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Hot Jupiters exhibit large day-night temperature contrasts. Their cooler nightsides are thought to host clouds. However, the exact nature of these clouds, their spatial distribution, and their impact on atmospheric dynamics, thermal structure, and spectra is still unclear. We investigate the atmosphere of WASP-43 b, a short period hot Jupiter recently observed with JWST, to understand the radiative and dynamical impact of clouds on the atmospheric circulation and thermal structure. We aim to understand the impact of different kinds of condensates potentially forming in WASP-43 b, with various sizes and atmospheric metallicities. We used a 3D global climate model (GCM) with a new temperature-dependent cloud model that includes radiative feed-backs coupled with hydrodynamical integrations to study the atmospheric properties of WASP-43 b. We produced observables from our simulations and compared them to spectral phase curves from various observations. We show that clouds have a net warming effect, meaning that the greenhouse effect caused by clouds is stronger than the albedo cooling effect. We show that the radiative effect of clouds has various impacts on the dynamical and thermal structure of WASP-43 b. Depending on the type of condensates and their sizes, the radiative-dynamical feedback will modify the horizontal and vertical temperature gradient and reduce the wind speed. For super-solar metallicity atmospheres, fewer clouds form in the atmosphere, leading to a weaker feedback. Comparisons with spectral phase curves observed with HST, Spitzer, and JWST indicate that WASP-43 b s nightside is cloudy and rule out sub-micron Mg2SiO4 cloud particles as the main opacity source. Distinguishing between cloudy solar and cloudy super-solar-metallicity atmospheres is not straightforward, and further observations of both reflected light and thermal emission are needed., Comment: 23 pages, 15 Figures + 7 Figures in appendix. Accepted for publication in Astronomy & Astrophysics

We report the results of the trajectory-based simulation of far-infrared collision-induced absorption (CIA) due to CH$_4-$N$_2$ pairs at temperatures between 70 and 400 K. Our analysis utilizes recently calculated high-level potential energy (PES) and induced dipole surfaces (IDS) [Finenko, A. A., Chistikov, D. N., Kalugina, Y. N., Conway E. K., Gordon, I. E., Phys. Chem. Chem. Phys., 2021, doi: 10.1039/d1cp02161c]. Treating collision partners as rigid rotors, the time evolution of interaction-induced dipole is accumulated over a vast ensemble of classical trajectories and subsequently transformed into CIA spectrum via Fourier transform. In our calculations, both bound and unbound states are properly accounted for, and the rigorous theory of lower-order spectral moments is addressed to check the accuracy of simulated profiles. Classically derived trajectory-based profiles are subject to two approximate desymmetrization procedures so that resulting profiles conform to the quantum principle of detailed balance. The simulated profiles are compared to laboratory measurements and employed for modeling Titan's spectra in the 50-500 cm$^{-1}$ range. Based on the desymmetrized simulated profiles, a new semi-empirical model for CH$_4-$N$_2$ CIA is proposed for modeling Titan's infrared spectra. Synthetic spectra derived using this model yield an excellent agreement with the data recorded by the Composite Infrared Spectrometer (CIRS) aboard the Cassini spacecraft at low and high emission angles., Comment: 18 pages, 11 figures, accepted for publication in ApJS . The semiempirical CH4-N2 profiles tabulated in the HITRAN CIA format are available on the ApJS website or upon reasonable request

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

Hubble (HST) spectroscopic transit observations of the temperate sub-Neptune K2-18b were interpreted as the presence of water vapour with potential water clouds. 1D modelling studies also predict the formation of water clouds at some conditions. However, such models cannot predict the cloud cover, driven by atmospheric dynamics and thermal contrasts, and thus their real impact on spectra. The main goal of this study is to understand the formation, distribution and observational consequences of water clouds on K2-18b and other temperate sub-Neptunes. We simulated the atmospheric dynamics, water cloud formation and spectra of K2-18b for H2-dominated atmosphere using a 3D GCM. We analysed the impact of atmospheric composition (with metallicity from 1*solar to 1000*solar), concentration of cloud condensation nuclei and planetary rotation rate. Assuming that K2-18b has a synchronous rotation, we show that the atmospheric circulation in the upper atmosphere essentially corresponds to a symmetric day-to-night circulation. This regime preferentially leads to cloud formation at the substellar point or at the terminator. Clouds form for metallicity >100*solar with relatively large particles. For 100-300*solar metallicity, the cloud fraction at the terminators is small with a limited impact on transit spectra. For 1000*solar metallicity, very thick clouds form at the terminator. The cloud distribution appears very sensitive to the concentration of CCN and to the planetary rotation rate. Fitting HST transit data with our simulated spectra suggests a metallicity of ~100-300*solar. In addition, we found that the cloud fraction at the terminator can be highly variable, leading to a potential variability in transit spectra. This effect could be common on cloudy exoplanets and could be detectable with multiple transit observations., Comment: Accepted for publication in A&A. 16 pages, 14 figures

The atmospheric composition of exoplanets with masses between 2 and 10 M$_\oplus$ is poorly understood. In that regard, the sub-Neptune K2-18b, which is subject to Earth-like stellar irradiation, offers a valuable opportunity for the characterisation of such atmospheres. Previous analyses of its transmission spectrum from the Kepler, Hubble (HST), and Spitzer space telescopes data using both retrieval algorithms and forward-modelling suggest the presence of H$_2$O and an H$_2$--He atmosphere, but have not detected other gases, such as CH$_4$. We present simulations of the atmosphere of K2-18 b using Exo-REM, our self-consistent 1D radiative-equilibrium model, using a large grid of atmospheric parameters to infer constraints on its chemical composition. We show that our simulations favour atmospheric metallicities between 40 and 500 times solar and indicate, in some cases, the formation of H$_2$O-ice clouds, but not liquid H$_2$O clouds. We also confirm the findings of our previous study, which showed that CH$_4$ absorption features nominally dominate the transmission spectrum in the HST spectral range. We compare our results with results from retrieval algorithms and find that the H$_2$O-dominated spectrum interpretation is either due to the omission of CH$_4$ absorptions or a strong overfitting of the data. Finally, we investigated different scenarios that would allow for a CH$_4$-depleted atmosphere. We were able to fit the data to those scenarios, finding, however, that it is very unlikely for K2-18b to have a high internal temperature. A low C/O ratio ($\approx$ 0.01--0.1) allows for H$_2$O to dominate the transmission spectrum and can fit the data but so far, this set-up lacks a physical explanation. Simulations with a C/O ratio $<$ 0.01 are not able to fit the data satisfactorily., Comment: Accepted for publication in Astronomy & Astrophysics ; Exo-REM code and k-coefficients tables available here: https://gitlab.obspm.fr/dblain/exorem

In their Letter, Tsiaras et al. reported the detection of water vapour in the atmosphere of K2-18 b, an exoplanet of 7 to 10 Earth masses located in the habitable zone of an M-dwarf star. The detection is based on an absorption feature seen at 1.4 $\mu$m in observations of the transiting exoplanet with the Hubble Space Telescope/Wide Field Camera 3. We have simulated the mean temperature structure and composition of K2-18b using a radiative-convective equilibrium model and we present here the corresponding transit spectroscopy calculations. We argue that the reported absorption is most likely due to methane, a gas expected to be abundant in the hydrogen-helium atmosphere of cold sub-Neptunes. More generally, we show that the 1.4-$\mu$m absorption seen in transit spectra is not diagnostic of the presence of water vapour for sub-Neptunes having an effective temperature less than 600 K and that water vapour dominates over methane at this wavelength only at larger temperatures., Comment: This preprint has not undergone any post-submission improvements or corrections. The Version of Record of this article is published in Nature Astronomy (Matters Arising), and is available online at https://www.nature.com/articles/s41550-022-01678-z

The Cassini/Composite InfraRed Spectrometer (CIRS) instrument has been observing the middle atmosphere of Titan over almost half a Saturnian year. We used the CIRS dataset processed through the up-to-date calibration pipeline to characterize seasonal changes of temperature and abundance profiles in the middle atmosphere of Titan, from mid-northern winter to early northern summer all around the satellite. We used limb spectra from 590 to 1500 cm$^{-1}$ at 0.5-cm$^{-1}$ spectral resolution, which allows us to probe different altitudes. We averaged the limb spectra recorded during each flyby on a fixed altitude grid to increase the signal-to-noise ratio. These thermal infrared data were analyzed by means of a radiative transfer code coupled with an inversion algorithm, in order to retrieve vertical temperature and abundance profiles. These profiles cover an altitude range of approximately 100 to 600 km, at 10- or 40-km vertical resolution (depending on the observation). Strong changes in temperature and composition occur in both polar regions where a vortex is in place during the winter. At this season, we observe a global enrichment in photochemical compounds in the mesosphere and stratosphere and a hot stratopause located around 0.01 mbar, both linked to downwelling in a pole-to-pole circulation cell. After the northern spring equinox, between December 2009 and April 2010, a stronger enhancement of photochemical compounds occurred at the north pole above the 0.01-mbar region, likely due to combined photochemical and dynamical effects. During the southern autumn in 2015, above the South pole, we also observed a strong enrichment in photochemical compounds that contributed to the cooling of the stratosphere above 0.2 mbar. Close to the northern spring equinox, in December 2009, the thermal profile at 74{\deg}N exhibits an oscillation that we interpret in terms of an inertia-gravity wave.

We have analyzed spectra recorded between 50 and 650 cm$^{-1}$ by the Composite Infrared Spectrometer (CIRS) aboard the Cassini spacecraft at low and high emission angles to determine simultaneously the H$_2$ mole fraction and ortho-to-para ratio in Titan's troposphere. We used constraints from limb spectra between 50 and 900 cm$^{-1}$ and from in situ measurements by the Huygens probe to characterize the temperature, haze and gaseous absorber profiles. We confirm that the N$_2$-CH$_4$ collision-induced absorption (CIA) coefficients used up to now need to be increased by about 52% at temperatures of 70-85 K. We find that the N$_2$-N$_2$ CIA coefficients are also too low in the N$_2$ band far wing, beyond 110 cm$^{-1}$, in agreement with recent quantum mechanical calculations. We derived a H$_2$ mole fraction equal to (0.88 $\pm$ 0.13) $\times$ 10$^{-3}$, which pertains to the $\sim$1-34 km altitude range probed by the S$_0$(0) and S$_0$(1) lines. The H$_2$ para fraction is close to equilibrium in the 20-km region. We have investigated different mechanisms that may operate in Titan's atmosphere to equilibrate the H$_2$ o-p ratio and we have developed a one-dimensional model that solves the continuity equation in presence of such conversion mechanisms. We conclude that exchange with H atoms in the gas phase or magnetic interaction of H$_2$ in a physisorbed state on the surface of aerosols are too slow compared with atmospheric mixing to play a significant role. On the other hand, magnetic interaction of H$_2$ with CH$_4$, and to a lesser extent N$_2$, can operate on a timescale similar to the vertical mixing time in the troposphere. This process is thus likely responsible for the o-p equilibration of H$_2$ in the mid-troposphere implied by CIRS measurements.

The atmosphere of Titan, the largest moon of Saturn, is rich in organic molecules, and it has been suggested that the moon may serve as an analog for the pre-biotic Earth due to its highly reducing chemistry and existence of global hazes. Photochemical models of Titan have predicted the presence of propadiene (historically referred to as allene), CH$_{2}$CCH$_{2}$, an isomer of the well-measured propyne (also called methylacetylene) CH$_{3}$CCH, but its detection has remained elusive due to insufficient spectroscopic knowledge of the molecule - which has recently been remedied with an updated spectral line list. Here we present the first unambiguous detection of the molecule in any astronomical object, observed with the Texas Echelle Cross Echelle Spectrograph (TEXES) on the NASA Infrared Telescope Facility (IRTF) in July 2017. We model its emission line near 12 $\mu$m and measure a volume mixing ratio (VMR) of (6.9 $\pm$ 0.8) $\times$10$^{-10}$ at 175 km, assuming a vertically increasing abundance profile as predicted in photochemical models. Cassini measurements of propyne made during April 2017 indicate that the abundance ratio of propyne to propadiene is 8.2$\pm$1.1 at the same altitude. This initial measurement of the molecule in Titan's stratosphere paves the way towards constraining the amount of atomic hydrogen available on Titan, as well as future mapping of propadiene on Titan from 8 meter and larger ground based observatories, and future detection on other planetary bodies.

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

Recent analysis of Gemini-North/NIFS H-band (1.45 - 1.8 $\mu$m) observations of Uranus, recorded in 2010, with recently updated line data has revealed the spectral signature of hydrogen sulphide (H$_2$S) in Uranus's atmosphere (Irwin et al., 2018). Here, we extend this analysis to Gemini-North/NIFS observations of Neptune recorded in 2009 and find a similar detection of H$_2$S spectral absorption features in the 1.57 - 1.58 $\mu$m range, albeit slightly less evident, and retrieve a mole fraction of $\sim1-3$ ppm at the cloud tops. We find a much clearer detection (and much higher retrieved column abundance above the clouds) at southern polar latitudes compared with equatorial latitudes, which suggests a higher relative humidity of H$_2$S here. We find our retrieved H$_2$S abundances are most consistent with atmospheric models that have reduced methane abundance near Neptune's south pole, consistent with HST/STIS determinations (Karkoschka and Tomasko, 2011). We also conducted a Principal Component Analysis (PCA) of the Neptune and Uranus data and found that in the 1.57 - 1.60 $\mu$m range, some of the Empirical Orthogonal Functions (EOFs) mapped closely to physically significant quantities, with one being strongly correlated with the modelled H$_2$S signal and clearly mapping the spatial dependence of its spectral detectability. Just as for Uranus, the detection of H$_2$S at the cloud tops constrains the deep bulk sulphur/nitrogen abundance to exceed unity (i.e. $ > 4.4 - 5.0$ times the solar value) in Neptune's bulk atmosphere, provided that ammonia is not sequestered at great depths, and places a lower limit on its mole fraction below the observed cloud of (0.4 - 1.3) $\times 10^{-5}$. The detection of gaseous H$_2$S at these pressure levels adds to the weight of evidence that the principal constituent of the 2.5 - 3.5-bar cloud is likely to be H$_2$S ice., Comment: 13 Figures

We report on results of an observing campaign to support the Juno mission. At the beginning of 2016, using TEXES (Texas Echelon cross-dispersed Echelle Spectrograph), mounted on the NASA Infrared Telescope Facility (IRTF), we obtained data cubes of Jupiter in the 1930--1943 cm$^{-1}$ spectral ranges (around 5 $\mu$m), which probe the atmosphere in the 1--4 bar region, with a spectral resolution of $\approx$ 0.15 cm$^{-1}$ and an angular resolution of $\approx$ 1.4". This dataset is analysed by a code that combines a line-by-line radiative transfer model with a non-linear optimal estimation inversion method. The inversion retrieves the vertical abundance profiles of NH$_3$ - which is the main contributor at these wavelengths - with a maximum sensitivity at $\approx$ 1--3 bar, as well as the cloud transmittance. This retrieval is performed on over more than one thousand pixels of our data cubes, producing maps of the disk, where all the major belts are visible. We present our retrieved NH$_3$ abundance maps which can be compared with the distribution observed by Juno's MWR (Bolton et al., 2017; Li et al., 2017) in the 2 bar region and discuss their significance for the understanding of Jupiter's atmospheric dynamics. We are able to show important latitudinal variations - such as in the North Equatorial Belt (NEB), where the NH$_3$ abundance is observed to drop down to 60 ppmv at 2 bar - as well as longitudinal variability. In the zones, we find the NH$_3$ abundance to increase with depth, from 100 $\pm$ 15 ppmv at 1 bar to 500 $\pm$ 30 ppmv at 3 bar. We also display the cloud transmittance--NH$_3$ abundance relationship, and find different behaviour for the NEB, the other belts and the zones. Using a simple cloud model, we are able to fit this relationship, at least in the NEB, including either NH$_3$-ice or NH$_4$SH particles with sizes between 10 and 100 $\mu$m., Comment: 21 pages, 15 figures, 4 tables, preprint accepted by Icarus the 04 June 2018

We present simultaneous HST WFC3 + Spitzer IRAC variability monitoring for the highly-variable young ($\sim$20 Myr) planetary-mass object PSO J318.5-22. Our simultaneous HST + Spitzer observations covered $\sim$2 rotation periods with Spitzer and most of a rotation period with HST. We derive a period of 8.6$\pm$0.1 hours from the Spitzer lightcurve. Combining this period with the measured $v sin i$ for this object, we find an inclination of 56.2$\pm 8.1^{\circ}$. We measure peak-to-trough variability amplitudes of 3.4$\pm$0.1$\%$ for Spitzer Channel 2 and 4.4 - 5.8$\%$ (typical 68$\%$ confidence errors of $\sim$0.3$\%$) in the near-IR bands (1.07-1.67 $\mu$m) covered by the WFC3 G141 prism -- the mid-IR variability amplitude for PSO J318.5-22 one of the highest variability amplitudes measured in the mid-IR for any brown dwarf or planetary mass object. Additionally, we detect phase offsets ranging from 200--210$^{\circ}$ (typical error of $\sim$4$^{\circ}$) between synthesized near-IR lightcurves and the Spitzer mid-IR lightcurve, likely indicating depth-dependent longitudinal atmospheric structure in this atmosphere. The detection of similar variability amplitudes in wide spectral bands relative to absorption features suggests that the driver of the variability may be inhomogeneous clouds (perhaps a patchy haze layer over thick clouds), as opposed to hot spots or compositional inhomogeneities at the top-of-atmosphere level., Comment: 48 pages, 22 figures, accepted to AJ

We developed a simple, physical and self-consistent cloud model for brown dwarfs and young giant exoplanets. We compared different parametrisations for the cloud particle size, by either fixing particle radii, or fixing the mixing efficiency (parameter fsed) or estimating particle radii from simple microphysics. The cloud scheme with simple microphysics appears as the best parametrisation by successfully reproducing the observed photometry and spectra of brown dwarfs and young giant exoplanets. In particular, it reproduces the L-T transition, due to the condensation of silicate and iron clouds below the visible/near-IR photosphere. It also reproduces the reddening observed for low-gravity objects, due to an increase of cloud optical depth for low gravity. In addition, we found that the cloud greenhouse effect shifts chemical equilibriums, increasing the abundances of species stable at high temperature. This effect should significantly contribute to the strong variation of methane abundance at the L-T transition and to the methane depletion observed on young exoplanets. Finally, we predict the existence of a continuum of brown dwarfs and exoplanets for absolute J magnitude=15-18 and J-K color=0-3, due to the evolution of the L-T transition with gravity. This self-consistent model therefore provides a general framework to understand the effects of clouds and appears well-suited for atmospheric retrievals., Comment: 24 pages, 20 figures, submitted to ApJ. Comments welcome

Given the forthcoming launch of the James Webb Space Telescope (JWST) which will allow observ- ing exoplanet atmospheres with unprecedented signal-over-noise ratio, spectral coverage and spatial resolution, the uncertainties in the atmosphere modelling used to interpret the data need to be as- sessed. As the first step, we compare three independent 1D radiative-convective models: ATMO, Exo-REM and petitCODE. We identify differences in physical and chemical processes taken into ac- count thanks to a benchmark protocol we developed. We study the impact of these differences on the analysis of observable spectra. We show the importance of selecting carefully relevant molecular linelists to compute the atmospheric opacity. Indeed, differences between spectra calculated with Hitran and ExoMol exceed the expected uncertainties of future JWST observations. We also show the limitation in the precision of the models due to uncertainties on alkali and molecule lineshape, which induce spectral effects also larger than the expected JWST uncertainties. We compare two chemical models, Exo-REM and Venot Chemical Code, which do not lead to significant differences in the emission or transmission spectra. We discuss the observational consequences of using equilibrium or out-of- equilibrium chemistry and the major impact of phosphine, detectable with the JWST.Each of the models has benefited from the benchmarking activity and has been updated. The protocol developed in this paper and the online results can constitute a test case for other models., Comment: 36 pages, 30 figures, accepted by ApJ

The James Webb Space Telescope (JWST), scheduled for launch in 2018, is the successor to the Hubble Space Telescope (HST) but with a significantly larger aperture (6.5 m) and advanced instrumentation focusing on infrared science (0.6-28.0 $\mu$m ). In this paper we examine the potential for scientific investigation of Titan using JWST, primarily with three of the four instruments: NIRSpec, NIRCam and MIRI, noting that science with NIRISS will be complementary. Five core scientific themes are identified: (i) surface (ii) tropospheric clouds (iii) tropospheric gases (iv) stratospheric composition and (v) stratospheric hazes. We discuss each theme in depth, including the scientific purpose, capabilities and limitations of the instrument suite, and suggested observing schemes. We pay particular attention to saturation, which is a problem for all three instruments, but may be alleviated for NIRCam through use of selecting small sub-arrays of the detectors - sufficient to encompass Titan, but with significantly faster read-out times. We find that JWST has very significant potential for advancing Titan science, with a spectral resolution exceeding the Cassini instrument suite at near-infrared wavelengths, and a spatial resolution exceeding HST at the same wavelengths. In particular, JWST will be valuable for time-domain monitoring of Titan, given a five to ten year expected lifetime for the observatory, for example monitoring the seasonal appearance of clouds. JWST observations in the post-Cassini period will complement those of other large facilities such as HST, ALMA, SOFIA and next-generation ground-based telescopes (TMT, GMT, EELT)., Comment: 50 pages, including 22 figures and 2 tables

We present a radiative-convective equilibrium model for young giant exoplanets. Model predictions are compared with the existing photometric measurements of planet beta Pictoris b in the J, H, Ks, L', NB 4.05, M' bands . This model will be used to interpret future photometric and spectroscopic observations of exoplanets with SPHERE, mounted at the VLT with a first light expected mid-2014., Comment: 2 pages, 1 figure, Proceedings IAUS 299

The Voyager 1 flyby of Titan in 1980 gave a first glimpse of the chemical complexity of Titan's atmosphere, detecting many new molecules with the infrared spectrometer (IRIS). These included propane (C3H8) and propyne (CH3C2H), while the intermediate-sized C3Hx hydrocarbon (C3H6) was curiously absent. Using spectra from the Composite Infrared Spectrometer (CIRS) on Cassini, we show the first positive detection of propene (C3H6) in Titan's stratosphere (5-sigma significance), finally filling the three-decade gap in the chemical sequence. We retrieve a vertical abundance profile from 100-250 km, that varies slowly with altitude from ~2 ppbv at 100 km, to ~5 ppbv at 200 km. The abundance of C3H6 is less than both C3H8 and CH3C2H, and we remark on an emerging paradigm in Titan's hydrocarbon abundances whereby: alkanes > alkynes > alkenes within the C2Hx and C3Hx chemical families in the lower stratosphere. More generally, there appears to be much greater ubiquity and relative abundance of triple-bonded species than double-bonded, likely due to the greater resistance of triple bonds to photolysis and chemical attack., Comment: 16 pages; 4 figures; 1 table. Withdrawn due to journal policy

In this paper we describe a first quantitative search for several molecules in Titan's stratosphere in Cassini CIRS infrared spectra. These are: ammonia (NH3), methanol (CH3OH), formaldehyde (H2CO), and acetonitrile (CH3CN), all of which are predicted by photochemical models but only the last of which observed, and not in the infrared. We find non-detections in all cases, but derive upper limits on the abundances from low-noise observations at 25{\deg}S and 75{\deg}N. Comparing these constraints to model predictions, we conclude that CIRS is highly unlikely to see NH3 or CH3OH emissions. However, CH3CN and H2CO are closer to CIRS detectability, and we suggest ways in which the sensitivity threshold may be lowered towards this goal., Comment: 11 pages plus 6 figure files

We present the results of the Infrared Space Observatory Short Wavelength Spectrometer (ISO-SWS) observations of Jupiter related to ammonia. We focus on two spectral regions; the first one (the 10-micron region), probes atmospheric levels between 1 and 0.2 bar, while the second one (the 5-micron window), sounds the atmosphere between 8 and 2 bars. The two spectral windows cannot be fitted with the same ammonia vertical distribution. From the 10-micron region we infer an ammonia distribution of about half the saturation profile above the 1-bar level, where the N/H ratio is roughly solar. A totally different picture is derived from the 5-micron window, where we determine an upper limit of 3.7E-5 at 1 bar and find an increasing NH3 abundance at least down to 4 bar. This profile is similar to the one measured by the Galileo probe (Folkner et al. 1998). The discrepancy between the two spectral regions most likely arises from the spatial heterogeneity of Jupiter, the 5-micron window sounding dry areas unveiled by a locally thin cloud cover (the 5-micron hot spots), and the 10-micron region probing the mean jovian atmosphere above 1 bar. The 15NH3 mixing ratio is measured around 400 mbar from nu_2 band absorptions in the 10-micron region. We find the atmosphere of Jupiter highly depleted in 15N at this pressure level (15N/14N)_{Jupiter}=(1.9^{+0.9}_{-1.))E-3, while (15N/14N)_{earth}=3.68E-3. It is not clear whether this depletion reveals the global jovian (15N/14N) ratio. Instead an isotopic fractionation process, taking place during the ammonia cloud condensation, is indicated as a possible mechanism. A fractionation coefficient alpha higher than 1.08 would explain the observed isotopic ratio, but the lack of laboratory data does not allow us to decide unambiguously on the origin of the observed low (15N/14N) ratio., Comment: 57 pages, 2 tables, 15 figures. Accepted for publication in ICARUS