Comets are made of several distinct parts: first of all, there is a solid snowball, called nucleus, made of dust and ice. When the comet comes near the Sun, the nucleus heats up and becomes active, causing volatile gas to sublime. The released gas and dust form a cloud, or coma, and the dust element of the tail.
Let's see in detail the different parts of a comet :


  • The nucleus is the relatively solid and stable centrally located part, mostly formed of ice and gas with a small amount of dust and other solids like hydrocarbons. The sizes of cometary nuclei are mostly unknown since measuring them is a difficult task, but the few reliable measurements that have been done, show diameters that range from a few to 20 km.
    The composition of the nucleus is believed to be made by two parts: an interior structure embedded in a mantle.
    About the interior structure not much is known: the dominant volatile is water, followed by CO and CO2. Other minor species are believed to be present in low percentage ( <1% ). The relative abundance of the species, however varies among comets. For example, the CO/H2O ratio, which is an indication of the composition, reached 0.2 to 0.3 in Hale-Bopp but is typically 4 or 5 times smaller in other comets.
    The upper layers of the nucleus (the mantle) contain non-volatile dust, consisting of some silicate minerals and carbon rich grains. The ratio of volatile mass to refractory mass is probably near 1.
  • The coma is a dense atmosphere surrounding the nucleus, made of a cloud of water, carbon dioxide and other neutral gases as well as dust grains. It is formed when the nucleus is heated by the sun, making the gases sublimate, and is later swept into the elongated tails and a huge but very sparse hydrogen cloud (that can measure up to millions of km in diameter)
  • The dust tail is the most prominent part of a comet: it can be up to 10 million km long, composed of smoke-sized dust particles driven off the nucleus by escaping gases;
  • the ion tail is composed of plasma and laced with rays and streamers caused by interactions with the solar wind, which can be as much as several hundred million km long.



Back to the
general dictionary

Back to
current issue