Why is the surface of Halley’s Comet’s nucleus so dark?

The nucleus of Halley’s Comet is exceptionally dark because approximately 70 percent of its surface is covered by an insulating "crust" composed of less-volatile organic compounds and silicates. This dark coating is extremely black—reflecting only about 4 percent of the sunlight it receives—and effectively prevents the water ice beneath it from sublimating.1

While the term "dirty snowball" is often used to describe the nucleus, the dark surface is a result of this surface coating of nonvolatile materials. This layer develops as the comet approaches the Sun; as ices sublimate and escape into space, they carry away smaller particles, leaving behind a lag deposit of larger dust grains that are too heavy to be lifted by the escaping gases. This process creates a thick, dark layer that insulates the interior of the nucleus.123