The presence of clathrates in comet 67P/Churyumov-Gerasimenko

Methanol Formation in Hyperthermal Oxygen Collisions with Methane Clathrate Ice

The presence of small organic molecules at airless icy bodies may be significant for prebiotic chemistry, yet uncertainties remain about their origin. Here, we consider the role of hyperthermal reactive ions in modifying the organic inventory of ice. We employ molecular dynamics using the ReaxFF formalism to simulate bombardment of carbon-bearing ice by hyperthermal water group molecules (HxO, x = 0-2) with kinetic energy between 2 and 58 eV. Methanol is the dominant closed-shell organic product for a CH4 clathrate irradiated at low dose by atomic oxygen. It is produced at yields as high as 10%, primarily by a novel hot-atom reaction mechanism, while radiolysis makes a secondary contribution. At high irradiation doses (≳1.4 × 1015 cm-2), the composition is driven toward greater carbon oxidation states with formaldehyde being favored over methanol production. Other water group impactors are less efficient at inducing chemistry in the ice, and alternate clathrate guest species (CO, CO2) are very robust against hydrogenation.

A nearly terrestrial D/H for comet 67P/Churyumov-Gerasimenko

Cometary comae are a mixture of gas and ice-covered dust. Processing on the surface and in the coma change the composition of ice on dust grains relative to that of the nucleus. As the ice on dust grains sublimates, the local coma composition changes. Rosetta observations of 67P/Churyumov-Gerasimenko previously reported one of the highest D/H values for a comet. However, reanalysis of more than 4000 water isotope measurements over the full mission shows that dust markedly increases local D/H. The isotope ratio measured at a distance from the nucleus where the gas is well mixed is close to terrestrial, like that of other Jupiter family comets. This lower D/H has implications for understanding comet formation and the role of comets in delivering water to Earth.

Encapsulation of charged halogens by the 512 water cage

This study focuses on the encapsulation of the entire series of halides by the 512 cage of twenty water molecules and on the characterization of water to water and water to anion interactions. State-of-the-art computations are used to determine equilibrium geometries, energy related quantities, and thermal stability towards dissociation and to dissect the nature and strength of intermolecular interactions holding the clusters as stable units. Two types of structures are revealed: heavily deformed cages for F- indicating a preference for microsolvation, and slightly deformed cages for the remaining anions indicating a preference for encapsulation. The primary variable dictating the properties of the clusters is the charge density of the central halide, with the most severe effects observed for the F- case. For the remaining halides, the anion may be safely viewed as a sort of "big electron" with little local disruptive power, enough to affect the network of non-covalent hydrogen bonds in the cage, but not enough to break it. Gibbs energies for dissociation either into cavity and halide or into water molecules and halide suggest that, in a similar way as to methane clathrate, a more weakly bonded complex that has been detected in the gas phase, all halide containing clathrate-like structures should be amenable to experimental detection in the gas phase at moderate temperature and pressure conditions.

Three-body aggregation of guest molecules as a key step in methane hydrate nucleation and growth

Gas hydrates have an important role in environmental and astrochemistry, as well as in energy materials research. Although it is widely accepted that gas accumulation is an important and necessary process during hydrate nucleation, how guest molecules aggregate remains largely unknown. Here, we have performed molecular dynamics simulations to clarify the nucleation path of methane hydrate. We demonstrated that methane gather with a three-body aggregate pattern corresponding to the free energy minimum of three-methane hydrophobic interaction. Methane molecules fluctuate around one methane which later becomes the central gas molecule, and when several methanes move into the region within 0.8 nm of the potential central methane, they act as directional methane molecules. Two neighbor directional methanes and the potential central methane form a three-body aggregate as a regular triangle with a distance of ~6.7 Å which is well within the range of typical methane-methane distances in hydrates or in solution. We further showed that hydrate nucleation and growth is inextricably linked to three-body aggregates. By forming one, two, and three three-body aggregates, the possibility of hydrate nucleation at the aggregate increases from 3/6, 5/6 to 6/6. The results show three-body aggregation of guest molecules is a key step in gas hydrate formation.

To be or not to be? that is the entropic, enthalpic, and molecular interaction dilemma in the formation of (water)20 clusters and methane clathrate

A detailed analysis under a comprehensive set of theoretical and computational tools of the thermodynamical factors and of the intermolecular interactions behind the stabilization of a well known set of (water)20 cavities and of the methane clathrate is offered in this work. Beyond the available reports of experimental characterization at extreme conditions of most of the systems studied here, all clusters should be amenable to experimental detection at 1 atm and moderate temperatures since 280 K marks the boundary at which, ignoring reaction paths, formation of all clusters is no longer spontaneous from the 20H2O → (H2O)20 and CH4 + 20H2O → CH4@512 processes. As a function of temperature, a complex interplay leading to the free energy of formation occurs between the destabilizing entropic contributions, mostly due to cluster vibrations, and the stabilizing enthalpic contributions, due to intermolecular interactions and the PV term, is best illustrated by the highly symmetric 512 cage consistently showing signs of stronger intermolecular bonding despite having smaller binding energy than the other clusters. A fluxional wall of attractive non-covalent interactions, arising because of the cumulative effect of a large number of tiny individual charge transfers to the interstitial region, plays a pivotal role stabilizing the CH4@512 clathrate.

Formation of Cubic Ice via Clathrate Hydrate, Prepared in Ultrahigh Vacuum under Cryogenic Conditions

Cubic ice (ice I_c) is a crystalline phase of solid water, which exists in the earth's atmosphere and extraterrestrial environments. We provide experimental evidence that dissociation of acetone clathrate hydrate (CH) makes ice I_c in ultrahigh vacuum (UHV) at 130-135 K. In this process, we find that crystallization of ice I_c occurs below its normal crystallization temperature. Time-dependent reflection absorption infrared spectroscopy (RAIRS) and reflection high-energy electron diffraction (RHEED) were utilized to confirm the formation of ice I_c. Associated crystallization kinetics and activation energy (E_a) for the process were evaluated. We suggest that enhanced mobility or diffusion of water molecules during acetone hydrate dissociation enabled crystallization. Moreover, this finding implied that CHs might exist in extreme low-pressure environments present in comets. These hydrates, subjected to prolonged thermal annealing, transform into ice I_c. This unique process of crystallization hints at a possible mechanistic route for the formation of ice I_c in comets.

Gas hydrates in sustainable chemistry

This review includes the current state of the art understanding and advances in technical developments about various fields of gas hydrates, which are combined with expert perspectives and analyses.

Comparison of neutral outgassing of comet 67P/Churyumov-Gerasimenko inbound and outbound beyond 3 AU from ROSINA/DFMS

CONTEXT

Pre-equinox measurements of comet 67P/Churyumov-Gerasimenko with the mass spectrometer ROSINA/DFMS on board the Rosetta spacecraft revealed a strongly heterogeneous coma. The abundances of major and various minor volatile species were found to depend on the latitude and longitude of the nadir point of the spacecraft. The observed time variability of coma species remained consistent for about three months up to equinox. The chemical variability could be generally interpreted in terms of surface temperature and seasonal effects superposed on some kind of chemical heterogeneity of the nucleus.

AIMS

We compare here pre-equinox (inbound) ROSINA/DFMS measurements from 2014 to measurements taken after the outbound equinox in 2016, both at heliocentric distances larger than 3 AU. For a direct comparison we limit our observations to the southern hemisphere.

METHODS

We report the similarities and differences in the concentrations and time variability of neutral species under similar insolation conditions (heliocentric distance and season) pre- and post-equinox, and interpret them in light of the previously published observations. In addition, we extend both the pre- and post-equinox analysis by comparing species concentrations with a mixture of CO₂ and H₂O.

RESULTS

Our results show significant changes in the abundances of neutral species in the coma from pre- to post-equinox that are indicative of seasonally driven nucleus heterogeneity.

CONCLUSIONS

The observed pre- and post-equinox patterns can generally be explained by the strong erosion in the southern hemisphere that moves volatile-rich layers near the surface.

Clathrate hydrates in interstellar environment

Clathrate hydrates (CHs) are ubiquitous in earth under high-pressure conditions, but their existence in the interstellar medium (ISM) remains unknown. Here, we report experimental observations of the formation of methane and carbon dioxide hydrates in an environment analogous to ISM. Thermal treatment of solid methane and carbon dioxide-water mixture in ultrahigh vacuum of the order of 10^-10 mbar for extended periods led to the formation of CHs at 30 and 10 K, respectively. High molecular mobility and H bonding play important roles in the entrapment of gases in the in situ formed 5¹² CH cages. This finding implies that CHs can exist in extreme low-pressure environments present in the ISM. These hydrates in ISM, subjected to various chemical processes, may act as sources for relevant prebiotic molecules.

Water Reservoirs in Small Planetary Bodies: Meteorites, Asteroids, and Comets

Asteroids and comets are the remnants of the swarm of planetesimals from which the planets ultimately formed, and they retain records of processes that operated prior to and during planet formation. They are also likely the sources of most of the water and other volatiles accreted by Earth. In this review, we discuss the nature and probable origins of asteroids and comets based on data from remote observations, in situ measurements by spacecraft, and laboratory analyses of meteorites derived from asteroids. The asteroidal parent bodies of meteorites formed ≤4 Ma after Solar System formation while there was still a gas disk present. It seems increasingly likely that the parent bodies of meteorites spectroscopically linked with the E-, S-, M- and V-type asteroids formed sunward of Jupiter's orbit, while those associated with C- and, possibly, D-type asteroids formed further out, beyond Jupiter but probably not beyond Saturn's orbit. Comets formed further from the Sun than any of the meteorite parent bodies, and retain much higher abundances of interstellar material. CI and CM group meteorites are probably related to the most common C-type asteroids, and based on isotopic evidence they, rather than comets, are the most likely sources of the H and N accreted by the terrestrial planets. However, comets may have been major sources of the noble gases accreted by Earth and Venus. Possible constraints that these observations can place on models of giant planet formation and migration are explored.

Fast methane diffusion at the interface of two clathrate structures

Methane hydrates naturally form on Earth and in the interiors of some icy bodies of the Universe, and are also expected to play a paramount role in future energy and environmental technologies. Here we report experimental observation of an extremely fast methane diffusion at the interface of the two most common clathrate hydrate structures, namely clathrate structures I and II. Methane translational diffusion—measured by quasielastic neutron scattering at 0.8 GPa—is faster than that expected in pure supercritical methane at comparable pressure and temperature. This phenomenon could be an effect of strong confinement or of methane aggregation in the form of micro-nanobubbles at the interface of the two structures. Our results could have implications for understanding the replacement kinetics during sI–sII conversion in gas exchange experiments and for establishing the methane mobility in methane hydrates embedded in the cryosphere of large icy bodies in the Universe.

Methane dynamics at the interface of ice clathrate structures is expected to play a role in phenomena ranging from gas exchange to methane mobility in planetary cryospheres. Here, the authors observe extremely fast methane diffusion at the interface of the two most common clathrate hydrate structures.

The Rosetta mission orbiter science overview: the comet phase

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'.