dirty snowball model

The "dirty snowball" model describes comets as small, solid bodies composed of roughly equal parts volatile ices, fine silicate dust, and organic materials. These nuclei formed in the solar nebula billions of years ago through the collision and sticking of dust and ice particles.1

When a comet approaches the Sun, its surface ices sublimate, turning directly from solid to gas. This process releases gas molecules and embedded dust, forming the comet's atmosphere, known as the coma. Due to the comet's weak gravity, this atmosphere escapes into space, often forming two tails: one of ionized molecules and another of dust. The composition of these volatiles is similar to that found in interstellar clouds, suggesting comets are frozen remnants from the early solar system.123