Digital Vision./Photodisc/Getty Images
Comets are often described as “dirty snowballs,” a nickname that reflects their composition of frozen volatiles and refractory dust. The core, or nucleus, is a compact mixture of ice and dust. The ice component can be largely water, but substantial fractions of carbon dioxide, methane, and ammonia are also common. The dust is rich in organic compounds—such as the amino acid glycine—as well as metals (iron), clays, carbonates, and silicates.
When a comet resides in the outer reaches of the Solar System, its nucleus is largely intact. As it swings closer to the Sun, solar radiation heats the ices, turning them into gas. Some of the dust adheres to the nucleus, forming a protective crust, while in regions with thinner dust the released gases escape into space, creating the coma—a diffuse envelope around the nucleus.
Interaction with the solar wind produces two distinct tails that always point away from the Sun. The plasma tail, composed of ionized particles, is longer and straighter, whereas the dust tail is shorter, curved, and made of solid grains.
