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The tides of Venus

Methane hydrates – evil under the Antarctic or a force for good in the lab?

A study last week suggested that there could be up to 400 billion metric tonnes of methane under the Antarctic ice sheet.

The concern this study flagged up was the effect on the climate such a large amount of methane could have should it begin to leach out. Though the catastrophic outpouring of the methane in the near future seems unlikely, the study pointed out that that even small fluctuations in temperature and pressure of the ice sheet could cause an outpouring that current climate models don’t take account of.

And that’s where my interest (aside from the stability of the climate!) comes in. Methane hydrates are sometimes known as ‘fire ice’. When you pressurise methane and water and cool it down, the water forms a cage which traps the methane molecules, known as a clathrate structure.

This is a host-guest structure, with the water ‘host’ molecules containing the methane ‘guest’ molecules. To contain methane with water in this, delicate, structure requires temperatures to be below 25°C and the molecules to be buried by about 300m of sediment or ice.

This is why they are a continuing concern for deep sea hydrocarbon exploration. Moving the methane hydrates out of this setting, either by warming them up or taking them out of their buried environment, causes the water cage to destabilise and the methane to be released as a gas.

Methane hydrate, is often known as ‘fire ice’

This latest study isn’t the first to warn of the effect that the release of methane from methane hydrates can have on our climate. There’s some evidence that a warming of the ocean in the geological past caused a massive release of methane into the atmosphere.

This, affectionately termed an ‘ocean fart’, caused a large scale change in the carbon chemistry in the atmosphere and could have accelerated the warming in ancient times.

Humphry Davy (a rather interesting character who wrote romantic poetry aside from being a leading chemist) discovered clathrate materials in 1810, when he pressurised water with chlorine gas. Since then we have discovered that these structures form in a number of gases: Nitrogen, Argon even hydrogen to name a few.

There are also a number of ways that the water forms a cage about the gas molecule it’s hosting. These differ in the size and variety of cages that they contain. The research into these has opened up a number of intriguing possibilities of how we can engineer and store a lot of gas in a small amount of space.

Humphry Davy, a man of many talents. Engraving from about 1830, based on a portrait by Sir Thomas Lawrence (1769 - 1830)

But the problem remains that you need quite a bit of pressure or very cold temperatures to form these materials, and that’s not too possible for long term storage. There’s much work underway trying to understand how we can form these materials at surface pressure and room temperature.

Last year’s work at the European Synchrotron suspended droplets of a solution in a stream of air and used natural cooling from evaporation, adding a little bit of water to form their clathrate structures. Using the beam from the synchrotron, the researchers could monitor how the materials formed from the atomic scale.

A high speed video of the clathrate forming from a droplet, taken for the European Synchrotron study. Sarfraz et. al. Chemical Communications (2011)

My big hope, and that of many other scientists, is that really understanding how this super simple and elegant material forms may help efforts towards gas storage. Hopefully methane hydrates can be part of the solution, and not just part of the problem.

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