Comet nucleus

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. 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 0.5 g cm-3) were derived. The nucleus was still assumed to be majority-ice, perhaps overwhelmingly so.

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, and fragile on micro- and macro-scales.

Refractory-to-ice ratios are much higher, at least 3:1, possibly ~5:1, ~6:1, or more.

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

Main article: Accretion (astrophysics)#Accretion of comets

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

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 |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 |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.

The nucleus of comet Hale–Bopp was estimated to be 60 ± 20 km in diameter. Hale-Bopp appeared bright to the unaided eye because its unusually large nucleus gave off a great deal of dust and gas.

The nucleus of P/2007 R5 is probably only 100–200 meters in diameter.

The largest centaurs (unstable, planet crossing, icy asteroids) are estimated to be 250 km to 300 km in diameter. Three of the largest would include 10199 Chariklo (258 km), 2060 Chiron (230 km), and (≈220 km).

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

Composition

It was once thought that water-ice was the predominant constituent of the nucleus. In the dirty snowball model, dust is ejected when the ice retreats. Based on this, about 80% of the Halley's Comet nucleus would be water-ice, and frozen carbon monoxide (CO) makes up another 15%. Much of the remainder is frozen carbon dioxide, methane, and ammonia. Scientists think that other comets are chemically similar to Halley's Comet. The nucleus of Halley's Comet is also an extremely dark black. Scientists think that the surface of the comet, and perhaps most other comets, is covered with a black crust of dust and rock that covers most of the ice. These comets release gas only when holes in this crust rotate toward the Sun, exposing the interior ice to the warming sunlight.

This assumption was shown to be naive, starting at Halley. Coma composition does not represent nucleus composition, as activity selects for volatiles, and against refractories, including heavy organic fractions. Our understanding has evolved more toward mostly rock; recent estimates show that water is perhaps only 20-30% of the mass in typical nuclei. Instead, comets are predominantly organic materials and minerals. 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

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

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, 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 |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. 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 1 km 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)

References

References

1. (10 March 2006). "ESA Science & Technology: Halley". [[ESA]].
2. Bauer, Markus. (14 April 2015). "Rosetta and Philae Find Comet Not Magnetised". European Space Agency.
3. Schiermeier, Quirin. (14 April 2015). "Rosetta's comet has no magnetic field". [[Nature (journal).
4. (2 June 2015). "NASA Instrument on Rosetta Makes Comet Atmosphere Discovery". [[NASA]].
5. (2 June 2015). "Measurements of the near-nucleus coma of comet 67P/Churyumov–Gerasimenko with the Alice far-ultraviolet spectrograph on Rosetta". [[Astronomy and Astrophysics]].
6. Rickman, H. (2017). "Origin and Evolution of Comets: 10 years after the Nice Model, and 1 year after Rosetta". World Scientific Publishing Co Singapore.
7. Lyttleton, RA. (1948). "On the Origin of Comets". Mon. Not. R. Astron. Soc..
8. (1951). "On the Structure of Comets and the Formation of Tails". Mon. Not. R. Astron. Soc..
9. Lyttleton, R. (1972). "The Comets and Their Origin". Cambridge University Press New York.
10. (1990). "The Origin of Comets". Pergamon Press.
11. Whipple, F. (1950). "A Comet Model. I: the Acceleration of Comet Encke". Astrophysical Journal.
12. Whipple, F. (1951). "A Comet Model. II: Physical Relations for Comets and Meteors". Astrophysical Journal.
13. Delsemme, A. (1 Jul 1972). "Present Understanding of Comets". Comets: Scientific Data and Missions.
14. Wood, J. (Dec 1986). "Comet nucleus models: a review".
15. (1987). "ESA SP-278: Symposium on the Diversity and Similarity of Comets". ESA.
16. Rickman, H. (2017). "Origin and Evolution of Comets: 10 years after the Nice Model, and 1 year after Rosetta". World Scientific Publishing Co Singapore.
17. (1982). "Remote comets and related bodies: VJHK colorimetry and surface materials". Icarus.
18. (1984). "An idealized short-period comet model". Icarus.
19. (1985). "Color, albedo, and nucleus size of Halley's comet". Nature.
20. Greenberg, J. (May 1986). "Predicting that comet Halley is dark". Nature.
21. Rickman, H. (2017). "Origin and Evolution of Comets: 10 years after the Nice Model, and 1 year after Rosetta". World Scientific Publishing Co Singapore.
22. (1986). "ESA SP-250 Vol. III". ESA.
23. Whipple, F. (Oct 1987). "The Cometary Nucleus - Current Concepts". Astronomy & Astrophysics.
24. (2008). "Deep Impact and the Origin and Evolution of Cometary Nuclei". Space Science Reviews.
25. (Feb 2009). "Tensile strength as an indicator of the degree of primitiveness of undifferentiated bodies". Planet. Space Sci..
26. (2004). "Comets II". University of Arizona Press.
27. (Feb 2019). "Experiment on cometary activity: ejection of dust aggregates from a sublimating water-ice surface". Mon. Not. R. Astron. Soc..
28. (23 Jan 2015). "Dust Measurements in the coma of comet 67P/Churyumov–Gerasimenko inbound to the Sun". Science.
29. (2017). "The dust-to-ices ratio in comets and Kuiper belt objects". Mon. Not. R. Astron. Soc..
30. (Apr 2016). "Evolution of the dust size distribution of comet 67P/C–G from 2.2au to perihelion". Astrophysical Journal.
31. (Nov 2016). "Unexpected and significant findings in comet 67P/Churyumov–Gerasimenko: an interdisciplinary view". Mon. Not. R. Astron. Soc..
32. (Jan 2019). "The refractory-to-ice mass ratio in comets". Mon. Not. R. Astron. Soc..
33. (2020). "Dust-to-Gas and Refractory-to-ice Mass Ratios of Comet 67P/Churyumov–Gerasimenko from Rosetta Obs". Space Sci Rev.
34. (29 May 2015). "How comets were assembled". University of Bern.
35. (June 2015). "The shape and structure of cometary nuclei as a result of low-velocity accretion". [[Science (journal).
36. Weidenschilling, S. J.. (June 1997). "The Origin of Comets in the Solar Nebula: A Unified Model". Icarus.
37. Choi, Charles Q.. (15 November 2014). "Comets: Facts About The 'Dirty Snowballs' of Space". Space.com.
38. (20 July 2000). "Determining the ages of comets from the fraction of crystalline dust". [[Nature (journal).
39. "How Asteroids and Comets Formed". Science Clarified.
40. (2007). "Encyclopedia of the Solar System". Academic Press.
41. (2015). "Origi and Evolu of the Cometar Reserv". Space Science Reviews.
42. Meech, K. (2017). "Setting the scene: what did we know before Rosetta?". Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences.
43. (2020). "Potential Themis-family Asteroid Contribution to the Jupiter-family Comet Population". The Astronomical Journal.
44. Fernández, Yanga R.. (2002). "The Nucleus of Comet Hale-Bopp (C/1995 O1): Size and Activity". Earth, Moon, and Planets.
45. (25 September 2007). "SOHO's new catch: its first officially periodic comet". European Space Agency.
46. (1988). "Is the nucleus of Comet Halley a low density body?". Nature.
47. "Comet 9P/Tempel 1". The Planetary Society.
48. "Comet 81P/Wild 2". The Planetary Society.
49. Baldwin, Emily. (6 October 2014). "Measuring Comet 67P/C-G". European Space Agency.
50. Baldwin, Emily. (21 August 2014). "Determining the mass of comet 67P/C-G". European Space Agency.
51. National Aeronautics and Space Administration. (15 May 2023). "Comet Spectra Comparison".
52. (Dec 1986). "ESA Proceedings of an ESA workshop on the Comet Nucleus Sample Return Mission". ESA.
53. (Feb 2019). "Experiments on cometary activity: ejection of dust aggregates from a sublimating water-ice surface". Mon. Not. R. Astron. Soc..
54. (May 2017). "The composition of cometary ices". Philos. Trans. R. Soc. A.
55. (Feb 2019). "Water Reservoirs in Small Planetary Bodies: Meteorites, Asteroids, and Comets". Space Science Reviews.
56. (May 2017). "Comets: looking ahead". Philos. Trans. R. Soc. A.
57. (2007). "Protostars and Planets V". University of Arizona Press.
58. (2017). "The dust-to-ices ratio in comets and Kuiper belt objects". Mon. Not. R. Astron. Soc..
59. (2019). "Comet 67P/CG Nucleus Composition and Comparison to Other Comets". Space Science Reviews.
60. (2018). "Comet formation in collapsing pebble clouds What cometary bulk density implies for the cloud mass and dust-to-ice ratio". Astronomy & Astrophysics.
61. Borenstein, Seth. (10 December 2014). "The mystery of where Earth's water came from deepens". Excite News.
62. (10 December 2014). "Rosetta Instrument Reignites Debate on Earth's Oceans". [[NASA]].
63. astrophysique, Observatoire de Paris-PSL- Centre de recherche en astronomie et. (13 September 2017). "Is the organic matter in comets older than the solar system ?".
64. Jordans, Frank. (30 July 2015). "Philae probe finds evidence that comets can be cosmic labs". The Washington Post.
65. (30 July 2015). "Science on the Surface of a Comet". European Space Agency.
66. (31 July 2015). "Philae's First Days on the Comet – Introduction to Special Issue". [[Science (journal).
67. (13 January 2016). "Exposed ice on Rosetta's comet confirmed as water". European Space Agency.
68. (13 January 2016). "Exposed water ice on the nucleus of comet 67P/Churyumov–Gerasimenko". [[Nature (journal).
69. Baldwin, Emily. (18 November 2014). "Philae settles in dust-covered ice". European Space Agency.
70. Krishna Swamy, K. S.. (May 1997). "Physics of Comets". World Scientific.
71. Khan, Amina. (31 July 2015). "After a bounce, Rosetta". [[Los Angeles Times]].
72. (2017). "How primordial is the structure of comet 67P? Combined collisional and dynamical models suggest a late formation". Astronomy & Astrophysics.
73. (2018). "Catastrophic Disruptions As The Origin Of 67PC-G And Small Bilobate Comets".
74. (2020). "Cometary Nuclei- From Giotto to Rosetta". Space Science Reviews.
75. Yeomans, Donald K.. (2005). "Comets (World Book Online Reference Center 125580)". NASA.
76. Donald K. Yeomans. (1998). "Great Comets in History". Jet Propulsion Laboratory.
77. (2006). "The Size-Frequency Distribution of Dormant Jupiter Family Comets". Icarus.
78. esa. "Giotto overview". European Space Agency.
79. The data was a revelation, showing for the first time the jets, the low-albedo surface, and [[organic compound]]s.Organic compounds (usually referred to as organics) does not imply life, it is just a class of chemicals: see [[Organic chemistry]].