Oxygen clathrate
Oxygen clathrate or oxygen hydrate is a clathrate hydrate consisting of a water-ice framework with regular crystalline cavities that contain oxygen (O2) molecules.[1] It has been proposed to occur naturally in the surface and near-surface ices of oxygen-atmosphere icy moons such as Europa and Ganymede, where the oxygen is thought to originate from radiolysis of water ice by charged particles.[1]
Structure
[edit]At low pressure, oxygen clathrate adopts the cubic CS-II (also called sII) clathrate structure, with two distinct cage types: 16 smaller dodecahedral (12-faced) cages and 8 larger hexakaidecahedral (16-faced) cages per unit cell, first characterized by neutron powder diffraction.[2][3] As with hydrogen clathrate, multiple oxygen molecules can occupy the larger cage, with occupancy increasing from about 1 O2 per large cage at low pressure to as many as 2.8 O2 per cage near 1.4 GPa.[1]
Under pressure at room temperature, oxygen clathrate undergoes a sequence of structural transformations before decomposing into ice and free oxygen:[1]
- CS-II clathrate (stable from ambient pressure to about 1.0 GPa)
- a tetragonal ST clathrate structure (about 1.0–1.2 GPa), also observed in the argon–water and nitrogen–water systems
- a chiral C0 hydrate structure (about 1.2–1.6 GPa), in which oxygen occupies open channels in a water network unrelated to any stable ice phase, also seen in the hydrogen–water and carbon dioxide–water systems
- a filled ice phase isostructural with methane hydrate III (about 1.6–2.6 GPa), in which oxygen occupies channels within a water framework resembling ice Ih
Above 2.6 GPa at room temperature, this filled-ice phase decomposes into separate oxygen and ice VII.[1] This decomposition pressure is low compared to other filled-ice gas hydrates: hydrogen-filled ice remains stable to at least 90 GPa and methane-filled ice to at least 150 GPa.[4][5]
At low temperature (66 K), representative of icy-moon surface conditions, the CS-II clathrate remains stable up to about 1.4 GPa, above which the host lattice undergoes pressure-induced amorphization rather than a further ordered structural transition.[1]
Occurrence and planetary relevance
[edit]Oxygen has been detected spectroscopically on the surface of Ganymede, where it is thought to be held largely as a low-pressure clathrate,[6] and oxygen clathrates have been proposed as a component of subsurface ices on Europa.[7] Because oxygen hydrate is stable to at least 2.6 GPa, it can in principle penetrate to depths within icy moons both above and below any subsurface liquid-water ocean.[1]
Oxygen and hydrogen clathrates share structural similarities, both forming CS-II and C0 phases with multiple guest occupancy of the larger cage.[1] Since both O2 and H2 are produced together by radiolysis of water ice, their preferential retention in close proximity within a shared host lattice has been proposed as a mechanism for enhanced recombination back into water, offering a possible explanation for the discrepancy between modelled and measured rates of oxygen production on Europa.[1]
See also
[edit]References
[edit]- 1 2 3 4 5 6 7 8 9 Frost, Mungo; Kuzovnikov, Mikhail A.; Dalladay-Simpson, Philip; Howie, Ross T.; Loveday, John S.; Ranieri, Umbertoluca; Gregoryanz, Eugene (2025). "Implications of high-pressure oxygen hydrates on radiolytic oxygen in Jovian icy moons". Communications Chemistry. 8 (1): 128. doi:10.1038/s42004-025-01509-y. PMC 12041459.
- ↑ Tse, J.; Handa, Y.; Ratcliffe, C.; Powell, B. (1986). "Structure of oxygen clathrate hydrate by neutron powder diffraction". Journal of Inclusion Phenomena. 4: 235–240. doi:10.1007/BF00657996.
- ↑ Chazallon, B.; Kuhs, W. F. (2002). "In situ structural properties of N2-, O2-, and air-clathrates by neutron diffraction". The Journal of Chemical Physics. 117 (1): 308–320. doi:10.1007/BF00657996.
- ↑ Ranieri, Umbertoluca; Di Cataldo, Simone; Rescigno, Maria; Monacelli, Lorenzo; Gaal, Richard; Santoro, Mario; Andriambariarijaona, Leon; Parisiades, Paraskevas; De Michele, Cristiano; Bove, Livia Eleonora (2023). "Observation of the most H2-dense filled ice under high pressure". Proceedings of the National Academy of Sciences. 120 (52) e2312665120. doi:10.1073/pnas.2312665120. hdl:11573/1702315. PMC 10756306.
- ↑ Schaack, Sofiane; Ranieri, Umbertoluca; Depondt, Philippe; Gaal, Richard; Kuhs, Werner F.; Gillet, Philippe; Finocchi, Fabio; Bove, Livia E. (2019). "Observation of methane filled hexagonal ice stable up to 150 GPa". Proceedings of the National Academy of Sciences. 116 (33): 16204–16209. doi:10.1073/pnas.1904911116. hdl:11573/1415878. PMC 6697897.
- ↑ Spencer, J. R.; Calvin, W. M.; Person, M. J. (1995). "Charge-coupled device spectra of the Galilean satellites: molecular oxygen on Ganymede". Journal of Geophysical Research: Planets. 100 (E9): 19049–19056. doi:10.1029/95JE01503.
- ↑ Hand, K. P.; Chyba, C. F.; Carlson, R. W.; Cooper, J. F. (2006). "Clathrate hydrates of oxidants in the ice shell of Europa". Astrobiology. 6 (3): 463–482. doi:10.1089/ast.2006.6.463.