Comet nucleus

Source: Wikipedia, the free encyclopedia.

Comet Tempel 1
.

The nucleus is the solid, central part of a

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]

Results from the

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,

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

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

Main article: Accretion (astrophysics) § Accretion of comets
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

Main Belt.[45][46][47]

Size

Mimas and Phobos
for scale.
Tempel 1 and Hartley 2 compared

Most cometary nuclei are thought to be no more than about 16 kilometers (10 miles) across.

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

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.

Name Dimensions
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 67P 0.4[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

Arrokoth, and laboratory experiments on accretion, suggest refractories-to-ices ratios less than 1 may not be possible.[68]

The composition of

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

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

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.

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

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

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

coma.[5][6]
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

References

  1. ^ Robert Roy Britt (29 November 2001). "Comet Borrelly Puzzle: Darkest Object in the Solar System". Space.com. Archived from the original on 22 January 2009. Retrieved 26 October 2008.
  2. ^
    ESA
    . 10 March 2006. Retrieved 22 February 2009.
  3. ^ a b Bauer, Markus (14 April 2015). "Rosetta and Philae Find Comet Not Magnetised". European Space Agency. Retrieved 14 April 2015.
  4. ^
    S2CID 123964604
    .
  5. ^ a b Agle, DC; Brown, Dwayne; Fohn, Joe; Bauer, Markus (2 June 2015). "NASA Instrument on Rosetta Makes Comet Atmosphere Discovery". NASA. Retrieved 2 June 2015.
  6. ^
    S2CID 119104807
    .
  7. ^ Jordans, Frank (30 July 2015). "Philae probe finds evidence that comets can be cosmic labs". The Washington Post. Associated Press. Archived from the original on 23 December 2018. Retrieved 30 July 2015.
  8. ^ "Science on the Surface of a Comet". European Space Agency. 30 July 2015. Retrieved 30 July 2015.
  9. PMID 26228139
    .
  10. ISBN 978-9813222571
    .
  11. doi:10.1093/mnras/108.6.465
    .
  12. doi:10.1093/mnras/111.3.268
    .
  13. ISBN 9781107615618
    .
  14. ISBN 0-08-034859-9
    .
  15. doi:10.1086/145272
    .
  16. doi:10.1086/145416
    .
  17. Bibcode:1972csdm.conf..174D
    .
  18. ^ a b Wood, J (December 1986). Comet nucleus models: a review. ESA Workshop on the Comet Nucleus Sample Return Mission. pp. 123–31.
  19. ^ a b Kresak, L; Kresakova, M (1987). ESA SP-278: Symposium on the Diversity and Similarity of Comets. ESA. p. 739.
  20. ISBN 978-9813222571
    . "It has been a long time since comet nuclei could be imagined as frozen snowballs"
  21. doi:10.1016/0019-1035(82)90002-1
    .
  22. doi:10.1016/0019-1035(84)90157-X
    .
  23. S2CID 4357619
    .
  24. S2CID 46708189
    .
  25. ^
    ISBN 978-9813222571
    . "the term dust mantling has been in common use since more than 35 years"
  26. ^ 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.
  27. ^ Whipple, F (October 1987). "The Cometary Nucleus - Current Concepts". Astronomy & Astrophysics. 187 (1): 852.
  28. S2CID 123621097
    .
  29. doi:10.1016/j.pss.2008.02.011
    .
  30. ^ Weissman, P; Asphaug, E; Lowry, S (2004). "Structure and Density of Cometary Nuclei". Comets II. Tucson: University of Arizona Press. p. 337.
  31. S2CID 119278016
    .
  32. S2CID 206634190
    .
  33. doi:10.1093/mnras/stx983
    .
  34. S2CID 125072014
    .
  35. ^
    doi:10.1093/mnras/stw1663
    .
  36. doi:10.1093/mnras/sty2926
    .
  37. S2CID 216338717
    .
  38. ^ "How comets were assembled". University of Bern. 29 May 2015. Retrieved 8 January 2016 – via Phys.org.
  39. S2CID 36638785
    .
  40. doi:10.1006/icar.1997.5712
    .
  41. ^ Choi, Charles Q. (15 November 2014). "Comets: Facts About The 'Dirty Snowballs' of Space". Space.com. Retrieved 8 January 2016.
  42. S2CID 4430764
    .
  43. ^ "How Asteroids and Comets Formed". Science Clarified. Retrieved 16 January 2016.
  44. ISBN 978-0-12-088589-3
    .
  45. S2CID 123931232
    .
  46. PMID 28554969
    . Special issue: Cometary science after Rosetta
  47. S2CID 211252398
    .
  48. ^ a b c d e f g Yeomans, Donald K. (2005). "Comets (World Book Online Reference Center 125580)". NASA. Archived from the original on 29 April 2005. Retrieved 20 November 2007.
  49. ^ a b "What Have We Learned About Halley's Comet?". Astronomical Society of the Pacific (No. 6 – Fall 1986). 1986. Retrieved 14 December 2008.
  50. ^
    doi:10.1086/375752
    . Retrieved 14 December 2008.
  51. S2CID 189899565
    .
  52. ^ "SOHO's new catch: its first officially periodic comet". European Space Agency. 25 September 2007. Retrieved 20 November 2007.
  53. ^ a b c d D. T. Britt; G. J. Consol-magno SJ; W. J. Merline (2006). "Small Body Density and Porosity: New Data, New Insights" (PDF). Lunar and Planetary Science XXXVII. Archived from the original (PDF) on 17 December 2008. Retrieved 14 December 2008.
  54. ^ 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.
  55. S2CID 4335780
    .
  56. ^ "Comet 9P/Tempel 1". The Planetary Society. Archived from the original on 9 February 2006. Retrieved 15 December 2008.
  57. ^ "Comet 81P/Wild 2". The Planetary Society. Archived from the original on 6 January 2009. Retrieved 20 November 2007.
  58. ^ Baldwin, Emily (6 October 2014). "Measuring Comet 67P/C-G". European Space Agency. Retrieved 16 November 2014.
  59. ^ Baldwin, Emily (21 August 2014). "Determining the mass of comet 67P/C-G". European Space Agency. Retrieved 21 August 2014.
  60. ^ 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
  61. ^
    doi:10.1093/mnras/sty3182
    . 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
  62. S2CID 2207751
    . 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.
  63. PMID 30842688
    . While the coma is clearly heterogeneous in composition, no firm statement can be made about the compositional heterogeneity of the nucleus at any given time." "what can be measured in their comas remotely may not be representative of their bulk compositions.
  64. PMID 28554980
    . our understanding has been evolving more toward mostly rock
  65. ^ 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).
  66. doi:10.1093/mnras/stx983
    .
  67. S2CID 127214832
    . a predominance of organic materials and minerals.
  68. S2CID 119208933
    .
  69. ^ Borenstein, Seth (10 December 2014). "The mystery of where Earth's water came from deepens". Excite News. Associated Press. Retrieved 14 December 2014.
  70. ^ Agle, D. C.; Bauer, Markus (10 December 2014). "Rosetta Instrument Reignites Debate on Earth's Oceans". NASA. Retrieved 10 December 2014.
  71. S2CID 122405233
    .
  72. S2CID 186245081
    .
  73. ^ a b Filacchione, Gianrico; Capaccioni, Fabrizio; Taylor, Matt; Bauer, Markus (13 January 2016). "Exposed ice on Rosetta's comet confirmed as water" (Press release). European Space Agency. Archived from the original on 18 January 2016. Retrieved 14 January 2016.
  74. S2CID 4446724
    .
  75. ^ Baldwin, Emily (18 November 2014). "Philae settles in dust-covered ice". European Space Agency. Retrieved 18 December 2014.
  76. ISBN 981-02-2632-2
    .
  77. ^ Khan, Amina (31 July 2015). "After a bounce, Rosetta". Los Angeles Times. Retrieved 22 January 2016.
  78. ^ "Rosetta's frequently asked questions". European Space Agency. 2015. Retrieved 22 January 2016.
  79. S2CID 118394879
    .
  80. S2CID 119347364
    .
  81. ^ Michel, P.; Schwartz, S.; Jutzi, M.; Marchi, S.; Zhang, Y.; Richardson, D. C. (2018). Catastrophic Disruptions As The Origin Of 67PC-G And Small Bilobate Comets. 42nd COSPAR Scientific Assembly. p. B1.1–0002–18.
  82. S2CID 213437916
    . Sec. 6.3 Major Open Points Remain "data are not conclusive concerning the collisional environment during the formation and right afterwards"
  83. ^ JPL Public Information Office. "Comet Shoemaker-Levy Background". JPL/NASA. Retrieved 25 October 2008.
  84. ^ Whitney Clavin (10 May 2006). "Spitzer Telescope Sees Trail of Comet Crumbs". Spitzer Space Telescope at Caltech. Retrieved 25 October 2008.
  85. ^ Donald K. Yeomans (1998). "Great Comets in History". Jet Propulsion Laboratory. Retrieved 15 March 2007.
  86. ^ H. Boehnhardt. "Split Comets" (PDF). Lunar and Planetary Institute (Max-Planck-Institut für Astronomie Heidelberg). Retrieved 25 October 2008.
  87. ^ J. Pittichova; K.J. Meech; G.B. Valsecch; E.M. Pittich (1–6 September 2003). "Are Comets 42P/Neujmin 3 and 53P/Van Biesbroeck Parts of one Comet?". Bulletin of the American Astronomical Society, 35 #4. Archived from the original on 13 August 2009.
  88. ^ a b "Comet May Be the Darkest Object Yet Seen". The New York Times. 14 December 2001. Retrieved 9 May 2011.
  89. S2CID 14026673
    .
  90. ^ a b c d e esa. "Giotto overview". European Space Agency.
  91. ^ Organic compounds (usually referred to as organics) does not imply life, it is just a class of chemicals: see Organic chemistry.
  92. S2CID 122092751
    .

External links
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