Carbon dioxide clathrate

Carbon dioxide hydrate is a Type I gas clathrate (Sloan 1998). However, there has been some experimental evidence for the development of a metastable Type II phase at temperature near the ice melting point (Fleyfel and Devlin 1990, Staykova "et al." 2003).

Some history

As a matter of fact, probably the first evidence for the existence of CO₂ hydrates dates back to the year 1882, when Wróblewski (1882a, b and c) reported clathrate formation while studying carbonic acid. He noted that gas hydrate was a white material resembling snow and could be formed by raising the pressure above certain limit in his H₂O - CO₂ system. He was the first to estimate the CO₂ hydrate composition, finding it to be approximately CO₂·8H₂O. He also mentions that "...the hydrate is only formed either on the walls of the tube, where the water layer is extremely thin or on the free water surface... "(from French)" This already indicates the importance of the surface available for reaction, i.e. the larger the surface the better. Later on in 1894, Villard deduced the hydrate composition as CO₂·6H₂O. Three years later, he published the hydrate dissociation curve in the range 267 K to 283 K (Villard 1897). Tamman & Krige (1925) measured the hydrate decomposition curve from 253 K down to 230 K and Frost & Deaton (1946) determined the dissociation pressure between 273 and 283 K. Takenouchi & Kennedy (1965) measured the decomposition curve from 45 bars up to 2 kbar (4.5 to 200 MPa). For the first time the CO₂ hydrate was classified as a Type I clathrate by von Stackelberg & Muller (1954).

Importance

Here on Earth CO₂ hydrate is almost only of academic interest. It has been proposed to deposit atmospheric carbon dioxide in the form of clathrate on the ocean floor. On first sight it seems that the thermodynamic conditions there favor the existence of hydrates. Yet given that the pressure is created by sea water rather than by CO₂, the hydrate will decompose. Fact|date=October 2007

However, it is believed that CO₂ clathrate might be of significant importance for planetology. CO₂ is an abundant volatile on Mars. It dominates in the atmosphere and covers the polar ice caps much of the time. In the early seventies, the possible existence of CO₂ hydrates on Mars was proposed (Miller & Smythe 1970). Recent consideration of the temperature and pressure of the regolith and of the thermally insulating properties of dry ice and CO₂ clathrate (Ross and Kargel, 1998) suggested that dry ice, CO₂ clathrate, liquid CO₂, and carbonated groundwater are common phases even at Martian temperatures (Lambert and Chamberlain 1978, Hoffman 2000, Kargel "et al." 2000).

If CO₂ hydrates are present in the Martian polar caps, as some authors suggest (e.g. Clifford "et al." 2000, Nye "et al." 2000, Jakosky "et al." 1995, Hoffman 2000), then the cap will not melt as readily as it would if consisting only of water ice. This is because of the clathrate’s lower thermal conductivity, higher stability under pressure and higher strength (Durham 1998), compared to pure water ice.

The question of a possible diurnal and annual CO₂ hydrate cycle on Mars also stays, since the large temperature amplitudes observed there cause leaving and reentering the clathrate stability field on daily and seasonal basis. The question is can the gas hydrate be detected by any means, being deposited on the surface. Probably yes, probably no. The OMEGA spectrometer on board Mars Express returned some data, which were used by the OMEGA team to produce images of the south polar cap, as it was visible in terms of CO₂ and H₂O. No clearcut answer has been found yet.

The decomposition of CO₂ hydrate is believed to play a significant role in the terra-forming processes on Mars. Many of the observed surface features are partly attributed to it. For instance, Musselwhite "et al." (2001) argued that the Martian gullies had been formed not by liquid water but by liquid CO₂ since the present Martian climate does not allow liquid water existence on the surface in general. Especially this is true for the southern hemisphere where most of the gully structures occur. However, water can be present there as ice Ih, CO₂ hydrates or hydrates of other gases (e.g. Max & Clifford 2001, Pellenbarg "et al." 2003). All these can be melted under certain conditions and result in the gullies formation. There might also be liquid water at depths > 2 km under the surface (see geotherms in the phase diagram). It is believed that the melting of ground-ice by high heat fluxes has formed the Martian chaotic terrains (Mckenzie & Nimmo 1999). Milton (1974) suggested the decomposition of CO₂ clathrate had caused rapid water outflows and formation of chaotic terrains. Cabrol "et al." (1998) proposed that the physical environment and the morphology of the south polar domes on Mars suggest for possible cryovolcanism. The surveyed region consisted of 1.5 km-thick-layered deposits covered seasonally by CO₂ frost (Thomas "et al." 1992) underlain by H₂O ice and CO₂ hydrate at depths > 10 m (Miller and Smythe, 1970). When the pressure and the temperature are raised above the stability limit, clathrate is decomposed into ice and gases, resulting in explosive eruptions.

Still a lot more examples of the possible importance of the CO₂ hydrate on Mars can be given. One thing remains unclear: is it really possible to form hydrate there? Kieffer (2000) suggests no significant amount of clathrates could exist near the surface of Mars. Stewart & Nimmo (2002) find it is extremely unlikely that CO₂ clathrate is present in the Martian regolith in quantities that would affect surface modification processes. They argue that long term storage of CO₂ hydrate in the crust, hypothetically formed in an ancient warmer climate, is limited by the removal rates in the present climate. Other authors (e.g. Baker "et al." 1991) suggest that, if not today, at least in the early Martian geologic history the clathrates may have played an important role for the climate changes there. Since not too much is known about the CO₂ hydrates formation and decomposition kinetics, their physical and structural properties, it becomes clear that all the above mentioned speculations rest on extremely unstable basis.

Phase diagram

The hydrate structures are stable at different pressure-temperature conditions depending on the guest molecule. Here is given one Mars-related phase diagram of CO₂ hydrate, combined with those of pure CO₂ and water (Genov 2005). CO₂ hydrate has two quadruple points: (I-Lw-H-V) ("T" = 273.1 K; "p" = 12.56 bar or 1.256 MPa) and (Lw-H-V-LHC) ("T" = 283.0 K; "p" = 44.99 bar or 4.499 MPa) (Sloan, 1998). CO₂ itself has a triple point at "T" = 216.58 K and "p" = 5.185 bar (518.5 kPa) and a critical point at "T" = 304.2 K and "p" = 73.858 bar (7.3858 MPa). The dark gray region (V-I-H) represents the conditions at which CO₂ hydrate is stable together with gaseous CO₂ and water ice (below 273.15 K). On the horizontal axes the temperature is given in kelvins and degrees Celsius (bottom and top respectively). On the vertical ones are given the pressure (left) and the estimated depth in the Martian regolith (right). The horizontal dashed line at zero depth represents the average Martian surface conditions. The two bent dashed lines show two theoretical Martian geotherms after Stewart & Nimmo (2002) at 30° and 70° latitude.

References

*Baker, V. R., Strom, R. G., Gulicvk, V. C., Kargel, J. S., Komatsu, G. & Kale, V. S. (1991) Ancient oceans, ice sheets and the hydrological cycle on Mars. Nature 352, pp. 589-599
*Cabrol, N.A., Grin, E.A., Landheim, R. & McKay, C.P. (1998) Cryovolcanism as a possible origin for pancake-domes in the Mars 98 landing site area: relevance for climate reconstruction and exobiology exploration. In Lunar and Planetary Science XXIX, Lunar and Planetary Institute, Houston, Texas, abstract No. 1249.
*Clifford, S., "et al." (2000) The state and future of Mars polar science and exploration. Icarus 144, pp. 210–242.
*Durham W. B. (1998) Factors affecting the rheologic properties of Martian polar ice. In First International Conference on Mars Polar Science ICMPS, Houston, Texas, abstract No. 3024
*Fleyfel, F. and Devlin, J. P. (1990) Carbon Dioxide Clathrate Hydrate Epitaxial Growth: Spectroscopic Evidence for Formation of the Simple Type-II CO₂ Hydrate. J. Phys. Chem. 95, pp. 3811-3815
*Frost, E. M. & Deaton, W. M. (1946) Gas hydrate composition and equilibrium data. Oil and Gas Journal, 45, pp. 170-178
*Genov, G. Y. (2005) Physical processes of CO2 hydrate formation and decomposition at conditions relevant to Mars. Ph. D. Thesis, University of Göttingen.
*Hoffman, N. (2000) White Mars: A new model for Mars’ surface and atmosphere based on CO₂. Icarus, 146, pp. 326–342.
*Jakosky, B., B. Henderson, & M. Mellon (1995) Chaotic obliquity and the nature of the Martian climate. J. Geophys. Res., 100, pp. 1579 – 1584
*Kargel, J.S., Tanaka, K.L., Baker, V.R., Komatsu, G. & MacAyeal, D.R., 2000, Formation and dissociation of clathrate hydrates on Mars: Polar caps, northern plains, and highlands. In Lunar and Planetary Science XXX, Lunar and Planetary Institute, Houston, Texas, abstract No. 1891
*Kieffer, H. H. (2000) Clathrates Are Not the Culprit. Science, 287, 5459, pp. 1753-1754
*Lambert, R.S. & Chamberlain, V.E. (1978) CO₂ permafrost and Martian topography. Icarus, 34, p. 568–580.
*Max, M. D. & Clifford, S. M. (2001) Initiation of Martian outflow channels: Related to the dissociation of gas hydrate? G. Res. Lett. 28, 9, pp. 1787-1790
*Mckenzie, D. & Nimmo, F. (1999) The generation of Martian floods by the melting of ground ice above dykes. Nature, 397, pp. 231-233.
*Miller S. L. & Smythe W. D. (1970) Carbon Dioxide Clathrate in the Martian Ice Cap, Science 170, pp 531-533
*Milton, D.J. (1974) Carbon Dioxide Hydrate and Floods on Mars. Science, 183, pp. 654-656.
*Musselwhite D. S., Swindle T. D. & Lunine J. I. (2001) Liquid CO2 breakout and the formation of recent small gullies on Mars. . In Lunar and Planetary Science XXX, Lunar and Planetary Institute, Houston, Texas, abstract No. 1030
*Nye, J., Durham, W., Schenk, P. & Moore J. (2000) The instability of a south polar cap on Mars composed of carbon dioxide. Icarus 144, pp. 449–455.
*Pellenbarg, R. E., Max, M. D. & Clifford, S. M. (2003) Methane and carbon dioxide hydrates on Mars: Potential origins, distribution, detection, and implications for future in situ resource utilization. J. G. R. - planet, 108, E4, pp. 23-1 – 23-5.
*Ross, R.G. & Kargel, J.S. (1998) Thermal conductivity of solar system ices, with special reference to Martian polar caps. in Schmitt, B., "et al.", eds., Solar system ices: Dordrecht, Netherlands, Kluwer Academic Publishers, p. 33–62.
*Sloan E. D., Jr. (1998) Clathrate hydrates of natural gases. Second edition, Marcel Dekker Inc.:New York.
*Staykova, D.K., Kuhs, W.F., Salamatin, A.N. & Hansen, Th. (2003) Formation of porous gas hydrates from ice powders: Diffraction experiments and multistage model. J. Phys. Chem. B, 107, 10299- 10311.
*Stewart, S. T. & Nimmo, F. (2002)Surface runoff features on Mars: Testing the carbon dioxide formation hypothesis. J. Geoph. Res. 107, E9, pp. 5069
*Takenouchi, S. & Kennedy, G. C. (1965) Dissociation pressures of the phase CO₂·5 ¾ H₂O. J. Geology, 73, pp. 383-390
*Tamman, G. & Krige, G. J. (1925) Equilibrium pressures of gas hydrates. Zeit. Anorg. Und Algem. Chem., 146, pp. 179-195 (Original language German)
*Villard, M., P. (1894) On the carbonic hydrate and the composition of gas hydrates. Acad. Sci. Paris, Comptes rendus, 119, pp. 368-371 (Original language French)
*Thomas, P., K. Herkenhoff, A. Howard, B. Murray & S. Squyres (1992) Polar deposits on Mars. In Mars, pp. 767–795. Univ. of Arizona Press, Tucson.
*Villard, M., P. (1897) Experimental study of gas hydrates. Ann. Chim. Phys. (7), 11, pp. 353-360 (Original language French)
*Von Stackelberg, M. & Müller, H. R. (1954) Feste Gashydrate II. Structur und Raumchemie. Z. Electrochem. 58, 25-39.
*Wroblewski, S. (1882a) On the combination of carbonic acid and water. Acad. Sci. Paris, Comptes rendus, 94, pp. 212-213 (Original language French)
*Wroblewski, S. (1882b) On the composition of the hydrate of the carbonic acid. Acad. Sci. Paris, ibid., pp. 954-958 (Original language French)
*Wroblewski, S. (1882c) On the laws of solubility of the carbonic acid in water at high pressures. Acad. Sci. Paris, ibid., pp. 1355-1357 (Original language French)