Comet nucleus

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The nucleus of Comet Tempel 1. Tempel 1 (PIA02127).jpg
The nucleus of Comet Tempel 1.

The nucleus is the solid, central part of a comet, formerly 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 sublime 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] This is blacker than coal, and may be caused by a covering of dust. [2]

Contents

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. [3] [4] Further, the ALICE spectrograph on Rosetta determined that electrons (within 1 km (0.62 mi) 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. [5] [6] On 30 July 2015, scientists reported that the Philae spacecraft, that landed on comet 67P/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. [7] [8] [9]

Paradigm

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. [10] 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. [11] [12] [13] [14]

"Dirty snowball"

Beginning in the 1950s, Fred Lawrence Whipple published his "icy conglomerate" model. [15] [16] 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).[ citation needed ] 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. [17] 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. [18] [19]

"Icy dirtball"

It has been a long time since comet nuclei could be imagined as frozen snowballs. [20] 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. [21] [22] [23] [24] The term dust mantling has been in common use since more than 35 years. [25]

The Halley results exceeded even these- comets are not merely dark, but among the darkest objects in the Solar System [26] 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. [27] The nucleus was still assumed to be majority-ice, [18] perhaps overwhelmingly so. [19]

Modern theory

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 further to human understanding.

Densities were confirmed as quite low, ~0.6 g cm3. Comets were highly porous, [28] and fragile on micro- [29] and macro-scales. [30]

Refractory-to-ice ratios are much higher, [31] at least 3:1, [32] possibly ~5:1, [33] ~6:1, [34] [25] or more. [35] [36] [37]

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. [35]

Manx comets, Damocloids, and active asteroids demonstrate that there may be no bright line separating the two categories of objects.

Origin

The Helix Nebula has a cometary Oort cloud Comets Kick up Dust in Helix Nebula (PIA09178).jpg
The Helix Nebula has a cometary Oort cloud

Comets, or their precursors, formed in the outer Solar System, possibly millions of years before planet formation. [38] 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. [39] [40] 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. [41] [42] [43]

Astronomers think that comets originate in the Oort cloud, the scattered disk, [44] and the outer Main Belt. [45] [46] [47]

Size

Size and color comparison of the largest known comets, including dwarf planet Pluto and natural satellites Mimas and Phobos for scale. Largest comets size comparison.png
Size and color comparison of the largest known comets, including dwarf planet Pluto and natural satellites Mimas and Phobos for scale.
Tempel 1 and Hartley 2 compared Tempel 1 Hartley 2 comparison.jpg
Tempel 1 and Hartley 2 compared

Most cometary nuclei are thought to be no more than about 16 kilometers (10 miles) across. [48] The largest comets that have come inside the orbit of Saturn are 95P/Chiron (≈200 km), C/2002 VQ94 (LINEAR) (≈100 km), Comet of 1729 (≈100 km), Hale–Bopp (≈60 km), 29P (≈60 km), 109P/Swift–Tuttle (≈26 km), and 28P/Neujmin (≈21 km).

The potato-shaped nucleus of Halley's comet (15 × 8 × 8 km) [48] [49] 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) [50] of the nucleus of Halley's Comet. [48] Borrelly's nucleus was also potato-shaped and had a dark black surface. [48] 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 PIA20119-CometChristensen-C2006W3-CO2-WISE-20100420.jpg
C/2006 W3 (Chistensen) – emitting carbon gas

The nucleus of comet Hale–Bopp was estimated to be 60 ± 20 km in diameter. [51] 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. [52]

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 (523727) 2014 NW65 (≈220 km).

Known comets have been estimated to have an average density of 0.6 g/cm3. [53] Below is a list of comets that have had estimated sizes, densities, and masses.

NameDimensions
km		Density
g/cm3		Mass
kg [54]
Halley's Comet 15 × 8 × 8 [48] [49] 0.6 [55] 3×1014
Tempel 1 7.6×4.9 [56] 0.62 [53] 7.9×1013
19P/Borrelly 8×4×4 [50] 0.3 [53] 2×1013
81P/Wild 5.5×4.0×3.3 [57] 0.6 [53] 2.3×1013
67P/Churyumov–Gerasimenko See article on 67P0.4 [58] (1.0±0.1)×1013 [59]

Composition

It was once thought that water-ice was the predominant constituent of the nucleus. [60] In the dirty snowball model, dust is ejected when the ice retreats. [61] 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. [48] 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. [62] [63] Our understanding has evolved more toward mostly rock; [64] recent estimates show that water is perhaps only 20-30% of the mass in typical nuclei. [65] [66] [61] Instead, comets are predominantly organic materials and minerals. [67] Data from Churyumov-Gerasimenko and Arrokoth, and laboratory experiments on accretion, suggest refractories-to-ices ratios less than 1 may not be possible. [68]

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. [69] [70]

Organics

"Missing Carbon" [71] [72]

Structure

Surface of the nucleus of Comet 67P from 10 km away as seen by Rosetta spacecraft NAVCAM top 10 at 10 km - 7 (15763681495).jpg
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. [73] 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. [73] [74]

Fragment B of Comet 73P/Schwassmann-Wachmann 3 disintegrating, as seen by the Hubble Space Telescope Schwassman-Wachmann3-B-HST.gif
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. [75] 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, [76] the Rosetta mission dispelled the idea that comets are "rubble piles" of disparate material. [77] [78] [ dubious – discuss ] The Rosetta mission indicated that comets may be "rubble piles" of disparate material. [79] Data were not conclusive concerning the collisional environment during the formation and right afterwards. [80] [81] [82]

Splitting

The nucleus of some comets may be fragile, a conclusion supported by the observation of comets splitting apart. [48] Splitting comets include 3D/Biela in 1846, Shoemaker–Levy 9 in 1992, [83] and 73P/Schwassmann–Wachmann from 1995 to 2006. [84] Greek historian Ephorus reported that a comet split apart as far back as the winter of 372–373 BC. [85] Comets are suspected of splitting due to thermal stress, internal gas pressure, or impact. [86]

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. [87]

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, [88] and Deep Space 1 discovered that Comet Borrelly's surface reflects only 2.5–3.0% of the light that falls on it; [88] 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. [89] Two near-Earth asteroids with albedos this low include 14827 Hypnos and 3552 Don Quixote.[ dubious – discuss ]

Discovery and exploration

The first relatively close mission to a comet nucleus was space probe Giotto. [90] This was the first time a nucleus was imaged at such proximity, coming as near as 596 km. [90] The data was a revelation, showing for the first time the jets, the low-albedo surface, and organic compounds. [90] [91]

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. [90] Halley was calculated to be ejecting three tonnes of material per second [92] from seven jets, causing it to wobble over long time periods. [2] Comet Grigg–Skjellerup's nucleus was visited after Halley, with Giotto approaching 100–200 km. [90]

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. [3] [4] Further, the ALICE spectrograph on Rosetta determined that electrons (within 1 km (0.62 mi) 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. [5] [6]

Tempel 1 (PIA02127).jpg
StardustTemple1.jpg
Comet Borrelly Nucleus.jpg
Wild2 3.jpg
Comet Hartley 2 (super crop).jpg
Comet 67P on 19 September 2014 NavCam mosaic.jpg
Tempel 1
Deep Impact		Tempel 1
Stardust		 Borrelly
Deep Space 1		 Wild 2
Stardust		 Hartley 2
Deep Impact		 C-G
Rosetta

Comets already visited are:

See also

Related Research Articles

<span class="mw-page-title-main">Comet</span> Natural object in space that releases gas

A comet is an icy, small Solar System body that warms and begins to release gases when passing close to the Sun, a process called outgassing. This produces an extended, gravitationally unbound atmosphere or coma surrounding the nucleus, and sometimes a tail of gas and dust gas blown out from the coma. These phenomena are due to the effects of solar radiation and the outstreaming solar wind plasma acting upon the nucleus of the comet. Comet nuclei range from a few hundred meters to tens of kilometers across and are composed of loose collections of ice, dust, and small rocky particles. The coma may be up to 15 times Earth's diameter, while the tail may stretch beyond one astronomical unit. If sufficiently close and bright, a comet may be seen from Earth without the aid of a telescope and can subtend an arc of up to 30° across the sky. Comets have been observed and recorded since ancient times by many cultures and religions.

<span class="mw-page-title-main">Halley's Comet</span> Short-period comet visible every 75–79 years

Halley's Comet, Comet Halley, or sometimes simply Halley, officially designated 1P/Halley, is a short-period comet visible from Earth every 75–79 years. Halley is the only known short-period comet that is regularly visible to the naked eye from Earth, and thus the only naked-eye comet that can appear twice in a human lifetime. It last appeared in the inner parts of the Solar System in 1986 and will next appear in mid-2061.

<i>Giotto</i> (spacecraft) Halleys comet fly-by space mission

Giotto was a European robotic spacecraft mission from the European Space Agency. The spacecraft flew by and studied Halley's Comet and in doing so became the first spacecraft to make close up observations of a comet. On 13 March 1986, the spacecraft succeeded in approaching Halley's nucleus at a distance of 596 kilometers. It was named after the Early Italian Renaissance painter Giotto di Bondone. He had observed Halley's Comet in 1301 and was inspired to depict it as the star of Bethlehem in his painting Adoration of the Magi in the Scrovegni Chapel.

<i>Rosetta</i> (spacecraft) European orbiter sent to study a comet

Rosetta was a space probe built by the European Space Agency launched on 2 March 2004. Along with Philae, its lander module, Rosetta performed a detailed study of comet 67P/Churyumov–Gerasimenko (67P). During its journey to the comet, the spacecraft performed flybys of Earth, Mars, and the asteroids 21 Lutetia and 2867 Šteins. It was launched as the third cornerstone mission of the ESA's Horizon 2000 programme, after SOHO / Cluster and XMM-Newton.

<span class="mw-page-title-main">67P/Churyumov–Gerasimenko</span> Periodic contact binary comet

67P/Churyumov–Gerasimenko is a Jupiter-family comet, originally from the Kuiper belt, with a current orbital period of 6.45 years, a rotation period of approximately 12.4 hours and a maximum velocity of 135,000 km/h. Churyumov–Gerasimenko is approximately 4.3 by 4.1 km at its longest and widest dimensions. It was first observed on photographic plates in 1969 by Soviet astronomers Klim Ivanovych Churyumov and Svetlana Ivanovna Gerasimenko, after whom it is named. It most recently came to perihelion on 2 November 2021, and will next come to perihelion on 9 April 2028.

<span class="mw-page-title-main">46P/Wirtanen</span> Periodic comet with 5 year orbit

46P/Wirtanen is a small short-period comet with a current orbital period of 5.4 years. It was the original target for close investigation by the Rosetta spacecraft, planned by the European Space Agency, but an inability to meet the launch window caused Rosetta to be sent to 67P/Churyumov–Gerasimenko instead. It belongs to the Jupiter family of comets, all of which have aphelia between 5 and 6 AU. Its diameter is estimated at 1.4 kilometres (0.9 mi). In December 2019, astronomers reported capturing an outburst of the comet in substantial detail by the TESS space telescope.

<i>Philae</i> (spacecraft) Robotic European Space Agency lander that accompanied the Rosetta spacecraft

Philae was a robotic European Space Agency lander that accompanied the Rosetta spacecraft until it separated to land on comet 67P/Churyumov–Gerasimenko, ten years and eight months after departing Earth. On 12 November 2014, Philae touched down on the comet, but it bounced when its anchoring harpoons failed to deploy and a thruster designed to hold the probe to the surface did not fire. After bouncing off the surface twice, Philae achieved the first-ever "soft" (nondestructive) landing on a comet nucleus, although the lander's final, uncontrolled touchdown left it in a non-optimal location and orientation.

Timeline of <i>Rosetta</i> (spacecraft)

Rosetta is a space probe designed to rendezvous with the comet 67P/Churyumov–Gerasimenko, perform flybys of two asteroids, and carry lander Philae until its landing on 67P. This page records a detailed timeline of this mission.

<span class="mw-page-title-main">Coma (comet)</span> Cloud of gas or a trail around a comet or asteroid

The coma is the nebulous envelope around the nucleus of a comet, formed when the comet passes near the Sun in its highly elliptical orbit. As the comet warms, parts of it sublimate; this gives a comet a diffuse appearance when viewed through telescopes and distinguishes it from stars. The word coma comes from the Greek κόμη (kómē), which means "hair" and is the origin of the word comet itself.

<span class="mw-page-title-main">Bow shock</span> Boundary between a magnetosphere and an ambient magnetized medium

In astrophysics, a bow shock occurs when the magnetosphere of an astrophysical object interacts with the nearby flowing ambient plasma such as the solar wind. For Earth and other magnetized planets, it is the boundary at which the speed of the stellar wind abruptly drops as a result of its approach to the magnetopause. For stars, this boundary is typically the edge of the astrosphere, where the stellar wind meets the interstellar medium.

Comet dust refers to cosmic dust that originates from a comet. Comet dust can provide clues to comets' origin. When the Earth passes through a comet dust trail, it can produce a meteor shower.

<span class="mw-page-title-main">Gerhard Schwehm</span>

Gerhard Schwehm is Head of Solar System Science Operations Division for the European Space Agency (ESA). He was Mission Manager for the Rosetta mission until his retirement.

<span class="mw-page-title-main">Extinct comet</span> Comet that lacks typical activity

An extinct comet is a comet that has expelled most of its volatile ice and has little left to form a tail and coma. In a dormant comet, rather than being depleted, any remaining volatile components have been sealed beneath an inactive surface layer.

Eberhard Grün is a German planetary scientist who specialized in cosmic dust research. He is an active emeritus at the Max Planck Institute for Nuclear Physics (MPIK), Heidelberg (Germany), research associate at the Laboratory for Atmospheric and Space Physics (LASP) in Boulder (Colorado), and was a professor at the University of Heidelberg until his retirement in 2007. Eberhard Grün has had a leading role in international cosmic dust science for over 40 years.

The Micro-Imaging Dust Analysis System (MIDAS) is one of several instruments on the European Space Agency's Rosetta mission which studied in-situ the environment around the active comet 67P/Churyumov–Gerasimenko as it flew into the inner Solar System. MIDAS is an atomic force microscope (AFM) designed to collect dust particles emitted from the comet, and then scan them with a very sharp needle-like tip to determine their 3D structure, size and texture with very high resolution.

<span class="mw-page-title-main">Hanna von Hoerner</span> German astrophysicist and physicist (1942-2014)

Hanna von Hoerner was a German astrophysicist. She founded the company von Hoerner & Sulger which produces scientific instruments, notably cosmic dust analyzers used on space missions by the European Space Agency (ESA) and NASA.

CAESAR (spacecraft) Proposed sample-return mission to a comet

CAESAR is a sample-return mission concept to comet 67P/Churyumov–Gerasimenko. The mission was proposed in 2017 to NASA's New Frontiers program mission 4, and on 20 December 2017 it was one of two finalists selected for further concept development. On 27 June 2019, the other finalist, the Dragonfly mission, was chosen instead.

Chimera is a NASA mission concept to orbit and explore 29P/Schwassmann-Wachmann 1 (SW1), an active, outbursting small icy body in the outer Solar System. The concept was developed in response to the 2019 NASA call for potential missions in the Discovery-class, and it would have been the first spacecraft encounter with a Centaur and the first orbital exploration of a small body in the outer Solar System. The Chimera proposal was ranked in the first tier of submissions, but was not selected for further development for the programmatic reason of maintaining scientific balance.

<span class="mw-page-title-main">Space dust measurement</span> Space dust measurements

Space dust measurement refers to the study of small particles of extraterrestrial material, known as micrometeoroids or interplanetary dust particles (IDPs), that are present in the Solar System. These particles are typically of micrometer to sub-millimeter size and are composed of a variety of materials including silicates, metals, and carbon compounds. The study of space dust is important as it provides insight into the composition and evolution of the Solar System, as well as the potential hazards posed by these particles to spacecraft and other space-borne assets. The measurement of space dust requires the use of advanced scientific techniques such as secondary ion mass spectrometry (SIMS), optical and atomic force microscopy (AFM), and laser-induced breakdown spectroscopy (LIBS) to accurately characterize the physical and chemical properties of these particles.

<span class="mw-page-title-main">Dust astronomy</span> Branch of astronomy

Dust astronomy is a subfield of astronomy that uses the information contained in individual cosmic dust particles ranging from their dynamical state to its isotopic, elemental, molecular, and mineralogical composition in order to obtain information on the astronomical objects occurring in outer space. Dust astronomy overlaps with the fields of Planetary science, Cosmochemistry, and Astrobiology.

References

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