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This article needs to be updated. Please help update this article to reflect recent events or newly available information.(July 2020)
The nucleus is the solid, central part of a comet, once termed a dirty snowball or an icy dirtball. 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. This is blacker than coal, and may be caused by a covering of dust.
Comet nuclei, at ~1 km to at times tens of kilometers, could not be resolved by telescopes. Even current giant telescopes would give just a few pixels on target, assuming nuclei were not obscured by comae when near Earth. An understanding of the nucleus, versus the phenomenon of the coma, had to be deduced, from multiple lines of evidence.
The "flying sandbank" model, first proposed in the late-1800s, posits a comet as a swarm of bodies, not a discrete object at all. Activity is the loss of both volatiles, and population members. This model was championed in midcentury by Lyttleton, along with an origin. As the Sun passed through interstellar nebulosity, material would clump in wake eddies. Some would be lost, but some would remain in heliocentric orbits. The weak capture explained long, eccentric, inclined comet orbits. Ices per se were lacking; volatiles were stored by adsorption on grains.
Soon after Lyttleton, Fred Whipple published his "icy conglomerate" model. This was soon popularized as "dirty snowball." Comet orbits had been determined quite precisely, yet comets were at times recovered "off-schedule," by as much as days. Early comets could be explained by a "resisting medium"- such as "the aether", or the cumulative action of meteoroids against the front of the comet(s). But comets could return both early and late. Whipple argued that a gentle thrust from asymmetric emissions (now "nongravitational forces") better explained comet timing. This required that the emitter have cohesive strength- a single, solid nucleus with some proportion of volatiles. Lyttleton continued publishing flying-sandbank works as late as 1972. The death knell for the flying sandbank was Halley's Comet. Vega 2 and Giotto images showed a single body, emitting through a small number of jets.
It has been a long time since comet nuclei could be imagined as frozen snowballs. Whipple had already postulated a separate crust and interior. Before Halley's 1986 apparition, it appeared that an exposed ice surface would have some finite lifetime, even behind a coma. Halley's nucleus was predicted to be dark, not bright, due to preferential destruction/escape of gases, and retention of refractories. The term dust mantling has been in common use since more than 35 years.
The Halley results exceeded even these- comets are not merely dark, but among the darkest objects in the Solar System  Furthermore, prior dust estimates were severe undercounts. Both finer grains and larger pebbles appeared in spacecraft detectors, but not ground telescopes. The volatile fraction also included organics, not merely water and other gases. Dust-ice ratios appeared much closer than thought. Extremely low densities (0.1 to 0.5 g cm-3) were derived. The nucleus was still assumed to be majority-ice, perhaps overwhelmingly so.
Three rendezvous missions aside, Halley was one example. Its unfavorable trajectory also caused brief flybys at extreme speed, at one time. More frequent missions broadened the sample of targets, using more advanced instruments. By chance, events such as the breakups of Shoemaker-Levy 9 and Schwassmann-Wachmann 3 contributed to our understanding.
Densities were confirmed as quite low, ~0.6 g cm3. Comets were highly porous, and fragile on micro- and macro-scales.
Comets, or their precursors, formed in the outer Solar System, possibly millions of years before planet formation. How and when comets formed is debated, with distinct implications for Solar System formation, dynamics, and geology. Three-dimensional computer simulations indicate the major structural features observed on cometary nuclei can be explained by pairwise low velocity accretion of weak cometesimals. The currently favored creation mechanism is that of the nebular hypothesis, which states that comets are probably a remnant of the original planetesimal "building blocks" from which the planets grew.
During a flyby in September 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) of the nucleus of Halley's Comet. Borrelly's nucleus was also potato-shaped and had a dark black surface. Like Halley's Comet, Comet Borrelly only released gas from small areas where holes in the crust exposed the ice to sunlight.
C/2006 W3 (Chistensen) - emitting carbon gas
The nucleus of comet Hale-Bopp was estimated to be 60 ± 20 km in diameter. 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.
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 (?220 km).
Known comets have been estimated to have an average density of 0.6 g/cm3. Below is a list of comets that have had estimated sizes, densities, and masses.
This article needs to be updated. Please help update this article to reflect recent events or newly available information.(July 2020)
It was once thought that water-ice was the predominant constituent of the nucleus. In the dirty snowball model, dust is ejected when the ice retreats. Based on this, about 80% of the Halley's Comet nucleus would be water ice, and frozen carbon monoxide (CO) makes up another 15%. Much of the remainder is frozen carbon dioxide, methane, and ammonia. Scientists think that other comets are chemically similar to Halley's Comet. The nucleus of Halley's Comet is also an extremely dark black. Scientists think 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.
This assumption was shown to be naive, starting at Halley. Coma composition does not represent nucleus composition, as activity selects for volatiles, and against refractories, including heavy organic fractions. Our understanding has evolved more toward mostly rock; recent estimates show that water is perhaps only 20-30% of the mass in typical nuclei. Instead, comets are predominantly organic materials and minerals.
The composition of water vapor from Churyumov-Gerasimenko comet, as determined by the Rosetta mission, is substantially different from that found on Earth. The ratio of deuterium to hydrogen in the water from the comet was determined to be three times that found for terrestrial water. This makes it unlikely that water on Earth came from comets such as Churyumov-Gerasimenko.
Surface of the nucleus of Comet 67P from 10 km away as seen by Rosetta spacecraft
On 67P/Churyumov-Gerasimenko comet, some of the resulting water vapour may escape from the nucleus, but 80% of it recondenses in layers beneath the surface. This observation implies that the thin ice-rich layers exposed close to the surface may be a consequence of cometary activity and evolution, and that global layering does not necessarily occur early in the comet's formation history.
Fragment B of Comet 73P/Schwassmann-Wachmann 3 disintegrating, as seen by the Hubble Space Telescope
Measurements carried out by the Philae lander on 67P/Churyumov-Gerasimenko comet, indicate that the dust layer could be as much as 20 cm (7.9 in) thick. Beneath that is hard ice, or a mixture of ice and dust. Porosity appears to increase toward the center of the comet. While most scientists thought that all the evidence indicated that the structure of nuclei of comets is processed rubble piles of smaller ice planetesimals of a previous generation, the Rosetta mission dispelled the idea that comets are "rubble piles" of disparate material.[dubious – discuss] The Rosetta mission indicated that comets may be "rubble piles" of disparate material. Data are not conclusive concerning the collisional environment during the formation and right afterwards.
The nucleus of some comets may be fragile, a conclusion supported by the observation of comets splitting apart. Splitting comets include 3D/Biela in 1846, Shoemaker-Levy 9 in 1992, and 73P/Schwassmann-Wachmann from 1995 to 2006. Greek historian Ephorus reported that a comet split apart as far back as the winter of 372-373 BC. Comets are suspected of splitting due to thermal stress, internal gas pressure, or impact.
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.
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, and Deep Space 1 discovered that Comet Borrelly's surface reflects only 2.5-3.0% of the light that falls on it; by comparison, fresh 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.
The first relatively close mission to a comet nucleus was space probe Giotto. This was the first time a nucleus was imaged at such proximity, coming as near as 596 km. The data was a revelation, showing for the first time the jets, the low-albedo surface, and organic compounds.
During its flyby, Giotto was hit at least 12,000 times by particles, including a 1-gram fragment that caused a temporary loss of communication with Darmstadt. Halley was calculated to be ejecting three tonnes of material per second from seven jets, causing it to wobble over long time periods.Comet Grigg-Skjellerup's nucleus was visited after Halley, with Giotto approaching 100-200 km.
^ abWood, J (December 1986). Comet nucleus models: a review. ESA Workshop on the Comet Nucleus Sample Return Mission. pp. 123-31.
^ abKresak, L; Kresakova, M (1987). ESA SP-278: Symposium on the Diversity and Similarity of Comets. ESA. p. 739.
^Rickman, H (2017). "2.2.3 Dust Production Rates". Origin and Evolution of Comets: 10 years after the Nice Model, and 1 year after Rosetta. World Scientific Publishing Co Singapore. ISBN978-9813222571. "It has been a long time since comet nuclei could be imagined as frozen snowballs"
^ abRickman, H (2017). "4.2 Dust Mantling". Origin and Evolution of Comets: 10 years after the Nice Model, and 1 year after Rosetta. World Scientific Publishing Co Singapore. ISBN978-9813222571. "the term dust mantling has been in common use since more than 35 years"
^Tholen, D; Cruikshank, D; Hammel, H; Hartmann, W; Lark, N; Piscitelli, J (1986). "A comparison of the continuum colours of P/Halley, other comets and asteroids". ESA SP-250 Vol. III. ESA. p. 503.
^Whipple, F (October 1987). "The Cometary Nucleus - Current Concepts". Astronomy & Astrophysics. 187 (1): 852.
Halley: Using the volume of an ellipsoid of 15x8x8km * a rubble pile density of 0.6 g/cm3 yields a mass (m=d*v) of 3.02E+14 kg.
Tempel 1: Using a spherical diameter of 6.25 km; volume of a sphere * a density of 0.62 g/cm3 yields a mass of 7.9E+13 kg.
19P/Borrelly: Using the volume of an ellipsoid of 8x4x4km * a density of 0.3 g/cm3 yields a mass of 2.0E+13 kg.
81P/Wild: Using the volume of an ellipsoid of 5.5x4.0x3.3 km * a density of 0.6 g/cm3 yields a mass of 2.28E+13 kg.
^Wood, J A (December 1986). "Comet nucleus models: a review.". ESA Proceedings of an ESA workshop on the Comet Nucleus Sample Return Mission. ESA. pp. 123-31. water-ice as the predominant constituent
^ abBischoff, D; Gundlach, B; Neuhaus, M; Blum, J (February 2019). "Experiments on cometary activity: ejection of dust aggregates from a sublimating water-ice surface". Mon. Not. R. Astron. Soc. 483 (1): 1202-10. In the past, it was believed that comets are dirty snowballs and that the dust is ejected when the ice retreats." "...it has become evident that comets have a much higher dust-to-ice ratio than previously thought
^Bockelée-Morvan, D; Biver, N (May 2017). "The composition of cometary ices". Phil. Trans. R. Astron. Soc. A-Mathematics Physics and Engineering Science. 375 (2097). Molecular abundances are measured in cometary atmospheres. The extent to which they are representative of the nucleus composition has been the subject of many theoretical studies.
^A'Hearn, M (May 2017). "Comets: looking ahead". Phil. Trans. R. Astron. Soc. A-Mathematics Physics and Engineering Science. 375 (2097). our understanding has been evolving more toward mostly rock
^Jewitt, D; Chizmadia, L; Grimm, R; Prialnik, D (2007). "Water in the Small Bodies of the Solar System". Protostars and Planets V. University of Arizona Press. pp. 863-78. Recent estimates... show that water is less important, perhaps carrying only 20-30% of the mass in typical nuclei (Sykes et al., 1986).
^Fulle, M; Della Corte, V; Rotundi, A; Green, S; Accolla, M; Colangeli, L; Ferrari, M; Ivanovski, S; Sordini, R; Zakharov, V (2017). "The dust-to-ices ratio in comets and Kuiper belt objects". Mon. Not. R. Ast. Soc. 469: S45-49.
^Filacchione, G; Groussin, O; Herny, C; Kappel, D; Mottola, S; Oklay, N; Pommerol, A; Wright, I; Yoldi, Z; Ciarniello, M; Moroz, L; Raponi, A (2019). "Comet 67P/CG Nucleus Composition and Comparison to Other Comets". Space Science Reviews. 215: Article number 19. a predominance of organic materials and minerals.