The nucleus of a comet is the central portion of the head of a comet. It is a solid part of the comet, made of a special sort of dust which is called "fluffy" because it could be as light weight and full of holes as a sponge. The holes of this "sponge" are filled with ices like water, carbon dioxide (dry ice), and carbon monoxide (what comes out of your car).
Observations of the nuclei of comet Hale-Bopp and Hyakutake have given scientists fresh ideas about comet composition and evolution. But, scientists still don't know whether the nucleus is very hard, like solid ground, or very soft and breakable, like a snowball. The Rosetta mission hopes to land a probe on the surface of a comet to find out just how hard it is!
As a comet approaches the Sun, it begins to evaporate, forming the coma and a spectacular comet tail. This picture shows that evaporation may happen only in specific places on the nucleus. These spots of evaporation are called "jets". Halley's comet had three distinct jets on its surface as it approached the Sun in 1986.
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