Carbon dioxide clathrate

Carbon dioxide hydrate or carbon dioxide clathrate is a snow-like crystalline substance composed of water ice and carbon dioxide. It normally is a Type I gas clathrate. There has also been some experimental evidence for the development of a metastable Type II phase at a temperature near the ice melting point. The clathrate can exist below 283K (10 °C) at a range of pressures of carbon dioxide. CO₂ hydrates are widely studied around the world due to their promising prospects of carbon dioxide capture from flue gas and fuel gas streams relevant to post-combustion and pre-combustion capture. It is also quite likely to be important on Mars due to the presence of carbon dioxide and ice at low temperatures.

History

The first evidence for the existence of CO₂ hydrates dates back to the year 1882, when Zygmunt Florenty Wróblewski 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 a 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, M. P. 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 (-6 to 10°C). Tamman & Krige measured the hydrate decomposition curve from 253 K down to 230 K in 1925 and Frost & Deaton (1946) determined the dissociation pressure between 273 and 283 K (0 and 10°C). Takenouchi & Kennedy (1965) measured the decomposition curve from 45 bars up to 2 kbar (4.5 to 200 MPa). The CO₂ hydrate was classified as a Type I clathrate for the first time by von Stackelberg & Muller (1954).

Importance

Earth

On Earth, CO₂ hydrate is mostly of academic interest. Tim Collett of the United States Geological Survey (USGS) proposed pumping carbon dioxide into subsurface methane clathrates, thereby releasing the methane and storing the carbon dioxide. As of 2009, ConocoPhillips is working on a trial on the Alaska North Slope with the US Department of Energy to release methane in this way. At first glance, 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. Recently, Professor Praveen Linga and his group in collaboration with ExxonMobil have demonstrated the first-ever experimental evidence of the stability of carbon dioxide hydrate in deep-oceanic sediments.

Mars

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 its polar ice caps much of the time. In the early seventies, the possible existence of CO₂ hydrates on Mars was proposed. Recent consideration of the temperature and pressure of the regolith and of the thermally insulating properties of dry ice and CO₂ clathrate suggested that dry ice, CO₂ clathrate, liquid CO₂, and carbonated groundwater are common phases, even at Martian temperatures.

If CO₂ hydrates are present in the Martian polar caps, as some authors suggest, then the polar cap can potentially melt at depth. Melting of the polar cap would not be possible if it was composed entirely of pure water ice (Mellon ''et al.'' 1996). This is because of the clathrate’s lower thermal conductivity, higher stability under pressure, and higher strength, as compared to pure water ice.

The question of a possible diurnal and annual CO₂ hydrate cycle on Mars remains, since the large temperature amplitudes observed there cause exiting and reentering the clathrate stability field on a daily and seasonal basis. The question is, then, can gas hydrate being deposited on the surface be detected by any means? The OMEGA spectrometer on board Mars Express returned some data, which were used by the OMEGA team to produce CO₂ and H₂O-based images of the South polar cap. No definitive answer has been rendered with respect to Martian CO₂ clathrate formation.

The decomposition of CO₂ hydrate is believed to play a significant role in the terraforming processes on Mars, and 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. This is especially true in 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. All these can be melted under certain conditions and result in gully 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 formed the Martian chaotic terrains. Milton (1974) suggested the decomposition of CO₂ clathrate 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 possible cryovolcanism. The surveyed region consisted of 1.5 km-thick-layered deposits covered seasonally by CO₂ frost underlain by H₂O ice and CO₂ hydrate at depths > 10 m. 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. Baker ''et al.'' 1991 suggests 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, or their physical and structural properties, it becomes clear that all the above-mentioned speculations rest on extremely unstable bases.

Moons

On Enceladus decomposition of carbon dioxide clathrate is a possible way to explain the formation of gas plumes.

In Europa (moon), clathrate should be important for storing carbon dioxide. In the conditions of the subsurface ocean in Europa, carbon dioxide clathrate should sink, and therefore not be apparent at the surface.

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. 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). 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

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Clathrate hydrates
Natural gas
Industrial gases