Ice That Burns. How Does a Cage of Water Trap Methane?
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Water molecules can freeze into cages that hold a gas molecule inside each one, producing a solid that looks like ice, burns when lit and holds more gas than seems possible.
What the structure is
The compound consists of a host substance whose molecules form a rigid framework enclosing cavities, with molecules of a second substance trapped inside those cavities. Nothing is chemically bonded between host and guest. The guest simply cannot get out, and the framework only holds its shape because the guest is there propping it open. Water is the commonest host, forming cages of hydrogen-bonded molecules, and the guest may be methane, carbon dioxide, hydrogen or various other small molecules.
Where they form naturally
The conditions required are cold and high pressure together:
- •Deep ocean sediments on continental margins worldwide
- •Beneath permafrost in the Arctic
- •In the interiors of comets and outer solar system bodies
- •Wherever methane rises into cold water under pressure
- •The stability zone is a band, bounded above and below
- •Below it the sediment is too warm and the gas escapes
The quantities involved
One reason these attract so much attention is how much gas they concentrate. A unit volume of the solid releases well over one hundred times its own volume of methane at surface pressure, which is why a lump can be lit and will burn while melting into a puddle. Global estimates of how much carbon is held this way have varied enormously and remain uncertain by an order of magnitude, and even conservative figures are comparable with all conventional fossil fuel reserves combined. That has produced sustained interest in extracting them and sustained difficulty in doing so economically.
The useful versions
Trapping one substance inside a framework built by another is a technique as well as a nuisance, and several applications exploit it deliberately. Carbon dioxide forms these structures more readily than methane does under some conditions, which has been proposed as a way of storing it in sediments while releasing the methane already there, exchanging one gas for the other. Desalination has been attempted by forming them from seawater, since salt is excluded from the cage and the remaining brine can be drained off. Hydrogen storage in cages has been investigated for the same reason, as a way of holding a gas at manageable pressures.
Why engineers and climate scientists both worry
The same property causes trouble in two quite separate contexts. In oil and gas production, these solids form inside pipelines where cold high pressure gas meets water, and a plug can block a line completely or be ejected at speed, which is a serious hazard and consumes a great deal of chemical inhibitor to prevent. Attempts to cap a deepwater well in 2010 failed initially because a containment dome filled with them. In climate science, the concern is that warming oceans and thawing permafrost could release methane currently locked away, though how fast and how much reaches the atmosphere is heavily debated.
The takeaway
A rigid cage of water molecules holds a gas molecule in each cavity without any chemical bond, and the cage collapses if the guest leaves. The solid holds over a hundred times its own volume of methane, so it burns while melting. It forms in cold high pressure conditions in ocean sediments and under permafrost, blocks gas pipelines, and is a debated climate risk.