. Finally, we studied the impact of this absorption on the retrieval of the surface reflectivity and found that it is significant.

International audience; Venus is a terrestrial planet enshrouded by 20 km-thick clouds, which are composed of sulphuric acid-water solution droplets. To understand the Venus atmosphere, LMD and LATMOS laboratories have developed a 3D IPSL Venus Global Climate Model (Lebonnois et al. 2010). In this GCM, the cloud description is simplified. As clouds play a crucial role in radiative transfer, dynamics and generally the climate of Venus, it is necessary to improve the VGCM with a microphysical representation. To this end, we develop a Modal Aerosol Dynamics of Venusian Liquid Aerosol cloud model (MAD-VenLA). This model uses an implicit moment scheme to describe the particle size distribution and the microphysical processes in 0D. The particle size distribution is described by its first moments: total particle number (zeroth moment) and total particle volume (third moment) of the size distribution (Seigneur et al. 1986, Burgalat et al. 2014). Moreover, with this representation, the form of the size distribution is assumed to be a log-normal function. To represent a source of aerosol particles and the sedimentation of our cloud droplets, we have developed a 1D extension to our model. We are currently coupling MAD-VenLA with the 1D version of the IPSL Venus GCM.First, we will describe MAD-VenLA. Then, for validation, we will compare it with the sectional model SALSA in 0D (Kokkola et al. 2008), and with the CARMA Venus model (McGouldrick et al. 2007) and the Pioneer Venus LCPS observations (Knollenberg and Hunten, 1980) in 1D.In case of successful validation in 1D, we will be able to conduct 3D simulations with full microphysics in the future.

UV observations with Cassini ISS Narrow Angle Camera of Titan’s detached haze is an excellent tool to probe its aerosols content without being affected by the gas or the multiple scattering. Unfortunately, its low extent in altitude requires a high resolution calibration and limits the number of images available in the Cassini dataset. However, we show that it is possible to extract on each profile the local maximum of intensity of this layer and confirm its stability at 500 ± 8 km during the 2005–2007 period for all latitudes lower than 45°N. Using the fractal aggregate scattering model of Tomasko et al. (2008) and a single scattering radiative transfer model, it is possible to derive the optical properties required to explain the observations made at different phase angles. Our results indicates that the aerosols have at least ten monomers of 60 nm radius, while the typical tangential column number density is about 2 · 1010 agg m − 2 . Moreover, we demonstrate that these properties are constant within the error bars in the southern hemisphere of Titan over the observed time period. In the northern hemisphere, the size of the aerosols tends to decrease relatively to the southern hemisphere and is associated with a higher tangential opacity. However, the lower number of observations available in this region due to the orbital constraints is a limiting factor in the accuracy of these results. Assuming a fixed homogeneous content we notice that the tangential opacity can fluctuate up to a factor 3 among the observations at the equator. These variations could be linked with short scale temporal and/or longitudinal events changing the local density of the layer.

"The solar irradiance sensor (SIS) was included in the DREAMS package onboard the ExoMars 2016 Entry Descent and Landing Demonstrator Module, and has been selected in the METED meteorological station onboard the ExoMars 2020 Lander. This instrument is designed to measure at different time intervals the scattered flux or the sum of direct flux and scattered flux in UVA (315-400 nm) and NIR (700-1100 nm) bands. For SIS'16, these measurements are performed by a total of 3 sensors per band placed at the faces of a truncated tetrahedron with face inclination angles of 60. The principal goal of SIS'16 design is to perform measurements of the dust opacity in UVA and NIR wavelengths ranges, crucial parameters in the understanding of the Martian dust cycle. The retrieval procedure is based on the use of radiative transfer simulations to reproduce SIS observations acquired during daytime as a function of dust opacity. Based on different sensitivity analysis, the retrieval procedure also requires to include as free parameters (1) the, dust effective radius; (2) the dust effective variance; and (3) the imaginary part of the refractive index of dust particles in UVA band. We found that the imaginary part of the refractive index of dust particles does not have a big impact on NIR signal, and hence we can kept constant this parameter in the retrieval of dust opacity at this channel. In addition to dust opacity measurements, this instrument is also capable to detect and characterize clouds by looking at the time variation of the color index (CI), defined as the ratio between the observations in NIR and UVA channels, during daytime or twilight. By simulating CI signals with a radiative transfer model, the cloud opacity and cloud altitude (only during twilight) can be retrieved. Here the different retrieval procedures that are used to analyze SIS measurements, as well as the results obtained in different sensitivity analysis, are presented and discussed." Part of this work has been funded with the help of the Spanish National Research, Development and Innovation Program, through the grant ESP2014-54256-C4-3-R. Peerreview

Observations of Titan’s emissions during its 2009 eclipse by Saturn revealed a weak airglow around the moon, as well as a brighter emission from its disk (West et al. [2012]. Geophys. Res. Lett. 39 (1), 18204). We explore here the potential mechanisms that could generate these emissions and more specifically the role of magnetospheric plasma and cosmic rays in the upper and lower atmosphere, respectively. We consider excitation of N2 by these energy sources and calculate the resulting emissions through a detailed model of N2 airglow followed by careful radiation transfer of the emitted photons through the atmosphere, and into the UVIS and ISS instruments. Our results indicate that the observed limb emissions are consistent with magnetospheric plasma energy input, while emissions instigated by cosmic ray excitation are very weak and cannot explain the observed disk emissions. We discuss possible contributions from other sources that could potentially explain the disk observations. The most likely scenario is that of scattered stellar light from Titan’s disk.

In planetary atmospheres, adsorption of volatile molecules occurs on aerosols prior to nucleation and condensation. Therefore, the way adsorption occurs affects the subsequent steps of cloud formation. In the classical theory of heterogeneous nucleation, several physical quantities are needed for gas condensing on a substrate like aerosols, such as the desorption energies of the condensing gases on the substrate and the wetting parameters of the condensed phases on the substrate. For most planetary atmospheres, the values of such quantities are poorly known. In cloud models, these values are often approximately defined from more or less similar cases or simply fixed to reproduce macroscopic observable quantities such as cloud opacities. In this work, we used the results of a laboratory experiment in which methane and ethane adsorption isotherms on tholin, an analog of photochemical aerosols, are determined. This experiment also permits determination of the critical saturation ratio of nucleation. With this information we then retrieved the desorption energies of methane and ethane, which are the quantitative functions describing the adsorption isotherms and wetting parameters of these two condensates on tholin. We find that adsorption of methane on tholin is well explained by a Langmuir isotherm and a desorption energy ΔFo = 1.519 ± 0.0715 × 10−20 J. Adsorption of ethane tholin can be represented by a Brunauer-Emmett-Teller isotherm of type III. The desorption energy of ethane on tholin that we retrieved is ΔFo = 2.35 ± 0.03 × 10−20 J. We also determine that the wetting coefficients of methane and ethane on tholin are m = 0.994 ± 0.001 and m = 0.966 ± 0.007, respectively. Although these results are obtained from experiments representative of the Titan case, they are also of general value in cases of photochemical aerosols in other planetary atmospheres.

International audience; A lightweight and sophisticated optical depth sensor (ODS) able to measure alternatively scattered flux at zenith and the sum of the direct flux and the scattered flux in blue and red has been developed to work in martian environment. The principal goals of ODS are to perform measurements of the daily mean dust opacity and to retrieve the altitude and optical depth of high altitude clouds at twilight, crucial parameters in the understanding of martian meteorology. The retrieval procedure of dust opacity is based on the use of radiative transfer simulations reproducing observed changes in the solar flux during the day as a function of 4 free parameters: dust opacity in blue and red, and effective radius and effective width of dust size distribution. The detection of clouds is undertaken by looking at the time variation of the color index (CI), defined as the ratio between red and blue ODS channels, at twilight. The retrieval of altitude and optical depth of clouds is carried out using a radiative transfer model in spherical geometry to simulate the CI time variation at twilight. Here the different retrieval procedures to analyze ODS signals, as well as the results obtained in different sensitivity analysis are presented and discussed.

Numerous geological features that could be evaporitic in origin have been identified on the surface of Titan. Although they seem to be water–ice poor, their main properties – chemical composition, thickness, stratification – are essentially unknown. In this paper, which follows on a previous one focusing on the surface composition (Cordier, D., Barnes, J.W., Ferreira, A.G. [2013b]. Icarus 226(2),1431–1437), we provide some answers to these questions derived from a new model. This model, based on the up-to-date thermodynamic theory known as “PC-SAFT”, has been validated with available laboratory measurements and specifically developed for our purpose. 1-D models confirm the possibility of an acetylene and/or butane enriched central layer of evaporitic deposit. The estimated thickness of this acetylene–butane layer could explain the strong RADAR brightness of the evaporites. The 2-D computations indicate an accumulation of poorly soluble species at the deposit’s margin. Among these species, HCN or aerosols similar to tholins could play a dominant role. Our model predicts the existence of chemically trimodal “bathtub rings” which is consistent with what it is observed at the south polar lake Ontario Lacus. This work also provides plausible explanations to the lack of evaporites in the south polar region and to the high radar reflectivity of dry lakebeds.

Titan is an icy satellite of Saturn with a dense atmosphere and covered by a global photochemical organic haze. Ground based observations and the Huygens descent probe allowed to retrieve the main spectral signature of the water ice ( Griffith et al., 2003 , Coustenis et al., 2005 ) at the surface, possibly covered by a layer of sedimented organic material (Tomasko, M.G. et al. [2005]. Nature 438(7069), 765–778). However, the spectrum of the surface is not yet understood. In this study, we find that the surface reflectivity at the Huygens Landing Site (HLS) is well modeled by a layer of water ice grains overlaid by a moist layer of weakly compacted photochemical aggregated aerosols. Moist soils have spectra shifted toward short wavelengths relatively to spectra of dry soils. Cassini observations of Shangri-La region from orbit also show a very dark surface with a reflectivity peak shifted toward short wavelengths in respect to the reflectivity peak of bright surfaces, revealing a dichotomy between terrains based to their spectra in visible.

A small and sophisticated optical depth sensor (ODS) has been designed to work in the atmosphere of Mars. The instrument measures alternatively the diffuse radiation from the sky and the attenuated direct radiation from the Sun on the surface. The principal goals of ODS are to retrieve the daily mean aerosol optical depth (AOD) and to detect very high and optically thin clouds, crucial parameters in understanding the Martian meteorology and climatology. The detection of clouds is undertaken at twilight, allowing the detection and characterization of clouds with opacities below 0.03 (sub-visual clouds). In addition, ODS is capable to retrieve the aerosol optical depth during nighttime from moonlight measurements. Recently, ODS has been selected at the METEO meteorological station on board the ExoMars 2018 Lander. In order to study the performance of ODS under Mars-like conditions as well as to evaluate the retrieval algorithms for terrestrial measurements, ODS was deployed in Ouagadougou (Africa) between November 2004 and October 2005, a Sahelian region characterized by its high dust aerosol load and the frequent occurrence of Saharan dust storms. The daily average AOD values retrieved by ODS were compared with those provided by a CIMEL sunphotometer of the AERONET (Aerosol Robotic NETwork) network localized at the same location. Results represent a good agreement between both ground-based instruments, with a correlation coefficient of 0.77 for the whole data set and 0.94 considering only the cloud-free days. From the whole data set, a total of 71 sub-visual cirrus (SVC) were detected at twilight with opacities as thin as 1.10−3 and with a maximum of occurrence at altitudes between 14 and 20 km. Although further optimizations and comparisons of ODS terrestrial measurements are required, results indicate the potential of these measurements to retrieve the AOD and detect sub-visual clouds.

Né à Morlaix en 1914, Michel Mohrt n’est pas, loin s’en faut, l’écrivain breton de langue française le plus connu. Son élection à l’Académie française, en 1975, ne l’a pas rendu plus populaire dans une Bretagne que son œuvre n’a pourtant jamais cessé de célébrer. Est-ce l’éloignement d’un homme que sa vie a appelé à travailler dans la capitale depuis de longues années? C’était pourtant aussi le cas de Guillevic, c’est encore celui de Yann Queffélec, c’était celui de Jean Rouaud, avant son ret...

著者人数：156名, Accepted: 2008-05-27, 資料番号: SA1000998000

International audience; The atmosphere of Titan is a complex system, where thermal structure, radiative transfer, dynamics, microphysics and photochemistry are strongly coupled together. The global climate model developed over the past 15 years at the Pierre-Simon Laplace Institute has been exploring these different couplings, and has demonstrated how they can help to interpret the observed atmospheric structure of Titan's lower atmosphere (mainly in the stratosphere and troposphere). This review discusses these interactions, and our current understanding of their role in the context of this model, but also of other available works. The recent Cassini results, and the importance of the production mechanisms for Titan's haze, have put forward the need to explore the mesosphere and the couplings between upper and lower atmosphere, as well as the current limits of available models.

Spectroscopy for the investigation of the characteristics of the atmosphere of Venus (SPICAV) is a suite of three spectrometers in the UV and IR range with a total mass of 13.9 kg flying on the Venus Express (VEX) orbiter, dedicated to the study of the atmosphere of Venus from ground level to the outermost hydrogen corona at more than 40,000 km. It is derived from the SPICAM instrument already flying on board Mars Express (MEX) with great success, with the addition of a new IR high-resolution spectrometer, solar occultation IR (SOIR), working in the solar occultation mode. The instrument consists of three spectrometers and a simple data processing unit providing the interface of these channels with the spacecraft. A UV spectrometer (118–320 nm, resolution 1.5 nm) is identical to the MEX version. It is dedicated to nadir viewing, limb viewing and vertical profiling by stellar and solar occultation. In nadir orientation, SPICAV UV will analyse the albedo spectrum (solar light scattered back from the clouds) to retrieve SO2, and the distribution of the UV-blue absorber (of still unknown origin) on the dayside with implications for cloud structure and atmospheric dynamics. On the nightside, γ and δ bands of NO will be studied, as well as emissions produced by electron precipitations. In the stellar occultation mode the UV sensor will measure the vertical profiles of CO2, temperature, SO2, SO, clouds and aerosols. The density/temperature profiles obtained with SPICAV will constrain and aid in the development of dynamical atmospheric models, from cloud top (not, vert, similar60 km) to 160 km in the atmosphere. This is essential for future missions that would rely on aerocapture and aerobraking. UV observations of the upper atmosphere will allow studies of the ionosphere through the emissions of CO, CO+, and CO2+, and its direct interaction with the solar wind. It will study the H corona, with its two different scale heights, and it will allow a better understanding of escape mechanisms and estimates of their magnitude, crucial for insight into the long-term evolution of the atmosphere. The SPICAV VIS-IR sensor (0.7–1.7 μm, resolution 0.5–1.2 nm) employs a pioneering technology: an acousto-optical tunable filter (AOTF). On the nightside, it will study the thermal emission peeping through the clouds, complementing the observations of both VIRTIS and Planetary Fourier Spectrometer (PFS) on VEX. In solar occultation mode this channel will study the vertical structure of H2O, CO2, and aerosols. The SOIR spectrometer is a new solar occultation IR spectrometer in the range λ=2.2–4.3 μm, with a spectral resolution λ/Δλ>15,000, the highest on board VEX. This new concept includes a combination of an echelle grating and an AOTF crystal to sort out one order at a time. The main objective is to measure HDO and H2O in solar occultation, in order to characterize the escape of D atoms from the upper atmosphere and give more insight about the evolution of water on Venus. It will also study isotopes of CO2 and minor species, and provides a sensitive search for new species in the upper atmosphere of Venus. It will attempt to measure also the nightside emission, which would allow a sensitive measurement of HDO in the lower atmosphere, to be compared to the ratio in the upper atmosphere, and possibly discover new minor atmospheric constituents.

The near-infrared spectrum of Titan, Saturn's largest moon and one of the Cassini/Huygens' space mission primary targets, covers the 0.8 to 5 micron region in which it shows several weak CH 4 absorption regions, and in particular one centered near 2.75 micron. Due to the interference of telluric absorption, only part of this window region (2.9–3.1 μm) has previously been observed from the ground [Noll, K.S., Geballe, T.R., Knacke, R., Pendleton, F., Yvonne, J., 1996. Icarus 124, 625–631; Griffith, C.A., Owen, T., Miller, G.A., Geballe, T., 1998. Nature 395, 575–578; Griffith, C.A., Owen, T., Geballe, T.R., Rayner, J., Rannou, P., 2003. Science 300, 628–630; Geballe, T.R., Kim, S.J., Noll, K.S., Griffith, C.A., 2003. Astrophys. J. 583, L39–L42]. We report here on the first spectroscopic observations of Titan covering the whole 2.4–4.9 μm region by two instruments on board the Infrared Space Observatory (ISO) in 1997. These observations show the 2.75-μm window in its complete extent for the first time. In this study we have also used a high-resolution Titan spectrum in the 2.9–3.6 μm region taken with the Keck [Geballe, T.R., Kim, S.J., Noll, K.S., Griffith, C.A., 2003. Astrophys. J. 583, L39–L42; Kim, S.J., Geballe, T.R., Noll, K.S., Courtin, R., 2005. Icarus 173, 522–532] to infer information on the atmospheric parameters (haze extinction, single scattering albedo, methane abundance, etc.) by fitting the methane bands with a detailed microphysical model of Titan's atmosphere (updated from Rannou, P., McKay, C.P., Lorenz, R.D., 2003. Planet. Space Sci. 51, 963–976). We have included in this study an updated version of a database for the CH 4 absorption coefficients [STDS, Wenger, Ch., Champion, J.-P., 1998. J. Quant. Spectrosc. Radiat. Transfer 59, 471–480. See also http://www.u-bourgogne.fr/LPUB/TSM/sTDS.html for latest updates; Boudon, V., Champion, J.-P., Gabard, T., Loete, M., Michelot, F., Pierre, G., Rotger, M., Wenger, Ch., Rey, M., 2004. J. Mol. Spectrosc. 228, 620–634]. For the atmosphere we find that (a) the haze extinction profile that best matches the data is one with higher (by 40%) extinction in the atmosphere with respect to Rannou et al. (2003) down to about 30 km where a complete cut-off occurs; (b) the methane mixing ratio at Titan's surface cannot exceed 3% on a disk-average basis, yielding a maximum CH 4 column abundance of 2.27 km-am in Titan's atmosphere. From the derived surface albedo spectrum in the 2.7–3.08 micron region, we bring some constraints on Titan's surface composition. The albedo in the center of the methane window varies from 0.01 to 0.08. These values, compared to others reported in the other methane windows, show a strong compatibility with the water ice spectrum in the near-infrared. Without confirming its existence from this work alone, our data then appear to be compatible with water ice. A variety of other ices, such as CO 2 , NH 3 , tholin material or hydrocarbon liquid cannot be excluded from our data, but an additional unidentified component with a signature around 2.74 micron is required to satisfy the data.

SPectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (SPICAM) Light, a light-weight (4.7 kg) UV–IR instrument to be flown on Mars Express orbiter, is dedicated to the study of the atmosphere and ionosphere of Mars. A UV spectrometer (118–320 nm, resolution 0.8 nm) is dedicated to nadir viewing, limb viewing and vertical profiling by stellar and solar occultation (3.8 kg). It addresses key issues about ozone, its coupling with H2O, aerosols, atmospheric vertical temperature structure and ionospheric studies. UV observations of the upper atmosphere will allow studies of the ionosphere through the emissions of CO, CO+, and CO 2 + , and its direct interaction with the solar wind. An IR spectrometer (1.0–1.7 μm, resolution 0.5–1.2 nm) is dedicated primarily to nadir measurements of H2O abundances simultaneously with ozone measured in the UV, and to vertical profiling during solar occultation of H2O, CO2, and aerosols. The SPICAM Light near-IR sensor employs a pioneering technology acousto-optical tunable filter (AOTF), leading to a compact and light design. Overall, SPICAM Light is an ideal candidate for future orbiter studies of Mars, after Mars Express, in order to study the interannual variability of martian atmospheric processes. The potential contribution to a Mars International Reference Atmosphere is clear.

We have developed in the last decade a two-dimensional version of the Titan global circulation model LMDZ. This model accounts for multiple coupling occuring on Titan between dynamics, haze, chemistry and radiative transfer. It was successful at explaining many observed features related to atmosphere state (wind, temperature), haze structure and chemical species distributions, recently, an important step in our knowledge about Titan has been done with Cassini and Huygens visits to Titan. In this context, we want to make the results of our model available for the scientific community which is involved in the study of Titan. Such a tool should be useful to give a global frame (spatial and time behaviour of physical quantities) for interpreting ground based telescope observations.

We use Titan's geometric albedo to constrain the vertical distribution of the haze. Microphysical models incorporating fractal aggregates do not readily fit the methane features at 0.62 μm band and the dark 0.88 μm of the albedo spectrum simultaneously. We take advantage of this apparent discrepancy to constrain the haze vertical profile. We used the geometric albedo and several results and constraints from other works to better constrain the vertical haze extinction profile, especially in the low stratosphere. The objective of this model is to give a solution that simultaneously fits the main constraints known to apply to the haze. We find that the haze extinction increases with decreasing altitude with a scale height about equal to the atmospheric scale height down to 100 km . Below this altitude, extinction must decrease down to 30 km . This is necessary in order to have enough haze to sustain a relatively high albedo (0.076) in the dark 0.88 μm methane band and to show the 0.62 μm band in the haze continuum. We set the haze production rate around 7×10 −14 kg m −2 s −1 , and the aerosols production altitude around 400 km (or at pressure 1.5 Pa ). The physical processes which generate such a profile are not clear. However, purely one-dimensional effects such as condensation, sedimentation, and rainout can be ruled out, and we believe that this relative clearing in Titan's troposphere and lower stratosphere is due to particle horizontal transport by the mean circulation.