Methane is widespread in the Universe, and its occurrence is intimately connected with that of water, often as clathrate hydrate, likely the priority form in which methane is stored in icy moons, water-rich exoplanets but also in the depths of Earth’s oceans. Arrangement and stability range of the crystalline structures, decomposition conditions, and miscibility of the resulting dense fluid mixtures are crucial for modeling the static and dynamic properties of these complex extraterrestrial environments and for identifying possible prebiotic reactive events under transient favorable conditions of pressure, temperature, and irradiation. Here, we report a high-pressure study of methane hydrate up to 4 GPa and 550 K. Ex situ synthesis of crystalline methane hydrate allowed the analysis of homogeneous samples by state of the art Raman and FTIR spectroscopy, accessing information which considerably expands and modifies our knowledge of the crystalline structures, of the decomposition conditions, and of the molten fluid’s characteristics in a wide pressure and temperature range.
Among the ice mixtures that can be found in our Universe, those involving ethylene are poorly studied even though ethylene reportedly exists in the presence of water in several astrochemical domains. Here we report about the chemistry of ethylene and water mixtures in both pressure (0-15 GPa) and temperature (300-370 K) ranges relevant to celestial bodies conditions. The behaviour of the binary mixture has been tracked starting from the ethylene clathrate hydrate and following its evolution through two different crystalline phases up to 2.10 GPa, where it decomposes into a solid mixture of water ice and crystalline ethylene. The pressure and temperature evolution of this mixture has been studied up to the complete transformation of ethylene into polyethylene and compared with that of the pure hydrocarbon, reporting here for the first time its spectroscopic features upon compression. The spectroscopic analysis of the recovered polymers from the ice mixtures provided hints about the reactivity of the monomer under the environmental stress exerted by the water network. The results of this study are expected to be significant in a variety of fields ranging from astrochemistry to material science and also to fundamental chemistry, particularly regarding the study and modelisation of the behaviour of complex mixtures.
The structural evolution with pressure of icy mixtures of simple molecules is a poorly explored field despite the fundamental role they play in setting the properties of the crustal icy layer of the outer planets and of their satellites. Water and ammonia are the two major components of these mixtures, and the crystal properties of the two pure systems and of their compounds have been studied at high pressures in a certain detail. On the contrary, the study of their heterogeneous crystalline mixtures whose properties, due to the strong N–H⋯O and O–H⋯N hydrogen bonds, can be substantially altered with respect to the individual species has so far been overlooked. In this work, we performed a comparative Raman study with a high spatial resolution of the lattice phonon spectrum of both pure ammonia and water–ammonia mixtures in a pressure range of great interest for modeling the properties of icy planets’ interiors. Lattice phonon spectra represent the spectroscopic signature of the molecular crystals’ structure. The activation of a phonon mode in plastic NH3-III attests to a progressive reduction in the orientational disorder, which corresponds to a site symmetry reduction. This spectroscopic hallmark allowed us to solve the pressure evolution of H2O–NH3–AHH (ammonia hemihydrate) solid mixtures, which present a remarkably different behavior from the pure crystals likely to be ascribed to the role of the strong H-bonds between water and ammonia molecules characterizing the crystallites’ surface.
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