Old Gas, New Gas
Methane—made and taken apart by microbes, in the Earth, by people
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 The proposal was withdrawn after strong opposition from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.