The nucleus is the solid, central part of a comet, popularly termed a dirty snowball. A cometary nucleus is composed of rock, dust, and frozen gases. When heated by the Sun, the gases sublimate and produce an atmosphere surrounding the nucleus known as the coma. The force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the Sun. A typical comet nucleus has an albedo of 0.04.[1]
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The nucleus of some comets may be fragile, a conclusion supported by the observation of comets splitting apart.[2] Splitting comets include 3D/Biela in 1846, Shoemaker–Levy 9 in 1992,[3] and 73P/Schwassmann–Wachmann from 1995 to 2006.[4] Greek historian Ephorus reported that a comet split apart as far back as the winter of 372–373 BC.[5] Comets are suspected of splitting due to thermal stress, internal gas pressure, or impact.[6]
Comets 42P/Neujmin and 53P/Van Biesbroeck appear to be fragments of a parent comet. Numerical integrations have shown that both comets had a rather close approach to Jupiter in January 1850, and that, before 1850, the two orbits were nearly identical.[7]
Most cometary nuclei are thought to be no more than about 10 miles (16 kilometers) across.[2] The largest comets that have come inside the orbit of Saturn are Hale–Bopp (~60 km), 29P (~30.8 km), 109P/Swift–Tuttle (~26 km), and 28P/Neujmin (~21.4 km).
The potato-shaped nucleus of Halley's comet (15 × 8 × 8 km)[2][8] contains equal amounts of ice and dust. About 80 percent of the ice is water ice, and frozen carbon monoxide makes up another 15 percent. Much of the remainder is frozen carbon dioxide, methane, and ammonia.[2] Scientists believe that other comets are chemically similar to Halley's Comet. The nucleus of Halley's Comet is also extremely dark black. Scientists believe that the surface of the comet, and perhaps most other comets, is covered with a black crust of dust and rock that covers most of the ice. These comets release gas only when holes in this crust rotate toward the sun, exposing the interior ice to the warming sunlight.
During a flyby in 2001, the Deep Space 1 spacecraft observed the nucleus of Comet Borrelly and found it to be about half the size (8×4×4 km)[9] of the nucleus of Halley's Comet.[2] Borrelly's nucleus was also potato-shaped and had a dark black surface.[2] Like Halley's Comet, Comet Borrelly only released gas from small areas where holes in the crust exposed the ice to sunlight.
The nucleus of comet Hale–Bopp was estimated to be 60 ± 20 km in diameter.[10] Hale-Bopp appeared bright to the unaided eye because its unusually large nucleus gave off a great deal of dust and gas.
The nucleus of P/2007 R5 is probably only 100–200 meters in diameter.[11]
The largest centaurs (unstable, planet crossing, icy asteroids) are estimated to be 250 km to 300 km in diameter. Three of the largest would include 10199 Chariklo (258 km), 2060 Chiron (230 km), and the currently lost 1995 SN55 (~300 km).
Known comets have been estimated to have an average density of 0.6 g/cm3.[12] Below is a list of comets that have had estimated sizes, densities, and masses.
| Name | Dimensions km |
Density g/cm3 |
Mass kg[13] |
|---|---|---|---|
| Halley's Comet | 15 × 8 × 8[2][8] | 0.6[14] | 3×1014 |
| Tempel 1 | 7.6×4.9[15] | 0.62[12] | 7.9×1013 |
| 19P/Borrelly | 8×4×4[9] | 0.3[12] | 2×1013 |
| 81P/Wild | 5.5×4.0×3.3[16] | 0.6[12] | 2.3×1013 |
Comets are often described as "dirty snowballs", though recent observations have revealed dry dusty or rocky surfaces, suggesting that the ices are hidden beneath the crust. It has been suggested that comets should be referred to as "Icy dirtballs".[1] Cometary nuclei are among the darkest objects known to exist in the solar system. The Giotto probe found that Comet Halley's nucleus reflects approximately 4% of the light that falls on it,[17] and Deep Space 1 discovered that Comet Borrelly's surface reflects only 2.5–3.0% of the light that falls on it;[17] by comparison, asphalt reflects 7% of the light that falls on it. It is thought that complex organic compounds are the dark surface material. Solar heating drives off volatile compounds leaving behind heavy long-chain organics that tend to be very dark, like tar or crude oil. The very darkness of cometary surfaces allows them to absorb the heat necessary to drive their outgassing.
Roughly six percent of the near-earth asteroids are thought to be extinct nuclei of comets (see Extinct comets) which no longer experience outgassing.[18] Two near-earth asteroids with albedos this low include 14827 Hypnos and 3552 Don Quixote.
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