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The atmosphere of Titan, Saturn's largest moon, exhibits interesting UV- and radiation-driven chemistry between nitrogen and methane, resulting in dipolar, nitrile-containing molecules. The assembly and subsequent solvation of such molecules in the alkane lakes and seas found on the moon's surface are of particular interest for investigating the possibility of prebiotic chemistry in Titan's hydrophobic seas. Here we characterize the solvation of acetonitrile, a product of Titan's atmospheric radiation chemistry tentatively detected on Titan's surface [H. B. Niemann et al., Nature 438, 779-784 (2005)], in an alkane mixture estimated to match a postulated composition of the smaller lakes during cycles of active drying and rewetting. Molecular dynamics simulations are employed to determine the potential of mean force of acetonitrile (CH3CN) clusters moving from the alkane vapor into the bulk liquid. We find that the clusters prefer the alkane liquid to the vapor and do not dissociate in the bulk liquid. This opens up the possibility that acetonitrile-based microscopic polar chemistry may be possible in the otherwise nonpolar Titan lakes. [ABSTRACT FROM AUTHOR]

The likely existence of aromatic anions in many important extraterrestrial environments, from the atmosphere of Titan to the interstellar medium (ISM), is attracting increasing attention. Nitrogen and oxygen atoms are also widely observed in the ISM and in the ionospheres of planets and moons. In the current work, we extend previous studies to explore the reactivity of prototypical aromatic anions (deprotonated toluene, aniline, and phenol) with N and O atoms both experimentally and computationally. The benzyl and anilinide anions both exhibit slow associative electron detachment (AED) processes with N atom, and moderate reactivity with O atom in which AED dominates but ionic products are also formed. The reactivity of phenoxide is dramatically different; there is no measurable reaction with N atom, and the moderate reactivity with O atom produces almost exclusively ionic products. The reaction mechanisms are studied theoretically by employing density functional theory calculations, and spin conversion is found to be critical for understanding some product distributions. This work provides insight into the rich gas-phase chemistry of aromatic ion-atom reactions and their relevance to ionospheric and interstellar chemistry. [ABSTRACT FROM AUTHOR]

We performed laboratory experiments to study the photochemical evolution induced by long‐UV irradiation of benzene ices in Titan's atmosphere. The aim of this study was to investigate whether photo‐processed benzene ices could lead to the formation of aerosols analogs to those observed in Titan's stratosphere. Prior to that, spectroscopic properties of amorphous and crystalline benzene ices were studied as a function of temperature, using infrared spectroscopy. UV photolysis experiments (λ > 230 nm) of benzene ices led to the formation of volatile photo‐products, among which fulvene is identified, and of a residue dominated by νCH IR features, demonstrating that pure aromatic‐based polymeric structures are not sufficient to explain the composition of Titan's stratospheric haze layer. However, we provide a characterization of long‐UV‐induced benzene‐containing aerosol analogs, which will contribute to Titan's surface organics layer. These data are of prime interest in the context of the future Dragonfly space mission. Plain Language Summary: Titan, often compared with the early Earth, is the only moon in the solar system to have a dense atmosphere, mainly composed of nitrogen and methane. In the upper part of the atmosphere (>1,000 km), UV photons, photoelectrons, energetic ions and magnetospheric electrons induce the dissociation and the ionization of nitrogen and methane. These reactions lead to the formation of complex organic molecules—including hydrocarbons such as benzene—and aerosols in the high atmosphere (an organic haze responsible for Titan's brownish color), which are subject to different UV radiation classes depending on the altitude. Therefore, during their sedimentation toward the surface, these organic photoproducts are expected to be modified. Once the tropopause is reached, molecules like benzene (C6H6) condense and could evolve under FUV radiations (λ > 200 nm) and contribute to aerosol formation. Key Points: Laboratory experiments demonstrate that the interaction between FUV photons and benzene ice leads to a solid‐state photochemical activityThe in situ monitoring by infrared spectroscopy of the benzene ice photolysis leads to identify the formation of one of benzene's isomer, fulveneThe photo‐produced residue presents some IR features similar to those of Titan's aerosols probed by Cassini/VIMS [ABSTRACT FROM AUTHOR]

Following the arrival of Cassini at Titan in 2004, the Titan atmosphere has been shown to contain large complex polycyclic-aromatic hydrocarbons. Since Cassini has provided a great deal of data, there exists a need for kinetic rate data to help with modeling this atmosphere. One type of kinetic data needed is electron-ion dissociative recombination (e-IDR) rate constants. These data are not readily available for larger compounds, such as naphthalene, or oxygen containing compounds, such as 1,4 dioxane or furan. Here, the rate constants for naphthalene, 1,4 dioxane, and furan have been measured and their temperature dependencies are determined when possible, using the University of Georgia's Variable Temperature Flowing Afterglow. The rate constants are compared with those previously published for other compounds; these show trends which illustrate the effects which multirings and oxygen heteroatoms substitutions have upon e-IDR rate constants. [ABSTRACT FROM AUTHOR]

Like Earth, the atmosphere of Titan, Saturn's largest moon, is affected by the ability of the surface to store and release heat. Modeling efforts have shown that global temperature distributions and wind profiles differ between simulations describing Titan's surface with a uniform thermal inertia. Cassini‐Huygens demonstrated, however, that a variety of morphologies and compositions make up the Titanian landscape. Using data from Cassini RADAR and the Visible and Infrared Mapping Spectrometer, we classified the surface into five terrain types: dune, lake, hummocky, plains, and labyrinth. We estimated the thermal and physical properties (conductivity, specific heat, and density) for each type, creating a 1° × 1° global map of thermal inertia values. For the lakes, as the depth of convection is not yet known, we considered both still and convective bodies. Four simulations of the Titan Atmospheric Model were run with different surface thermal properties: a low homogeneous thermal inertia, a moderate homogeneous, the heterogeneous map with still lakes, and the heterogeneous map with convective lakes. In dry regimes (i.e., without the hydrological cycle), the differences between the four cases were generally minimal, suggesting that general circulation models can use a single (moderate) value for surface thermal inertia value for large‐scale investigations that do not consider diurnal variations. Given the importance of hydrological processes and regional spatial diversity on Titan, future work should consider the effects of nonuniform thermal inertia on Titan's climate on local and regional scales. Key Points: Titan's heterogeneous surface morphologies and compositions suggest a diversity of thermal inertia valuesLarge‐scale investigations by general circulation models can use a single‐valued moderate thermal inertiaRegional and diurnal variations probably warrant considering heterogeneous thermal inertia [ABSTRACT FROM AUTHOR]

Titan's ionosphere is an important component of the moon's environment. Ionospheric densities above ≈900 km have been studied with in situ measurements, but few density profiles for lower altitudes has been reported. Here we report the generation of the complete set of twenty ionospheric electron density profiles from the Cassini Radio Science Subsystem instrument, including seven profiles not previously reported (inbound and outbound profiles for flybys T101, T102, and T117 and outbound profile for flyby T119). Uncertainties on average electron density profiles are generally 100–250 cm−3. On the whole, the main features of the new profiles are generally consistent with previous studies. Specifically, a single layer of plasma with peak density of 1,000–3,000 cm−3, peak altitude of 1,000–1,300 km, and full‐width at half maximum of a few hundred kilometers. Two new profiles are "disturbed" similar to three identified by previous studies. Based on inspection of the underlying individual profiles, which generally come from two ground stations on opposite sides of Earth, disturbed terrestrial ionospheric conditions are not responsible for the disturbed nature of these Titan ionospheric profiles. A significant plasma layer is present at 500–700 km in several profiles. This low‐altitude plasma layer can occur in profiles that are not disturbed. Key Points: A complete set of Cassini radio occultation profiles of ionospheric electron density at Titan is generatedThese data products are useful for a wide range of studies of Titan's environmentA low‐altitude plasma layer is present in some undisturbed profiles [ABSTRACT FROM AUTHOR]

Titan, the biggest moon of Saturn, has a thick atmosphere which presents similarities with the one thought to be on Earth at its beginning. The study of Titan's photochemical haze is thus a precious tool in gaining knowledge of the primitive atmosphere of Earth. The chemistry occurring in Titan's atmosphere and the exact processes at act in the formation of the hazes remain largely unknown. The production of analogs samples on Earth has proved to be a useful tool to improve our knowledge of the aerosols formation on Titan. Such solid organic analogs samples, named tholins, were produced with the PAMPRE experiment (French acronym for Aerosols Microgravity Production by Reactive Plasma). PAMPRE tholins were found to be mostly insoluble, with only one-third of the bulk sample that can be dissolved in methanol. This partial solubility limited the previous studies in mass spectrometry, which were done only on the soluble fraction. The goal of the present study is to compare the two fractions of PAMPRE's tholins (insoluble and soluble) using a ultra-high resolution Fourier transform ion cyclotron resonance mass spectrometer (FTICR) equipped with a laser desorption/ionization source. Using modified Van Krevelen diagrams, we compare the global distribution of the molecules within the samples according to their Hydrogen/Carbon ratio and Nitrogen/Carbon ratio. Major differences are observed in the molecular composition of the soluble and the insoluble fraction. The soluble fraction of tholins was previously identified as a set of polymers of average formula (C 2 H 3 N) n . In this work we observe that the insoluble fraction of tholins is comprised of a significantly different set of polymers with an average composition of (C 4 H 3 N 2 ) n . [ABSTRACT FROM AUTHOR]

The tidal response of Titan′s two largest northern seas, Ligeia Mare and Kraken Mare, is studied by means of a two-dimensional, depth-averaged, shallow water model, SLIM (http://www.climate.be/slim). Kraken Mare is formed of two basins, the northern one being assumed to be linked by a single strait, Trevize Fretum, to Ligeia Mare. The tidal motions tend to be independent of each other in each basin (i.e., Ligeia Mare, Kraken 1 and Kraken 2) which results in sharp transitions in the straits. Our results are overall rather similar to those of Tokano et al. (2014), suggesting that a 2D model such as SLIM is adequate for modelling Titan′s tides and, since it is (presumably) less computationally demanding, may be better for sensitivity studies. For instance, the maximum tidal range in Kraken and Ligeia Maria respectively are 0.29 m and 0.14 m, which is within the range predicted by Lorenz et al. (2014) although smaller by 0.07 m and larger by 0.04 m than the estimates of Tokano et al. (2014) (but it occurs at the same location). The tidal currents are faster (by about one order of magnitude) in the straits linking those Maria than in the basins themselves (with a maximum of 0.36 m/s in the strait linking Kraken 1 and Kraken 2, Seldon Fretum). A decomposition of the tidal history into different harmonic components is carried out. Except in specific areas such as the straits and the amphidromic point(s), the main tidal component has a period of 1 Titan Day. We also briefly studied the eigenmodes of the northern seas whose period is close to the tidal period: such modes are very local. Indeed, their magnitude is significant (with respect to the magnitude of the modes elsewhere in the seas) in small bay(s) or near the islands of Kraken 2 and Ligeia Mare. They are not excited by the tides as they do not appear in the tidal motion. A sensitivity analysis to poorly constrained parameters (bottom friction coefficient, depth of Trevize Fretum and attenuation factor – the latter is briefly discussed with respect to the values of the Love numbers found in the literature) is also conducted. The model parameters are seen to have a significant impact on the liquid exchanges between the basins and, consequently, on the tidal range and phase, fluid velocity and location of amphidromic point(s). [ABSTRACT FROM AUTHOR]

Formation of organic aerosols driven by photochemical reactions has been observed and suggested in CH 4 -containing atmospheres, including Titan and early Earth. However, the detailed production and growth mechanisms of organic aerosols driven by solar far ultraviolet (FUV) light remain poorly constrained. We conducted laboratory experiments simulating photochemical reactions in a CH 4 CO 2 atmosphere driven by the FUV radiations dominated by the Lyman-α line. In the experiments, we analyzed time variations in thickness and infrared spectra of solid organic film formed on an optical window in a reaction cell. Gas species formed by FUV irradiation were also analyzed and compared with photochemical model calculations. Our experimental results show that the growth rate of the organic film decreases as the CH 4 /CO 2 ratio of reactant gas mixture decreases, and that the decrease becomes very steep for CH 4 /CO 2  < 1. Comparison with photochemical model calculations suggests that polymerizations of gas-phase hydrocarbons, such as polyynes and aromatics, cannot account for the growth rate of the organic film but that the addition reaction of CH 3 radicals onto the organic film with the reaction probability around 10 −2 can explain the growth rate. At CH 4 /CO 2  < 1, etching by O atom formed by CO 2 photolysis would reduce or inhibit the growth of the organic film. Our results suggest that organic aerosols would grow through CH 3 addition onto the surface during the precipitation of aerosol particles in the middle atmosphere of Titan and early Earth. On Titan, effective CH 3 addition would reduce C 2 H 6 production in the atmosphere. On early Earth, growth of aerosol particles would be less efficient than those on Titan, possibly resulting in small-sized monomers and influencing UV shielding. [ABSTRACT FROM AUTHOR]

Submillimeter emission lines of carbon monoxide (CO) in Titan’s atmosphere provide excellent probes of atmospheric temperature due to the molecule’s long chemical lifetime and stable, well constrained volume mixing ratio. Here we present the analysis of 4 datasets obtained with the Atacama Large Millimeter/Submillimeter Array (ALMA) in 2012, 2013, 2014, and 2015 that contain strong CO rotational transitions. Utilizing ALMA’s high spatial resolution in the 2012, 2014, and 2015 observations, we extract spectra from 3 separate regions on Titan’s disk using datasets with beam sizes ranging from 0.35 × 0.28″ to 0.39 × 0.34″. Temperature profiles retrieved by the NEMESIS radiative transfer code are compared to Cassini Composite Infrared Spectrometer (CIRS) and radio occultation science results from similar latitude regions. Disk-averaged temperature profiles stay relatively constant from year to year, while small seasonal variations in atmospheric temperature are present from 2012 to 2015 in the stratosphere and mesosphere ( ∼ 100–500 km) of spatially resolved regions. We measure the stratopause (320 km) to increase in temperature by 5 K in northern latitudes from 2012 to 2015, while temperatures rise throughout the stratosphere at lower latitudes. We observe generally cooler temperatures in the lower stratosphere ( ∼ 100 km) than those obtained through Cassini radio occultation measurements, with the notable exception of warming in the northern latitudes and the absence of previous instabilities; both of these results are indicators that Titan’s lower atmosphere responds to seasonal effects, particularly at higher latitudes. While retrieved temperature profiles cover a range of latitudes in these observations, deviations from CIRS nadir maps and radio occultation measurements convolved with the ALMA beam-footprint are not found to be statistically significant, and discrepancies are often found to be less than 5 K throughout the atmosphere. ALMA’s excellent sensitivity in the lower stratosphere (60–300 km) provides a highly complementary dataset to contemporary CIRS and radio science observations, including altitude regions where both of those measurement sets contain large uncertainties. The demonstrated utility of CO emission lines in the submillimeter as a tracer of Titan’s atmospheric temperature lays the groundwork for future studies of other molecular species – particularly those that exhibit strong polar abundance enhancements or are pressure-broadened in the lower atmosphere, as temperature profiles are found to consistently vary with latitude in all three years by up to 15 K. [ABSTRACT FROM AUTHOR]

Nitrogen isotopologues could give in principle valuable constraints on the formation and evolution of Titan’s atmosphere and its interior over geological time. For this purpose, we developed the first photochemical model dedicated to the study of the fractionation of several nitrogen isotopologues. Emphasis has been placed on several nitriles: HCN, CH 3 CN, HC 3 N, C 2 H 3 CN, C 2 H 5 CN. We show that the HCN/HC 15 N and HC 3 N/HC 3 15 N ratios are very sensitive to the production of magnetospheric electrons. So, these compounds can serve as probes to study the putative evolution with time of the production of magnetospheric electrons throughout the atmosphere. We also show that the CH 3 CN/CH 3 C 15 N and C 2 H 5 CN/C 2 H 5 C 15 N ratios are highly sensitive to cosmic rays. So, they can serve as probes to estimate their effect in the lower atmosphere of Titan (100–300 km). Detection of new isotopologues (particularly CH 3 C 15 N) could give strong constraints to photochemical models and could improve our understanding of the main physical and chemical processes at work in Titan’s atmosphere. [ABSTRACT FROM AUTHOR]

The Permittivity Wave and Altimetry system on board the Huygens probe observed an ionospheric hidden layer at a much lower altitude than the main ionosphere during its descent through the atmosphere of Titan, the largest satellite of Saturn. Previous studies predicted a similar ionospheric layer. However, neither previous nor post-Huygens theoretical models have been able to reproduce the measurements of the electrical conductivity and charge densities reported by the Mutual Impedance (MI) and Relaxation Probe (RP) sensors. The measurements were made from an altitude of 140 km down to the ground and show a maximum of charge densities of ≈ 2 × 10 9 m −3 positive ions and ≈ 450 × 10 6 m −3 electrons at approximately 65 km. Such a large difference between positive and negative charge densities has not yet been understood. Here, by making use of electron and ion capture processes in to aerosols, we are able to model both electron and positive ion number densities and to reconcile experimental data and model results. [ABSTRACT FROM AUTHOR]

We have developed a seasonal radiative–dynamical model of Titan's stratosphere to investigate the temporal variation of temperatures in the 0.2–4 mbar range observed by the Cassini/CIRS spectrometer. The model incorporates gas and aerosol vertical profiles derived from Cassini/CIRS and Huygens/DISR data to calculate the radiative heating and cooling rate profiles as a function of time and latitude. At 20°S in 2007, the heating rate is larger than the cooling rate at all altitudes, and more specifically by 20–35% in the 0.1–5 mbar range. A new calculation of the radiative relaxation time as a function of pressure level is presented, leading to time constants significantly lower than previous estimates. At 6°N around spring equinox, the radiative equilibrium profile is warmer than the observed one at all levels. Adding adiabatic cooling in the energy equation, with a vertical upward velocity profile approximately constant in pressure coordinates below the 0.02-mbar level (corresponding to 0.03–0.05 cm s −1 at 1 mbar), allows us to reproduce the observed profile quite well. The velocity profile above the ∼0.5-mbar level is however affected by uncertainties in the haze density profile. The model shows that the change in insolation due to Saturn's orbital eccentricity is large enough to explain the observed 4-K decrease in equatorial temperatures around 1 mbar between 2009 and 2016. At 30°N and S, the radiative model predicts seasonal variations of temperature much larger than observed. A seasonal modulation of adiabatic cooling/heating is needed to reproduce the temperature variations observed from 2005 to 2016 between 0.2 and 4 mbar. At 1 mbar, the derived vertical velocities vary in the range −0.05 (winter solstice) to 0.16 (summer solstice) cm s −1 at 30°S, −0.01 (winter solstice) to 0.14 (summer solstice) cm s −1 at 30°N, and 0.03–0.07 cm s −1 at the equator. [ABSTRACT FROM AUTHOR]

Saturn's moon Titan has surface liquids of liquid hydrocarbons and a thick, cold, nitrogen atmosphere, and is a target for future exploration. Critical to the design and operation of vehicles for this environment is knowledge of the amount of dissolved nitrogen gas within the cryogenic liquid methane and ethane seas. This paper rigorously reviews experimental data on the vapor-liquid equilibrium of nitrogen/methane/ethane mixtures, noting the possibility for split liquid phases, and presents simple analytical models for conveniently predicting solubility of nitrogen in pure liquid ethane, pure liquid methane, and a mixture of liquid ethane and methane. Model coefficients are fit to three temperature ranges near the critical point, intermediate range, and near the freezing point to permit accurate predictions across the full range of thermodynamic conditions. The models are validated against the consolidated database of 2356 experimental data points, with mean absolute error between data and model less than 8% for both binary nitrogen/methane and nitrogen/ethane systems, and less than 17% for the ternary nitrogen/methane/ethane system. The model can be used to predict the mole fractions of ethane, methane, and nitrogen as a function of location within the Titan seas. [ABSTRACT FROM AUTHOR]

A one dimensional photochemical model for the dayside ionosphere of Titan has been developed for calculating the density profiles of ions and electrons under steady state photochemical equilibrium condition. We concentrated on the T40 flyby of Cassini orbiter and used the in-situ measurements from instruments onboard Cassini as input to the model. An energy deposition model is employed for calculating the attenuated photon flux and photoelectron flux at different altitudes in Titan’s ionosphere. We used the Analytical Yield Spectrum approach for calculating the photoelectron fluxes. Volume production rates of major primary ions, like, N 2 + , N + , CH 4 + , CH 3 + , etc due to photon and photoelectron impact are calculated and used as input to the model. The modeled profiles are compared with the Cassini Ion Neutral Mass Spectrometer (INMS) and Langmuir Probe (LP) measurements. The calculated electron density is higher than the observation by a factor of 2 to 3 around the peak. We studied the impact of different model parameters, viz. photoelectron flux, ion production rates, electron temperature, dissociative recombination rate coefficients, neutral densities of minor species, and solar flux on the calculated electron density to understand the possible reasons for this discrepancy. Recent studies have shown that there is an overestimation in the modeled photoelectron flux and N 2 + ion production rates which may contribute towards this disagreement. But decreasing the photoelectron flux (by a factor of 3) and N 2 + ion production rate (by a factor of 2) decreases the electron density only by 10 to 20%. Reduction in the measured electron temperature by a factor of 5 provides a good agreement between the modeled and observed electron density. The change in HCN and NH 3 densities affects the calculated densities of the major ions (HCNH + , C 2 H 5 + , and CH 5 + ); however the overall impact on electron density is not appreciable ( < 20%). Even though increasing the dissociative recombination rate coefficients of the ions C 2 H 5 + and CH 5 + by a factor of 10 reduces the difference between modeled and observed densities of the major ions, the modeled electron density is still higher than the observation by  ∼ 60% at the peak. We suggest that there might be some unidentified chemical reactions that may account for the additional loss of plasma in Titan’s ionosphere. [ABSTRACT FROM AUTHOR]

Cassini has shown that CH 2 NH 2 + is likely present in relatively high abundance in Titan’s upper atmosphere. Relatively little is known about this molecule even though it contains the same number of electrons as ethylene, a molecule of significance to Titan’s chemistry. Any studies on CH 2 NH 2 + with application to Titan or its atmospheric chemistry will have to be done remotely at this point with the end of the fruitful Cassini mission. Consequently, trusted quantum chemical techniques are utilized here to produce the rotational, vibrational, and rovibrational spectroscopic constants for CH 2 NH 2 + for the first time. The methodology produces a tightly fit potential energy surface here that is well-behaved indicating a strong credence in the accuracy for the produced values. Most notably, the 884.1 cm − 1 NH 2 out-of-plane bend is the brightest of the vibrational frequencies reported here for CH 2 NH 2 + , and an observed and unattributed feature in this spectral region has been documented but never assigned to a molecular carrier. Follow-up IR or radio observations making use of the 540 GHz to 660 GHz range with the 0.45 D molecular dipole moment will have to be undertaken in order to confirm this or any attribution, but the data provided in this work will greatly assist in any such studies related to CH 2 NH 2 + . [ABSTRACT FROM AUTHOR]

During the last 30 years, many works have focused on the determination of the CO abundance in Titan’s atmosphere, but no measurement above 300 km has been done yet due to the faint signal of CO. Nevertheless, such measurements are particularly awaited as a confirmation of photochemical models predictions that CO is uniformly mixed in the whole atmosphere. Moreover, since CO is the main atmospheric reservoir of oxygen, its actual abundance has implications on the origins of Titan’s atmosphere. In this work, we analyse a set of Cassini VIMS daytime limb observations of Titan at 4.7 µm, which is dominated by solar-pumped non-LTE (non-local thermodynamic equilibrium) emission of CO ro-vibrational bands. In order to retrieve the CO abundance from these observations, we developed a non-LTE model for the CO vibrational levels. The retrieval of the CO concentration is performed following a bayesian approach and using the calculated non-LTE populations. The data set analysed consists of 47 limb scanning sequences -about 1500 spectra- acquired by VIMS in 2006 and 2007. CO relative abundance profiles from 200 to 500 km are obtained, for each set analysed. The mean result shows no significant variations with altitude and is consistent with the prediction of a well-mixed vertical profile. However, if compared with Earth-based mm measurements, a small vertical gradient is plausible. [ABSTRACT FROM AUTHOR]

In situ data from the GCMS instrument on the Huygens probe indicate that Titan's atmosphere contains small amounts of the primordial noble gases 36 Ar and 22 Ne (tentative detection), but it is unknown how they were obtained by the satellite. Based on the apparent similarity in the 22 Ne/ 36 Ar (atom) ratio between Titan's atmosphere and the solar composition, a previously neglected hypothesis for the origin of primordial noble gases in Titan's atmosphere is suggested – these species may have been acquired near the end of Titan's formation, when the moon could have gravitationally captured some nebular gas that would have been present in its formation environment (the Saturnian subnebula). These noble gases may be remnants of a primary atmosphere. This could be considered the simplest hypothesis to explain the 22 Ne/ 36 Ar ratio observed at Titan. However, the 22 Ne/ 36 Ar ratio may not be exactly solar if these species can be fractionated by external photoevaporation in the solar nebula, atmospheric escape from Titan, or sequestration on the surface of Titan. While the GCMS data are consistent with a 22 Ne/ 36 Ar ratio of 0.05 to 2.5 times solar (1σ range), simple estimates that attempt to account for some of the effects of these evolutionary processes suggest a sub-solar ratio, which may be depleted by approximately one order of magnitude. Models based on capture of nebular gas can explain why the GCMS did not detect any other primordial noble gas isotopes, as their predicted abundances are below the detection limits (especially for 84 Kr and 132 Xe). It is also predicted that atmospheric Xe on Titan should be dominated by radiogenic 129 Xe if the source of primordial Xe is nebular gas. Of order 10 −2 –10 −1 bar of primordial H 2 may have been captured along with the noble gases from a gas-starved disk, but this H 2 would have quickly escaped from the initial atmosphere. To have the opportunity to capture nebular gas, Titan should have formed within ∼10 Myr of the formation of the solar system, before the ultimate source of gas (the solar nebula) dissipated. More specifically, if photoevaporative fractionation happened, the time-evolution for the depletion of permanent gases in the solar nebula can be parameterized to the ∼3 times solar noble gas enrichments of Jupiter for an assumed Jupiter formation time of ∼2 Myr after calcium-aluminum-rich inclusions (CAIs). This allows the construction of a consistent chronology with a Titan formation time of ∼3–4 Myr after CAIs. Because the models presented in this work are pushing the limits of the data from Titan, future mass spectrometric measurements of the noble gases and their isotopes (to at least ppt sensitivity) will be essential to confirm the Huygens detection of 22 Ne, and to constrain the roles of evolutionary processes and mixed sources in determining the noble gas geochemistry of Titan's atmosphere. The clearest indication of a nebular gas source for noble gases on Titan would be a solar-like isotopic ratio of 20 Ne/ 22 Ne ≈ 14. [ABSTRACT FROM AUTHOR]

The origin of CO and the external source of H 2 O in the atmosphere of Titan is still a matter of debate. We investigated the chemical fractionation of oxygen isotopologues in order to give new constraints on the origin of oxygen species. We created a new chemical scheme and we developed a 1-D time-dependent photochemical model to compute the mole fraction profiles of various oxygen isotopologues. We show that the photochemical fractionation of oxygen isotopologues is quite low. Observations of C 18 O and CO 18 O are compatible with both an external origin or an internal origin of CO considering that the various sources of oxygen have a cometary 16 O/ 18 O ratio ( 16 O/ 18 O ≈ 500). Improvement of the measurements of the 16 O/ 18 O ratio in both Enceladus’ plumes and atmospheric CO 2 could give a valuable constraint on the origin of oxygen in Titan’s atmosphere. [ABSTRACT FROM AUTHOR]

The formation of aerosols in the atmosphere of Titan is based extensively on ion-neutral chemistry and physical condensation processes. Herein it is shown that the formation of aerosols may also occur through an alternative pathway that involves the physical aggregation of negatively charged particles, which are known to be abundant in the satellite’s atmosphere. It is shown that, given the right circumstances, like-charged particles with a dielectric constant characteristic of nitrated hydrocarbons have sufficient kinetic energy to overcome any repulsive electrostatic barrier that separates them and can subsequently experience an attractive interaction at very short separation. Aerosol growth can then unfold through a charge scavenging process, whereby nitrated aggregates preferentially grow by assimilating smaller like-charged particles. Since hydrocarbon aerosols have much lower dielectric constants, it is shown that a similar mechanism involving hydrocarbon particles will not be as efficient. As a consequence of this proposed growth mechanism, it is suggested that the lower atmosphere of Titan will be enriched in nitrogen-containing aerosols. [ABSTRACT FROM AUTHOR]

Titania nanofillers were used to reinforce nanofibers in composite mats produced by electrospinning of poly(vinyl butyral) with two different concentrations of polymers. The titania nanoparticles and titania nanotubes were added into an acetic acid/ethanol solution in different contents of 3 and 1 wt%, respectively. The effect of the processing system on the morphology of the produced fibers was analyzed. The antimicrobial poly(vinyl butyral) composite fibers with titanium dioxide nanoparticles and titanium dioxide nanotubes were produced by single and multineedle electrospinning systems. This study reports fabrication of composite nanofibrous mats with significant mechanical and antimicrobial properties at a high production speed, which is promising for commercial applications (health care, photocatalysis, protective clothing, etc.). The reported result revealed an outstanding correlation between values of elastic modulus derived from nanoindentation and dynamic mechanical techniques. Copyright © 2017 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Temperature profiles of Titan’s atmosphere obtained by Cassini radio occultations approach the nitrogen condensation temperature curve at some places. This raises the question as to whether nitrogen, Titan’s main atmospheric constituent, might condense in some seasons and areas contrary to previous perception. To address this question, possible areas and seasons of nitrogen condensation are searched for by a global climate model. The model is run under the present atmospheric pressure and composition but under various orbital configurations including the present one. Under the present orbital configuration the polar temperature at either pole becomes lowest around the northern autumnal equinox one season after aphelion. Liquid nitrogen clouds may appear in this season between 30 and 40 km altitude at least near the south pole, presumably embedded in icy methane clouds. Any falling nitrogen rain is likely to entirely evaporate before reaching the surface and thus does not affect the seasonal cycle of surface pressure. Seasonal nitrogen condensation is more frequent and intense when Saturn’s orbital eccentricity is larger and hence the heliocentric distance at aphelion is larger. Nevertheless, orbital parameter variations alone are not capable of flooding the surface with liquid nitrogen or causing large fluctuations of the surface pressure. [ABSTRACT FROM AUTHOR]

This paper presents the first results of the solid phase analysis of the Titan tholins generated in the Titan Haze Simulation (THS) experiments. This study complements the gas phase analysis study that was presented in a previous publication introducing the THS capabilities. In the THS experiment, the chemistry is simulated by plasma in the stream of a supersonic jet expansion. With this unique design, the gas is jet-cooled to Titan-like temperature (∼150 K) before inducing the chemistry by plasma, and remains at low temperature in the plasma discharge (∼200 K). Here, we present and discuss the results of scanning electron microscopy and infrared spectroscopy studies of THS solid aerosols produced in the four gas mixtures already studied by mass spectrometry in the gas phase: N 2 CH 4 , N 2 CH 4 C 2 H 2 , N 2 CH 4 C 6 H 6 and N 2 CH 4 C 2 H 2 C 6 H 6 . Differences in the morphology of the grains and aggregates produced in the volume of the gas phase in the plasma cavity, depending on the initial precursors, have been observed by scanning electron microscopy, that appear to be linked to differences in the growth processes and might have an impact on microphysical models. The mid-infrared spectroscopic analysis highlights changes in the nitrogen chemistry, and the abundance of aromatic compounds, depending on the initial gas mixture. A preliminary study of the aging and degradation of the THS samples with time and exposure to air and light has shown the importance, for future studies of laboratory-generated planetary aerosol analogs, of collecting, storing and characterizing samples under controlled environment. A comparison to VIMS data shows that the THS tholins produced in simpler mixtures, i.e., with a higher level of nitrogen incorporation, are more representative of Titan's aerosols. [ABSTRACT FROM AUTHOR]

Titanium-bearing (hydro)andradites from hydrothermally altered mantle section of the Nagaland ophiolite, India have been investigated for mineral chemical study. In serpentinized peridotite they occur in association with accessory chromites or dusty magnetites and the TiO 2 content in them is relatively low (< 0.35 wt%). (Hydro)andradites in magnetite pods, hosted within serpentinized peridotite have a range of TiO 2 content (up to 13.77 wt%) and occur typically within veins and veinlets intricately traversing the pods. The TiO 2 -rich variants of the (hydro)andradites within the veins occur either as large, polyhedral, margin-parallel zoned, mostly isolated grains within an andradite-rich porous matrix, or in clusters of equant, complexly zoned spherulites, resembling a fissure-fill. Besides (hydro)andradites as the most abundant component, the remaining of the veins consists mostly of serpentine and chlorite with relict titanites and perovskites. Magnetite pods are almost monomineralic, composed of large chromian magnetite crystals with accessory ilmenites occurring at the grain boundaries. Textural and mineralogical evidences suggest the transformation of these pods prior to the formation of (hydro)andradites from chromitite protoliths that have undergone extensive Ca-metasomatism together with host peridotites. We envisage at least two hydrothermal episodes for the mineralogical modifications where the formation of (hydro)andradites is related to the last event. The textural and mineral chemical characteristics of the entire assemblage as a whole demonstrate a gradual, temporal evolution in the composition of the metasomatic fluid from the onset of serpentinization to the end. This new occurrence of titanian andradites from chromitite protolith, hitherto not reported has major implications on the mobility of elements in hydrothermal environments. [ABSTRACT FROM AUTHOR]

Benzene detection has been reported in Titan's atmosphere both in the stratosphere at ppb levels by remote sensing ( Coustenis et al., 2007 ; Vinatier et al., 2007 ) and in the thermosphere at ppm levels by the Cassini's Ion and Neutral Mass Spectrometer (Waite et al., 2007). This detection supports the idea that aromatic and heteroaromatic reaction pathways may play an important role in Titan's atmospheric chemistry, especially in the formation of aerosols. Indeed, aromatic molecules are easily dissociated by ultraviolet radiation and can therefore contribute significantly to aerosol formation. It has been shown recently that aerosol analogs produced from a gas mixture containing a low concentration of aromatic and/or heteroaromatic molecules (benzene, naphthalene, pyridine, quinoline and isoquinoline) have spectral signatures below 500 cm −1 , a first step towards reproducing the aerosol spectral features observed by Cassini's Composite InfraRed Spectrometer (CIRS) in the far infrared ( Anderson and Samuelson 2011 , and references therein). In this work we investigate the influence of environmental temperature on the absorption spectra of such aerosol samples, simulating the temperature range to which aerosols, once formed, are exposed during their transport through Titan's stratosphere. Our results show that environmental temperature does not have any major effect on the spectral shape of these aerosol analogs in the far-infrared, which is consistent with the CIRS observations. [ABSTRACT FROM AUTHOR]

A low-temperature flow tube and ultra-high vacuum apparatus were used to explore the uptake and heterogeneous chemistry of acetylene (C 2 H 2 ) on cosmic dust analogues over the temperature range encountered in Titan's atmosphere below 600 km. The uptake coefficient, γ , was measured at 181 K to be (1.6 ± 0.4) × 10 -4 , (1.9 ± 0.4) × 10 −4 and (1.5 ± 0.4) × 10 −4 for the uptake of C 2 H 2 on Mg 2 SiO 4 , MgFeSiO 4 and Fe 2 SiO 4 , respectively, indicating that γ is independent of Mg or Fe active sites. The uptake of C 2 H 2 was also measured on SiO 2 and SiC as analogues for meteoric smoke particles in Titan's atmosphere, but was found to be below the detection limit (γ < 6 × 10 −8 and < 4 × 10 -7 , respectively). The rate of cyclo-trimerization of C 2 H 2 to C 6 H 6 was found to be 2.6 × 10 -5 exp(-741/ T ) s −1 , with an uncertainty ranging from ± 27 % at 115 K to ± 49 % at 181 K. A chemical ablation model was used to show that the bulk of cosmic dust particles (radius 0.02–10 µm) entering Titan's atmosphere do not ablate (< 1% mass loss through sputtering), thereby providing a significant surface for heterogeneous chemistry. A 1D model of dust sedimentation shows that the production of C 6 H 6 via uptake of C 2 H 2 on cosmic dust, followed by cyclo-trimerization and desorption, is probably competitive with gas-phase production of C 6 H 6 between 80 and 120 km. [ABSTRACT FROM AUTHOR]

Chamberlain ([1963] Planet. Space Sci., 11, 901–960) described the use of the exobase layer to determine escape from planetary atmospheres, below which it is assumed that molecular collisions maintain thermal equilibrium and above which collisions are deemed negligible. De La Haye et al. ([2007] Icarus., 191, 236–250) used this approximation to extract the energy deposition and non-thermal escape rates for Titan's atmosphere by fitting the Cassini Ion Neutral Mass Spectrometer (INMS) density data. De La Haye et al. assumed the gas distributions were composed of an enhanced population of super-thermal molecules ( E >> kT ) that could be described by a kappa energy distribution function (EDF), and they fit the data using the Liouville theorem. Here we fitted the data again, but we used the conventional form of the kappa EDF. The extracted kappa EDFs were then used with the Direct Simulation Monte Carlo (DSMC) technique (Bird [1994] Molecular Gas Dynamics and the Direct Simulation of Gas Flows) to evaluate the effect of collisions on the exospheric profiles. The INMS density data can be fit reasonably well with thermal and various non-thermal EDFs. However, the extracted energy deposition and escape rates are shown to depend significantly on the assumed exobase altitude, and the usefulness of such fits without directly modeling the collisions is unclear. Our DSMC results indicate that the kappa EDFs used in the Chamberlain approximation can lead to errors in determining the atmospheric temperature profiles and escape rates. Gas kinetic simulations are needed to accurately model measured exospheric density profiles, and to determine the altitude ranges where the Liouville method might be applicable. [ABSTRACT FROM AUTHOR]

Ion chemistry appears to be deeply involved in the formation of heavy molecules in the upper atmosphere of Titan. These large species form the seeds of the organic aerosols responsible for the opaque haze surrounding the biggest satellite of Saturn. The chemical pathways involving individual anions remain however mostly unknown. The determination of the rates of the elementary reactions with ions and the identification of the products are essential to the progress in our understanding of Titan’s upper atmosphere. We have taken steps in that direction through the investigation of the low temperature reactivity of C 3 N − , which was tentatively identified in the spectra measured by the CAPS-ELS instrument of the Cassini spacecraft during its high altitude flybys. The reaction of this anion with HC 3 N, one of the most abundant trace organics in the atmosphere, has been studied over the 49–294 K temperature range in uniform supersonic flows using the CRESU technique. The proton transfer is found to be the main exit channel (>91%) of the C 3 15 N − + HC 3 N reaction. It remains however indistinguishable with the non-isotopically labeled C 3 14 N − reactant. The T − 1 / 2 temperature dependence of this proton transfer reaction and its global rate are reasonably well reproduced theoretically using an average dipole orientation model. A minor exit channel, reactive detachment (< 9%), has also been uncovered, although the nature of the neutral products has not been determined. It is concluded that the C 3 14 N − + HC 3 N reaction cannot contribute to the growth of molecular anions in the upper atmosphere of Titan. Due to the low branching into the neutral exit channel, it cannot contribute either to the growth of neutrals even assuming a complete mass transfer. [ABSTRACT FROM AUTHOR]

Rapid escape from a proposed early CH 4 /NH 3 atmosphere on Titan could, in principle, limit the amount of NH 3 that is converted by photolysis into the present N 2 atmosphere. Assuming that this conversion occurred, a recent estimate of escape driven by the surface temperature and pressure was used to constrain Titan's accretion temperature. Here we show that for the range of temperatures of interest, heating of the surface is not the primary driver for escape. Atmospheric loss from a thick Titan atmosphere is predominantly driven by heating of the upper atmosphere; therefore, the loss rate cannot be used to easily constrain the accretion temperature. We give an estimate of the solar driven escape rate from an early atmosphere on Titan, and then briefly discuss its relevance to the cooling rate, isotope ratios, and the time period suggested to convert NH 3 to the present N 2 atmosphere. [ABSTRACT FROM AUTHOR]

In support of atmospheric remote sensing of Titan and jovian planets, we measured absorption cross sections of benzene (C 6 H 6 ) in the 7–15 µm region at temperatures between 235 K and 297 K. For this, high-resolution laboratory spectra of C 6 H 6 were obtained using two cold cells (80 cm and 2.07 cm path length) configured to a high resolution Fourier-transform infrared (FT-IR) spectrometer, Bruker IFS-125HR, at the Jet Propulsion Laboratory (JPL). The spectrum sets include 15 pure and 15 N 2 -broadened benzene spectra in the 630–1534 cm −1 region, along with four additional spectra broadened by an H 2 (85%) and He(15%) gas mixture for the 630–740 cm -1 region. From these spectra, temperature dependent benzene cross sections were obtained for gas phase benzene in the presence of N 2 and (H 2 +He) at ambient pressures and temperatures down to 235 K. In addition, we generated two independent sets of pseudolines: one of N 2 -broadened benzene for Titan and the other of (H 2 +He)-broadened benzene for jovian planets. It is shown that the benzene pseudolines can reproduce the observed features to ∼ 5% in transmittance, including the continuum-like absorption formed by numerous overlapping weak and hot band transitions. Based on the pseudoline parameters, the integrated band intensities at 296 K for the three strongest bands in the region were measured to be 177.0(73), 14.0(10), 27.2(9)×10 − 17 cm −1 /(molecule·cm −2 ) in the region of v 4 at 674 cm −1 , v 14 at 1038.267, and v 13 at 1483.985 cm −1 , respectively, from the combined set of pure and N 2 -broadened benzene spectra. For the (H 2 +He) mixture-broadened benzene spectra, the integrated band intensity for v 4 band in the 630–735 cm −1 region was measured to be 168.8(17)×10 − 17 cm −1 /(molecule·cm −2 ) at 296 K, which is in agreement with the intensity derived from the N 2 -broadened benzene spectra within the combined measurement uncertainties. The results from this work show an excellent agreement (2%) with one of the latest experimental studies by Rinsland et al. (2008) . Furthermore, additional characteristics carried by the pseudolines approach are discussed. Finally, we provide the two sets of pseudo line list (PLL) as electronic supplements. [ABSTRACT FROM AUTHOR]

Titan, with its thick, nitrogen-dominated atmosphere, has been seen from satellite and terrestrial observations to harbour methane clouds. To investigate whether atmospheric features such as clouds could also be visible from the surface of Titan, data taken with the Side Looking Imager (SLI) on-board the Huygens probe after landing have been analysed to identify any potential atmospheric features. In total, 82 SLI images were calibrated, processed and examined for features. The calibrated images show a smooth vertical radiance gradient across the images, with no other discernible features. After mean-frame subtraction, six images contained an extended, horizontal feature that had a radiance value that lay outside the 95% confidence limit of the predicted radiance when compared to regions higher and lower in the images. The change in optical depth of these features were found to be between 0.005 and 0.014. It is considered that these features most likely originate from the presence of a fog bank close to the horizon that rises and falls during the period of observation. [ABSTRACT FROM AUTHOR]

In the context of the emergence of extra-terrestrial oceanography, we adapted an existing oceanographic model, SLIM (), to the conditions of Titan, a moon of Saturn. The tidal response of the largest southern lake at Titan's surface, namely Ontario Lacus, is simulated. SLIM solves the 2D, depth-averaged shallow water equations on an unstructured mesh using the discontinuous Galerkin finite element method, which allows for high spatial resolution wherever needed. The impact of the wind forcing, the bathymetry, and the bottom friction is also discussed. The predicted maximum tidal range is about 0.56 m in the southern part of the lake, which is more than twice as large as the previous estimates (see Tokano, Ocean Dyn 60:(4) 803-817 (Tokano )). The patterns and magnitude of the current are also markedly different from those of previous studies: the tidal motion is not aligned with the major axis of the lake and the speed is larger nearshore. Indeed, the main tidal component rotates clockwise in an exact period of one Titan day and the tidal currents can reach 0.046 ms close to the shores depending on the geometry and the bathymetry. Except for these specific nearshore regions, the current speed is less than 0.02 ms . Circular patterns can be observed offshore, their rotational direction and size varying along the day. [ABSTRACT FROM AUTHOR]

The reactions of the CN − and C 3 N − anions with cyanoacetylene HC 3 N, of special interest for the chemistry of Titan’s upper atmosphere, have been investigated by means of FTICR mass-spectrometry. Primary ions, CN − and C 3 N − , have been produced by dissociative electron attachment (DEA) from BrCN and BrC 3 N, and prepared in a clean way before reaction. Total rate constants have been measured for both reactions at 300 K and are found to be as follows: (3.9 ± 0.5) × 10 −9 and (1.0 ± 0.2) × 10 −10 cm 3 s −1 for the reaction of HC 3 N with CN − and C 3 N − , respectively. For the CN − + HC 3 N reaction, proton transfer is found to be the only reactive channel within our detection limits. Proton transfer is also dominant for the C 3 N − + HC 3 N reaction but the resulting ionic product being identical to the primary ion C 3 N − , this process is transparent for the kinetics of the C 3 N − + HC 3 N reaction and the kinetic rate retrieved corresponds to a slow and competitive detachment pathway. Yet the nature and energetics of the neutral product(s) formed through this process remain unknown. Additional experiments using isotopic products have allowed to retrieve specific rate constants associated with the proton transfer channel in the C 3 15 N − + HC 3 N and C 3 N − + HC 3 15 N reactions and the measured rates are found to be significantly lower than for the CN − + HC 3 N system. This decrease and the evolution of reactivity when going from CN − to C 3 N − and the opening of a new detachment pathway are finally discussed. [ABSTRACT FROM AUTHOR]

Titan is the only world we know, other than Earth, that has a liquid on its surface. It also has a thick atmosphere composed of nitrogen and methane with a thick organic haze. There are lakes, rain, and clouds of methane and ethane. Here, we address the question of carbon-based life living in Titan liquids. Photochemically produced organics, particularly acetylene, in Titan's atmosphere could be a source of biological energy when reacted with atmospheric hydrogen. Light levels on the surface of Titan are more than adequate for photosynthesis, but the biochemical limitations due to the few elements available in the environment may lead only to simple ecosystems that only consume atmospheric nutrients. Life on Titan may make use of the trace metals and other inorganic elements produced by meteorites as they ablate in its atmosphere. It is conceivable that H2O molecules on Titan could be used in a biochemistry that is rooted in hydrogen bonds in a way that metals are used in enzymes by life on Earth. Previous theoretical work has shown possible membrane structures, azotosomes, in Titan liquids, azotosomes, composed of small organic nitrogen compounds, such as acrylonitrile. The search for a plausible information molecule for life in Titan liquids remains an open research topic--polyethers have been considered and shown to be insoluble at Titan temperatures. Possible search strategies for life on Titan include looking for unusual concentrations of certain molecules reflecting biological selection. Homochirality is a special and powerful example of such biology selection. Environmentally, a depletion of hydrogen in the lower atmosphere may be a sign of metabolism. A discovery of life in liquid methane and ethane would be our first compelling indication that the universe is full of diverse and wondrous life forms. [ABSTRACT FROM AUTHOR]

Over the past decade, the Cassini-Huygens mission to the Saturn system has revolutionized our understanding of Titan and its climate. Veiled in a thick organic haze, Titan's visible appearance belies an active, seasonal weather cycle operating in the lower atmosphere. Here we review the climate of Titan, as gleaned from observations and models. Titan's cold surface temperatures (∼90 K) allow methane to form clouds and precipitation analogously to Earth's hydrologic cycle. Because of Titan's slow rotation and small size, its atmospheric circulation falls into a regime resembling Earth's tropics, with weak horizontal temperature gradients. A general overview of how Titan's atmosphere responds to seasonal forcing is provided by estimating a number of climate-related timescales. Titan lacks a global ocean, but methane is cold-trapped at the poles in large seas, and models indicate that weak baroclinic storms form at the boundary of Titan's wet and dry regions. Titan's saturated troposphere is a substantial reservoir of methane, supplied by deep convection from the summer poles. A significant seasonal cycle, first revealed by observations of clouds, causes Titan's convergence zone to migrate deep into the summer hemispheres, but its connection to polar convection remains undetermined. Models suggest that downwelling of air at the winter pole communicates upper-level radiative cooling, reducing the stability of the middle troposphere and priming the atmosphere for spring and summer storms when sunlight returns to Titan's lakes. Despite great gains in our understanding of Titan, many challenges remain. The greatest mystery is how Titan is able to retain an abundance of atmospheric methane with only limited surface liquids, while methane is being irreversibly destroyed by photochemistry. A related mystery is how Titan is able to hide all the ethane that is produced in this process. Future studies will need to consider the interactions between Titan's atmosphere, surface, and subsurface in order to make further progress in understanding Titan's complex climate system. [ABSTRACT FROM AUTHOR]

Analogous to Earth's water cycle, Titan's methane-based hydrologic cycle supports standing bodies of liquid and drives processes that result in common morphologic features including dunes, channels, lakes, and seas. Like lakes on Earth and early Mars, Titan's lakes and seas preserve a record of its climate and surface evolution. Unlike on Earth, the volume of liquid exposed on Titan's surface is only a small fraction of the atmospheric reservoir. The volume and bulk composition of the seas can constrain the age and nature of atmospheric methane, as well as its interaction with surface reservoirs. Similarly, the morphology of lacustrine basins chronicles the history of the polar landscape over multiple temporal and spatial scales. The distribution of trace species, such as noble gases and higher-order hydrocarbons and nitriles, can address Titan's origin and the potential for both prebiotic and biotic processes. Accordingly, Titan's lakes and seas represent a compelling target for exploration. [ABSTRACT FROM AUTHOR]

We present a detailed model for the vibrational population of all non pre-dissociating excited electronic states of N 2 , as well as for the ground and ionic states, in Titan’s atmosphere. Our model includes the detailed energy deposition calculations presented in the past (Lavvas, P. et al. [2011]. Icarus 213(1), 233–251) as well as the more recent developments in the high resolution N 2 photo-absorption cross sections that allow us to calculate photo-excitation rates for different vibrational levels of singlet nitrogen states, and provide information for their pre-dissociation yields. In addition, we consider the effect of collisions and chemical reactions in the population of the different states. Our results demonstrate that above 600 km altitude, collisional processes are efficient only for a small sub-set of the excited states limited to the A and W ( ν = 0) triplet states, and to a smaller degree to the a ′ singlet state. In addition, we find that a significant population of vibrationally excited ground state N 2 survives in Titan’s upper atmosphere. Our calculations demonstrate that this hot N 2 population can improve the agreement between models and observations for the emission of the c 4 ′ state that is significantly affected by resonant scattering. Moreover we discuss the potential implications of the vibrationally excited population on the ionospheric densities. [ABSTRACT FROM AUTHOR]

Bimolecular diffusion coefficients are important parameters used by atmospheric models to calculate altitude profiles of minor constituents in an atmosphere. Unfortunately, laboratory measurements of these coefficients were never conducted at temperature conditions relevant to the atmosphere of Titan. Here we conduct a detailed uncertainty analysis of the bimolecular diffusion coefficient parameters as applied to Titan's upper atmosphere to provide a better understanding of the impact of uncertainty for this parameter on models. Because temperature and pressure conditions are much lower than the laboratory conditions in which bimolecular diffusion parameters were measured, we apply a Bayesian framework, a problem-agnostic framework, to determine parameter estimates and associated uncertainties. We solve the Bayesian calibration problem using the open-source QUESO library which also performs a propagation of uncertainties in the calibrated parameters to temperature and pressure conditions observed in Titan's upper atmosphere. Our results show that, after propagating uncertainty through the Massman model, the uncertainty in molecular diffusion is highly correlated to temperature and we observe no noticeable correlation with pressure. We propagate the calibrated molecular diffusion estimate and associated uncertainty to obtain an estimate with uncertainty due to bimolecular diffusion for the methane molar fraction as a function of altitude. Results show that the uncertainty in methane abundance due to molecular diffusion is in general small compared to eddy diffusion and the chemical kinetics description. However, methane abundance is most sensitive to uncertainty in molecular diffusion above 1200 km where the errors are nontrivial and could have important implications for scientific research based on diffusion models in this altitude range. [ABSTRACT FROM AUTHOR]

Electron-induced polymerization processes and charging events that can occur within Titan’s atmosphere or on its surface were simulated using electron irradiation and dissociative electron attachment (DEA) studies of nitrogen-containing organic condensates. The DEA studies probe the desorption of H − from hydrogen cyanide (HCN), acetonitrile (CH 3 CN), and aminoacetonitrile (NH 2 CH 2 CN) ices, as well as from synthesized tholin materials condensed or deposited onto a graphite substrate maintained at low temperature (90–130 K). The peak cross sections for H − desorption during low-energy (3–15 eV) electron irradiation were measured and range from 3 × 10 − 21 to 2 × 10 −18 cm 2 . Chemical and structural transformations of HCN ice upon 2 keV electron irradiation were investigated using X-ray photoelectron and Fourier-transform infrared spectroscopy techniques. The electron-beam processed materials displayed optical properties very similar to tholins produced by conventional discharge methods. Electron and negative ion trapping lead to 10 11 charges cm −2 on a flat surface which, assuming a radius of 0.05 μm for Titan aerosols, is ∼628 charges/radius (in μm). The facile charge trapping indicates that electron interactions with nitriles and complex tholin-like molecules could affect the conductivity of Titan’s atmosphere due to the formation of large negative ion complexes. These negatively charged complexes can also precipitate onto Titan’s surface and possibly contribute to surface reactions and the formation of dunes. [ABSTRACT FROM AUTHOR]

The article presents short summaries of articles published in the same issue and in other journals, including an article on hybridization of snowshoe hare camouflage and a test of physicist Albert Einstein's theory of relativity using a gravitational lens system, both in the same issue, and an article on Benzene ice clouds detected in the atmosphere of Saturn moon's Titan in issue 310 of "Icarus."

Theoretical nitrogen-broadening coefficients and their temperature exponents for methyl cyanide lines in parallel (Δ K = 0) bands are calculated by a semi-empirical approach suitable for CH 3 X-type absorbers with large dipole moments. Together with the standard four-parameter correction factor determined from fitting on experimental line widths for each fixed K and requiring an extrapolation for higher K inaccessible experimentally, a new form accounting explicitly for the K -dependence and reducing the number of required parameters is proposed and successfully tested. Extensive (0 ⩽ J ⩽ 70, 0 ⩽ K ⩽ 20) R - and P -line lists are provided for the Earth and Titan atmosphere temperature ranges, which could be useful for atmospheric/astrophysical applications and spectroscopic databases. [ABSTRACT FROM AUTHOR]

Based on a validated model for cryogenic chemical systems, referred to as CRYOCHEM (“Cryogenic Chemistry Model”), surface liquids on Titan are shown to exhibit exotic behavior of density increase with temperature but decrease with pressure, unless the temperature falls below 89.8 K. It is also the case for the atmospheric liquid condensates below an altitude where the liquid density is minimum. The exotic behavior is of compositional origin, which does not have an analog in the atmosphere and liquid water on Earth. As the latitudinal and seasonal variations of surface temperature are known, it is possible to map out the global liquid and vapor density variations as well as the equilibrium phase compositions, which will be useful as inputs for atmospheric general circulation models (GCMs) and investigations of Titan’s methane-equivalent of Earth’s hydrological cycle, local subsurface alkanology (equivalent to hydrology on Earth), lake convection, and clastic and chemical sedimentation in the lakes. Further, the density variations can be used to derive a general idea about global fluid circulation in the upper crust based on averaged conditions on Titan. The surface liquid should tend to flow toward the hottest spot on Titan and a return flow occurs beneath the surface, thus providing analogies with thermohaline circulation in Earth’s oceans. The vapor phase, on the other hand, has ordinary properties that make the global atmospheric circulation similar to the Hadley cell on Earth, but Titan’s cycle reaches the polar regions. The calculated compositions of surface liquids are more methane-rich than other models indicated, thus qualitatively in the right direction to satisfy polar-lake compositions deduced from loss tangents. However, quantitatively there remains a need to find yet more accurate liquid compositions and an optimum equilibrium within constraints of the atmospheric measurements. [ABSTRACT FROM AUTHOR]

Titan’s thick N 2 –CH 4 atmosphere is unlike any in the Solar System, and its origin has been shrouded in mystery for over half a century. Here, I perform a detailed analysis of chemical and isotopic data from the Cassini–Huygens mission to develop the hypothesis that Titan’s (non-photochemical) atmospheric gases came from deep within. It is suggested that Titan’s CH 4 , N 2 , and noble gases originated in a rocky core buried inside the giant satellite, and hydrothermal and cryovolcanic processes were critical to the creation of Titan’s atmosphere. Mass balance and chemical equilibrium calculations demonstrate that all aspects of this hypothesis can be considered geochemically plausible with respect to contemporary observational, experimental, and theoretical knowledge. Specifically, I show that a rocky core with a bulk noble gas content similar to that in CI carbonaceous meteorites would contain sufficient 36 Ar and 22 Ne to explain their reported abundances. I also show that Henry’s law constants for noble gases in relevant condensed phases can be correlated with the size of their atoms, which leads to expected mixing ratios for 84 Kr (∼0.2 ppbv) and 132 Xe (∼0.01 ppbv) that can explain why these species have yet to be detected (Huygens upper limit <10 ppbv). The outgassing of volatiles into Titan’s atmosphere may be restricted by the stability of clathrate hydrates in Titan’s interior. The noble gas geochemistry also provides significant new insights into the origin of N 2 and CH 4 on Titan, as I find that Ar and N 2 , and Kr and CH 4 should exhibit similar phase partitioning behavior on Titan. One implication is that over 95% of Titan’s N 2 may still reside in the interior. Another key result is that the upper limit from the Huygens GC–MS on the Kr/CH 4 ratio in Titan’s atmosphere is far too low to be consistent with accretion of primordial CH 4 clathrate, which motivates me to consider endogenic production of CH 4 from CO 2 as a result of geochemical reactions between liquid water and anhydrous rock (i.e., serpentinization). I show that sufficient CH 4 can be produced to replenish Titan’s atmosphere many times over in the face of irreversible photolysis and escape of CH 4 , which is consistent with the favored model of episodic cryovolcanic outgassing. There should also have been enough NH 3 inside Titan so that its thermal decomposition in a hot rocky core can generate the observed atmospheric N 2 , and if correct this model would imply that Titan’s interior has experienced vigorous hydrothermal processing. The similarity in 14 N/ 15 N between cometary NH 3 and Titan’s N 2 is consistent with this picture. As for the isotopes in CH 4 , I show that their observed relative abundances can be explained by low-temperature (∼20 °C) equilibria with liquid water (D/H) and the expected aqueous alteration mineral calcite ( 12 C/ 13 C), provided that nickel was present to catalyze isotopic exchange over geologic timescales. The present hypothesis is chemically and isotopically consistent with the Cassini–Huygens data, and it implies that the formation of Titan’s atmosphere would have been an unavoidable consequence of volatile processing that was driven by the geophysical evolution of the interior. If all of the atmospheric N 2 and CH 4 have an endogenic origin, then no more than ∼1.6 times the present amount of N 2 can be lost by photochemistry and escape over the history of the atmosphere; and the D/H ratio in Titan’s water should be much lower than that in Enceladus’ plume. Given its important implications to the origin and evolution of volatiles in the outer Solar System, we must go back to Titan to acquire additional isotopic data that will allow more rigorous tests of models of the origin of its atmosphere. I predict the following isotopic ratios: 20 Ne/ 22 Ne ≈ 8.9, 36 Ar/ 38 [ABSTRACT FROM AUTHOR]

We analyzed spectra acquired at the limb of Titan in the 2006–2013 period by the Cassini/Composite Infrared Spectrometer (CIRS) in order to monitor the seasonal evolution of the thermal, gas composition and aerosol spatial distributions. We are primarily interested here in the seasonal changes after the northern spring equinox and interpret our results in term of global circulation seasonal changes. Data cover the 600–1500 cm −1 spectral range at a resolution of 0.5 or 15.5 cm −1 and probe the 150–500 km vertical range with a vertical resolution of about 30 km. Retrievals of the limb spectra acquired at 15.5 cm −1 resolution allowed us to derive eight global maps of temperature, aerosols and C 2 H 2 , C 2 H 6 and HCN molecular mixing ratios between July 2009 and May 2013. In order to have a better understanding of the global changes taking place after the northern spring equinox, we analyzed 0.5 cm −1 resolution limb spectra to infer the mixing ratio profiles of 10 molecules for some latitudes. These profiles are compared with CIRS observations performed during the northern winter. Our observations are compatible with the coexistence of two circulation cells upwelling at mid-latitudes and downwelling at both poles from at last January 2010 to at least June 2010. One year later, in June 2011, there are indications that the global circulation had reversed compared to the winter situation, with a single pole-to-pole cell upwelling at the north pole and downwelling at the south pole. Our observations show that in December 2011, this new pole-to-pole cell has settled with a downward velocity of 4.4 mm/s at 450 km above the south pole. Therefore, in about two years after the equinox, the global circulation observed during the northern winter has totally reversed, which is in agreement with the predictions of general circulation models. We observe a sudden unexpected temperature decrease above the south pole in February 2012, which is probably related to the strong enhancement of molecular gas in this region, acting as radiative coolers. In July and November 2012, we observe a detached haze layer located around 320–330 km, which is comparable to the altitude of the detached haze layer observed by the Cassini Imaging Science Subsystem (ISS) in the UV. [ABSTRACT FROM AUTHOR]

Simulation results are presented from a new general circulation model (GCM) of Titan, the Titan Atmospheric Model (TAM), which couples the Flexible Modeling System (FMS) spectral dynamical core to a suite of external/sub-grid-scale physics. These include a new non-gray radiative transfer module that takes advantage of recent data from Cassini–Huygens, large-scale condensation and quasi-equilibrium moist convection schemes, a surface model with “bucket” hydrology, and boundary layer turbulent diffusion. The model produces a realistic temperature structure from the surface to the lower mesosphere, including a stratopause, as well as satisfactory superrotation. The latter is shown to depend on the dynamical core’s ability to build up angular momentum from surface torques. Simulated latitudinal temperature contrasts are adequate, compared to observations, and polar temperature anomalies agree with observations. In the lower atmosphere, the insolation distribution is shown to strongly impact turbulent fluxes, and surface heating is maximum at mid-latitudes. Surface liquids are unstable at mid- and low-latitudes, and quickly migrate poleward. The simulated humidity profile and distribution of surface temperatures, compared to observations, corroborate the prevalence of dry conditions at low latitudes. Polar cloud activity is well represented, though the observed mid-latitude clouds remain somewhat puzzling, and some formation alternatives are suggested. [ABSTRACT FROM AUTHOR]

Theoretical self-broadening coefficients and associated temperature dependences for methyl cyanide lines in parallel (Δ K = 0) bands are reported for large ranges of rotational quantum numbers (0 ⩽ J ⩽ 70, K ⩽ 20) requested by spectroscopic databases. The calculations are performed by a semi-empirical method, particularly suitable for active molecules with large dipole moments, which needs only a few experimental data for model parameters fitting. Since the common power law for the temperature-dependence exponents is invalid for wide temperature ranges, two separate sets of temperature exponents are provided for Earth and Titan atmospheres applications. [ABSTRACT FROM AUTHOR]

We present an analysis of the VIMS solar occultations dataset, which allows us to extract vertically resolved information on the characteristics of Titan’s atmosphere between ∼100 and 700 km with a vertical resolution of ∼10 km. After a series of data treatment procedures to correct problems in pointing stability and parasitic light, 4 occultations out of 10 are retained. This sample covers different seasons and latitudes of Titan. The transmittances show clearly the evolution of the haze, with the detection of the detached layer at ∼310 km in September 2011 at mid-northern latitudes. Through the inversion of the transmission spectra with a line-by-line radiative transfer code we retrieve the vertical distribution of CH 4 and CO mixing ratio. For methane inversion we use its 1.4, 1.7 and 2.3 μm bands. The first two bands are always in good agreement and yield an average stratospheric abundance of 1.28 ± 0.08%, after correcting for forward-scattering effects, with no significant differences between the occultations. This is significantly less than the value of 1.48% obtained by the GCMS/Huygens instrument. We find that the 2.3 μm band cannot be used for the extraction of methane abundance because it is blended with other absorptions, not included in our atmospheric model. The analysis of the residual spectra after the inversion shows that such additional absorptions are present through a great part of the VIMS wavelength range. We attribute many of these bands, including the one at 2.3 μm, to gaseous ethane, whose near-infrared spectrum is not well modeled yet. Ethane also contributes significantly to the strong absorption at 3.2–3.5 μm that was previously attributed only to C–H stretching bands from aerosols. Ethane bands may affect the surface windows too, especially at 2.7 μm. Other residual bands are generated by stretching modes of C–H, C–C and C–N bonds. In addition to the C–H stretch from aliphatic hydrocarbons at 3.4 μm, we detect a strong and narrow absorption at 3.28 μm which we tentatively attribute to the presence of PAHs in the stratosphere. C–C and C–N stretching bands are possibly present between 4.3 and 4.5 μm. Finally, we obtain the CO mixing ratio between 70 and 170 km, through the inversion of its 4.7 μm band. The average result of 46 ± 16 ppm is in good agreement with previous studies. [ABSTRACT FROM AUTHOR]

Dicyanoacetylene has not been observed so far in the gas phase in Titan’s atmosphere but this molecule is still on the list of the detected species, on the basis of the correspondence between a solid phase feature measured at 478 cm −1 in the laboratory and a spectral feature observed by Voyager. In this work, the infrared spectrum of gaseous C 4 N 2 has been investigated to improve our knowledge of the band intensities and the line parameters for this molecule. Results of previously investigated bands have been revised and the intensity of the ν 9 band at 107 cm −1 , measured for the first time, was found to be the strongest absorption in the whole infrared domain. We have also improved the analysis of the complex rotational and hot band structure of C 4 N 2 in order to obtain the first line lists for both bending modes ν 8 and ν 9 . Using our radiative transfer code including the new line list of the strong ν 9 band, we have searched for the signature of C 4 N 2 at 107 cm −1 in the atmosphere of Titan utilizing Titan CIRS far infrared spectra. Despite averaging a large number of CIRS spectra at northern latitudes during the very favorable Titan winter, no gaseous C 4 N 2 could be detected. At the 1- σ level we obtain an abundance upper limit of 5.3 × 10 −10 for the limb average which is lower than or comparable to previously inferred values. As a consequence, the absence or very low amount of gaseous C 4 N 2 makes quite puzzling its presence in the solid phase with an abundance compatible with the observed spectral feature at 478 cm −1 . [ABSTRACT FROM AUTHOR]