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Old Gas, New Gas

Methane—made and taken apart by microbes, in the Earth, by people

Roald Hoffmann

Methane Hydrates

Under pressure and low temperature, methane (which normally boils at -161 degrees Celsius) forms a thermodynamically stable association with water. These solids are called methane hydrates, examples of a broader class of structures, the clathrates.

What's stable at one temperature and pressure may not be at another. Under ambient conditions at sea level, methane is a gas, water a liquid. But in the permafrost and deep at sea, the weak hydrogen bonds between water molecules reinforce the still weaker forces between CH4 and H2O to create an aggregate made of a water cage around one or more methane molecules.

Methane hydrates are white solids, less dense than water. They remain on the seafloor only because they are agglomerated with rocks and mud. (There, opportunistic evolution has led a variety of species to use the methane in situ, as a carbon and energy source.) Under the weight of 1,000 meters of ocean, methane hydrate is stable to about 12 degrees, and because the seafloor is colder than that, the ice-like hydrates form spontaneously wherever methane is available. Brought up to the surface the hydrates fall apart to methane and water.

In a previous issue of American Scientist (May-June 2001), Robert L. Kleinberg and Peter G. Brewer looked at how gas hydrate deposits might be exploited. Current guesses of the quantity of methane contained in hydrates are around 1016 cubic meters—exceeding by a factor of 100, roughly, our estimates of "normal" natural gas resources. Could they be mined? Not easily. Much of that methane hydrate is tied up in inaccessible clays and pores. Moreover, one would have to get it out very carefully, as methane is a most effective greenhouse gas. The existing atmospheric burden of methane from natural-gas leaks, cows and termites is consequential enough.

(An aside: In 2003, as part of its adherence to the Kyoto protocol, the government of New Zealand proposed a flatulence tax on its 54 million sheep and cattle. New Zealand also has a population of approximately 4 million people, but belching and farting ruminants are responsible for roughly half of the country's greenhouse-gas burden. The proposal was withdrawn after strong opposition from farmers, but it still has supporters.)

One more lesson from the methane hydrates, part of my ongoing struggle against the seductive forces of simplicity: The structure at right, a dodecahedron of water molecules called 512 (referring to its twelve pentagonal faces), is only one building block of the methane hydrates. The three most common hydrate structures contain repeating units of 46, 136 and 34 water molecules, some making up the 512 cavities. But the common structures also incorporate polyhedra with four-membered and six-membered hydrogen-bonded water rings of substantially greater complexity, for instance 51268 and 435663.

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