Final answer:
Comets are called "dirty snowballs" because they are made up of ice and dust, including hydrocarbons and silicates, which vaporize and form a gas head and tail when near the Sun.
Step-by-step explanation:
Comets are sometimes referred to as "dirty snowballs" because they are composed of both icy and dusty matter. When a comet comes near the Sun, the increase in temperature causes some of its icy material to vaporize, which releases both gas forming a large head and often a tail. The dirt referred to in the dirty snowball model consists of dark, primitive hydrocarbons and silicates, similar to materials found on dark, primitive asteroids.
The composition of comets was described by Whipple in 1950 and has been confirmed by various spacecraft studies, including the ESA Rosetta mission. However, comet dust, being fluffy and inconsequential, usually disintegrates before reaching Earth's surface, unlike more substantial fragments from asteroids.