Comet nucleus

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. "Flying sandbank" 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 Raymond 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. "Dirty snowball" Beginning in the 1950s, Fred Lawrence 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. "Icy dirtball" 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 ) were derived. The nucleus was still assumed to be majority-ice, and fragile on micro- and macro-scales. Refractory-to-ice ratios are much higher, at least 3:1, possibly ~5:1, ~6:1, This is a full reversal from the dirty snowball model. The Rosetta science team has coined the term "mineral organices," for minerals and organics with a minor fraction of ices. Manx comets, Damocloids, and active asteroids demonstrate that there may be no bright line separating the two categories of objects. == Origin ==

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. Astronomers think that comets originate in the Oort cloud, the scattered disk, and the outer Main Belt. == Size ==

Pluto and natural satellites Mimas and Phobos for scale Most cometary nuclei are thought to be no more than about 16 kilometers (10 miles) across. contains equal amounts of ice and dust. 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. 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. == Composition ==

image of Comet Read, the first comet in the main asteroid belt to have water vapor detected around it. Unlike most comets, Comet Read has no detectable carbon dioxide. 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. Our understanding has evolved more toward mostly rock; recent estimates show that water is perhaps only 20-30% of the mass in typical nuclei. Data from Churyumov–Gerasimenko and Arrokoth, and laboratory experiments on accretion, suggest refractories-to-ices ratios less than 1 may not be possible. 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. Organics Rosetta found that around 40% of Churyumov–Gerasimenko nucleus is organic molecules. Philae spacecraft, that landed on comet Churyumov–Gerasimenko in November 2014, detected at least 16 organic compounds, of which four (including acetamide, acetone, methyl isocyanate and propionaldehyde) were detected for the first time on a comet. == Structure ==

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. Measurements carried out by the Philae lander on 67P/Churyumov–Gerasimenko comet, indicate that the dust layer could be as much as 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 results of Rosetta mission show that not all cometary nucleus are "rubble piles". Data were not conclusive concerning the collisional environment during the formation and right afterwards. Splitting 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. == Albedo ==

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, 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. Roughly six percent of the near-Earth asteroids are thought to be extinct nuclei of comets (see Extinct comets) which no longer experience outgassing. Two near-Earth asteroids with albedos this low include 14827 Hypnos and 3552 Don Quixote. == Discovery and exploration ==

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. 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. 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. Results from the Rosetta and Philae spacecraft show that the nucleus of 67P/Churyumov–Gerasimenko has no magnetic field, which suggests that magnetism may not have played a role in the early formation of planetesimals. Further, the ALICE spectrograph on Rosetta determined that electrons (within above the comet nucleus) produced from photoionization of water molecules by solar radiation, and not photons from the Sun as thought earlier, are responsible for the degradation of water and carbon dioxide molecules released from the comet nucleus into its coma. Comets already visited are: • Halley's Comet • 26P/Grigg–Skjellerup • Tempel 1 (also hit with impactor) • 19P/Borrelly • 81P/Wild • 103P/Hartley • C/2013 A1 (Siding Spring) – unplanned encounter with Mars spacecraft • 67P/Churyumov–Gerasimenko (also landed on) == See also ==