Comets and comet clouds form early in the life span of a solar system, and are primarily composed of frozen gases, rocks and dust, coalescing out of clouds of smaller particles into the cosmic bodies with trademark tails we're familiar with. (science.howstuffworks.com)
Bold Idea: Extract frozen gas from permafrost or from the ocean floor to power new, ultraefficient turbines. (discovermagazine.com)
Interior Astronomers hoped the probe would confirm that comets fit the popular model of a «dirty snowball»: an icy core made up of a solid mix of water ice, dirt, and frozen gases like carbon dioxide and carbon monoxide, coated with a thin crust of debris. (discovermagazine.com)