Search

Abstract Liquid ethane is present in the lakes and seas observed on Titan's surface by the Cassini‐Huygens mission. While interaction between liquid hydrocarbons and water ice is expected to result in the formation of clathrate hydrates, such reaction (and its kinetics) has not yet been demonstrated for cryogenic liquids under relevant planetary conditions. In this paper, we report the first experimental evidence for rapid formation of clathrates upon direct contact of liquid ethane with water ice at 1 bar using micro‐Raman spectroscopy. Kinetics experiments conducted in the temperature range 150–173 K yield an activation energy of 14.8 ± 2.2 kJ/mol, which is suggestive of a diffusion‐controlled mechanism for clathrate formation. This implies that a clathrate reservoir can form within seasonal time scales on Titan if liquid ethane comes into contact with a pre‐existing icy bedrock, which may hold important implications for the structure and dynamics of Titan's crust and its global evolution.

Synchrotron radiation facilities provide highly polarized x-ray beams across a wide energy range. However, the exact type and degree of polarization vary according to the beamline and experimental setup. To accurately determine the angle and degree of linear polarization, a portable x-ray polarimeter has been developed. This setup consists of a silicon drift detector that rotates around a target made of high-density polyethylene. The imprint generated in the angular distribution of scattered photons from the target at a 90-degree angle between the incident x-rays and detector has been exploited to determine the beam polarization. Measurements were conducted at the GALAXIES beamline of the SOLEIL synchrotron. The expected angular distribution of the scattered photons for a given beam polarization was obtained through simulations using the Geant4 simulation toolkit. An excellent agreement between simulations and the collected data has been obtained, validating the setup and enabling a precise determination of the beam polarization., (© 2024 Author(s). Published under an exclusive license by AIP Publishing.)

Robust thermodynamic data are essential for the development of geodynamic and geochemical models of ocean worlds. The water-ammonia system is of interest in the study of ocean worlds due to its purported abundance in the outer solar system, geological implications, and potential importance for origins of life. In support of developing new equations of state, we conducted 1 bar specific heat capacity measurements ( C p ) using a differential scanning calorimeter (DSC) at low temperatures (184-314 K) and low mass fractions of ammonia (5.2-26.9 wt %) to provide novel data in the parameter space most relevant for planetary studies. This is the first known set of data with sufficient fidelity to investigate the trend of specific heat capacity with respect to temperature. The obtained C p in the liquid phase domain above the liquidus generally increases with temperature. Deviations of our data from the currently adopted equation of state by Tillner-Roth and Friend[Tillner-Roth R.; Friend D. G.J. Phys. Chem. Ref. Data1998, 27, 63-96]. are generally negative (ranging from +1 to -10%) and larger at lower temperatures. This result suggests that suppression of the critical behavior of supercooled water (rapid increase in specific heat with decreasing temperature) by ammonia starts at a smaller concentration than that set by Tillner-Roth and Friend.[Tillner-Roth R.; Friend D. G.J. Phys. Chem. Ref. Data1998, 27, 63-96]. C p measurements of the liquid were also obtained in the partial melting domain between the eutectic and liquidus. This novel data set will be useful in future investigations of conditions where such partial melt may exist, such as the ice shell-ocean boundary or the interiors of ocean worlds that may contain relatively large proportions of dissolved ammonia., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

The ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) instrument aboard the Rosetta mission revolutionized our understanding of cometary material composition. One of Rosetta's key findings is the complexity of the composition of comet 67P/Churyumov-Gerasimenko. Here, we used ROSINA data to analyze dust particles that were volatilized during a dust event in September 2016 and report the detection of large organosulfur species and an increase in the abundances of sulfurous species previously detected in the coma. Our data support the presence of complex sulfur-bearing organics on the surface of the comet. In addition, we conducted laboratory simulations that show that this material may have formed from chemical reactions that were initiated by the irradiation of mixed ices containing H 2 S. Our findings highlight the importance of sulfur chemistry in cometary and precometary materials and the possibility of characterizing organosulfur materials in other comets and small icy bodies using the James Webb Space Telescope.

Titan, Saturn's largest moon, has a plethora of organic compounds in the atmosphere and on the surface that interact with each other. Cryominerals such as co-crystals may influence the geologic processes and chemical composition of Titan's surface, which in turn informs our understanding of how Titan may have evolved, how the surface is continuing to change, and the extent of Titan's habitability. Previous works have shown that a pyridine:acetylene (1:1) co-crystal forms under specific temperatures and experimental conditions; however, this has not yet been demonstrated under Titan-relevant conditions. Our work here demonstrates that the pyridine:acetylene co-crystal is stable from 90 K, Titan's average surface temperature, up to 180 K under an atmosphere of N 2 . In particular, the co-crystal forms via liquid-solid interactions within minutes upon mixing of the constituents at 150 K, as evidenced by distinct, new Raman bands and band shifts. X-ray diffraction (XRD) results indicate moderate anisotropic thermal expansion (about 0.5-1.1%) along the three principal axes between 90-150 K. Additionally, the co-crystal is detectable after being exposed to liquid ethane, implying stability in a residual ethane "wetting" scenario on Titan. These results suggest that the pyridine:acetylene co-crystal could form in specific geologic contexts on Titan that allow for warm environments in which liquid pyridine could persist, and as such, this cryomineral may preserve the evidence of impact, cryovolcanism, or subsurface transport in surface materials., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

Our recent Communication suggested that ammonia in aqueous solution may preferentially destabilize large cages in methane clathrate hydrates. A Comment favored ammonia incorporation instead, but it did not accurately describe our proposed mechanism and relied primarily on studies conducted in different chemical systems and/or which used other preparation methods.

The incorporation of ammonia inside methane clathrate hydrate is of great interest to the hydrate chemistry community. We investigated the phase behavior of methane clathrate formed from aqueous ammonia solution. Ammonia's presence decreases methane occupancy in the large cages, without definitive Raman spectroscopic evidence for its incorporation inside the structure.

The vibrational signatures for the υ 2 C≡C and υ 1 symmetric C-H stretches of acetylene in cubic structure I clathrate, synthesized under ambient pressure, are reported for the first time. The most diagnostic features are at 1966 for υ 2 and 3353 cm -1 for υ 1 , respectively, and are assigned to acetylene trapped in the large 5 12 6 2 cages. In addition, the υ 2 mode for acetylene occupying the small 5 12 cages is observed at 1972.5 cm -1 , a red shift of 1.5 cm -1 from its gas phase frequency. Unit cell parameters and thermal expansion coefficients are determined via powder X-ray diffraction between 195 and 225 K and are found to be in good correlation with previous single crystal data at 143 K. The calculated density for acetylene clathrate is also reported, with values ranging from 0.985 g/cm 3 at 195 K to 0.976 g/cm 3 at 225 K. These results are relevant for spectral detection of acetylene-containing compounds on planetary bodies, as well as providing additional insights on the thermal behavior and physical properties of acetylene clathrate.

Ice sintering is a form of metamorphism that drives the microstructural evolution of an aggregate of grains through surface and volume diffusion. This leads to an increase in the grain-to-grain contact area ("neck") and density of the aggregate over time, resulting in the evolution of its strength, porosity, thermal conductivity, and other properties. This process plays an important role in the evolution of icy planetary surfaces, though its rate and nature are not well constrained. In this study, we explore the model of Swinkels and Ashby (1981), and assess the extent to which it can be used to quantify sintering timescales for water ice. We compare predicted neck growth rates to new and historical observations of ice sintering, and find agreement to some studies at the order of magnitude level. First-order estimates of neck growth timescales on planetary surfaces show that ice may undergo significant modification over geologic timescales, even in the outer solar system. Densification occurs over much longer timescales, suggesting some surfaces may develop cohesive, but porous, crusts. Sintering rates are extremely sensitive to temperature and grain size, occurring faster in warmer aggregates of smaller grains. This suggests that the microstructural evolution of ices may vary not only throughout the solar system, but also spatially across the surface and in the near-surface of a given body. Our experimental observations of complex grain growth and mass redistribution in ice aggregates point to components of the model that may benefit from improvement, and areas where additional laboratory studies are needed.

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.

The international Rosetta mission was launched in 2004 and consists of the orbiter spacecraft Rosetta and the lander Philae. The aim of the mission is to map the comet 67P/Churyumov-Gerasimenko by remote sensing, and to examine its environment in situ and its evolution in the inner Solar System. Rosetta was the first spacecraft to rendezvous with and orbit a comet, accompanying it as it passes through the inner Solar System, and to deploy a lander, Philae, and perform in situ science on the comet's surface. The primary goals of the mission were to: characterize the comet's nucleus; examine the chemical, mineralogical and isotopic composition of volatiles and refractories; examine the physical properties and interrelation of volatiles and refractories in a cometary nucleus; study the development of cometary activity and the processes in the surface layer of the nucleus and in the coma; detail the origin of comets, the relationship between cometary and interstellar material and the implications for the origin of the Solar System; and characterize asteroids 2867 Steins and 21 Lutetia. This paper presents a summary of mission operations and science, focusing on the Rosetta orbiter component of the mission during its comet phase, from early 2014 up to September 2016.This article is part of the themed issue 'Cometary science after Rosetta'., (© 2017 The Authors.)

Heat transport and ice sublimation in comets are interrelated processes reflecting properties acquired at the time of formation and during subsequent evolution. The Microwave Instrument on the Rosetta Orbiter (MIRO) acquired maps of the subsurface temperature of comet 67P/Churyumov-Gerasimenko, at 1.6 mm and 0.5 mm wavelengths, and spectra of water vapor. The total H2O production rate varied from 0.3 kg s(-1) in early June 2014 to 1.2 kg s(-1) in late August and showed periodic variations related to nucleus rotation and shape. Water outgassing was localized to the "neck" region of the comet. Subsurface temperatures showed seasonal and diurnal variations, which indicated that the submillimeter radiation originated at depths comparable to the diurnal thermal skin depth. A low thermal inertia (~10 to 50 J K(-1) m(-2) s(-0.5)), consistent with a thermally insulating powdered surface, is inferred., (Copyright © 2015, American Association for the Advancement of Science.)

Clathrate hydrates, ice-like crystalline compounds in which small guest molecules are enclosed inside cages formed by tetrahedrally hydrogen-bonded water molecules, are naturally abundant on Earth and are generally expected to exist on icy celestial bodies. A prototypical example is Saturn's moon Titan, where dissociation of methane clathrates, a major crustal component, could contribute significantly to the replenishment of atmospheric methane. Ammonia is an important clathrate inhibiting agent that may be present (potentially at high concentrations) in Titan's interior. In this study, low-temperature Raman experiments are conducted to examine the dissociation point of tetrahydrofuran clathrates, an ambient-pressure analogue of methane clathrates, over a wide range of ammonia concentrations from 0 to 25 wt %. A phase diagram for the H2O-THF-NH3 system is generated, showing two main results: (i) ammonia lowers the dissociation point of clathrate hydrates to a similar extent compared to the melting of water ice and (ii) THF clathrate exhibits a "liquidus-like" behavior in the presence of ammonia, with a eutectic temperature of about 203.6 K. As temperatures higher than this estimated eutectic are anticipated within Titan's icy crust, these results imply that partial dissociation of clathrates can occur readily and may contribute to outgassing from the interior.

We report the first experimental finding of a solid molecular complex between benzene and ethane, two small apolar hydrocarbons, at atmospheric pressure and cryogenic temperatures. Considerable amounts of ethane are found to be incorporated inside the benzene lattice upon the addition of liquid ethane onto solid benzene at 90-150 K, resulting in formation of a distinctive co-crystalline structure that can be detected via micro-Raman spectroscopy. Two new features characteristic of these co-crystals are observed in the Raman spectra at 2873 and 1455 cm(-1), which are red-shifted by 12 cm(-1) from the υ1 (a1g) and υ11 (eg) stretching modes of liquid ethane, respectively. Analysis of benzene and ethane vibrational bands combined with quantum mechanical modeling of isolated molecular dimers reveal an interaction between the aromatic ring of benzene and the hydrogen atoms of ethane in a C-H···π fashion. The most favored configuration for the benzene-ethane dimer is the monodentate-contact structure, with a calculated interaction energy of 9.33 kJ/mol and an equilibrium bonding distance of 2.66 Å. These parameters are comparable to those for a T-shaped co-crystalline complex between benzene and acetylene that has been previously reported in the literature. These results are relevant for understanding the hydrocarbon cycle of Titan, where benzene and similar organics may act as potential hydrocarbon reservoirs due to this incorporation mechanism.

We present new experimental data on the liquidus of ice polymorphs in the H(2)O-NH(3) system under pressure, and use all available data to develop a new thermodynamic model predicting the phase behavior in this system in the ranges (0-2.2 GPa; 175-360 K; 0-33 wt % NH(3)). Liquidus data have been obtained with a cryogenic optical sapphire-anvil cell coupled to a Raman spectrometer. We improve upon pre-existing thermodynamic formulations for the specific volumes and heat capacities of the solid and liquid phase in the pure H(2)O phase diagram to ensure applicability of the model in the low-temperature metastable domain down to 175 K. We compute the phase equilibria in the pure H(2)O system with this new model. Then we develop a pressure-temperature dependent activity model to describe the effect of ammonia on phase transitions. We show that aqueous ammonia solutions behave as regular solutions at low pressures, and as close-to-ideal solutions at pressure above 600 MPa. The computation of phase equilibria in the H(2)O-NH(3) system shows that ice III cannot exist at concentrations above 5-10 wt % NH(3) (depending on pressure), and ice V is not expected to form above 25%-27% NH(3). We eventually address the applications of this new model for thermal and evolution models of icy satellites.

Measurements of the carbon and nitrogen isotopic ratios as well as the detection of 40Ar and 36Ar by the gas chromatograph mass spectrometer (GCMS) instrument on board the Huygens probe have provided key constraints on the origin and evolution of Titan's atmosphere, and indirectly on the evolution of its interior. Those data combined with models of Titan's interior can be used to determine the story of volatile outgassing since Titan's formation. In the absence of an internal source, methane, which is irreversibly photodissociated in Titan's stratosphere, should be removed entirely from the atmosphere in a time-span of a few tens of millions of years. The episodic destabilization of methane clathrate reservoir stored within Titan's crust and subsequent methane outgassing could explain the present atmospheric abundance of methane, as well as the presence of argon in the atmosphere. The idea that methane is released from the interior through eruptive processes is also supported by the observations of several cryovolcanic-like features on Titan's surface by the mapping spectrometer (VIMS) and the radar on board Cassini. Thermal instabilities within the icy crust, possibly favoured by the presence of ammonia, may explain the observed features and provide the conditions for eruption of methane and other volatiles. Episodic resurfacing events associated with thermal and compositional instabilities in the icy crust can have major consequences on the hydrocarbon budget on Titan's surface and atmosphere.

We propose a thermodynamic model of the properties of liquid water and ices I, III, V, and VI that can be used in the ranges of 0-2200 MPa and 180-360 K. This model is the first to be applicable to all H(2)O phases in these wide ranges, which exceed the stability domain of all phases. Developing empirical or semiempirical expressions for the specific volumes of liquid water or ices has been necessary. The model has been tested on available experimental data sets. The specific volume of liquid water is reproduced with an accuracy better than 1%. The error on the specific volume of ices remains within 2%. The model has also been used to describe the melting curves of high-pressure ice polymorphs and compared with new Simon equations fitting available data. Our calculations suggest a slight revision of the triple point positions in the H(2)O phase diagram. We have ensured the reliability of our model up to 1.5 GPa, and we have shown that it can be used with good confidence up to 2.2 GPa. In order to show the validity of this model in the low-temperature domains, the melting curve of ice Ih in the water-ammonia system has been modeled. This curve is reproduced with good accuracy down to 180 K, at a 1 bar pressure. It shows that this model can be used in further studies for modeling equilibriums involving liquid or solid phases of H(2)O under pressure and for investigating the effect of inhibitors in complex water-rich systems.