Why are coments called the dirty snowballs?

Comets are often referred to as "dirty snowballs" because they are composed primarily of ice, dust, and rocky material. When they approach the Sun, the heat causes the ice to vaporize, releasing gas and dust that form a glowing coma and tail. This combination of icy components and solid debris creates the analogy of a snowball that has become "dirty" due to the presence of non-icy materials.