Why are comet nuclei often described as dirty snowballs?

The term "dirty snowball" is a popular description for a comet's nucleus that originated from the "icy conglomerate" model proposed by American astronomer Fred L. Whipple in 1950.1

This model describes the nucleus as a solid body composed of a mixture of volatile ices—primarily water ice, along with carbon monoxide, carbon dioxide, methanol, and other compounds—embedded with fine silicate dust and organic materials.32

The "dirty" aspect refers to the significant presence of these rocky dust grains and organic compounds mixed throughout the ice. As a comet approaches the Sun, solar heating causes the ices to sublimate, releasing gas and carrying the embedded dust into space to form the comet's coma and tails. Some comets may contain such a high proportion of this dust and organic material that they are described as having more "dirt" than ice.32