Carbon dioxide clathrate

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Zygmunt Florenty Wróblewski

CO₂ 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).

[edit] 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 – 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. For the first time the CO₂ hydrate was classified as a Type I clathrate by von Stackelberg & Muller (1954).

[edit] Importance

In this mosaic taken by the Mars Global Surveyor: Aram Chaos - top left and Iani Chaos - bottom right. A river-bed-like outflow channel can be seen, originating from Iani Chaos and extending towards the top of the image.

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.^{[ citation needed ]}

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.

Changing gully deposits on Mars

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.

[edit] Phase diagram

CO₂ hydrate phase diagram. The black squares show experimental data (after Sloan, 1998 and references therein). The lines of the CO₂ phase boundaries are calculated according to the Intern. thermodyn. tables (1976). The H₂O phase boundaries are only guides to the eye. The abbreviations are as follows: L - liquid, V - vapor, S - solid, I - water ice, H - hydrate.

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) and (Lw-H-V-LHC) (T = 283.0 K; p = 44.99 bar) (Sloan, 1998). CO₂ itself has a triple point at T = 216.58 K and p = 5.185 bar and a critical point at T = 304.2 K and p = 73.858 bar. 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 Kelvin and 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^o and 70^o latitude.

[edit] References

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