Comets have two major components -- ice and dust -- which have earned them the nickname "dirty snowballs." They also contain various gases and organic materials, although the composition of the ice can vary. Some ice is made from water, but some is likely formed from substances such as carbon dioxide, methane and ammonia. Studies of comet samples found that the dust contained amino acids such as glycine, as well as iron, clays, carbonates and silicates.

The nucleus of a comet is made up of dust and ice, and that's the entire comet when it's far out in the solar system. As it approaches the sun, the ice begins to take a gaseous form. Some of the dust is left to create a protective coating on the nucleus. In thinner areas, gases break though the dust forming a cloud called the coma. Solar wind emitted by the sun blows the dust and gases into two tails. The plasma tail is longer and straight and is made of electrically charged particles. The dust tail is shorter and curved and made of dust particles. The tails always point away from the sun.