What are these cristalline systems made of?
Clathrate hydrates are crystalline materials consisting of guest molecules surrounded by water molecules forming a cage-like structure such as an “ice-like” hydrogen bond host network. Specifically, the nanometer scale cages are formed from polygonal rings of water molecules connected at their edges by means of hydrogen bonds. According to structural analyses , each oxygen atom of the host sub-structure is surrounded by four equiprobable (half occupied) hydrogen sites, so that the hydrogen atoms of the aqueous sub-matrix are dynamically disordered. The most stable water ring structure is a pentagon (examples of cages are shown in the Figure). The arrangement of such three-dimensional crystalline cage-like structures greatly depends on the nature of the encapsulated molecule and its dynamics, which has the property to stabilize the clathrate edifice. Clathrate hydrates may encapsulate more than one component, and are in such case referred to as “mixed” or “mixed guests” clathrate hydrates. Under suitable thermodynamic conditions, clathrates hydrates crystallize into three major types of structure: two cubic structures termed type I and type II and one hexagonal structure termed type H.
Representation of types I, II and H structures adopted by clathrate hydrates (the water’s oxygen atoms are located at the corner of the polygone and the H-bonds are contained at the edge). The 5^12 cage refers to a dodecahedron since this cavity is constituted of 12 pentagonal faces.
Where are they?
Clathrate hydrates are found on Earth in deep ocean seafloors along continental margins, on subsurface layers of soil that remain frozen during the year (permafrost), and ice cores. For marine systems, the gas (mainly methane) hydrate stability zone begins below 300-600 m of water depth (where 100 m water column is approximately equivalent to 10 bar), while for permafrost systems the stability zone generally occurs around 100–300 m depth. Hydrates have been recovered mainly from deep sea drilling and sediment coring, but also have been studied in situ using, for instance, a sea-going Raman spectrometer.
Beyond our own planet, the combination of water-ice rich extraterrestrial bodies, appropriated temperatures and very low pressures suggests that clathrate hydrates may have taken part in the assemblage of the building blocks of many bodies that form now the solar system. Various volatiles could be involved in pure or mixed hydrate stable structures, formed from shallow depths to the deepest interiors of icy planetary bodies, or even possibly at the surface on condition that the protective atmosphere generates compatible pressure-temperature conditions (e.g., on Saturn’s largest moon Titan). The nature of the guest molecules in clathrate hydrates from cometary and planetary origin likely resemble those detected in comet tails and planetary outgassing plumes (such as Enceladus), respectively, or those that make up their icy bulk and that are detected by infrared spectroscopy. Comparison of clathrate hydrates predicted stability fields with the derived P-T conditions of astrophysical objects suggests that such structures could be present on comets and in the Martian permafrost. They may also exist mixed with water ice in the outer icy shell of Saturn’s icy moons (Europa, Enceladus, Titan), and/or within the high pressure ice layer of Jupiter’s moon (Callisto, Ganymede), and that of Titan. In addition, clathrate hydrates are expected to occur within the undifferentiated rock-ice mixture on Callisto, and on the dwarf planet Pluto. Besides, clathrate hydrates formation and subsequent gas trapping could explain the general enrichments in volatiles in both Jupiter and Saturn.
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