Comet nucleus#"Dirty snowball"

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.{{cite book |last=Rickman |first=H |date=2017 |title=Origin and Evolution of Comets: 10 years after the Nice Model, and 1 year after Rosetta |isbn=978-9813222571 |publisher=World Scientific Publishing Co Singapore |chapter=1.1.1 The Comet Nucleus }} 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.{{cite journal |last=Lyttleton |first=RA |journal=Mon. Not. R. Astron. Soc. |date=1948 |title=On the Origin of Comets |volume=108 |issue=6 |pages=465–75 |doi=10.1093/mnras/108.6.465 |bibcode=1948MNRAS.108..465L |doi-access=free }}{{cite journal |last1=Lyttleton |first1=R |journal=Mon. Not. R. Astron. Soc. |date=1951 |title=On the Structure of Comets and the Formation of Tails |volume=111 |issue=3 |pages=268–77 |doi=10.1093/mnras/111.3.268 |bibcode=1951MNRAS.111..268L |doi-access=free }}{{cite book |last=Lyttleton |first=R |date=1972 |title=The Comets and Their Origin |isbn=9781107615618 |publisher= Cambridge University Press New York }}{{cite book |last1=Bailey |first1=M |last2=Clube |first2=S |last3=Napier |first3=W |date=1990 |title=The Origin of Comets |isbn=0-08-034859-9 |publisher=Pergamon Press |chapter=8.3 Lyttleton's Accretion Theory }}

Beginning in the 1950s, Fred Lawrence Whipple published his "icy conglomerate" model.{{cite journal |last=Whipple |first=F|journal=Astrophysical Journal |date=1950 |title=A Comet Model. I: the Acceleration of Comet Encke |volume=111 |pages=375–94 |doi=10.1086/145272|bibcode=1950ApJ...111..375W}}{{cite journal |last=Whipple |first=F |journal=Astrophysical Journal |date=1951 |title=A Comet Model. II: Physical Relations for Comets and Meteors |volume=113 |pages=464–74 |doi=10.1086/145416 |bibcode=1951ApJ...113..464W |doi-access=free }} 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|date=November 2022}} 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.{{cite journal |last=Delsemme |first=A |date=1 Jul 1972 |title=Present Understanding of Comets |journal=Comets: Scientific Data and Missions |page=174 |bibcode=1972csdm.conf..174D }} 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.{{cite conference |last=Wood |first=J |conference=ESA Workshop on the Comet Nucleus Sample Return Mission |date=Dec 1986 |title=Comet nucleus models: a review |pages=123–31 }}{{cite book |last1=Kresak |first1=L |last2=Kresakova |first2=M |date=1987 |title=ESA SP-278: Symposium on the Diversity and Similarity of Comets |publisher=ESA |page=739 }}

It has been a long time since comet nuclei could be imagined as frozen snowballs.{{cite book |last=Rickman |first=H |date=2017 |title=Origin and Evolution of Comets: 10 years after the Nice Model, and 1 year after Rosetta |isbn=978-9813222571 |publisher=World Scientific Publishing Co Singapore |chapter=2.2.3 Dust Production Rates}} "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.{{cite journal |last1=Hartmann |first1=W |last2=Cruikshank |first2=D |last3=Degewij |first3=J |journal=Icarus |date=1982 |title= Remote comets and related bodies: VJHK colorimetry and surface materials|volume=52 |issue=3 |pages=377–08 |doi=10.1016/0019-1035(82)90002-1 |bibcode=1982Icar...52..377H }}{{cite journal |last1=Fanale |first1=F |last2=Salvail |first2=J |journal=Icarus |date=1984 |title=An idealized short-period comet model |volume=60 |page=476 |doi=10.1016/0019-1035(84)90157-X }}{{cite journal |last1=Cruikshank |first1=D |last2=Hartmann |first2=W |last3=Tholen |first3=D |s2cid=4357619 |journal=Nature |date=1985 |title=Color, albedo, and nucleus size of Halley's comet |volume=315 |issue=6015 |page=122 |doi=10.1038/315122a0 |bibcode=1985Natur.315..122C }}{{cite journal |last=Greenberg |first=J |s2cid=46708189 |journal=Nature |date=May 1986 |title=Predicting that comet Halley is dark |volume=321 |issue=6068 |page=385 |doi=10.1038/321385a0 |bibcode=1986Natur.321..385G |doi-access=free }} The term dust mantling has been in common use since more than 35 years.{{cite book |last=Rickman |first=H |date=2017 |title=Origin and Evolution of Comets: 10 years after the Nice Model, and 1 year after Rosetta |isbn=978-9813222571 |publisher=World Scientific Publishing Co Singapore |chapter=4.2 Dust Mantling }} "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 {{cite book |last1=Tholen |first1=D |last2=Cruikshank |first2=D |last3=Hammel |first3=H |last4=Hartmann |first4=W |last5=Lark |first5=N |last6=Piscitelli |first6=J |date=1986 |title=ESA SP-250 Vol. III |publisher=ESA |page=503 |chapter=A comparison of the continuum colours of P/Halley, other comets and asteroids }} 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.{{cite journal |last=Whipple |first=F |journal=Astronomy & Astrophysics |date=Oct 1987 |title=The Cometary Nucleus - Current Concepts |volume=187 |issue=1 |page=852 }} 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 further to human understanding.

Densities were confirmed as quite low, ~0.6 g cm3. Comets were highly porous,{{cite journal |last1=A'Hearn |first1=M |s2cid=123621097 |journal=Space Science Reviews |date=2008 |title= Deep Impact and the Origin and Evolution of Cometary Nuclei|volume=138 |issue=1 |page=237 |doi=10.1007/s11214-008-9350-3 |bibcode=2008SSRv..138..237A }} and fragile on micro-{{cite journal |last1=Trigo-Rodriguez |first1=J |last2=Blum |first2=J |journal=Planet. Space Sci. |date=Feb 2009 |title=Tensile strength as an indicator of the degree of primitiveness of undifferentiated bodies |volume=57 |issue=2 |pages=243–49|doi=10.1016/j.pss.2008.02.011 |bibcode=2009P&SS...57..243T }} and macro-scales.{{cite book |last1=Weissman |first1=P |last2=Asphaug |first2=E |last3=Lowry |first3=S |date=2004 |title=Comets II |publisher= University of Arizona Press |location=Tucson |page=337 |chapter=Structure and Density of Cometary Nuclei }}

Refractory-to-ice ratios are much higher,{{cite journal |last1=Bischoff |first1=D |last2=Gundlach |first2=B |last3=Neuhaus |first3=M |last4=Blum |first4=J |s2cid=119278016 |journal=Mon. Not. R. Astron. Soc. |date=Feb 2019 |title=Experiment on cometary activity: ejection of dust aggregates from a sublimating water-ice surface |volume=483 |issue=1 |page=1202 |doi=10.1093/mnras/sty3182 |doi-access=free |arxiv=1811.09397 |bibcode=2019MNRAS.483.1202B }} at least 3:1,{{cite journal |last1=Rotundi |first1=A |last2=Sierks H |last3=Della Corte V |last4=Fulle M |last5=GutierrezP |s2cid=206634190 |display-authors=etal |journal=Science |date=23 Jan 2015 |title=Dust Measurements in the coma of comet 67P/Churyumov-Gerasimenko inbound to the Sun |volume=347 |issue=6220 |pages=aaa3905 |doi=10.1126/science.aaa3905 |pmid=25613898 |bibcode=2015Sci...347a3905R |doi-access=free }} possibly ~5:1,{{cite journal |last1=Fulle |first1=M |last2=Della Corte |first2=V |last3=Rotundi |first3=A |last4=Green |first4=S |last5=Accolla |first5=M |last6=Colangeli |first6=L |last7=Ferrari |first7=M |last8=Ivanovski |first8=S |last9=Sordini |first9=R |last10=Zakharov |first10=V | journal=Mon. Not. R. Astron. Soc. |date=2017 |title=The dust-to-ices ratio in comets and Kuiper belt objects |volume=469 |page=S45-49 |doi=10.1093/mnras/stx983 |bibcode=2017MNRAS.469S..45F |doi-access=free }} ~6:1,{{cite journal |last1=Fulle |first1=M |last2=Marzari |first2=F |last3=Della Corte |first3=V |last4=Fornasier |first4=S |journal=Astrophysical Journal |date=Apr 2016 |title=Evolution of the dust size distribution of comet 67P/C-G from 2.2au to perihelion |volume=821 |page=19 |doi=10.3847/0004-637X/821/1/19 |s2cid=125072014 |url=http://oro.open.ac.uk/46060/1/__science_science-users_users_sfg27_My%20Documents_CV_Pre-prints_of_papers_2016_Fulle_et_al_ApJ_accepted.pdf |doi-access=free |hdl=11577/3199084 }} or more.{{cite journal |last1=Fulle |first1=M |last2=Altobelli |first2=N |last3=Buratti |first3=B |last4=Choukroun |first4=M |last5=Fulchignoni |first5=M |last6=Grün |first6=E |last7=Taylor |first7=M |display-authors=etal |date=Nov 2016 |title=Unexpected and significant findings in comet 67P/Churyumov-Gerasimenko: an interdisciplinary view |journal=Mon. Not. R. Astron. Soc. |volume=462 |pages=S2-8 |doi=10.1093/mnras/stw1663 |bibcode=2016MNRAS.462S...2F |doi-access=free }}{{cite journal |last1=Fulle |first1=M |last2=Blum |first2=J |last3=Green |first3=S |last4=Gundlach |first4=B |last5=Herique |first5=A |last6=Moreno |first6=F |last7=Mottola |first7=S |last8=Rotundi |first8=A |last9=Snodgrass |first9=C | journal=Mon. Not. R. Astron. Soc. |date=Jan 2019 |title=The refractory-to-ice mass ratio in comets |volume=482 |issue=3 |pages=3326–40 |doi=10.1093/mnras/sty2926 |doi-access=free |bibcode=2019MNRAS.482.3326F |url=https://www.pure.ed.ac.uk/ws/files/76687259/Fulle_dust2gas_accepted.pdf }}{{cite journal |last1=Choukroun |first1=M |last2=Altwegg |first2=K |author2-link=Kathrin Altwegg|last3=Kührt |first3=E |last4=Biver |first4=N |last5=Bockelée-Morvan |first5=D|author5-link=Dominique Bockelée-Morvan |s2cid=216338717 |display-authors=etal |journal=Space Sci Rev |date=2020 |title=Dust-to-Gas and Refractory-to-ice Mass Ratios of Comet 67P/Churyumov-Gerasimenko from Rosetta Obs |volume=216 |page=44 |doi=10.1007/s11214-020-00662-1 |doi-access=free }}

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.

{{main|Accretion (astrophysics)#Accretion of comets}}

Comets, or their precursors, formed in the outer Solar System, possibly millions of years before planet formation.{{cite news |url=http://phys.org/news/2015-05-comets.html |title=How comets were assembled |publisher=University of Bern |via=Phys.org |date=29 May 2015 |access-date=8 January 2016}} 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.{{Cite journal |title=The shape and structure of cometary nuclei as a result of low-velocity accretion |journal=Science |first1=M. |last1=Jutzi |first2=E. |last2=Asphaug |s2cid=36638785 |volume=348 |issue=6241 |pages=1355–1358 |date=June 2015 |doi=10.1126/science.aaa4747 |bibcode=2015Sci...348.1355J |pmid=26022415|doi-access=free }}{{cite journal |title=The Origin of Comets in the Solar Nebula: A Unified Model |journal=Icarus |last=Weidenschilling |first=S. J. |volume=127 |issue=2 |pages=290–306 |date=June 1997 |doi=10.1006/icar.1997.5712 |bibcode=1997Icar..127..290W}} 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.{{cite news |url=http://www.space.com/53-comets-formation-discovery-and-exploration.html/ |title=Comets: Facts About The 'Dirty Snowballs' of Space |work=Space.com |last=Choi |first=Charles Q. |date=15 November 2014 |access-date=8 January 2016}}{{cite journal |title=Determining the ages of comets from the fraction of crystalline dust |journal=Nature |last1=Nuth |first1=Joseph A. |last2=Hill |first2=Hugh G. M. |last3=Kletetschka |first3=Gunther |s2cid=4430764 |volume=406 |issue=6793 |pages=275–276 |date=20 July 2000 |doi=10.1038/35018516 |bibcode=2000Natur.406..275N |pmid=10917522}}{{cite web |url=http://www.scienceclarified.com/scitech/Comets-and-Asteroids/How-Asteroids-and-Comets-Formed.html |title=How Asteroids and Comets Formed |work=Science Clarified |access-date=16 January 2016}}

Astronomers think that comets originate in the Oort cloud, the scattered disk,{{cite book |title=Encyclopedia of the Solar System |chapter=Comet Populations and Cometary Dynamics |edition=2nd |publisher=Academic Press |location=Amsterdam |last1=Levison |first1=Harold F. |first2=Luke |last2=Donnes |editor1-first=Lucy-Ann Adams |editor1-last=McFadden |editor2-first=Paul Robert |editor2-last=Weissman |editor3-first=Torrence V. |editor3-last=Johnson |pages=[https://archive.org/details/encyclopediaofso0000unse_u6d1/page/575 575–588] |date=2007 |isbn=978-0-12-088589-3 |chapter-url=https://archive.org/details/encyclopediaofso0000unse_u6d1/page/575 }} and the outer Main Belt.{{cite journal |last1=Dones |first1=L |last2=Brasser |first2=R |last3=Kaib |first3=N |last4=Rickman |first4=H |s2cid=123931232 |date=2015 |title=Origi and Evolu of the Cometar Reserv |journal=Space Science Reviews |volume=197 |pages=191–69 |doi=10.1007/s11214-015-0223-2 |bibcode=2015SSRv..197..191D }}{{cite journal |last=Meech |first=K |date=2017 |title=Setting the scene: what did we know before Rosetta? |journal=Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences |volume=375 |at=Section 6 |issue=2097 |doi=10.1098/rsta.2016.0247|pmid=28554969 |bibcode=2017RSPTA.37560247M |doi-access=free |pmc=5454221 }} Special issue: Cometary science after Rosetta{{cite journal |last1=Hsieh |first1=H |last2=Novaković |first2=B |last3=Walsh |first3=K |last4=Schörghofer |first4=N |s2cid=211252398 |journal= The Astronomical Journal|date=2020 |title=Potential Themis-family Asteroid Contribution to the Jupiter-family Comet Population |volume=159 |issue=4 |page= 179|doi=10.3847/1538-3881/ab7899 |pmid=32255816 |pmc=7121251 |arxiv=2002.09008 |bibcode=2020AJ....159..179H |doi-access=free }}

File:Largest comets size comparison.png Pluto and natural satellites Mimas and Phobos for scale.]]

File:Tempel 1 Hartley 2 comparison.jpg

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

|date=1986

|title=What Have We Learned About Halley's Comet?

|publisher=Astronomical Society of the Pacific (No. 6 – Fall 1986)

|url=http://www.astrosociety.org/education/publications/tnl/06/06.html

|access-date=14 December 2008}} 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){{cite journal

|last=Weaver |first=H. A.

|author2=Stern, S.A. |author3=Parker, J. Wm.

|title=Hubble Space Telescope STIS Observations of Comet 19P/BORRELLY during the Deep Space 1 Encounter

|journal=The Astronomical Journal

|volume=126 |pages=444–451 |date=2003

|doi=10.1086/375752

|url=http://www.iop.org/EJ/article/1538-3881/126/1/444/202321.text.html

|access-date=14 December 2008 |bibcode=2003AJ....126..444W |issue=1|doi-access=free}} 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.

File:PIA20119-CometChristensen-C2006W3-CO2-WISE-20100420.jpg

The nucleus of comet Hale–Bopp was estimated to be 60 ± 20 km in diameter.{{cite journal | last = Fernández | first=Yanga R. | s2cid=189899565 | title = The Nucleus of Comet Hale-Bopp (C/1995 O1): Size and Activity | date = 2002 | journal = Earth, Moon, and Planets | volume = 89 | issue=1 | pages = 3–25| doi = 10.1023/A:1021545031431|bibcode = 2002EM&P...89....3F }} 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.{{cite web|date=25 September 2007|title=SOHO's new catch: its first officially periodic comet|publisher=European Space Agency |url=http://www.esa.int/esaCP/SEMAU2C1S6F_index_0.html|access-date=20 November 2007}}

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 {{mpl|(523727) 2014 NW|65}} (≈220 km).

Known comets have been estimated to have an average density of 0.6 g/cm³.{{cite web

|date=2006

|title=Small Body Density and Porosity: New Data, New Insights

|publisher=Lunar and Planetary Science XXXVII

|author1=D. T. Britt

|author2=G. J. Consol-magno SJ

|author3=W. J. Merline

|url=http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2214.pdf

|access-date=14 December 2008

|archive-url=https://web.archive.org/web/20081217064607/http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2214.pdf

|archive-date=17 December 2008

|url-status=dead

}} Below is a list of comets that have had estimated sizes, densities, and masses.

{{Update section|date=July 2020}}

It was once thought that water-ice was the predominant constituent of the nucleus.{{cite book |last1=Wood |first1=J A |date=Dec 1986 |title=ESA Proceedings of an ESA workshop on the Comet Nucleus Sample Return Mission |publisher=ESA |pages=123–31 |chapter=Comet nucleus models: a review. |quote="water-ice as the predominant constituent" }} In the dirty snowball model, dust is ejected when the ice retreats.{{cite journal |last1=Bischoff |first1=D |last2=Gundlach |first2=B |last3=Neuhaus |first3=M |last4=Blum |first4=J |journal=Mon. Not. R. Astron. Soc. |date=Feb 2019 |title=Experiments on cometary activity: ejection of dust aggregates from a sublimating water-ice surface |volume=483 |issue=1 |pages=1202–10 |doi=10.1093/mnras/sty3182 |arxiv=1811.09397 |bibcode=2019MNRAS.483.1202B |quote="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" |doi-access=free }} 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.{{Citation needed|date=August 2024}}

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.{{cite journal |last1=Bockelée-Morvan |first1=D |author1-link=Dominique Bockelée-Morvan|last2=Biver |first2=N |journal=Philos. Trans. R. Soc. A |date=May 2017 |title=The composition of cometary ices |volume=375 |issue=2097 |doi=10.1098/rsta.2016.0252 |pmid=28554972 |bibcode=2017RSPTA.37560252B |s2cid=2207751 |quote="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." |doi-access=free }}{{cite journal |last1=O'D. Alexander |first1=C |last2=McKeegan |first2=K |last3=Altwegg |first3=K|author3-link=Kathrin Altwegg |journal=Space Science Reviews |date=Feb 2019 |title=Water Reservoirs in Small Planetary Bodies: Meteorites, Asteroids, and Comets |volume=214 |issue=1 |page=36 |doi=10.1007/s11214-018-0474-9 |pmid=30842688 |quote="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."|pmc=6398961 }} Our understanding has evolved more toward mostly rock;{{cite journal |last1=A'Hearn |first1=M |journal=Philos. Trans. R. Soc. A |date=May 2017 |title=Comets: looking ahead |volume=375 |issue=2097 |doi=10.1098/rsta.2016.0261 |pmid=28554980 |pmc=5454229 |bibcode=2017RSPTA.37560261A |quote="our understanding has been evolving more toward mostly rock" }} recent estimates show that water is perhaps only 20-30% of the mass in typical nuclei.{{cite book |last1=Jewitt |first1=D |last2=Chizmadia |first2=L |last3=Grimm |first3=R |last4=Prialnik |first4=D |date=2007 |title=Protostars and Planets V |publisher=University of Arizona Press |pages=863–78 |chapter=Water in the Small Bodies of the Solar System |quote="Recent estimates... show that water is less important, perhaps carrying only 20-30% of the mass in typical nuclei (Sykes et al., 1986)." }}{{cite journal |last1=Fulle |first1=M |last2=Della Corte |first2=V |last3=Rotundi |first3=A |last4=Green |first4=S |last5=Accolla |first5=M |last6=Colangeli |first6=L |last7=Ferrari |first7=M |last8=Ivanovski |first8=S |last9=Sordini |first9=R |last10=Zakharov |first10=V |journal=Mon. Not. R. Astron. Soc. |date=2017 |title=The dust-to-ices ratio in comets and Kuiper belt objects |volume=469 |page=S45-49 |doi=10.1093/mnras/stx983 |bibcode=2017MNRAS.469S..45F |doi-access=free }} Instead, comets are predominantly organic materials and minerals.{{cite journal |last1=Filacchione |first1=G |last2=Groussin |first2=O |last3=Herny |first3=C |last4=Kappel |first4=D |last5=Mottola |first5=S |last6=Oklay |first6=N |last7=Pommerol |first7=A |last8=Wright |first8=I |last9=Yoldi |first9=Z |last10=Ciarniello |first10=M |last11=Moroz |first11=L |last12=Raponi |first12=A |journal=Space Science Reviews |date=2019 |title=Comet 67P/CG Nucleus Composition and Comparison to Other Comets |volume=215 |issue=1 |page=Article number 19 |doi=10.1007/s11214-019-0580-3 |bibcode=2019SSRv..215...19F |s2cid=127214832 |url=https://hal.archives-ouvertes.fr/hal-02496584/file/Filacchione_etal_2019.pdf |quote="a predominance of organic materials and minerals." }} Data from Churyumov-Gerasimenko and Arrokoth, and laboratory experiments on accretion, suggest refractories-to-ices ratios less than 1 may not be possible.{{cite journal |last1=Lorek |first1=S. |last2=Gundlach |first2=B. |last3=Lacerda |first3=P.|last4=Blum |first4=J. |title=Comet formation in collapsing pebble clouds What cometary bulk density implies for the cloud mass and dust-to-ice ratio |journal=Astronomy & Astrophysics |date=2018 |volume=587 |page=A128|doi=10.1051/0004-6361/201526565 |s2cid=119208933 |doi-access=free |arxiv=1601.05726 }}

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.{{cite news |url=http://apnews.excite.com/article/20141210/us-sci-comet-water-67af853779.html |title=The mystery of where Earth's water came from deepens |work=Excite News |agency=Associated Press |first=Seth |last=Borenstein |date=10 December 2014 |access-date=14 December 2014}}{{cite web |url=http://www.jpl.nasa.gov/news/news.php?release=2014-423 |title=Rosetta Instrument Reignites Debate on Earth's Oceans |publisher=NASA |first1=D. C. |last1=Agle |first2=Markus |last2=Bauer |date=10 December 2014 |access-date=10 December 2014}}

"Missing Carbon"{{cite journal |last1=Kissel |first1=J. |last2=Sagdeev |first2=R. Z. |last3=Bertaux |first3=J. L. |last4=Angarov |first4=V. N. |last5=Audouze |first5=J. |last6=Blamont |first6=J. E. |last7=Buchler |first7=K. |last8=Evlanov |first8=E. N. |last9=Fechtig |first9=H. |last10=Fomenkova |first10=M. N. |last11=Hoerner |first11=von H. |last12=Inogamov |first12=N. A. |last13=Khromov |first13=V. N. |last14=Knabe |first14=W. |last15=Krueger |first15=F. R. |last16=Langevin |first16=Y. |last17=Leonasv |first17=B. |title=Composition of comet Halley dust particles from Vega observations |journal=Nature |date=1986 |volume=321 |page=280|doi=10.1038/321280a0 |bibcode=1986Natur.321..280K |s2cid=122405233 }}{{cite journal |last1=Kissel |first1=J. |last2=Brownlee |first2=D. E. |last3=Buchler |first3=K. |last4=Clark |first4=B. |last5=Fechtig |first5=H. |last6=Grun |first6=E. |last7=Hornung |first7=K. |last8=Igenbergs |first8=E. |title=Composition of comet Halley dust particles from Giotto observations |journal=Nature |date=1986 |volume=321 |page=336|doi=10.1038/321336a0 |bibcode=1986Natur.321..336K |s2cid=186245081 }}

File:NAVCAM top 10 at 10 km – 7 (15763681495).jpg 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.{{cite press release|url=http://www.esa.int/Our_Activities/Space_Science/Rosetta/Exposed_ice_on_Rosetta_s_comet_confirmed_as_water |title=Exposed ice on Rosetta's comet confirmed as water |publisher=European Space Agency |first1=Gianrico |last1=Filacchione |first2=Fabrizio |last2=Capaccioni |first3=Matt |last3=Taylor |first4=Markus |last4=Bauer |date=13 January 2016 |access-date=14 January 2016 |url-status=dead |archive-url=https://web.archive.org/web/20160118052157/http://www.esa.int/Our_Activities/Space_Science/Rosetta/Exposed_ice_on_Rosetta_s_comet_confirmed_as_water |archive-date=18 January 2016 }} 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.{{cite journal |title=Exposed water ice on the nucleus of comet 67P/Churyumov–Gerasimenko |journal=Nature |first1=G. |last1=Filacchione |first2=M. C. |last2=de Sanctis |first3=F. |last3=Capaccioni |first4=A. |last4=Raponi |first5=F. |last5=Tosi |s2cid=4446724 |display-authors=etal |date=13 January 2016 |doi=10.1038/nature16190|bibcode = 2016Natur.529..368F |volume=529 |issue=7586 |pages=368–372 |pmid=26760209}}

File:Schwassman-Wachmann3-B-HST.gif

Measurements carried out by the Philae lander on 67P/Churyumov–Gerasimenko comet, indicate that the dust layer could be as much as {{convert|20|cm|abbr=on}} thick. Beneath that is hard ice, or a mixture of ice and dust. Porosity appears to increase toward the center of the comet.{{cite web |url=http://blogs.esa.int/rosetta/2014/11/18/philae-settles-in-dust-covered-ice/ |title=Philae settles in dust-covered ice |publisher=European Space Agency |first=Emily |last=Baldwin |date=18 November 2014 |access-date=18 December 2014}} 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,{{cite book |title=Physics of Comets |series=World Scientific Series in Astronomy and Astrophysics, Volume 2 |publisher=World Scientific |edition=2nd |last=Krishna Swamy |first=K. S. |pages=364 |date=May 1997 |isbn=981-02-2632-2}} the Rosetta mission dispelled the idea that comets are "rubble piles" of disparate material.{{cite news |url=http://www.latimes.com/science/sciencenow/la-sci-sn-rosetta-philae-comet-67p-churyumov-gerasimenko-organic-bounce-20150730-story.html |title=After a bounce, Rosetta |work=Los Angeles Times |last=Khan |first=Amina |date=31 July 2015 |access-date=22 January 2016}}{{cite web |url=http://www.esa.int/Our_Activities/Space_Science/Rosetta/Frequently_asked_questions |title=Rosetta's frequently asked questions |publisher=European Space Agency |date=2015 |access-date=22 January 2016}}{{dubious|date=July 2020}} The Rosetta mission indicated that comets may be "rubble piles" of disparate material.{{cite journal |last1=Rickman |first1=H |last2=Marchi |first2=S |last3=AHearn |first3=M |last4=Barbieri |first4=C |last5=El-Maarry |first5=M |last6=Güttler |first6=C |last7=Ip |first7=W |s2cid=118394879 |journal=Astronomy & Astrophysics |date=2015 |title=Comet 67P/Churyumov-Gerasimenko: Constraints on its origin from OSIRIS observations |volume=583 |page=Article 44 |doi=10.1051/0004-6361/201526093 |arxiv=1505.07021 |bibcode=2015A&A...583A..44R }} Data were not conclusive concerning the collisional environment during the formation and right afterwards.{{cite journal |last1=Jutzi |first1=M |last2=Benz |first2=W |last3=Toliou |first3=A |last4=Morbidelli |first4=A |last5=Brasser |first5=R |s2cid=119347364 |journal=Astronomy & Astrophysics |date=2017 |title=How primordial is the structure of comet 67P? Combined collisional and dynamical models suggest a late formation |volume=597 |page=A# 61 |doi=10.1051/0004-6361/201628963 |arxiv=1611.02604 |bibcode=2017A&A...597A..61J }}{{cite conference |last1=Michel |first1=P. |last2=Schwartz |first2=S. |last3=Jutzi |first3=M. |last4=Marchi |first4=S. |last5=Zhang |first5=Y. |last6=Richardson |first6=D. C. |title=Catastrophic Disruptions As The Origin Of 67PC-G And Small Bilobate Comets |conference=42nd COSPAR Scientific Assembly |date=2018 |page=B1.1–0002–18}}{{cite journal |last1=Keller |first1=H |last2=Kührt |first2=E |s2cid=213437916 |journal=Space Science Reviews |date=2020 |title=Cometary Nuclei- From Giotto to Rosetta |volume=216 |issue=1 |page=Article 14 |doi=10.1007/s11214-020-0634-6 |bibcode=2020SSRv..216...14K |doi-access=free }} Sec. 6.3 Major Open Points Remain "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.{{cite web|last=Yeomans|first=Donald K.|date=2005|title=Comets (World Book Online Reference Center 125580)|publisher=NASA|url=http://www.nasa.gov/worldbook/comet_worldbook.html|archive-url=https://web.archive.org/web/20050429084558/http://www.nasa.gov/worldbook/comet_worldbook.html|url-status=dead|archive-date=29 April 2005|access-date=20 November 2007}} Splitting comets include 3D/Biela in 1846, Shoemaker–Levy 9 in 1992,{{cite web

|title=Comet Shoemaker-Levy Background

|publisher=JPL/NASA

|author=JPL Public Information Office

|url=http://www2.jpl.nasa.gov/sl9/background.html

|access-date=25 October 2008}} and 73P/Schwassmann–Wachmann from 1995 to 2006.{{cite web

|date=10 May 2006

|title=Spitzer Telescope Sees Trail of Comet Crumbs

|publisher=Spitzer Space Telescope at Caltech

|author=Whitney Clavin

|url=http://www.spitzer.caltech.edu/news/239-ssc2006-13-Spitzer-Telescope-Sees-Trail-of-Comet-Crumbs

|access-date=25 October 2008}} Greek historian Ephorus reported that a comet split apart as far back as the winter of 372–373 BC.{{cite web|title=Great Comets in History|author=Donald K. Yeomans|work=Jet Propulsion Laboratory|url=http://ssd.jpl.nasa.gov/?great_comets|date=1998|access-date=15 March 2007}} Comets are suspected of splitting due to thermal stress, internal gas pressure, or impact.{{cite web

|title=Split Comets

|publisher=Lunar and Planetary Institute (Max-Planck-Institut für Astronomie Heidelberg)

|author=H. Boehnhardt

|url=http://www.lpi.usra.edu/books/CometsII/7011.pdf

|access-date=25 October 2008}}

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.{{cite web

|date=1–6 September 2003

|title=Are Comets 42P/Neujmin 3 and 53P/Van Biesbroeck Parts of one Comet?

|publisher=Bulletin of the American Astronomical Society, 35 #4

|author1=J. Pittichova

|author2=K.J. Meech

|author3=G.B. Valsecch

|author4=E.M. Pittich

|url=http://aas.org/archives/BAAS/v35n4/dps2003/72.htm?q=publications/baas/v35n4/dps2003/72.htm

|archive-url=https://web.archive.org/web/20090813153039/http://aas.org/archives/BAAS/v35n4/dps2003/72.htm?q=publications%2Fbaas%2Fv35n4%2Fdps2003%2F72.htm

|archive-date=13 August 2009

|url-status=dead

}}

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;{{cite web

|date=14 December 2001

|title=Comet May Be the Darkest Object Yet Seen

|work=The New York Times

|url=https://www.nytimes.com/2001/12/14/us/comet-may-be-the-darkest-object-yet-seen.html

|access-date=9 May 2011}} 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.{{cite journal |first1=Kathryn|last1=Whitman |first2=Alessandro|last2=Morbidelli |first3=Robert|last3=Jedicke |s2cid=14026673 |title=The Size-Frequency Distribution of Dormant Jupiter Family Comets |date=2006 |arxiv=astro-ph/0603106|bibcode = 2006Icar..183..101W |doi = 10.1016/j.icarus.2006.02.016 |volume=183 |issue=1 |journal=Icarus |pages=101–114}} Two near-Earth asteroids with albedos this low include 14827 Hypnos and 3552 Don Quixote.{{dubious|date=July 2020}}

The first relatively close mission to a comet nucleus was space probe Giotto.{{cite web|url=http://www.esa.int/Our_Activities/Space_Science/Giotto_overview|title=Giotto overview|author=esa|work=European Space Agency}} 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.Organic compounds (usually referred to as organics) does not imply life, it is just a class of chemicals: see Organic chemistry.

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{{cite journal |title=Dust density and mass distribution near comet Halley from Giotto observations |author=J. A. M. McDonnell |s2cid=122092751 |journal=Nature |volume=321 |pages=338–341 |date=15 May 1986 |doi=10.1038/321338a0 |bibcode = 1986Natur.321..338M |display-authors=etal}} 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 {{convert|1|km|mi|abbr=on}} 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)

• Coma (cometary)
• Hypatia (stone)
• List of comets visited by spacecraft

{{Reflist|30em}}

• [http://antwrp.gsfc.nasa.gov/apod/image/9612/halley_hmc_big.gif Nucleus of Halley's Comet] (15×8×8 km)
• [http://apod.nasa.gov/apod/ap040103.html Nucleus of Comet Wild 2] (5.5×4.0×3.3 km)
• [https://web.archive.org/web/20110812112210/http://www.icq.eps.harvard.edu/ICQsplit.html International Comet Quarterly: Split Comets]
• [http://www.esa.int/var/esa/storage/images/esa_multimedia/images/2014/08/comet_on_3_august_2014/14707891-1-eng-GB/Comet_on_3_August_2014.png 67/P by Rosetta][http://www.esa.int/var/esa/storage/images/esa_multimedia/images/2014/08/comet_on_3_august_20142/14708015-1-eng-GB/Comet_on_3_August_2014.png 2] ([http://www.esa.int/spaceinimages/Images/2014/08/Comet_on_3_August_2014 ESA])

{{Comets|nonobject=yes}}

{{DEFAULTSORT:Comet Nucleus}}

Category:Comets