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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]

Moray Sinus is an estuary located at the northern end of Titan's Kraken Mare. The Cassini RADAR altimeter acquired three segments over this mare during the T104 flyby of Titan, on August 21, 2014. Herein, we present a detailed analysis of the received echoes. Some of these waveforms exhibit a reflection from the seafloor, from up to 85−18+28 m of depth (1σ error). Monte Carlo simulations have been performed in order to assess the most probable values and estimation errors for the seafloor depth. Insights from this study, featuring the synergic use of the synthetic aperture radar images coupled to the altimetry and passive radiometry datasets, have been used to constrain the dielectric properties (i.e., absorptivity of the liquid) and roughness of this region of Kraken Mare. The resulting Ku‐band specific attenuation of the liquid is 17−3+3 dB/μs, corresponding to a loss tangent of 4.6−0.9+0.9×10−5, which is very similar to the loss tangent estimated at Ligeia Mare. The data in hand do not permit us to discern the most likely explanation for the lack of a seafloor reflection from the main body of Kraken Mare: either a very deep sea or a more absorbing liquid composition. However, if the main body of Kraken Mare is characterized by an absorption similar to Moray Sinus, then based on models of the response to altimetry mode observations we can conclude that it exceeds 100 m of depth, which is also compatible with radiometry observations. Plain Language Summary: From 2013 to 2017, we have been probing the depth of Titan's methane‐dominated seas by penetrating the liquid with Cassini's radar altimeter. The depth and composition of each of the seas had already been derived, except for Titan's largest sea Kraken Mare. Herein, we describe the final observation of this campaign before the end of Cassini in 2017. Our analysis reveals that the seafloor at the center of Moray Sinus—an estuary located at the northern end of Kraken Mare, is up to 85 m deep. The radar waves are absorbed to an extent such that the liquid composition is compatible with 70% methane, 16% nitrogen, and 14% ethane (assuming ideal mixing). The analysis of the altimetry data in the main body of Kraken Mare showed no evidence for signal returns from the sea floor, suggesting the liquid is either too deep or too absorptive for Cassini's radio waves to penetrate. However, if the liquid in the main body of Kraken Mare is similar in composition to Moray Sinus, as one would expect, then its depth exceeds 100 m. This is compatible with a separate estimate using the radar as a "radiometer," sensing thermal energy from the sea at radio wavelengths. Key Points: Moray Sinus is an estuary located at the northern end of Titan's Kraken MareAnalysis of Cassini's radar altimeter data shows that the near‐shore local seafloor is up to 85 m deep in Moray Sinus estuaryThe composition of the sea, inferred from radar absorptivity, is very similar to that estimated at Ligeia MareThe main body of Kraken Mare exceeds 100 m of depth [ABSTRACT FROM AUTHOR]

Evidence for tectonic activity on Titan is provided by the presence of eroded mountain ranges. Xanadu is an equatorial region of Titan characterized by a complex topography, even though, overall, it has a lower average elevation compared to its surroundings. We investigated Xanadu's southwestern margin, a part of the region composed of heavily eroded and rugged terrains to the north and east, and of smoother, more uniform terrains to the west and south. The central portions of southwestern Xanadu (SW Xanadu) are characterized by an extensive fluvial network. The presence of such a distinctive feature was the main reason motivating the study of this area, given its potential to provide tectonic indications. Through detailed geomorphological mapping (map scale 1:700,000) on Synthetic Aperture Radar data and analysis of both fluvial drainage patterns and Digital Terrain Models, we identified several putative tectonic structures in this area: normal faulting to the west and east, thrust faulting to the north, and small‐scale strike‐slip faulting in its central parts. Pull‐apart basins are depressions bounded by both dip‐slip faults and by (overlapping and/or bending) segments of a major transcurrent fault, i.e., they are basins generated in transtensional tectonic settings. We propose that central SW Xanadu is a pull‐apart basin, bordered by both normal and thrust faults and formed by transtensional tectonics, which we consider to be the most recent tectonic phase active in this area. This basin is characterized by small‐scale strike‐slip faulting within it, on which a fluvial network has subsequently imposed. Plain Language Summary: Saturn's largest moon, Titan, has several mountain ranges on its surface. It is thought that they are formed by large‐scale processes related to Titan's interior, such as tectonic activity. Xanadu is an equatorial region characterized by a rugged topography, even though its surrounding regions stand higher. We investigated Xanadu's southwestern margin, which is characterized by many different terrain types and by the presence of a hydrocarbon‐related fluvial system. At Titan's surface pressure and temperature, hydrocarbons are in a liquid state and can therefore form rivers. Using radar data from the Cassini mission, we mapped in detail the differences in morphology of each terrain in the region. Furthermore, we analyzed local fluvial drainage patterns and different types of elevation data. We identified several areas that may testify the influence of Titan's internal processes on shaping SW Xanadu. We propose that different tectonic phases have been active in this region through time. Key Points: Detailed geomorphological mapping of southwestern Xanadu has been conducted using radar and newly available topographic dataSeveral putative tectonic structures have been identified in the study areaSouthwestern Xanadu is interpreted as a pull‐apart basin formed by transtensional tectonic activity [ABSTRACT FROM AUTHOR]

The Saturn‐orbiting Cassini spacecraft completed 126 close Titan flybys from 2004 until 2017. During almost all of them the Cassini Radio and Plasma Wave Science (RPWS) instrument was turned on to search for radio emissions attributed to Titan lightning. Here we report about their nondetection after close inspection of all Titan flybys throughout the Cassini mission. We also infer new and strong constraints on the permissible flash energy and flash rate of potential Titan lightning. The nondetection of lightning flashes by Cassini observations implies that any lightning on Titan must be either very weak, very rare, or does not exist at all, and the latter could be due to cloud electric fields being too low to initiate a discharge. This finding holds important implications for the prebiotic chemistry of Titan and also implies that lightning will not be a significant hazard to the upcoming Dragonfly mission. Plain Language Summary: During its Saturn tour the Cassini spacecraft performed 126 close Titan flybys. The RPWS (Radio and Plasma Wave Science) instrument would have been able to detect radio emissions from potential lightning in Titan's atmosphere, similar to the easy detection of Earth lightning during the Cassini Earth flyby in August 1999. A careful inspection of RPWS data has revealed no radio signals that could be attributed to Titan lightning. The long observation times make it very likely that Titan lightning does not exist, is very weak, or is very rare. Key Points: All Cassini Titan flybys were examined for radio emissions from Titan lightningNo radio emissions from Titan lightning were found in Cassini RPWS dataNew upper limits for potential but unlikely Titan lightning activity were set [ABSTRACT FROM AUTHOR]

Previous work at Titan presented a set of 85 flux ropes detected during Cassini flybys of Titan from 2005 to 2017. In that study a force‐free model was used to determine the radii and axial magnetic field of the flux ropes. In this work we apply non‐force‐free models. The non‐force‐free model shows an improvement in the number of flux ropes that can be fitted with a model, along with improved uncertainties and χ2 values. A number of asymmetries and features in the magnetometer data cannot be reproduced by either model; therefore, we deform the force‐free model to show that small deformations can replicate these features. One such deformation is to use an elliptical cross section, which replicates a plateau in magnetic field strength along with asymmetries on either side of the center of the flux ropes. Additionally, we explore the properties of bending a flux rope, where we find that minimum variance analysis becomes increasingly degenerate with bending, along with a slight bend causing the switching of the axial field direction from intermediate to maximum variance direction. We conclude that the flux ropes at Titan show aspects of developing flux ropes, compared to other planetary bodies, which exhibit more agreement to the force‐free assumptions of mature flux ropes. Key Points: Various models are fitted to 85 flux ropes that were detected in Titan's ionosphereA non‐force‐free model is a more suitable fit statistically compared to a force‐free model, implying that flux ropes are dynamic at TitanVarious deformations that act on a flux rope are explored, and in some cases, an elliptical cross section is more appropriate than circular [ABSTRACT FROM AUTHOR]

Titan's magnetic environment is a dynamic and unique place. We detect 85 flux ropes during all the Cassini flybys of Titan from 2005–2017. Analysis describing the location of flux ropes in Titan's ionosphere as well as where Titan is in Saturn's magnetosphere shows that the flux ropes are more often found when Titan is in the noon sector of Saturn's magnetosphere. A secondary peak of occurrence is found in the postmidnight area, where it is expected that Saturn's magnetosphere is highly dynamic. We also find that the flux rope occurrence is correlated with the average electron density profile of Titan's ionosphere. A force‐free model is utilized to estimate the radii and core magnetic field strength of the flux ropes. We find a large range of radii from 150–500 km with a small number of very large flux ropes (500–1,000 km) and a range of core field strengths of 1–15 nT, again with some much larger values(20–40 nT). The model also shows that more flux ropes are right‐handed than left‐handed in twist; however, we are unable to determine if there is a physical reason or if this is due to an observer bias. Additionally, we evaluate the goodness of fit for the model in each instance and conclude that, on average, the flux ropes may be better represented by a non‐force‐free model. Key Points: Eighty‐five flux ropes are detected in Titan's ionosphere using all of Cassini's flybys of TitanFlux ropes are present when Titan is in dynamic areas of Saturn's magnetosphereFitting a force‐free flux rope model leads to the conclusion that on average they are unlikely to be force free [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]

The Cassini-Huygens mission to Saturn provided a close-up study of the gas giant planet, as well as its rings, moons, and magnetosphere. The Cassini spacecraft arrived at Saturn in 2004, dropped the Huygens probe to study the atmosphere and surface of Saturn’s planet-sized moon Titan, and orbited Saturn for the next 13 years. In 2017, when it was running low on fuel, Cassini was intentionally vaporized in Saturn’s atmosphere to protect the ocean moons, Enceladus and Titan, where it had discovered habitats potentially suitable for life. Mission findings include Enceladus’ south polar geysers, the source of Saturn’s E ring; Titan’s methane cycle, including rain that creates hydrocarbon lakes; dynamic rings containing ice, silicates, and organics; and Saturn’s differential rotation. This Review discusses highlights of Cassini’s investigations, including the mission’s final year. [ABSTRACT FROM AUTHOR]

Titan's hydrocarbon lakes play an important role in the chemistry, geomorphology, and climate of the satellite. Our knowledge of their composition relies mainly on thermodynamic modeling and assumptions based on Cassini Radar and VIMS (Visible and Infrared Mapping Spectrometer) data. Several thermodynamic models have been used to calculate the composition of these lakes, and their results on even the major lake components (methane, ethane, propane, and nitrogen) exhibit large discrepancies. Recent Cassini radar observations revealed an echo from the lake's bottom. A low loss factor of attenuation is needed within the lakes to interpret these observations, and it has been suggested that the lakes are dominated by methane. Cassini VIMS data obtained on the North Pole lakes at three-year intervals showed no detectable surface level change, which is consistent with ethane being their primary constituent. This additional discrepancy between thermodynamic models and Cassini data strongly shows the need for experimental measurements under realistic Titan conditions in order to better constrain the thermodynamic models. We designed and built a cryogenic experimental platform allowing the simulation of Titan's lakes. This facility, named Titan Lakes Simulation System (TiLSS), produces liquid hydrocarbons in equilibrium with a gas phase mimicking Titan's atmosphere. Samples of the condensed liquid are injected directly into a gas chromatograph allowing the direct measurement of its chemical components and their abundances. To test the overall operation of the system, a gas mixture of methane and ethane was condensed under 1.5 bar of nitrogen and analyzed. Results from this proof of concept test are in good agreement with experimental studies previously published. [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]

Cassini's Visible and Infrared Mapping Spectrometer (VIMS) has provided evidence of several different hydrocarbons on the surface of Titan using seven atmospheric windows. Methane (CH) and ethane (CH) are suggested to exist in both the liquid and solid states on Titan. Even if the average surface temperature (90-94 K) of Titan is clearly in the liquid stability field of both CH and CH, the particles can condense in the atmosphere (∼65 km for CH and lower stratosphere for CH Anderson et al. in Icarus, 243:129-138, 2014) and precipitate allowing them to melt and/or sublimate. It is also suggested that these liquids can freeze on the surface due to evaporative cooling. We conducted a laboratory study at Titan surface conditions to determine the phase change of CH and CH and to test if they would freeze on the surface of Titan. Using NIR reflectance spectroscopy, we calculated the reflectivity ratio ( $R_{\mathrm{solid}}/R_{\mathrm{liquid}}$ ) of CH and CH of 1.08 and 1.36, respectively, suggesting an 8% increase in reflectivity for CH and a 36% increase for CH during phase change. The low albedo in liquid phase for both CH and CH is consistent with observations made by VIMS in both Titan's northern and southern latitudes. We also find the evaporation rate of amorphous CH close to Titan conditions, which is $9.0 \pm 0.3 \times 10^{-5}~\mbox{kg}\,\mbox{s}^{-1}\,\mbox{m}^{-2}$ at 87 K and we estimated a sublimation rate of $0.22 \times 10^{-5}~\mbox{kg}\,\mbox{s}^{-1}\,\mbox{m}^{-2}$ at 83 K for a 1.5 bar N atmosphere. The freezing rate of ∼46 m/year for CH was observed whereas, for CH, we observed that CH does not freeze at 87 K due to a high N dissolution rate. However, the viscosity of CH increases with a decreasing temperature that results in amorphous CH. The results show a remarkable difference between the formation of ice for two liquids with different N dissolution rates. Consequently, using the results obtained from the laboratory study we predict that the observed change is albedo during and after rainfall on Titan is caused by CH due to evaporative cooling processes. [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]

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]

Titan, the largest satellite of Saturn, exhibits extensive aeolian, that is, wind-formed, dunes, features previously identified exclusively on Earth, Mars and Venus. Wind tunnel data collected under ambient and planetary-analogue conditions inform our models of aeolian processes on the terrestrial planets. However, the accuracy of these widely used formulations in predicting the threshold wind speeds required to move sand by saltation, or by short bounces, has not been tested under conditions relevant for non-terrestrial planets. Here we derive saltation threshold wind speeds under the thick-atmosphere, low-gravity and low-sediment-density conditions on Titan, using a high-pressure wind tunnel refurbished to simulate the appropriate kinematic viscosity for the near-surface atmosphere of Titan. The experimentally derived saltation threshold wind speeds are higher than those predicted by models based on terrestrial-analogue experiments, indicating the limitations of these models for such extreme conditions. The models can be reconciled with the experimental results by inclusion of the extremely low ratio of particle density to fluid density on Titan. Whereas the density ratio term enables accurate modelling of aeolian entrainment in thick atmospheres, such as those inferred for some extrasolar planets, our results also indicate that for environments with high density ratios, such as in jets on icy satellites or in tenuous atmospheres or exospheres, the correction for low-density-ratio conditions is not required. [ABSTRACT FROM AUTHOR]

Highlights: [•] Titan tholin amino acids were measured using multi-dimensional chromatographic analysis. [•] We compared the tholin amino acids with those generated from Miller–Urey (MU) reactions. [•] The amino acids detected in MU reactions are similar to those generated in the Titan simulation. [•] Many Solar System environments may thus provide similar organic compounds. [•] Given reducing conditions, similar materials should be available throughout the universe. [Copyright &y& Elsevier]

Highlights: [•] The observed obliquity can be reproduced only for internal models with a dense ocean. [•] The Love number is more sensitive to the ocean density than to the shell thickness. [•] The MoI derived from obliquity and k 2 is lower than the hydrostatic estimate. [Copyright &y& Elsevier]

Titan's surface-atmosphere system bears remarkable similarities to Earth's, the most striking being an active, global methane cycle akin to Earth's water cycle. Like the hydrological cycle of Earth, Titan's seasonal methane cycle is driven by changes in the distribution of solar energy. The Cassini spacecraft, which arrived at Saturn in 2004 in the midst of northern winter and southern summer, has observed surface changes, including shoreline recession, at Titan's south pole and equator. However, active surface processes have yet to be confirmed in the lakes and seas in Titan's north polar region. As the 2017 northern summer solstice approaches, the onset of dynamic phenomena in this region is expected. Here we present the discovery of bright features in recent Cassini RADAR data that appeared in Titan's northern sea, Ligeia Mare, in July 2013 and disappeared in subsequent observations. We suggest that these bright features are best explained by the occurrence of ephemeral phenomena such as surface waves, rising bubbles, and suspended or floating solids. We suggest that our observations are an initial glimpse of dynamic processes that are commencing in the northern lakes and seas as summer nears in the northern hemisphere. [ABSTRACT FROM AUTHOR]

We combine derived ion-electron pair formation rates with Cassini Radio Plasma Wave Science Langmuir Probe measurements of electron and positive ion number densities in Titan's sunlit ionosphere. We show that positive ion number densities in Titan's sunlit ionosphere can increase toward significantly lower altitudes than the peak of ion-electron pair formation despite that the effective ion-electron recombination coefficient increases. This is explained by the increased mixing ratios of negative ions, which are formed by electron attachment to neutrals. While such a process acts as a sink for free electrons, the positive ions become longer-lived as the rate coefficients for ion-anion neutralization reactions are smaller than those for ion-electron dissociative recombination reactions. [ABSTRACT FROM AUTHOR]

Highlights: [•] Craters on Titan preferentially lie at higher than average elevations. [•] Distribution is consistent with presence of hydrocarbon wetlands at low elevations. [•] Marine impacts lack surface topography and would be difficult to observe with Cassini. [Copyright &y& Elsevier]

Abstract: Objective: Characterize and understand acoustic instrument performance on the surface of Titan. Methods: The Huygens probe measured the speed of sound in Titan's atmosphere with a 1MHz pulse time-of-flight transducer pair near the bottom of the vehicle. We examine the fraction of pulses correctly received as a function of time. Results: This system returned good data from about 11km altitude, where the atmosphere became thick enough to effectively transmit the sound, down to the surface just before landing: these data have been analyzed previously. After an initial transient at landing, the instrument operated nominally for about 10min, recording pulses much as during descent. The fraction of pulses detected then declined and the transmitted sound ceased to be detected altogether, despite no indication of instrument or probe configuration changes. Conclusions: The most likely explanation appears to be absorption of the signal by polyatomic gases with relaxation losses at the instrument frequency, such as ethane, acetylene and carbon dioxide. These vapors, detected independently by the GCMS instrument, were evolved from the surface material by the warmth leaking from the probe, and confirm the nature of the surface materials as ‘damp’ with a cocktail of volatile compounds. Some suggestions for future missions are considered. Practice implications: None. [Copyright &y& Elsevier]

Saturn's moon Titan is the only satellite in this solar system with a dense atmosphere and hydrocarbon seas. The Titan Mare Explorer (TiME) mission would splashdown a capsule to float for 3 months on Ligeia Mare, a several-hundred-kilometer wide sea near Titan's north pole. Among TiME's scientific goals is the determination of the depth of Ligeia, to be achieved with an acoustic depth sounder. Since Titan's surface temperature is known to vary around 92 K, all instruments must be ruggedized to operate at cryogenic temperatures. This paper's contributions include an approach to infer key acoustic properties of this remote environment and the extraterrestrial environment's influence on the development of a cryogenic depth sounder. Additionally, an approach is formulated to infer the transducer's response, sensitivity, and performance when in situ calibration is impossible or when replicating key environmental conditions is too costly. [ABSTRACT FROM AUTHOR]

Abstract: Titan's atmosphere has been extensively studied during the Cassini–Huygens Mission. The polar environment undergoes considerable seasonal variation and is distinct from the more quiescent equatorial environment experienced by the Huygens probe in 2005. The thermal structure of the upper atmosphere is affected by interaction of Titan's atmosphere with the plasma and energetic particles in Saturn's magnetosphere or in the solar wind. Titan's polar regions, and its hydrocarbon lakes in particular, are of interest for future exploration as is the complex organic chemistry that takes place in the upper atmosphere. Thus, specific environmental models are required for future exploration. Furthermore, the models developed to support the design of Huygens had to accommodate wide uncertainties, requiring large design margins. The extensive observations by the Cassini spacecraft (and indeed Huygens itself) provide a basis for narrowing these uncertainties, notably in composition. We provide a description of these new data sets, their scientific basis, and present limitations. The empirically constrained neutral temperatures and densities presented here also have important implications for fundamental chemical modeling. The empirical modeling presented here provides constraints on these models, which will lead to improved estimations of chemical products. Therefore, the paper describes a new model of Titan's atmospheric structure to guide both scientific studies and future engineering studies of exploration of the Saturn system. Of particular interest are aero-capture and aero-braking missions that may enable future exploration of the Saturnian satellite system. [Copyright &y& Elsevier]

The search for and detailed characterization of habitable environments on other worlds-- places where liquid water, heat/energy sources, and biologically important organic molecules exist or could have once existed -- is a major twenty-first-century goal for space exploration by NASA and other space agencies, motivated by intense public interest and highly ranked science objectives identified in recent National Academy decadal surveys. Through telescopic observations, terrestrial laboratory and field studies, and a "flyby, orbit, land, rove, and return" strategy for robotic exploration, particular emphasis will be placed on specific worlds already identified as potentially habitable: Mars, Jupiter's ocean moon Europa, and Saturn's icy and organic-bearing moons Titan and Enceladus. However, the potential abounds for surprising discoveries at many of our solar system's other planetary, satellite, and asteroidal destinations, as well as within newly discovered planetary systems around other stars. [ABSTRACT FROM AUTHOR]

The article focuses on the organic chemical composition of Titan based on the discovery report of the Cassini-Huygens mission to the Saturn system. It mentions that Titan's atmosphere was denser than the Earth's and consists of nitrogen, carbon monoxide, methane, and hydrogen, and the surface have various putative energy sources. Furthermore, information on Tholin, which is a sticky brownish residue formed in Titan atmospheric simulation experiments, is presented.

Photochemical models of Titan's atmosphere predict that three-body association reactions are the main production route for several major hydrocarbons. The kinetic rate constants of these reactions strongly depend on density and are therefore only important in Titan's lower atmosphere. However, radiative association reactions do not depend on pressure. The possible existence of large rates at low density suggests that association reactions could significantly affect the chemistry of Titan's upper atmosphere and better constraints for them are required. The kinetic parameters of these reactions are extremely difficult to constrain by experimental measurements as the low pressure of Titan's upper atmosphere cannot be reproduced in the laboratory. However, in the recent years, theoretical calculations of kinetics parameters have become more and more reliable. We therefore calculated several radical-radical and radical-molecule association reaction rates using transition state theory. The calculations indicate that association reactions are fast even at low pressure for adducts having as few as four C atoms. These drastic changes have however only moderate consequences for Titan's composition. Locally, mole fractions can vary by as much as one order of magnitude but the column-integrated production and condensation rates of hydrocarbons change only by a factor of a few. We discuss the impact of these results for the organic chemistry. It would be very interesting to check the impact of these new rate constants on other environments, such as giant and extrasolar planets as well as the interstellar medium. [ABSTRACT FROM AUTHOR]

Although there is evidence that liquids have flowed on the surface at Titan's equator in the past, to date, liquids have only been confirmed on the surface at polar latitudes, and the vast expanses of dunes that dominate Titan's equatorial regions require a predominantly arid climate. We report the detection by Cassini's Imaging Science Subsystem of a large low-latitude cloud system early in Titan's northern spring and extensive surface changes (spanning more than 500,000 square kilometers) in the wake of this storm. The changes are most consistent with widespread methane rainfall reaching the surface, which suggests that the dry channels observed at Titan's low latitudes are carved by seasonal precipitation. [ABSTRACT FROM AUTHOR]

Abstract: Titan is a very interesting target in deep space exploration. With its solid surface on which a rover can easily travel and its methane lakes which can be sailed it is the ideal target for a deep space mission which includes a mobile platform. In the present paper the general layout of a rover for a mission to Titan is studied, dealing with the mobility, power generation and trajectory control issues. A four-wheels configuration with slip steering was chosen; to compare this solution with the more conventional strategy based on steering wheels, simulations were performed on some trajectories computed through the well known ‘potential’ method, using both slip steering and conventional steering control, for different vehicle speeds. The comparison between the simulated trajectories allows to state the adequateness of the proposed approach. The results here obtained apply not only to a future mission to Titan, but also to other missions designed for the exploration of the satellites of the outer planets having a size comparable with that of Titan and the largest Kuiper belt objects like Pluto and 136472 Makemake. [ABSTRACT FROM AUTHOR]

Abstract: We apply a multivariate statistical method to Titan data acquired by different instruments onboard the Cassini spacecraft. We have searched through Cassini/VIMS hyperspectral cubes, selecting those data with convenient viewing geometry and that overlap with Cassini/RADAR scatterometry footprints with a comparable spatial resolution. We look for correlations between the infrared and microwave ranges the two instruments cover. Where found, the normalized backscatter cross-section obtained from the scatterometer measurement, corrected for incidence angle, and the calibrated antenna temperature measured along with the scatterometry echoes, are combined with the infrared reflectances, with estimated errors, to produce an aggregate data set, that we process using a multivariate classification method to identify homogeneous taxonomic units in the multivariate space of the samples. In medium resolution data (from 20 to 100km/pixel), sampling relatively large portions of the satellite’s surface, we find regional geophysical units matching both the major dark and bright features seen in the optical mosaic. Given the VIMS cubes and RADAR scatterometer passes considered in this work, the largest homogeneous type is associated with the dark equatorial basins, showing similar characteristics as each other on the basis of all the considered parameters. On the other hand, the major bright features seen in these data generally do not show the same characteristics as each other. Xanadu, the largest continental feature, is as bright as the other equatorial bright features, while showing the highest backscattering coefficient of the entire satellite. Tsegihi is very bright at 5μm but it shows a low backscattering coefficient, so it could have a low roughness on a regional scale and/or a different composition. Another well-defined region, located southwest of Xanadu beyond the Tui Regio, seems to be detached from the surrounding terrains, being bright at 2.69, 2.78 and 5μm but having a low radar brightness. In this way, other units can be found that show correlations or anti-correlations between the scatterometric response and the spectrophotometric behavior, not evident from the optical remote sensing data. [Copyright &y& Elsevier]

Abstract: This paper reports on the results from an extensive study of all nadir-looking spectra acquired by Cassini/CIRS during the 44 flybys performed in the course of the nominal mission (2004–2008). With respect to the previous study (Coustenis, A., and 24 colleagues [2007]. Icarus 189, 35–62, on flybys TB–T10) we present here a significantly richer dataset with, in particular, more data at high northern and southern latitudes so that the abundances inferred here at these regions are more reliable. Our enhanced high-resolution dataset allows us to infer more precisely the chemical composition of Titan all over the disk. We also include improved spectroscopic data for some molecules and updated temperature profiles. The latitudinal distributions of all of the gaseous species are inferred. We furthermore test vertical distributions essentially for acetylene (C2H2) from CIRS limb-inferred data and from current General Circulation Models for Titan and compare our results on all the gaseous abundances with predictions from 1-D photochemical-radiative models to check the reliability of the chemical reactions and pathways. [Copyright &y& Elsevier]

Abstract: Future planetary exploration of Titan will require higher degrees of on-board automation, including autonomous determination of sites where the probability of significant scientific findings is the highest. In this paper, a novel Artificial Intelligence (AI) method for the identification and interpretation of sites that yield the highest potential of cryovolcanic activity is presented. We introduce the theory of fuzzy cognitive maps (FCM) as a tool for the analysis of remotely collected data in planetary exploration. A cognitive model embedded in a fuzzy logic framework is constructed via the synergistic interaction of planetary scientists and AI experts. As an application example, we show how FCM can be employed to solve the challenging problem of recognizing cryovolcanism from Synthetic Aperture Radar (SAR) Cassini data. The fuzzy cognitive map is constructed using what is currently known about cryovolcanism on Titan and relies on geological mapping performed by planetary scientists to interpret different locales as cryovolcanic in nature. The system is not conceived to replace the human scientific interpretation, but to enhance the scientists’ ability to deal with large amounts of data, and it is a first step in designing AI systems that will be able, in the future, to autonomously make decisions in situations where human analysis and interpretation is not readily available or could not be sufficiently timely. The proposed FCM is tested on Cassini radar data to show the effectiveness of the system in reaching conclusions put forward by human experts and published in the literature. Four tests are performed using the Ta SAR image (October 2004 fly-by). Two regions (i.e. Ganesa Macula and the lobate high backscattering region East of Ganesa) are interpreted by the designed FCM as exhibiting cryovolcanism in agreement with the initial interpretation of the regions by . Importantly, the proposed FCM is shown to be flexible and adaptive as new data and knowledge are acquired during the course of exploration. Subsequently, the FCM has been modified to include topographic information derived from SAR stereo data. With this additional information, the map concludes that Ganesa Macula is not a cryovolcanic region. In conclusion, the FCM methodology is shown to be a critical and powerful component of future autonomous robotic spacecraft (e.g., orbiter(s), balloon(s), surface/lake lander(s), rover(s)) that will be deployed for the exploration of Titan. [Copyright &y& Elsevier]

Abstract: We note the geomorphological and meteorological processes at Racetrack Playa, Death Valley National Park, as analogs for those at Ontario Lacus on Titan. Although Ontario is ∼50× larger, the planforms of the two features are nearly identical, both are extremely flat, and are in environments where infrequent rainfall occurs against a climate, where evaporation exceeds precipitation. While the famous moving rocks on the Racetrack Playa may be exceptional on the Earth, the lower gravity and thicker atmosphere may render wind-induced rock transport comparatively common on Titan. Nearby Bonnie Claire Playa also provides field insights into the interpretation of remote sensing data from Titan. [Copyright &y& Elsevier]

Abstract: The Cassini Titan Radar Mapper is providing an unprecedented view of Titan’s surface geology. Here we use Synthetic Aperture Radar (SAR) image swaths (Ta–T30) obtained from October 2004 to December 2007 to infer the geologic processes that have shaped Titan’s surface. These SAR swaths cover about 20% of the surface, at a spatial resolution ranging from ∼350m to ∼2km. The SAR data are distributed over a wide latitudinal and longitudinal range, enabling some conclusions to be drawn about the global distribution of processes. They reveal a geologically complex surface that has been modified by all the major geologic processes seen on Earth – volcanism, tectonism, impact cratering, and erosion and deposition by fluvial and aeolian activity. In this paper, we map geomorphological units from SAR data and analyze their areal distribution and relative ages of modification in order to infer the geologic evolution of Titan’s surface. We find that dunes and hummocky and mountainous terrains are more widespread than lakes, putative cryovolcanic features, mottled plains, and craters and crateriform structures that may be due to impact. Undifferentiated plains are the largest areal unit; their origin is uncertain. In terms of latitudinal distribution, dunes and hummocky and mountainous terrains are located mostly at low latitudes (less than 30°), with no dunes being present above 60°. Channels formed by fluvial activity are present at all latitudes, but lakes are at high latitudes only. Crateriform structures that may have been formed by impact appear to be uniformly distributed with latitude, but the well-preserved impact craters are all located at low latitudes, possibly indicating that more resurfacing has occurred at higher latitudes. Cryovolcanic features are not ubiquitous, and are mostly located between 30° and 60° north. We examine temporal relationships between units wherever possible, and conclude that aeolian and fluvial/pluvial/lacustrine processes are the most recent, while tectonic processes that led to the formation of mountains and Xanadu are likely the most ancient. [Copyright &y& Elsevier]

Abstract: This paper reports on the analysis of the highest spatial resolution hyperspectral images acquired by the Visual and Infrared Mapping Spectrometer (VIMS) onboard the Cassini spacecraft during its prime mission. A bright area matches a flow-like feature coming out of a caldera-like feature observed in Synthetic Aperture Radar (SAR) data recorded by the Cassini radar experiment [Lopes et al., 2007. Cryovolcanic features on Titan''s surface as revealed by the Cassini Titan Radar Mapper. Icarus 186, 395–412, doi:10.1016/j.icarus.2006.09.006]. In this SAR image, the flow extends about 160km east of the caldera. The contrast in brightness between the flow and the surroundings progressively vanishes, suggesting alteration or evolution of the composition of the cryolava during the lifetime of the eruptions. Dunes seem to cover part of this flow on its eastern end. We analyze the different terrains using the Spectral Mixing Analysis (SMA) approach of the Multiple-Endmember Linear Unmixing Model (MELSUM, Combe et al., 2008). The study area can be fully modeled by using only two types of terrains. Then, the VIMS spectra are compared with laboratory spectra of known materials in the relevant atmospheric windows (from 1 to 2.78μm). We considered simple molecules that could be produced during cryovolcanic events, including H2O, CO2 (using two different grain sizes), CH4 and NH3. We find that the mean spectrum of the cryoflow-like feature is not consistent with pure water ice. It can be best fitted by linear combinations of spectra of the candidate materials, showing that its composition is compatible with a mixture of H2O, CH4 and CO2. [Copyright &y& Elsevier]

Abstract: By comparing observations from the Cassini imaging system, UV spectrometer, and Huygens atmospheric structure instrument, we determine an apparent radius of ∼40 nm, an imaginary index <0.3 at 187.5 nm and a number density of ∼30 particles cm−3 for the detached haze layer at 520 km in Titan''s mesosphere. We point out that the detached haze layer is coincident with a local maximum in the measured temperature profile and show that the temperature maximum is caused by absorption of sunlight in the detached haze layer. This rules out condensation as the source of the layer. The derived particle size is in good agreement with that estimated for the size of the monomers in the aggregate particles that make up the main haze layer. Calculations of the sedimentation velocity of the haze particles coupled with the derived number density imply a mass flux , which is approximately equal to the mass flux required to explain the main haze layer. Because the aerosol size and mass flux derived for the detached layer agree with those determined for the main layer, we suggest that the main haze layer in Titan''s stratosphere is formed primarily by sedimentation and coagulation of particles in the detached layer. This implies that high-energy radical and ion chemistry in the thermosphere is the main source of haze on Titan. [Copyright &y& Elsevier]

The analysis derived from the Cassini SAR imagery reflects the complex Titan's surface morphology with a wide range of backscattering coefficients and peculiar features such as periodic structures and lakelike features, which were observed on July 22, 2006, when polar areas were first imaged, and are considered good candidates to be filled with liquid hydrocarbons. In this paper, the modeling description of lakes is addressed by means of a double-layer model which considers an upper liquid-hydrocarbon layer and a lower layer compatible with the radar response of the neighboring areas. This model is introduced into a Bayesian framework for the purpose of inferring the likely ranges of some parameters and, in particular, of the optical thickness of the hypothesized liquid-hydrocarbon layer and of the wind speed. The main idea is to use the information contained in the parameter probability density function, which describes how probability is distributed among the different values of parameters according to the various scenarios considered. The analysis carried out on lakes and surrounding areas on flybys T16 and T19 determines optical thickness values from 0.2 to 6. For T25 flyby, the inferred values of optical thickness indicate that a limit value of optical thickness may be 9. Considering that, beyond these values, the signal from the bottom layer is completely attenuated, information on the wind speed on the upper layer can be inferred. The found mean values of wind speed are around 0.2-0.3 m/s according to different hypotheses on the upper layer dielectric constant. [ABSTRACT FROM AUTHOR]

The article features Titan, the largest satellite of planet Saturn. A brief historical overview of Titan's discovery is discussed. It explores relevant information brought by the Cassini Huygens mission showing Titan's remarkable Earth-like meteorology. Particular focus is given to Titan's significant atmosphere and landscapes and the resemblance to the Earth's greenhouse climate. It also analyzes the changing cloud patterns of Titan which shows equal basic physical processes as of the Earth.

Titan was once thought to have global oceans of light hydrocarbons on its surface, but after 40 close flybys of Titan by the Cassini spacecraft, it has become clear that no such oceans exist. There are, however, features similar to terrestrial lakes and seas, and widespread evidence for fluvial erosion, presumably driven by precipitation of liquid methane from Titan’s dense, nitrogen-dominated atmosphere. Here we report infrared spectroscopic data, obtained by the Visual and Infrared Mapping Spectrometer (VIMS) on board the Cassini spacecraft, that strongly indicate that ethane, probably in liquid solution with methane, nitrogen and other low-molecular-mass hydrocarbons, is contained within Titan’s Ontario Lacus. [ABSTRACT FROM AUTHOR]

Largest satellite of Saturn and the only in the solar system having a dense atmosphere, Titan is one of the key planetary bodies for astrobiological studies, due to several aspects. (i) Its analogies with planet Earth, in spite of much lower temperatures, with, in particular, a methane cycle on Titan analogous to the water cycle on Earth. (ii) The presence of an active organic chemistry, involving several of the key compounds of prebiotic chemistry. The recent data obtained from the Huygens instruments show that the complex organic matter in Titan’s low atmosphere is mainly concentrated in the aerosol particles. The formation of biologically interesting compounds may also occur in the deep water ocean, from the hydrolysis of complex organic material included in the chrondritic matter accreted during the formation of Titan. (iii) The possible emergence and persistence of Life on Titan. All ingredients which seem necessary for Life to appear and even develop – liquid water, organic matter and energy – are present on Titan. Consequently, it cannot be excluded that life may have emerged on or in Titan. In spite of the extreme conditions in this environment life may have been able to adapt and to persist. Many data are still expected from the Cassini-Huygens mission and future astrobiological exploration mission of Titan are now under consideration. Nevertheless, Titan already looks like another world, with an active organic chemistry, in the absence of permanent liquid water, on the surface: a natural laboratory for prebiotic-like chemistry. [ABSTRACT FROM AUTHOR]