||The solid, centrally located part of a comet. The nucleus is a mass of dust and frozen
gases. When heated by the Sun, the gases sublimate and produce an atmosphere surrounding
the nucleus known as the coma, which is later swept into an elongated tail. Reliable
measurements of cometary nuclei indicate sizes from a few km to 10 or 20 km. The nucleus
of Comet Hale-Bopp is one of the largest (perhaps 40 km). The composition of the nucleus
is determined by measuring the composition of the coma (except for 67P/Churyumov-Gerasimenko).
The dominant volatile is water, followed by carbon dioxide, carbon monoxide, formaldehyde,
methanol, methane at a few percent level (with respect to water) and many other molecules
at a lower level.