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The Soi crater region, with the well-preserved Soi crater in its center, covers almost 10% of Titan’s surface. Schoenfeld et al. (2023) [1] mapped this region at 1:800,000 scale and produced a geomorphological map showing that the area consists of 22 distinct geomorphological units. The region includes the boundaries between the equatorial regions of Titan and the mid-latitudes and extends into the high northern latitudes (above 50o). We analyzed 262 different locations from several Visual and Infrared Mapping Spectrometer (VIMS) datacubes using a radiative transfer technique [2] and a mixing model [3], yielding compositional constraints on Titan’s optical surface layer and near-surface substrate compositional constraints using RADAR microwave emissivity. We have derived combinations of top surface materials between dark materials, tholin-like materials, water-ice, and methane. We found no evidence of CO2 and NH3 ice. We discuss our results in terms of origin and evolution theories.[1] Schoenfeld, A., et al. (2023), JGR-Planets 128, e2022JE007499; [2] Solomonidou, A., et al., (2020a), Icarus, 344, 113338; [3] Solomonidou, A., et al. (2020b), A&A, 641, A16.

Some of the satellites of the gas giants Jupiter and Saturn, at orbits beyond the snow-line and the traditional “habitable zone”, have been revealed by missions like Galileo, Cassini-Huygens and Juno as unique and extremely interesting bodies with a strong astrobiological potential [1]. Jupiter’s Europa and Ganymede show indications of harboring liquid water oceans under their icy crusts, which, in the case of Europa, may be in direct contact with a silicate mantle floor and kept warm through time by tidally generated heat. Ganymede, the largest satellite in our Solar System, is unique in possessing an induced magnetic field and probably harbours an undersurface liquid water ocean contained between two ice layers. Thanks to the fabulous international cooperation that came behind the Cassini-Huygens mission, Saturn’s system was revealed and Titan [2] and Enceladus [3], were found to possess organic chemistry, unique geological features and internal liquid water oceans. I will describe my personal experience of the Cassini-Huygens mission.The icy satellites provide a conceptual basis within which new theories for understanding habitability can be constructed. In view of the many questions remaining unanswered [4], these bodies will be further investigated in the future by new missions to the giant planets systems. Future space exploration towards the Galilean satellites will be performed by missions such as ESA’s JUpiter Icy moons Explorer (JUICE, [5]) (whose main target is Ganymede and will be launched in April 2023) and NASA’s Europa Clipper mission to launch in 2024. For a return to Titan, NASA has recently selected the Dragonfly mission [6], while other concepts are being studied for these and other icy moons, also around the ice giants.Future in situ measurements will be extremely useful in unveiling these worlds. In the meantime, Juno data and ground-based observations can help complement the space discoveries.I will discuss what we currently know and what we expect to learn about habitable conditions in the outer solar system and how our perception of these worlds has changed, along with the need to better protect their environments [7].References: 1. Coustenis, A., Encrenaz, Th., 2013. Life beyond Earth: the search for habitable worlds in the Universe. CUP. ISBN: 9781107026179. 2. Coustenis, A., 2021. The Atmosphere of Titan. In Read, P. (Ed.), Oxford Research Encyclopedia of Planetary Science. DOI: 10.1093/acrefore/9780190647926.013.120 3. Lunine, J., Coustenis, A., Mitri, G., et al., 2018. “Future exploration of Enceladus and other Saturnian moons”. In “Enceladus and the Icy Moons of Saturn”. LPI/UA/Space Science Series, P. Schenk, R. Clark, C.J.A. Howett, A. Verbiscer, J.H. Waite Eds., ISBN 9780816537075. 4. Nixon, C. A., et al., 2018. PSS, 155, 50-72. 5. Coustenis, A., Witasse, O., Erd, C., 2021. The JUICE mission: expectations and challenges. Fall issue of The Bridge on space exploration, Sept. 2021, Vol. 51, issue #3, pp. 41-50. https://www.nae.edu/260902/The-JUICE-Mission-Challenges-and-Expectations 6. Barnes, J. et al., The Plan. Sci. J., 4, 18. 7. Fisk, L., Worms, J.-C., Coustenis, A., et al., 2021. Introductory Note to the June 2021 and Update of the COSPAR Policy on Planetary Protection. Space Research Today 211, Aug. 2021, 9-25, https://doi.org/10.1016/j.srt.2021.07.009 and Policy : https://doi.org/10.1016/j.srt.2021.07.010

In 2018 and 2022, we have published an analysis of the observability of temperate planets (with an equilibrium temperature of about 300-550 K) with ARIEL [1, 2]. This presentation is an update of this analysis which uses new targets identified in particular from the TESS database [3] and investigates their observability with ARIEL. Using the parameters of these new targets, and an atmospheric mean molecular weight of 2.3 g/mol (corresponding to a hydrogen-rich atmosphere), and using the ArielRad code [4], we give an estimate of the number of transits (T2) needed for these objects to be observed in the Tier 2 mode of the space mission, i. e. with a S/N @ 7 . The Tier 2 spectroscopic mode (R @10 for 1.1 mm < l < 1.95 mm, R = 50 for 1.95 mm < l < 3.9 mm, R @ 15 for 3.9 mm < l Preliminary results of this study are given below: Using the candidate list of Edwards and Tinetti (2022) [3], we find 22 targets with T2 < 35, observable during the lifetime of ARIEL. 7 among these targets, with a mass above 5 terrestrial masses, are presumably hydrogen-rich planets and should be observable with Ariel in the Tier 2 mode. In particular, the big Neptune TOI-3884 b, with T2 = 1, is expected to be observable also in the Tier 3 mode. 8 other targets, with a mass between 1.5 and 5 terrestrial masses, will be observable in the Tier 2 mode if they have a hydrogen-rich atmosphere. The Ariel observation will thus allow us to determine whether they are sub-Neptunes or super-Earths. A promising case is LC 98-59 c and d for which the value of T2 is low (T2 = 7 and 6 respectively). The 7 remaining candidates are exo-Earths, mostly from the TRAPPIST-1 system. Because their atmospheric mean molecular weight is expected to be as high as 18 g/mol (in the case of a H2O-rich atmosphere) or even more (in case of a CO/N2/CO2-rich atmosphere), they are not expected to be observable with Ariel in the Tier 2 mode. In the future, we plan to re-evaluate the observability of the candidates using different values of the atmospheric mean molecular weight. For the most favorable cases, we plan to calculate their synthetic spectra for different atmospheric compositions, considering also the possible effect of clouds. References: [1] Encrenaz et al., Exp. Astr. 46, 31 (2018) [2] Encrenaz et al., Exp. Astr. 53, 375 (2022). [3] Edwards, B. and Tinetti, G. Astron. J., 164, id.15, 25 pp. (2022) [4] Mugnai, L. V. et al. 2021b, Astron. J, 161, 284 (2021) &nbsp

Planetary protection guidance for martian exploration has become a notable point of discussion over the last decade. This is due to increased scientific interest in the habitability of the red planet with updated techniques, missions becoming more attainable by smaller space agencies, and both the private sector and governments engaging in activities to facilitate commercial opportunities and human-crewed missions. The international standards for planetary protection have been developed through consultation with the scientific community and the space agencies by the Committee on Space Research's (COSPAR) Panel on Planetary Protection, which provides guidance for compliance with the Outer Space Treaty of 1967. In 2021, the Panel evaluated recent scientific data and literature regarding the planetary protection requirements for Mars and the implications of this on the guidelines. In this paper, we discuss the COSPAR Planetary Protection Policy for Mars, review the new scientific findings and discuss the next steps required to enable the next generation of robotic missions to Mars.

The Cassini cameras and especially the Visual and infrared Mapping Spectrometer has provided a sequence of spectra showing the diversity of Titan’s surface spectrum from flybys performed during the 13 years of Cassini’s operation. The investigation of Titan’s surface chemical composition is of great importance for the understanding of the atmosphere-surface-interior system of the moon. The Soi crater region with the well-preserved Soi crater in its center, spans from Titan’s equatorial regions to high northern latitudes. This provides a rich diversity of landscapes, one that is also representative of the diversity encountered across Titan. We mapped this region at 1:800,000 scale using Cassini SAR and non-SAR data and produced a geomorphological map using the methodology presented by [1] and [2]. The VIMS coverage of the region allowed for detailed analysis of spectra of 262 different locations using a radiative transfer technique [3;4] and a mixing model [5;6], yielding compositional constraints on Titan’s optical surface layer. Additional constraints on composition on the near-surface substrate were obtained from microwave emissivity. We identified 22 geomorphological units, 3 of which were not previously described, and derived combinations of top surface materials between dark materials, tholin-like materials, water-ice, and methane. We found no evidence of CO2 and NH3 ice. We also observe empty lakes as far south as 40°N, which mark the most southern extent of Titan’s north polar lakes. We use the stratigraphic relations between our mapping units and the relation between the geomorphology and the composition of the surface layers to build hypotheses on the origin and evolution of the regional geology. [1] Malaska, M., et al. (2016), Icarus 270, 130; [2] Schoenfeld, A., et al. (2021), Icarus 366, 114516; [3] Solomonidou, A., et al. (2014), J. Geophys. Res. Planets, 119, 1729; [4] Solomonidou, A., et al. (2016), Icarus, 270, 85; [5] Solomonidou, A., et al. (2018), J. Geophys. Res. Planets, 123, 489; [6] Solomonidou, A., et al. (2020), A&A 641, A16.

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

We present a White Paper with a science theme concept of ocean world evolution and habitability proposed in response to ESA’s Voyage 2050 Call with a focus on Titan and Enceladus in the Saturn system. Ocean worlds in the outer Solar System that possess subsurface liquid water oceans are considered to be prime targets for extra-terrestrial life and offer windows into Solar System evolution and habitability. The Cassini-Huygens mission to the Saturn system (2004–2017) revealed Titan with its organic-rich evolving world with terrestrial features and Enceladus with its active aqueous environment to be ideal candidates to investigate ocean world evolution and habitability. Additionally, this White Paper presents a baseline for a multiple flyby mission with a focused payload as an example of how ocean world evolution and habitability in the Saturn system could be investigated building on the heritage of the Cassini-Huygens mission and complementing the recently selected NASA Dragonfly mission.

The Soi crater region, an extensive region covering almost 10% of Titan’s surface, has the Soi crater in its middle, which is a relatively well-preserved crater on Titan. This region includes the boundaries between the equatorial regions of Titan and the mid-latitudes, and extends into the high northern latitudes (above 50o). All these three Titan latitudes are dominated by different types of geomorphological units, such as dunes, mountains, and lakes, and are governed by different geological processes (such as lacustrine, aeolian and fluvial). An additional important and unique characteristic of the Soi crater region is that it includes 59 empty lakes, and the extent of these features reaches as far south as 40oN. We mapped this region at 1:800,000 scale and produced the first detailed geomorphological map of the region using the same methodology as presented by [1;2] and Schoenfeld et al. [3]. We included non-SAR (Synthetic Aperture Radar) data such as radiometry, ISS, and VIMS data in order to analyze vast areas not observed by SAR. We performed detailed VIMS analysis of hundreds of distinct regions for all geomorphological units with a radiative transfer technique [4] and a mixing model [5], to infer constraints on the composition. In our results, we introduce new geomorphological units, which were not seen in previous mapping of large Titan regions such as the Afekan and South Belet, and report the extensive presence of the scalloped plains units and their possible origin. A total of 10 craters, including Soi, are identified in this region, which are older than the plains and dune units. The radiative transfer analysis from VIMS showed that the major constituents covering the Soi crater region are compatible with water ice and organic alkane, alkene and alkyl-like stretch materials. We discuss our results in terms of origin and evolution theories.[1] Malaska, M., et al. (2016), Icarus 270, 130; [2] Malaska, M., et al. (2020), Icarus, 344, 113764. [3] Schoenfeld, A., et al. (2021), Icarus 366, 114516. [4] Solomonidou, A., et al. (2020a), Icarus, 344, 113338; [5] Solomonidou, A., et al. (2020b), A&A 641, A16.

Icy moons of Jupiter and Saturn are privileged targets in currently developed space missions by several space agencies and in particular ESA, NASA and their partners. One of these missions is ESA’s first large mission in the Cosmic Vision Programme, JUICE [1], which is being developed to address questions regarding the Jupiter system and its satellites, with a focus on the largest moon, Ganymede. The overarching theme for JUICE is the emergence of habitable worlds around gas giants taking into account the requirements involving the presence of organic compounds, trace elements, water, energy sources and a relative stability of the environment over time.Among other, JUICE will determine the characteristics (composition and dynamics) of the exospheres of the icy moons [2], in particular Ganymede and Europa, with for instance coordinated observations among sets of instruments like UVS, PEP, RPWI, MAJIS, 3GM, J-MAG, JANUS and SWI. The JUICE mission is scheduled to be launched in spring 2023 and arrive at Jupiter in mid-2031 and is foreseen to last nominally for 3 and a half years. JUICE investigations will benefit from current observations by JUNO and will be synergistic to NASA’s Europa Clipper mission. I will describe the foreseen investigations of the tenuous atmospheres of the icy moons around Jupiter.Cassini explored the dense and organic-laden atmosphere of Titan during several flybys over 13 years [2,3] and also determined the characteristics of the Enceladus plumes. However, new questions have risen and several cold cases [4] remain that will constitute major science objectives for future space missions to the satellites around Saturn, like Dragonfly [5] or an orbiter in the Saturn system or a dedicated Enceladus mission…In particular, Titan’s atmosphere has not yet revealed all its secrets, in particular for the chemical composition, which should be much more complex than what was detected by Cassini-Huygens. Future in situ measurements will be extremely useful in unveiling this unique complex world. In the meantime, ground-based observations with large telescopes like ALMA, elsewhere in Chile or the ones in Hawaii can help complement the past discoveries.References:[1] Coustenis, A., Witasse, O., Erd, C., 2021. The JUICE mission: expectations and challenges. Fall issue of The Bridge on space exploration, Sept. 2021, Vol. 51, issue #3, pp. 41-50.[2] Coustenis, A., Tokano, T., Burger, M. H., Cassidy, T. A., Lopes, R. M., Lorenz, R. D., Retherford, K. D., Schubert G., 2010. Atmospheres/exospheres characteristics of icy satellites. Space Sci. Rev., 153, 155-184. https://www.nae.edu/260902/The-JUICE-Mission-Challenges-and-Expectations[3] Coustenis, A., 2021. “The Atmosphere of Titan”. In Read, P. (Ed.), Oxford Research Encyclopedia of Planetary Science. Oxford University Press (August 31). doi: https://doi.org/10.1093/acrefore/9780190647926.013.120[4] Nixon, C. A., Lorenz, R. D., Achterberg, R. K., et al. (2018). Titan's cold case files - Outstanding questions after Cassini-Huygens. Planetary and Space Science, 155, 50-72.[5] Barnes, J. et al. (2021). Science Goals and Objectives for the Dragonfly Titan Rotorcraft Relocatable Lander. The Plan. Sci. J., VoL. 2, Issue 4, id.130, 18 pp.

International audience; While the Ariel mission is primarily designed for the study of warm and hot objects, with an equilibrium temperature above 500 K, in this paper we want to explore a larger sample of possible colder targets. We thus investigate the detectability with Ariel of “temperate” exoplanets (with an equilibrium temperature of 400 K). We first consider the case of hydrogen-rich exoplanets (from Jupiters to sub-Neptunes) and we calculate their infrared transmission spectrum for several classes of stars. We consider the Tier 2 mode of Ariel, for which the resolving power (R = 50 for l < 4 mm and R=15 for l > 4 mm) is sufficient to get information about the chemical composition of the objects. Results show that temperate Jupiters and sub-Neptunes around all types of stars from G2 to M8, with revolution periods of a few tens of days and transit durations of a few hours, could be observed with Ariel, up to distances of about 50 pc for Jupiters and 25 pc for sub-Neptunes. In the case of temperate super-Earths, we estimate that they will not be observable in the Ariel Tier 2 mode. In a study of currently available target candidates, we find one sub-Neptune (TOI-178 g) as possibly observable in Ariel’s Tier 2. This study, a follow-up of “Transit spectroscopy of temperate Jupiters with ARIEL: A feasibility study” (Encrenaz et al., Exp. Astr. 46:31-44, 2018), is submitted to the ARIEL special issue of Experimental Astronomy (manuscript under revision)

Titan, Saturn’s largest satellite, is one of the most intriguing moons in our Solar System, in particular because of its dense and extended nitrogen-based and organic-laden atmosphere. Other unique features include a methanological cycle similar to the Earth’s hydrological one, surface features similar to terrestrial ones, and a probable under-surface liquid water ocean. Besides the dinitrogen main component, the gaseous content includes methane and hydrogen, which, through photochemistry and photolysis, produce a host of trace gases such as hydrocarbons and nitriles. This very advanced organic chemistry creates layers of orange-brown haze surrounding the satellite. The chemical compounds diffuse downward in the form of aerosols and condensates and are finally deposited on the surface. There is very little oxygen in the atmosphere, mainly in the form of H2O, CO, and CO2. The atmospheric chemical and thermal structure varies significantly with seasons, much like on Earth, albeit on much longer time scales. Extensive analysis of Titan data from ground, Earth-orbiting observatories, and space missions, like those returned by the 13-year operating Cassini-Huygens spacecraft, reveals a complex system with strong interactions among the atmosphere, the surface, and the interior. The processes operating in the atmosphere are informative of what occurs on Earth and give hints as to the origin and evolution of our outer Solar System.

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

The investigation of Titan’s surface chemical composition is of great importance for the understanding of the atmosphere-surface-interior system of the moon. The Cassini cameras and especially the Visual and infrared Mapping Spectrometer has provided a sequence of spectra showing the diversity of Titan’s surface spectrum from flybys performed during the 13 years of Cassini’s operation. In the 0.8-5.2 μm range, this spectro-imaging data showed that the surface consists of a multivariable geological terrain hosting complex geological processes. The data from the seven narrow methane spectral “windows” centered at 0.93, 1.08, 1.27, 1.59, 2.03, 2.8 and 5 μm provide some information on the lower atmospheric context and the surface parameters. Nevertheless, atmospheric scattering and absorption need to be clearly evaluated before we can extract the surface properties. In various studies (Solomonidou et al., 2014; 2016; 2018; 2019; 2020a, 2020b; Lopes et al., 2016; Malaska et al., 2016; 2020), we used radiative transfer modeling in order to evaluate the atmospheric scattering and absorption and securely extract the surface albedo of multiple Titan areas including the major geomorphological units. We also investigated the morphological and microwave characteristics of these features using Cassini RADAR data in their SAR and radiometry mode. Here, we present a global map for Titan’s surface showing the chemical composition constraints for the various units. The results show that Titan’s surface composition, at the depths detected by VIMS, has significant latitudinal dependence, with its equator being dominated by organic materials from the atmosphere and a very dark unknown material, while higher latitudes contain more water ice. The albedo differences and similarities among the various geomorphological units give insights on the geological processes affecting Titan’s surface and, by implication, its interior. We discuss our results in terms of origin and evolution theories. References: [1] Solomonidou, A., et al. (2014), J. Geophys. Res. Planets, 119, 1729; [2] Solomonidou, A., et al. (2016), Icarus, 270, 85; [3] Solomonidou, A., et al. (2018), J. Geophys. Res. Planets, 123, 489; [4] Solomonidou, A., et al. (2020a), Icarus, 344, 113338; [5] Solomonidou, A., et al. (2020b), A&A 641, A16; [6] Lopes, R., et al. (2016) Icarus, 270, 162; [7] Malaska, M., et al. (2016), Icarus 270, 130; [8] Malaska, M., et al. (2020), Icarus, 344, 113764. Acknowledgements: This work was conducted at the California Institute of Technology (Caltech) under contract with NASA. Y.M. and A.S. (partly) was supported by the Czech Science Foundation (grant no. 20-27624Y). ©2021 California Institute of Technology. Government sponsorship acknowledged.

Titan’s atmosphere and surface (a complex system) evolve with season, as Titan follows Saturn in its orbit around the Sun for 30 years with an inclination of about 27°. We performed an analysis of spectra acquired by Cassini/CIRS at high resolution covering the range from 600 to 1500 cm-1 since the beginning and until the last flyby of Titan in 2017 and describe the temperature and chemical composition variations ([1-3]. By applying our radiative transfer code (ARTT) to the high-resolution CIRS spectra we study the stratospheric evolution over almost two Titan seasons [1,2], corresponding to the Cassini mission duration. CIRS nadir and limb spectral together show variations in temperature and chemical composition in the stratosphere during the Cassini mission, before and after the Northern Spring Equinox (NSE) and also during one Titan year. Since the 2010 equinox we have thus reported on monitoring of Titan’s stratosphere near the poles and in particular on the observed strong temperature decrease and compositional enhancement above Titan’s southern polar latitudes since 2012 and until 2014 of several trace species, such as complex hydrocarbons and nitriles, which were previously observed only at high northern latitudes. This effect followed the transition of Titan’s seasons from northern winter in 2002 to northern summer in 2017, while at that latter time the southern hemisphere was entering winter. Our data show a continued decrease of the abundances which we first reported to have started in 2015. The 2017 data we have acquired and analyzed here are important because they are the only ones recorded since 2014 close to the south pole in the far-infrared nadir mode at high resolution. A large temperature increase in the southern polar stratosphere (by 10-50 K in the 0.5 mbar-0.05 mbar pressure range) is found and a change in the temperature profile’s shape. The 2017 observations also show a related significant decrease in most of the abundances which must have started sometime between 2014 and 2017 [3]. In our work, we show that the equatorial latitudes remain rather constant throughout the Cassini mission. We have thus shown that the south pole of Titan is now losing its strong enhancement, while the north pole also slowly continues its decrease in gaseous opacities. It would have been interesting to see when this might happen, but the Cassini mission ended in September 2017. Perhaps future ground-based measurements and the Dragonfly mission can pursue this investigation and monitor Titan’s atmosphere to characterize the seasonal events. Our results set constraints on GCM and photochemical models. References: [1] Coustenis et al., 2016, Icarus 270, 409-420 [2] Coustenis et al., 2018, Astroph. J., Lett., 854, no2 [3] Coustenis et al., 2020. Titan’s neutral atmosphere seasonal variations up to the end of the Cassini mission. Icarus 344, 113413. https://doi.org/10.1016/j.icarus.2019.113413.

International audience

International audience; Ce chapitre présente successivement les satellites des quatre planètes géantes (Jupiter, Saturne, Uranus et Neptune) en insistant particulièrement sur les lunes potentiellement intéressantes pour l’exobiologie (Europe, Ganymède, Callisto, Titan et Encelade). Il décrit ensuite les anneaux des planètes géantes, puis des petits corps (Chariklo, Haumea) en analysant leurs mécanismes de formation et de destruction.

13 pages, white paper submitted to the NRC Decadal Survey for Planetary Science and Astrobiology; International audience; We outline a flagship-class mission concept focused on studying Titan as a global system, with particular emphasis on the polar regions. Investigating Titan from the unique standpoint of a polar orbit would enable comprehensive global maps to uncover the physics and chemistry of the atmosphere, and the topography and geophysical environment of the surface and subsurface. The mission includes two key elements: (1) an orbiter spacecraft, which also acts as a data relay, and (2) one or more small probes to directly investigate Titan's seas and make the first direct measurements of their liquid composition and physical environment. The orbiter would carry a sophisticated remote sensing payload, including a novel topographic lidar, a long-wavelength surface-penetrating radar, a sub-millimeter sounder for winds and for mesospheric/thermospheric composition, and a camera and near-infrared spectrometer. An instrument suite to analyze particles and fields would include a mass spectrometer to focus on the interactions between Titan's escaping upper atmosphere and the solar wind and Saturnian magnetosphere. The orbiter would enter a stable polar orbit around 1500 to 1800 km, from which vantage point it would make global maps of the atmosphere and surface. One or more probes, released from the orbiter, would investigate Titan's seas in situ, including possible differences in composition between higher and lower latitude seas, as well as the atmosphere during the parachute descent. The number of probes, as well as the instrument complement on the orbiter and probe, remain to be finalized during a mission study that we recommend to NASA as part of the NRC Decadal Survey for Planetary Science now underway, with the goal of an overall mission cost in the "small flagship" category of ~$2 bn. International partnerships, similar to Cassini-Huygens, may also be included for consideration.

Titan is a unique body in the solar system in particular because of its earth-like surface features, its putative undersurface liquid water ocean and its large organic content in the atmosphere and on the surface . These chemical species evolve with season, as Titan follows Saturn in its orbit around the Sun with an inclination of about 27°. We performed an analysis of spectra acquired by Cassini/CIRS at high resolution covering the range from 10 to 1500 cm-1 since the beginning and until the last flyby of Titan in 2017 and describe the temperature and composition variations ([1-3]. By applying our radiative transfer code (ARTT) to the high-resolution CIRS spectra we study the stratospheric evolution over almost two Titan seasons [1,2]. CIRS nadir and limb spectral together show variations in temperature and chemical composition in the stratosphere during the Cassini mission, before and after the Northern Spring Equinox (NSE) and also during one Titan year.Since the 2010 equinox we have thus reported on monitoring of Titan’s stratosphere near the poles and in particular on the observed strong temperature decrease and compositional enhancement above Titan’s southern polar latitudes since 2012 and until 2014 of several trace species, such as complex hydrocarbons and nitriles, which were previously observed only at high northern latitudes. This effect followed the transition of Titan’s seasons from northern winter in 2002 to northern summer in 2017, while at that latter time the southern hemisphere was entering winter.Our data show a continued decrease of the abundances which we first reported to have started in 2015. The 2017 data we have acquired and analyzed here are important because they are the only ones recorded since 2014 close to the south pole in the far-infrared nadir mode at high resolution. A large temperature increase in the southern polar stratosphere (by 10-50 K in the 0.5 mbar-0.05 mbar pressure range) is found and a change in the temperature profile’s shape. The 2017 observations also show a related significant decrease in most of the abundances which must have started sometime between 2014 and 2017 [3]. In our work, we show that the equatorial latitudes remain rather constant throughout the Cassini mission.We have thus shown that the south pole of Titan is now losing its strong enhancement, while the north pole also slowly continues its decrease in gaseous opacities. It would have been interesting to see when this might happen, but the Cassini mission ended in September 2017. Perhaps future ground-based measurements and the Dragonfly mission can pursue this investigation and monitor Titan’s atmosphere to characterize the seasonal events. Our results set constraints on GCM and photochemical models.References: [1] Coustenis et al., 2016, Icarus 270, 409-420; [2] Coustenis et al., 2018, Astroph. J., Lett., 854, no2; [3] Coustenis et al., 2020. Titan’s neutral atmosphere seasonal variations up to the end of the Cassini mission. Icarus 344, 113413. https://doi.org/10.1016/j.icarus.2019.113413.

The investigation of Titan’s surface chemical composition is of great importance for the understanding of the atmosphere-surface-interior system of the moon. The Cassini cameras and especially the Visual and infrared Mapping Spectrometer has provided a sequence of spectra showing the diversity of Titan’s surface spectrum from flybys performed during the 13 years of Cassini’s operation. In the 0.8-5.2 μm range, this spectro-imaging data showed that the surface consists of a multivariable geological terrain hosting complex geological processes. The data from the seven narrow methane spectral “windows” centered at 0.93, 1.08, 1.27, 1.59, 2.03, 2.8 and 5 μm provide some information on the lower atmospheric context and the surface parameters. Nevertheless, atmospheric scattering and absorption need to be clearly evaluated before we can extract the surface properties. In various studies (Solomonidou et al., 2014; 2016; 2018; 2019; 2020a, 2020b; Lopes et al., 2016; Malaska et al., 2016; 2020), we used radiative transfer modeling in order to evaluate the atmospheric scattering and absorption and securely extract the surface albedo of multiple Titan areas including the major geomorphological units. We also investigated the morphological and microwave characteristics of these features using Cassini RADAR data in their SAR and radiometry mode. Here, we present a global map for Titan’s surface showing the chemical composition constraints for the various units. The results show that Titan’s surface composition, at the depths detected by VIMS, has significant latitudinal dependence, with its equator being dominated by organic materials from the atmosphere and a very dark unknown material, while higher latitudes contain more water ice. The albedo differences and similarities among the various geomorphological units give insights on the geological processes affecting Titan’s surface and, by implication, its interior. We discuss our results in terms of origin and evolution theories.[1] Solomonidou, A., et al. (2014), J. Geophys. Res. Planets, 119, 1729; [2] Solomonidou, A., et al. (2016), Icarus, 270, 85; [3] Solomonidou, A., et al. (2018), J. Geophys. Res. Planets, 123, 489; [4] Solomonidou, A., et al. (2020a), Icarus, 344, 113338; [5] Solomonidou, A., et al. (2020b), A&A 641, A16; [6] Lopes, R., et al. (2016) Icarus, 270, 162; [7] Malaska, M., et al. (2016), Icarus 270, 130; [8] Malaska, M., et al. (2020), Icarus, 344, 113764.

Ariel, the Atmospheric Remote-sensing Infrared Exoplanet Large-survey, was adopted as the fourth medium-class mission in ESA's Cosmic Vision programme to be launched in 2029. During its 4-year mission, Ariel will study what exoplanets are made of, how they formed and how they evolve, by surveying a diverse sample of about 1000 extrasolar planets, simultaneously in visible and infrared wavelengths. It is the first mission dedicated to measuring the chemical composition and thermal structures of hundreds of transiting exoplanets, enabling planetary science far beyond the boundaries of the Solar System. The payload consists of an off-axis Cassegrain telescope (primary mirror 1100 mm x 730 mm ellipse) and two separate instruments (FGS and AIRS) covering simultaneously 0.5-7.8 micron spectral range. The satellite is best placed into an L2 orbit to maximise the thermal stability and the field of regard. The payload module is passively cooled via a series of V-Groove radiators; the detectors for the AIRS are the only items that require active cooling via an active Ne JT cooler. The Ariel payload is developed by a consortium of more than 50 institutes from 16 ESA countries, which include the UK, France, Italy, Belgium, Poland, Spain, Austria, Denmark, Ireland, Portugal, Czech Republic, Hungary, the Netherlands, Sweden, Norway, Estonia, and a NASA contribution., Comment: Ariel Definition Study Report, 147 pages. Reviewed by ESA Science Advisory Structure in November 2020. Original document available at: https://www.cosmos.esa.int/documents/1783156/3267291/Ariel_RedBook_Nov2020.pdf/

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

We investigate nine Titan impact craters using Visual and Infrared Mapping Spectrometer (VIMS) data and a radiative transfer code (RT) [e.g. 1] in addition to emissivity data, in order to constrain the spectral behavior and composition of the craters. Past studies have looked at the chemical composition of impact craters either by using qualitative comparisons between craters [e.g. 2;3] or by combining all craters into a single unit [4], rather than separating them by geographic location or degradation state. Here, we use a radiative transfer model to first estimate the atmospheric contribution to the data, then extract the surface albedos of the impact crater subunits, and finally constrain their composition by using a library of candidate Titan materials. Following the general characterization of the impact craters, we study two impact crater subunits, the ‘crater floor’, which refers to the bottom of a crater, and the ‘ejecta blanket’, which is the material thrown out of the crater during an impact event. The results show that Titan’s mid-latitude plain craters: Afekan, Soi, and Forseti, in addition to Sinlap and Menrva are enriched in an OH-bearing constituent (likely water-ice) in an organic based mixture, while the equatorial dune craters: Selk, Ksa, Guabonito, and Santorini, appear to be purely composed of organic material (mainly unknown dune dark material). This follows the pattern seen in [4], where midlatitude alluvial fans, undifferentiated plains, and labyrinths were found to consist of a tholin-like and water-ice mixture, while the equatorial undifferentiated plains, hummocky terrains, dunes, and variable plains were found to consist of a dark material and tholin-like mixture in their very top layers. These observations also agree with the evolution scenario proposed by [3], wherein the impact cratering process produces a mixture of organic material and water ice, which is later “cleaned” through fluvial erosion in the midlatitude plains; a cleaning process that does not appear to operate in the equatorial dunes, which seem to be quickly covered by a thin layer of sand sediment. This scenario agrees with other works that suggest that atmospheric deposition is similar in the low-latitudes and midlatitudes on Titan, but with more rain falling onto the higher latitudes causing additional processing of materials in those regions [e.g. 5]. In either case, it appears that active processes are working to shape the surface of Titan, and it remains a dynamic world in the present day.[1] Hirtzig, M., et al. (2013). Icarus, 226, 470–486; [2] Neish, C.D., et al. (2015), Geophys. Res. Lett. 42, 3746–3754; [3] Werynski, A., et al. (2019), Icarus, 321, 508-521; [4] Solomonidou, A., et al. (2018), J. Geophys. Res, 123, 2, 489-507; [5] Neish, A.C., et al. (2016), Icarus, 270, 114–129.

The international planetary science community met in London in January 2020, united in the goal of realising the first dedicated robotic mission to the distant Ice Giants, Uranus and Neptune, as the only major class of Solar System planet yet to be comprehensively explored. Ice-Giant-sized worlds appear to be a common outcome of the planet formation process, and pose unique and extreme tests of our understanding of planetary origins, exotic water-rich planetary interiors, dynamic seasonal atmospheres, complex magnetospheric configurations, geologically-rich icy satellites (both natural and captured), and delicate planetary rings. This article introduces a special issue of Philosophical Transactions of the Royal Society A on Ice Giant System exploration at the start of the 2020s. We review the scientific potential and existing mission design concepts for an ambitious international partnership for exploring Uranus and/or Neptune in the coming decades., Comment: 13 pages, 2 figures. Introductory article (accepted) for "Ice Giant Systems" special issue of Phil. Trans. A, based on Royal Society Discussion Meeting, Jan 2020

In our recent publication [1] we reported new results concerning the seasonal atmospheric evolution near Titan’s poles and equator in terms of temperature and composition using nadir spectra acquired by the Cassini Composite Infrared Spectrometer (CIRS) at high spectral resolution during the last year of the Cassini mission in 2017 complementing previous investigations covering almost two Titan seasons. In previous papers [2,3], we reported on monitoring of Titan’s stratosphere near the poles after the mid-2009 northern spring equinox. In particular we have reported on the observed strong temperature decrease and compositional enhancement above Titan’s southern polar latitudes since 2012 and until 2014 of several trace species, such as complex hydrocarbons and nitriles, which were previously observed only at high northern latitudes. This effect accompanied the transition of Titan’s seasons from northern winter in 2002 to northern summer in 2017, while at that latter time, the southern hemisphere was entering winter. Our new data, acquired in 2017 and analyzed here, are important because they are the only ones recorded since 2014 close to the south pole in the mid-infrared nadir mode at high resolution. A large temperature increase in the southern polar stratosphere (by 10-50 K in the 0.1 to 0.01 mbar pressure range) is found associated with a change in the temperature profile’s shape. The 2017 observations also show a related significant decrease in most of the southern abundances which must have started sometime between 2014 and 2017 [1]. For the north, the spectra indicate a continuation of the decrease of the abundances which we first reported to have started in 2015 and small temperature variations [1]. We discuss comparisons with other results and with current photochemical and dynamical models which could be updated and improved by the new constraints set by the findings presented here. [1] Coustenis et al., 2019, Icarus 344, 1 July 2020, 113413 ; [2] Coustenis et al., 2016, Icarus 270, 409-420; [3] Coustenis et al., 2018, Astroph. J. Lett. 854, no2.

The JUpiter ICy moons Explorer (JUICE) (ESA) and Europa Clipper (NASA) missions will be launched in the next decade in order to spend a number of years making detailed observations of the giant gaseous planet Jupiter and three of its largest moons, Ganymede, Callisto, and Europa. The focus of both missions is to scrutinize the nature of these icy moons and characterize the conditions that may have led to the emergence of habitable environment for life. A particular aspect of both JUICE and Europa Clipper missions is the exploration of the icy crusts, which comprises a new and very focused objective for outer solar system missions. A good example however of such investigations has been presented by Cassini’s RADAR instrument. One of JUICE’s tasks is to characterize the structure and properties of the ice shell by probing the subsurface of Ganymede down to a depth of a few kilometers. An additional task of JUICE’s ice penetrating RIME and Clipper’s radar REASON is to obtain profiles of subsurface thermal, compositional, and structural horizons down to a maximum depth of 1 to 9 km depending on the crust’s properties. However, the interpretation of radar sounding experiments relies on the ability to decipher the backscattering patterns. The icy moons of Jupiter have been reported to be efficient backscatters of cm-wavelength radiations, showing polarization effects that must be due to factors other than the only single scattering process at the space-body surface interface. The identification of the characteristics of the backscattering radiation results from the superposition of backscattering layers with different electrical properties as a function of depth. Amongst the properties likely to modify the backscattering efficiency are: the amount of silicate (partial absorbers), the ice salinity, and the distribution of cracks present in the layer (inhomogeneity of the medium at size scales comparable to the radar wavelengths). Consequently, direct measurements of the dielectric values (electrical conductivity and permittivity) of an icy layer with properties controlled in the lab could unveil the backscattering efficiency of the sample. We provide here the first version of a database of measurements, following a number of laboratory experiments made at CAB-CSIC-INTA, in collaboration with UCM and CSIC-ITEFI (Please see abstracts EPSC Muñoz-Iglesias et al. and EPSC Gonzalez-Díaz et al.), collected and classified based on the parameters and properties of the different ice samples, which are manufactured to resemble the Jovian moon ice compositions. During the experiments, different conditions of T and P were applied and the recorded measurements of electrical properties were collected in this database of ‘ice behavior’. Since the next remote sensing data are not expected anytime before the arrival of JUICE and Europa Clipper (around 2030), the database provides us with the opportunity to compare with the current literature and the data available for the Jovian icy moons from previous missions, but also add information that is currently missing and relate that information to relative surface processes. The ice database would facilitate the interpretation of future radar sounding measurements, providing a better constrain on the true nature of ices of the surfaces of JUICE’s and Europa Clipper’s future targets.

We outline a flagship-class mission concept focused on studying Titan as a global system, with particular emphasis on the polar regions. Investigating Titan from the unique standpoint of a polar orbit would enable comprehensive global maps to uncover the physics and chemistry of the atmosphere, and the topography and geophysical environment of the surface and subsurface. The mission includes two key elements: (1) an orbiter spacecraft, which also acts as a data relay, and (2) one or more small probes to directly investigate Titan's seas and make the first direct measurements of their liquid composition and physical environment. The orbiter would carry a sophisticated remote sensing payload, including a novel topographic lidar, a long-wavelength surface-penetrating radar, a sub-millimeter sounder for winds and for mesospheric/thermospheric composition, and a camera and near-infrared spectrometer. An instrument suite to analyze particles and fields would include a mass spectrometer to focus on the interactions between Titan's escaping upper atmosphere and the solar wind and Saturnian magnetosphere. The orbiter would enter a stable polar orbit around 1500 to 1800 km, from which vantage point it would make global maps of the atmosphere and surface. One or more probes, released from the orbiter, would investigate Titan's seas in situ, including possible differences in composition between higher and lower latitude seas, as well as the atmosphere during the parachute descent. The number of probes, as well as the instrument complement on the orbiter and probe, remain to be finalized during a mission study that we recommend to NASA as part of the NRC Decadal Survey for Planetary Science now underway, with the goal of an overall mission cost in the "small flagship" category of ~$2 bn. International partnerships, similar to Cassini-Huygens, may also be included for consideration., 13 pages, white paper submitted to the NRC Decadal Survey for Planetary Science and Astrobiology

International audience; Liquid water oceans are now predicted to exist beneath the icy shells of numerous worlds in the outer solar system. These ocean worlds are prime targets in our search for evidence of life beyond Earth, and specifically extant life. Here we review the conditions that may lead to several of these worlds being habitable, and provide a framework for the future exploration of these astrobiologically compelling targets.

International audience; We investigate nine Titan impact craters using Visual and Infrared Mapping Spectrometer (VIMS) data and a radiative transfer code (RT) [e.g. 1] in addition to emissivity data, in order to constrain the spectral behavior and composition of the craters. Past studies have looked at the chemical composition of impact craters either by using qualitative comparisons between craters [e.g. 2;3] or by combining all craters into a single unit [4], rather than separating them by geographic location or degradation state. Here, we use a radiative transfer model to first estimate the atmospheric contribution to the data, then extract the surface albedos of the impact crater subunits, and finally constrain their composition by using a library of candidate Titan materials. Following the general characterization of the impact craters, we study two impact crater subunits, the 'crater floor', which refers to the bottom of a crater, and the 'ejecta blanket', which is the material thrown out of the crater during an impact event. The results show that Titan's mid-latitude plain craters: Afekan, Soi, and Forseti, in addition to Sinlap and Menrva are enriched in an OH-bearing constituent (likely water-ice) in an organic based mixture, while the equatorial dune craters: Selk, Ksa, Guabonito, and Santorini, appear to be purely composed of organic material (mainly unknown dune dark material). This follows the pattern seen in [4], where midlatitude alluvial fans, undifferentiated plains, and labyrinths were found to consist of a tholin-like and water-ice mixture, while the equatorial undifferentiated plains, hummocky terrains, dunes, and variable plains were found to consist of a dark material and tholin-like mixture in their very top layers. These observations also agree with the evolution scenario proposed by [3], wherein the impact cratering process produces a mixture of organic material and water ice, which is later "cleaned" through fluvial erosion in the midlatitude plains; a cleaning process that does not appear to operate in the equatorial dunes, which seem to be quickly covered by a thin layer of sand sediment. This scenario agrees with other works that suggest that atmospheric deposition is similar in the low-latitudes and midlatitudes on Titan, but with more rain falling onto the higher latitudes causing additional processing of materials in those regions [e.g. 5]. In either case, it appears that active processes are working to shape the surface of Titan, and it remains a dynamic world in the present day.[1] Hirtzig, M., et al. (2013). Icarus, 226, 470-486; [2] Neish, C.D., et al. (2015), Geophys. Res. Lett. 42, 3746-3754; [3] Werynski, A., et al. (2019), Icarus, 321, 508-521; [4] Solomonidou, A., et al. (2018), J. Geophys. Res, 123, 2, 489-507; [5] Neish, A.C., et al. (2016), Icarus, 270, 114-129.&

Titan is one of the most promising bodies in the solar system from the astrobiological perspective in particular because of its large organic content in the atmosphere and on the surface. These chemical species evolve with time. We performed an analysis of spectra acquired by Cassini/CIRS at high resolution which cover the far-IR range from 10 to 1500 cm-1 since the beginning and until the last year of the Cassini mission in 2017 and describe the temperature and composition variations near Titan’s poles and at the equator over almost two Titan seasons ([1-3]. By applying our radiative transfer code (ARTT) to CIRS data and to the 1980 Voyager 1 flyby values inferred from the re-analysis of the Infrared Radiometer Spectrometer (IRIS) spectra, as well as to the intervening ground- and space-based observations (such as with ISO), we study the stratospheric evolution over a Titanian year (V1 encounter Ls=9° was reached in mid-2010) [1,2]. CIRS nadir and limb spectral together show variations in temperature and chemical composition in the stratosphere during the Cassini mission, before and after the Northern Spring Equinox (NSE) and also during one Titan year.After the 2010 equinox we have thus reported on monitoring of Titan’s stratosphere near the poles and in particular on the observed strong temperature decrease and compositional enhancement above Titan’s southern polar latitudes since 2012 and until 2014 of several trace species, such as complex hydrocarbons and nitriles, which were previously observed only at high northern latitudes. This effect followed the transition of Titan’s seasons from northern winter in 2002 to northern summer in 2017, while at that latter time the southern hemisphere was entering winter.Our data show a continued decrease of the abundances which we first reported to have started in 2015. The 2017 data we have acquired and analyzed here are important because they are the only ones recorded since 2014 close to the south pole in the far-infrared nadir mode at high resolution. A large temperature increase in the southern polar stratosphere (by 10-50 K in the 0.5 mbar-0.05 mbar pressure range) is found and a change in the temperature profile’s shape. The 2017 observations also show a related significant decrease in most of the abundances which must have started sometime between 2014 and 2017 [3]. In our work, we show that the equatorial latitudes remain rather constant throughout the Cassini mission.We have thus shown that the south pole of Titan is now losing its strong enhancement, while the north pole also slowly continues its decrease in gaseous opacities. It would have been interesting to see when this might happen, but the Cassini mission ended in September 2017. Perhaps future ground-based measurements can pursue this investigation and monitor Titan’s atmosphere to characterize the seasonal events. We have obtained thus significant results which set constraints on GCM and photochemical models. [1] Coustenis et al., 2016, Icarus 270, 409-420; [2] Coustenis et al., 2018, Astroph. J., Lett., 854, no2; [3] Coustenis et al., 2019, Icarus in press, https://doi.org/10.​1016/​j.​icarus.​2019.​113413.

International audience; For the Ice Giants, atmospheric entry probes provide critical measurements not attainable via remote observations. Including the 2013-2022 NASA Planetary Decadal Survey , there have been at least five comprehensive atmospheric probe engineering design studies performed in recent years by NASA and ESA. International science definition teams have assessed the science requirements, and each recommended similar measurements and payloads to meet science goals with current instrument technology. The probe system concept has matured and converged on general design parameters that indicate the probe would include a 1-meter class aeroshell and have a mass around 350 to 400-kg. Probe battery sizes vary, depending on the duration of a post-release coast phase, and assumptions about heaters and instrument power needs. The various mission concepts demonstrate the need for advanced power and thermal protection system development. 17 Page 2 of 13 A.A. Simon et al. show an Ice Giant mission with an in situ probe is feasible and would be welcomed by the international science community.

descripción no proporcionada por scopus, With funding from the Spanish government through the "María de Maeztu Unit of Excellence" accreditation (MDM-2017-0737)

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.

International audience

From 2004 to 2017, the Cassini spacecraft orbited Saturn, completing 127 close flybys of its largest moon, Titan. Cassini's Composite Infrared Spectrometer (CIRS), one of 12 instruments carried on board, profiled Titan in the thermal infrared (7-1000 microns) throughout the entire 13-year mission. CIRS observed on both targeted encounters (flybys) and more distant opportunities, collecting 8.4 million spectra from 837 individual Titan observations over 3633 hours. Observations of multiple types were made throughout the mission, building up a vast mosaic picture of Titan's atmospheric state across spatial and temporal domains. This paper provides a guide to these observations, describing each type and chronicling its occurrences and global-seasonal coverage. The purpose is to provide a resource for future users of the CIRS data set, as well as those seeking to put existing CIRS publications into the overall context of the mission, and to facilitate future inter-comparison of CIRS results with those of other Cassini instruments, and ground-based observations., Comment: 62 pages, 31 figures, 10 tables. CSV format table data included

The exploration and use of outer space is the province of all humankind. This principle in Article I of the UN Outer Space Treaty guarantees the freedom to explore outer space, including the Moon and other celestial bodies, without discrimination, and to carry out scientific investigations. This freedom, however, comes with a responsibility described in Article IX of the same Treaty. It states that space activities have to be conducted with due regard to the corresponding interests of all other States Parties to the Treaty. The avoidance of potentially harmful interference with activities of other States Parties is central. The harmful contamination of the Moon and other celestial bodies and the need to ensure safety of the Earth are highlighted in this context. With the entry into force of the Outer Space Treaty in 1967, planetary protection became part of international law. In observance of those treaty obligations, an international standard for planetary protection has been developed by the Committee on Space Research (COSPAR) which provides a forum for international consultation and has formulated a Planetary Protection Policy with associated requirements that are put in place after examination of the most updated relevant scientific studies and recommendations made by the COSPAR Panel on Planetary Protection.