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The Late Heavy Bombardment (LHB), or lunar cataclysm, is a hypothesized astronomical event thought to have occurred approximately 4.1 to 3.8 billion years (Ga) ago,^[1] at a time corresponding to the Neohadean and Eoarchean eras on Earth. According to the hypothesis, during this interval, a disproportionately large number of asteroids and comets collided into the terrestrial planets and their natural satellites in the inner Solar System, including Mercury, Venus, Earth (and the Moon) and Mars.^[2] These came from both post-accretion and planetary instability-driven populations of impactors.^[3] Although it gained widespread credence,^[4] definitive evidence remains elusive.^[5]

Evidence for the LHB derives from moon rock samples of Lunar craters brought back by the Apollo program astronauts. Isotopic dating showed that the rocks were last molten during impact events in a rather narrow interval of time, suggesting that a large proportion of craters were formed during this period. Several hypotheses attempt to explain this apparent spike in the flux of impactors in the inner Solar System, but no consensus yet exists. The Nice model, popular among planetary scientists, postulates that the giant planets underwent orbital migration, scattering objects from the asteroid belt, Kuiper belt, or both, into eccentric orbits and into the path of the terrestrial planets.^[3]

Other researchers doubt the heavy bombardment, arguing for example that the apparent clustering of lunar impact-melt ages is a statistical artifact produced by sampling rocks scattered from a single large impact.^[1] A range of evidence suggests that there may instead have been a more extended period of lunar bombardment, lasting from approximately 4.2 billion years ago to 3.5 billion years ago.^[6]

Evidence for a cataclysm

The main piece of evidence for a lunar cataclysm comes from the radiometric ages of impact melt rocks that were collected during the Apollo missions. The majority of these impact melts are thought to have formed during the collision of asteroids or comets tens of kilometres across, forming impact craters hundreds of kilometres in diameter. The Apollo 15, 16, and 17 landing sites were chosen as a result of their proximity to the Imbrium, Nectaris, and Serenitatis basins, respectively.^[7]

The apparent clustering of ages of these impact melts, between about 3.8 and 4.1 Ga, led investigators to postulate that those ages record an intense bombardment of the Moon.^[8] They named it the "lunar cataclysm" and proposed that it represented a dramatic increase in the rate of bombardment of the Moon around 3.9 Ga. If these impact melts were derived from these three basins, then not only did these three prominent impact basins form within a short interval of time, but so did many others based on stratigraphic grounds.^[7] At the time, the hypothesis was considered controversial.

As more data has become available, particularly from lunar meteorites, this hypothesis, while still controversial, has become more popular. The lunar meteorites are thought to randomly sample the lunar surface, and at least some of these should have originated from regions far from the Apollo landing sites. Many of the feldspathic lunar meteorites probably originated from the lunar far side, and impact melts within these have recently been dated. Consistent with the cataclysm hypothesis, none of their ages was found to be older than about 3.9 Ga.^[9]^[10] Nevertheless, the ages do not "cluster" at this date, but span between 2.5 and 3.9 Ga.^[11]

Dating of howardite, eucrite and diogenite (HED) meteorites and H chondrite meteorites originating from the asteroid belt reveal numerous ages from 3.4–4.1 Ga and an earlier peak at 4.5 Ga. The 3.4–4.1 Ga ages has been interpreted as representing an increase in impact velocities as computer simulations using hydrocode^[12] reveal that the volume of impact melt increases 100–1,000 times as the impact velocity increases from the current asteroid belt average of 5 km/s to 10 km/s. Impact velocities above 10 km/s require very high inclinations or the large eccentricities of asteroids on planet-crossing orbits. Such objects are rare in the current asteroid belt but the population would be significantly increased by the sweeping of resonances due to giant planet migration.^[13]

Studies of the highland crater size distributions suggest that the same family of projectiles struck Mercury and the Moon during the Late Heavy Bombardment.^[14] If the history of decay of late heavy bombardment on Mercury also followed the history of late heavy bombardment on the Moon, the youngest large basin discovered, Caloris, is comparable in age to the youngest large lunar basins, Orientale and Imbrium, and all of the plains units are older than 3 billion years.^[15]

Criticisms of the cataclysm hypothesis

While the cataclysm hypothesis has recently become more popular (in the last fifty years), particularly among dynamicists who have identified possible causes for such a phenomenon, it is still controversial and based on debatable assumptions. Two criticisms are that (1) the "cluster" of impact ages could be an artifact of sampling a single basin's ejecta, and (2) that the lack of impact melt rocks older than about 4.1 Ga is related to all such samples having been pulverized, or their ages being reset.^[3]^[7]

The first criticism concerns the origin of the impact melt rocks that were sampled at the Apollo landing sites. While these impact melts have been commonly attributed to having been derived from the closest basin, it has been argued that a large portion of these might instead be derived from the Imbrium basin.^[16] The Imbrium impact basin is the youngest and largest of the multi-ring basins found on the central nearside of the Moon, and quantitative modeling shows that significant amounts of ejecta from this event should be present at all of the Apollo landing sites. According to this alternative hypothesis, the cluster of impact melt ages near 3.9 Ga simply reflects material being collected from a single impact event, and not several. Additional criticism also argues that the age spike at 3.9 Ga identified in ⁴⁰Ar/³⁹Ar dating could also be produced by an episodic early crust formation followed by partial ⁴⁰Ar losses as the impact rate declined.^[17]

A second criticism concerns the significance of the lack of impact melt rocks older than about 4.1 Ga. One hypothesis for this observation that does not involve a cataclysm is that old melt rocks did exist, but that their radiometric ages have all been reset by the continuous effects of impact cratering over the past 4 billion years. Furthermore, it is possible that these putative samples could all have been pulverized to such small sizes that it is impossible to obtain age determinations using standard radiometric methods.^[18] Scientists continue to study the bombardment history of the moon in an attempt to clarify the history of the inner solar system.^[7]^[3]

Geological consequences on Earth

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*Ice Ages

If a cataclysmic cratering event truly occurred on the Moon, Earth would have been affected as well. Extrapolating lunar cratering rates^[19] to Earth at this time suggests that the following number of craters would have formed:^[20]

22,000 or more impact craters with diameters >20 km (12 mi),
about 40 impact basins with diameters about 1,000 km (620 mi),
several impact basins with diameters about 5,000 km (3,100 mi),

Before the formulation of the LHB hypothesis, geologists generally assumed that Earth remained molten until about 3.8 Ga. This date could be found in many of the oldest-known rocks from around the world, and appeared to represent a strong "cutoff point" beyond which older rocks could not be found. These dates remained fairly constant even across various dating methods, including the system considered the most accurate and least affected by environment, uranium–lead dating of zircons. As no older rocks could be found, it was generally assumed that Earth had remained molten until this date, which defined the boundary between the earlier Hadean and later Archean eons. Nonetheless, in 1999, the oldest known rock on Earth was dated to be 4.031 ± 0.003 billion years old, and is part of the Acasta Gneiss of the Slave Craton in northwestern Canada.^[21]

Older rocks could be found, however, in the form of asteroid fragments that fall to Earth as meteorites. Like the rocks on Earth, asteroids also show a strong cutoff point, at about 4.6 Ga, which is assumed to be the time when the first solids formed in the protoplanetary disk around the then-young Sun. The Hadean, then, was the period of time between the formation of these early rocks in space, and the eventual solidification of Earth's crust, some 700 million years later. This time would include the accretion of the planets from the disk and the slow cooling of Earth into a solid body as the gravitational potential energy of accretion was released.

Later calculations showed that the rate of collapse and cooling depends on the size of the rocky body. Scaling this rate to an object of Earth mass suggested very rapid cooling, requiring only 100 million years.^[22] The difference between measurement and theory presented a conundrum at the time.

The LHB offers a potential explanation for this anomaly. Under this model, the rocks dating to 3.8 Ga solidified only after much of the crust was destroyed by the LHB. Collectively, the Acasta Gneiss in the North American cratonic shield and the gneisses within the Jack Hills portion of the Narryer Gneiss Terrane in Western Australia are the oldest continental fragments on Earth, yet they appear to post-date the LHB. The oldest mineral yet dated on Earth, a 4.404 Ga zircon from Jack Hills, predates this event, but it is likely a fragment of crust left over from before the LHB, contained within a much younger (~3.8 Ga old) rock.^{[citation needed]}

The Jack Hills zircon led to an evolution in understanding of the Hadean eon.^[23] Older references generally show that Hadean Earth had a molten surface with prominent volcanos. The name "Hadean" itself refers to the "hellish" conditions assumed on Earth for the time, from the Greek Hades. Zircon dating suggested, albeit controversially, that the Hadean surface was solid, temperate, and covered by acidic oceans. This picture derives from the presence of particular isotopic ratios that suggest the action of water-based chemistry at some time before the formation of the oldest rocks (see Cool early Earth).^[24]

Of particular interest, Manfred Schidlowski argued in 1979 that the carbon isotopic ratios of some sedimentary rocks found in Greenland were a relic of organic matter: the ratio of carbon-12 to carbon-13 was unusually high, normally a sign of "processing" by life. There was much debate over the precise dating of the rocks, with Schidlowski suggesting they were about 3.8 Ga old, and others suggesting a more "modest" 3.6 Ga. In either case it was a very short time for abiogenesis to have taken place, and if Schidlowski was correct, arguably too short a time. The Late Heavy Bombardment and the "re-melting" of the crust that it suggests provides a timeline under which this would be possible: life either formed immediately after the Late Heavy Bombardment, or more likely survived it, having arisen earlier during the Hadean. A 2002 study suggest that the rocks Schidlowski found are indeed from the older end of the possible age range at about 3.85 Ga, suggesting the latter possibility is the most likely answer.^[25] Studies from 2005, 2006 and 2009 have found no evidence for the isotopically-light carbon ratios that were the basis for the original claims of early Hadean life.^[26]^[27]^[28] However, a similar study of Jack Hills rocks from 2008 shows traces of the same sort of potential organic indicators. Thorsten Geisler of the Institute for Mineralogy at the University of Münster studied traces of carbon trapped in small pieces of diamond and graphite within zircons dating to 4.25 Ga.^[29]

Three-dimensional computer models developed in May 2009 by a team at the University of Colorado at Boulder postulate that much of Earth's crust, and the microbes living in it, could have survived the bombardment. Their models suggest that although the surface of Earth would have been sterilized, hydrothermal vents below Earth's surface could have incubated life by providing a sanctuary for thermophile microbes.^[30] In April 2014, scientists reported finding evidence of the largest terrestrial meteor impact event to date near the Barberton Greenstone Belt. They estimated the impact occurred about 3.26 billion years ago and that the impactor was approximately 37 to 58 kilometres (23 to 36 miles) wide. The crater from this event, if it still exists, has not yet been found.^[31]

Possible causes

Giant-planet migration

Simulation showing outer planets and planetesimal belt: (a) Early configuration, before Jupiter (green) and Saturn (orange) reach 2:1 resonance; (b) Scattering of planetesimals into the inner Solar System after the orbital shift of Neptune (dark blue) and Uranus (light blue); (c) After ejection of planetesimals by planets.^[32]

In the Nice model, the Late Heavy Bombardment is the result of a dynamical instability in the outer Solar System. The original Nice model simulations by Gomes et al. began with the Solar System's giant planets in a tight orbital configuration surrounded by a rich trans-Neptunian belt. Objects from this belt stray into planet-crossing orbits, causing the orbits of the planets to migrate over several hundred million years. Jupiter and Saturn's orbits drift apart slowly until they cross a 2:1 orbital resonance, causing the eccentricities of their orbits to increase. The orbits of the planets become unstable and Uranus and Neptune are scattered onto wider orbits that disrupt the outer belt, causing a bombardment of comets as they enter planet-crossing orbits. Interactions between the objects and the planets also drive a faster migration of Jupiter and Saturn's orbits. This migration causes resonances to sweep through the asteroid belt, increasing the eccentricities of many asteroids until they enter the inner Solar System and impact the terrestrial planets.^[1]^[32]

The Nice model has undergone some modification since its initial publication. The giant planets now begin in a multi-resonant configuration due to an early gas-driven migration through the protoplanetary disk.^[33] Interactions with the trans-Neptunian belt allow their escape from the resonances after several hundred million years.^[34] The encounters between planets that follow include one between an ice giant and Saturn that propels the ice giant onto a Jupiter-crossing orbit followed by an encounter with Jupiter that drives the ice giant outward. This jumping-Jupiter scenario quickly increases the separation of Jupiter and Saturn, limiting the effects of resonance sweeping on the asteroids and the terrestrial planets.^[35]^[36] While this is required to preserve the low eccentricities of the terrestrial planets and avoid leaving the asteroid belt with too many high-eccentricity asteroids, it also reduces the fraction of asteroids removed from the main asteroid belt, leaving a now-nearly-depleted inner band of asteroids as the primary source of the impactors of the LHB.^[37] The ice giant is often ejected following its encounter with Jupiter, leading some to propose that the Solar System began with five giant planets.^[38] Recent^[when?] works, however, have found that impacts from this inner asteroid belt would be insufficient to explain the formation of ancient impact spherule beds and the lunar basins,^[39] and that the asteroid belt was probably not the source of the Late Heavy Bombardment.^[40]

Late formation of Uranus and Neptune

According to one planetesimal simulation of the establishment of the planetary system, the outermost planets Uranus and Neptune formed very slowly, over a period of several billion years.^[41] Harold Levison and his team have also suggested that the relatively low density of material in the outer Solar System during planet formation would have greatly slowed their accretion.^[42] The late formation of these planets has therefore been suggested as a different reason for the LHB. However, recent^[when?] calculations of gas-flows combined with planetesimal runaway growth in the outer Solar System imply that Jovian planets formed extremely rapidly, on the order of 10 My, which does not support this explanation for the LHB.

Planet V hypothesis

The Planet V hypothesis posits that a fifth terrestrial planet caused the Late Heavy Bombardment when its meta-stable orbit entered the inner asteroid belt. The hypothetical fifth terrestrial planet, Planet V, had a mass less than half of Mars and originally orbited between Mars and the asteroid belt. Planet V's orbit became unstable due to perturbations from the other inner planets causing it to intersect the inner asteroid belt. After close encounters with Planet V, many asteroids entered Earth-crossing orbits, causing the Late Heavy Bombardment. Planet V was ultimately lost, likely plunging into the Sun. In numerical simulations, an uneven distribution of asteroids, with the asteroids heavily concentrated toward the inner asteroid belt, has been shown to be necessary to produce the LHB via this mechanism.^[43] An alternate version of this hypothesis in which the lunar impactors are debris resulting from Planet V impacting Mars, forming the Borealis Basin, has been proposed to explain a low number of giant lunar basins relative to craters and a lack of evidence of cometary impactors.^[44]^[45]

Disruption of Mars-crossing asteroid

A hypothesis proposed by Matija Ćuk posits that the last few basin-forming impacts were the result of the collisional disruption of a large Mars-crossing asteroid. This Vesta-sized asteroid was a remnant of a population which initially was much larger than the current main asteroid belt. Most of the pre-Imbrium impacts would have been due to these Mars-crossing objects, with the early bombardment extending until 4.1 billion years ago. A period without many basin-forming impacts then followed, during which the lunar magnetic field decayed. Then, roughly 3.9 billion years ago, a catastrophic impact disrupted the Vesta-sized asteroid, significantly increasing the population of Mars-crossing objects. Many of these objects then evolved onto Earth-crossing orbits, producing a spike in the lunar impact rate during which the last few lunar impact basins are formed. Ćuk points to the weak or absent residual magnetism of the last few basins and a change in the size–frequency distribution of craters which formed during this late bombardment as evidence supporting this hypothesis.^[46] The timing^[47]^[48]^[49]^[50] and the cause^[51] of the change in the size–frequency distribution of craters is controversial.

Other potential sources

A number of other possible sources of the Late Heavy Bombardment have been investigated. Among these are additional Earth satellites orbiting independently or as lunar trojans, planetesimals left over from the formations of the terrestrial planets, Earth or Venus co-orbitals, and the breakup of a large main belt asteroid. Additional Earth satellites on independent orbits were shown to be quickly captured into resonances during the Moon's early tidally-driven orbital expansion and were lost or destroyed within a few million years.^[52] Lunar trojans were found to be destabilized within 100 million years by a solar resonance when the Moon reached 27 Earth radii.^[53] Planetesimals left over from the formation of the terrestrial planets were shown to be depleted too rapidly due to collisions and ejections to form the last lunar basins.^[54] The long-term stability of primordial Earth or Venus co-orbitals (trojans or objects with horseshoe orbits) in conjunction with the lack of current observations indicate that they were unlikely to have been common enough to contribute to the LHB.^[55] Producing the LHB from the collisional disruption of a main belt asteroid was found to require at minimum a 1,000–1,500 km parent body with the most favorable initial conditions.^[56] Debris produced by collisions among inner planets, now lost, has also been proposed as a source of the LHB.^[57]

Exosystem with possible Late Heavy Bombardment

Evidence has been found for Late Heavy Bombardment-like conditions around the star Eta Corvi.^[58]

References

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^ Johnson, Brandon C.; Collins, Garath S.; Minton, David A.; Bowling, Timothy J.; Simonson, Bruce M.; Zuber, Maria T. (2016). "Spherule layers, crater scaling laws, and the population of ancient terrestrial impactors". Icarus. 271: 350–359. Bibcode:2016Icar..271..350J. doi:10.1016/j.icarus.2016.02.023. hdl:10044/1/29965.
^ Nesvorný, David; Roig, Fernando; Bottke, William F. (2016). "Modeling the historical flux of planetary impactors". The Astronomical Journal. 153 (3): 103. arXiv:1612.08771. Bibcode:2017AJ....153..103N. doi:10.3847/1538-3881/153/3/103. S2CID 119028988.
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^ G. J. Taylor (August 21, 2001). "Uranus, Neptune, and the Mountains of the Moon". Planetary Science Research Discoveries.
^ Brasser, R; Morbidelli, A. (2011). "The terrestrial Planet V hypothesis as the mechanism for the origin of the late heavy bombardment". Astronomy & Astrophysics. 535: A41. Bibcode:2011A&A...535A..41B. doi:10.1051/0004-6361/201117336.
^ Minton, D. A.; Jackson, A. P.; Asphaug, E.; Fassett, C. I.; Richardson, J. E. (2015). "Debris from Borealis Basin Formation as the Primary Impactor Population of Late Heavy Bombardment" (PDF). Workshop on Early Solar System Impact Bombardment III. 1826: 3033. Bibcode:2015LPICo1826.3033M.
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^ Fassett, C. I.; Head, J. W.; Kadish, S. J.; Mazarico, E.; Neumann, G. A.; Smith, D. E.; Zuber, M. T. (2012). "Lunar impact basins: Stratigraphy, sequence and ages from superposed impact crater populations measured from Lunar Orbiter Laser Altimeter (LOLA) data". Journal of Geophysical Research. 117 (E12): n/a. Bibcode:2012JGRE..117.0H06F. doi:10.1029/2011JE003951. hdl:1721.1/85892. S2CID 17500741.
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^ Cuk, M. (2008). "Orbital Evolution of the Moon and the Lunar Cataclysm" (PDF). Workshop on the Early Solar System Impact Bombardment. 1439: 29. Bibcode:2008LPICo1439...29C.
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[22] Lithosphere-Hydrosphere Interactions on the Hadean (>4 Ga) Earth, covers many of the Hadean issues and timelines in depth

[23] Revising Earth's Early History

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[NS-20080702-29] Courtland, Rachel (July 2, 2008). "Did newborn Earth harbour life?". New Scientist. Retrieved April 13, 2014.

[RN-20090520-30] Steenhuysen, Julie (May 20, 2009). "Study turns back clock on origins of life on Earth". Reuters. Retrieved April 13, 2014.

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[Gomes05-32] Gomes, R.; Levison, H. F.; Tsiganis, K.; Morbidelli, A. (2005). "Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets". Nature. 435 (7041): 466–469. Bibcode:2005Natur.435..466G. doi:10.1038/nature03676. PMID 15917802.

[Morbidelli_etal_2007-33] Morbidelli, Alessandro; Tsiganis, Kleomenis; Crida, Aurélien; Levison, Harold F.; Gomes, Rodney (2007). "Dynamics of the Giant Planets of the Solar System in the Gaseous Protoplanetary Disk and Their Relationship to the Current Orbital Architecture". The Astronomical Journal. 134 (5): 1790–1798. arXiv:0706.1713. Bibcode:2007AJ....134.1790M. doi:10.1086/521705. S2CID 2800476.

[Levison_2011-34] Levison, Harold F.; Morbidelli, Alessandro; Tsiganis, Kleomenis; Nesvorný, David; Gomes, Rodney (2011). "Late Orbital Instabilities in the Outer Planets Induced by Interaction with a Self-gravitating Planetesimal Disk". The Astronomical Journal. 142 (5): 152. Bibcode:2011AJ....142..152L. doi:10.1088/0004-6256/142/5/152.

[Brasser_2009-35] Brasser, R.; Morbidelli, A.; Gomes, R.; Tsiganis, K.; Levison, H.F. (2009). "Constructing the secular architecture of the solar system II: the terrestrial planets". Astronomy and Astrophysics. 507 (2): 1053–1065. arXiv:0909.1891. Bibcode:2009A&A...507.1053B. doi:10.1051/0004-6361/200912878. S2CID 2857006.

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[Johnson_etal_2016-39] Johnson, Brandon C.; Collins, Garath S.; Minton, David A.; Bowling, Timothy J.; Simonson, Bruce M.; Zuber, Maria T. (2016). "Spherule layers, crater scaling laws, and the population of ancient terrestrial impactors". Icarus. 271: 350–359. Bibcode:2016Icar..271..350J. doi:10.1016/j.icarus.2016.02.023. hdl:10044/1/29965.

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[41] Nakano, Takenori (1 January 1987). "Formation of planets around stars of various masses. I – Formulation and a star of one solar mass". Monthly Notices of the Royal Astronomical Society. 224: 107–130. Bibcode:1987MNRAS.224..107N. doi:10.1093/mnras/224.1.107 – via NASA ADS.

[42] G. J. Taylor (August 21, 2001). "Uranus, Neptune, and the Mountains of the Moon". Planetary Science Research Discoveries.

[Brasser_2011-43] Brasser, R; Morbidelli, A. (2011). "The terrestrial Planet V hypothesis as the mechanism for the origin of the late heavy bombardment". Astronomy & Astrophysics. 535: A41. Bibcode:2011A&A...535A..41B. doi:10.1051/0004-6361/201117336.

[Minton_etal_2015-44] Minton, D. A.; Jackson, A. P.; Asphaug, E.; Fassett, C. I.; Richardson, J. E. (2015). "Debris from Borealis Basin Formation as the Primary Impactor Population of Late Heavy Bombardment" (PDF). Workshop on Early Solar System Impact Bombardment III. 1826: 3033. Bibcode:2015LPICo1826.3033M.

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[Cuk_etal_2010-47] Ćuk, Matija; Gladman, Brett J.; Stewart, Sarah T. (2010). "Constraints on the source of lunar cataclysm impactors". Icarus. 207 (2): 590–594. arXiv:0912.1847. Bibcode:2010Icar..207..590C. doi:10.1016/j.icarus.2009.12.013.

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[Cuk_etal_2011-49] Ćuk, Matija; Gladman, Brett J.; Stewart, Sarah T. (2011). "Rebuttal to the comment by Malhotra and Strom on "Constraints on the source of lunar cataclysm impactors"". Icarus. 216 (1): 363–365. Bibcode:2011Icar..216..363C. doi:10.1016/j.icarus.2011.08.011.

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[Cuk_etal_2012-55] Cuk, M.; Hamilton, D. P.; Holman, M. J. (2012). "Long-term stability of horseshoe orbits". Monthly Notices of the Royal Astronomical Society. 426 (4): 3051–3056. arXiv:1206.1888. Bibcode:2012MNRAS.426.3051C. doi:10.1111/j.1365-2966.2012.21964.x. S2CID 2614886.

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[Volk_Gladman_2015-57] Volk, Kathryn; Gladman, Brett (2015). "Consolidating and Crushing Exoplanets: Did It Happen Here?". The Astrophysical Journal Letters. 806 (2): L26. arXiv:1502.06558. Bibcode:2015ApJ...806L..26V. doi:10.1088/2041-8205/806/2/L26. S2CID 118052299.

[58] “Spitzer Observations of η Corvi : Evidence at ~1 Gyr for an LHB-Like Delivery of Organics & Water-Rich Material to the THZ of a Sun-Like Star.” C.M. Lisse, C.H. Chen, M.C. Wyatt, A. Morlok, P. Thebault, G. Bryden, D.M. Watson, P. Manoj, P. Sheehan, G. Sloan, T.M. Currie, Linar and Planetary Institute Science Conference Abstracts 42, (March 20, 2011), p. 2438, Bibcode:2011LPI....42.2438L.

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@@ Line 1: / Line 1: @@
+{{Short description|Hypothesized astronomical event}}
-{{short description|Interval when a disproportionately large number of asteroids are theorised to have collided with the inner planets}}
+[[File:Precambrian Evolution of Life.png|thumb|upright=1.1|Timescale]]
-[[File:Lunar cataclysm.jpg|thumb|upright=1.1|Artist's impression of the Moon during the ''Late Heavy Bombardment'' (above) and today (below).]]
+[[File:Lunar cataclysm.jpg|thumb|upright=1.1|Artist's impression of the Moon during the Late Heavy Bombardment (above) and today (below)]]The '''Late Heavy Bombardment''' ('''LHB'''), or '''lunar cataclysm''', is a hypothesized [[astronomical event]] thought to have occurred approximately 4.1 to 3.8 billion years (Ga) ago,<ref name=taylor1/> at a time corresponding to the [[Neohadean]] and [[Eoarchean]] eras on Earth. According to the hypothesis, during this interval, a disproportionately large number of [[asteroid]]s and [[comet]]s [[impact event|collide]]d into the [[terrestrial planet]]s and their [[natural satellite]]s in the [[inner Solar System]], including [[Mercury (planet)|Mercury]], [[Venus]], [[Earth]] (and the [[Moon]]) and [[Mars]].<ref>{{cite book|title= Encyclopedia of Astrobiology|first1= Philippe|last1= Claeys|first2= Alessandro|last2= Morbidelli|editor-first1= Muriel|editor-last1= Gargaud|editor-first2= Prof Ricardo|editor-last2= Amils|editor-first3= José Cernicharo|editor-last3= Quintanilla|editor-first4= Henderson James (Jim) |editor-last4= Cleaves II|editor-first5= William M.|editor-last5= Irvine|editor-first6= Prof Daniele L.|editor-last6= Pinti|editor-first7= Michel|editor-last7= Viso|date= 1 January 2011|publisher= Springer Berlin Heidelberg|pages= 909–912|doi= 10.1007/978-3-642-11274-4_869|chapter= Late Heavy Bombardment|isbn= 978-3-642-11271-3}}</ref> These came from both [[protoplanetary disk|post-accretion]] and [[formation and evolution of the Solar System#Planetary migration|planetary instability]]-driven populations of [[impact event|impactors]].<ref name="Bottke">{{cite journal |last1=Bottke |first1=William F. |last2=Norman |first2=Marc D. |title=The Late Heavy Bombardment |journal=Annual Review of Earth and Planetary Sciences |date=30 August 2017 |volume=45 |issue=1 |pages=619–647 |doi=10.1146/annurev-earth-063016-020131 |bibcode=2017AREPS..45..619B |url=https://doi.org/10.1146/annurev-earth-063016-020131 |access-date=11 August 2022 |language=en |issn=0084-6597}}</ref>  Although it gained widespread credence,<ref>{{cite journal |last1=Boehnke |first1=Patrick |last2=Harrison |first2=T. Mark |title=Illusory Late Heavy Bombardments |journal=Proceedings of the National Academy of Sciences |date=27 September 2016 |volume=113 |issue=39 |pages=10802–10806 |doi=10.1073/pnas.1611535113 |pmid=27621460 |pmc=5047187 |bibcode=2016PNAS..11310802B |language=en |issn=0027-8424|doi-access=free }}</ref> definitive evidence remains elusive.<ref>{{Cite journal|last=Mann|first=Adam|date=2018-01-24|title=Bashing holes in the tale of Earth's troubled youth|journal=Nature|language=EN|volume=553|issue=7689|pages=393–395|doi=10.1038/d41586-018-01074-6|pmid=29368708|bibcode=2018Natur.553..393M|doi-access=free}}</ref>
+Evidence for the LHB derives from [[moon rock]] samples of [[Lunar craters]] brought back by the [[Apollo program]] [[astronaut]]s. [[Radiometric dating|Isotopic dating]] showed that the rocks were last molten during impact events in a rather narrow interval of time, suggesting that a large proportion of craters were formed during this period. Several hypotheses attempt to explain this apparent spike in the flux of impactors in the inner Solar System, but no consensus yet exists. The [[Nice model]], popular among [[Planetary science|planetary scientists]], postulates that the [[giant planet]]s underwent [[Planetary migration|orbital migration]], scattering objects from the [[asteroid belt]], [[Kuiper belt]], or both, into eccentric orbits and into the path of the terrestrial planets.<ref name="Bottke"/>
-The '''Late Heavy Bombardment''' (abbreviated '''LHB''' and also known as the '''lunar cataclysm''') is a hypothesized event thought to have occurred approximately 4.1 to 3.8 billion [[year]]s (Ga) ago,<ref name=taylor1/> at a time corresponding to the [[Neohadean]] and [[Eoarchean]] eras on Earth. During this interval, a disproportionately large number of [[asteroid]]s are theorized to have collided with the early [[terrestrial planet]]s in the inner [[Solar System]], including [[Mercury (planet)|Mercury]], [[Venus]], [[Earth]], and [[Mars]].<ref>{{cite book|title= Encyclopedia of Astrobiology|first1= Philippe|last1= Claeys|first2= Alessandro|last2= Morbidelli|editor-first1= Muriel|editor-last1= Gargaud|editor-first2= Prof Ricardo|editor-last2= Amils|editor-first3= José Cernicharo|editor-last3= Quintanilla|editor-first4= Henderson James (Jim) |editor-last4= Cleaves II|editor-first5= William M.|editor-last5= Irvine|editor-first6= Prof Daniele L.|editor-last6= Pinti|editor-first7= Michel|editor-last7= Viso|date= 1 January 2011|publisher= Springer Berlin Heidelberg|pages= 909–912|doi= 10.1007/978-3-642-11274-4_869|chapter= Late Heavy Bombardment|isbn= 978-3-642-11271-3}}</ref> Since 2018, the existence of the late-heavy bombardment has been questioned.<ref>{{Cite journal|last=Mann|first=Adam|date=2018-01-24|title=Bashing holes in the tale of Earth’s troubled youth|url=http://www.nature.com/articles/d41586-018-01074-6|journal=Nature|language=EN|volume=553|pages=393|doi=10.1038/d41586-018-01074-6}}</ref>
-The Late Heavy Bombardment happened after the Earth and other rocky planets had formed and [[Accretion (astrophysics)|accreted]] most of their mass, but still quite early in Earth's history.
-Evidence for the LHB derives from lunar samples brought back by the [[Apollo program|Apollo]] astronauts. [[Radiometric dating|Isotopic dating]] of [[Moon]] rocks implies that most impact melts occurred in a rather narrow interval of time. Several hypotheses attempt to explain the apparent spike in the flux of impactors (i.e. [[asteroid]]s and [[comet]]s) in the inner Solar System, but no consensus yet exists. The [[Nice model]], popular among [[Planetary science|planetary scientists]], postulates that the [[giant planet]]s underwent [[Planetary migration|orbital migration]] and in doing so, scattered objects in the [[asteroid belt|asteroid]] and/or [[Kuiper belt]]s into eccentric orbits, and into the path of the terrestrial planets. Other researchers argue that the lunar sample data do not require a cataclysmic cratering event near 3.9&nbsp;Ga, and that the apparent clustering of impact-melt ages near this time is an artifact of sampling materials retrieved from a single large impact basin.<ref name=taylor1/> They also note that the rate of impact cratering could differ significantly between the outer and inner zones of the Solar System.<ref>
-{{cite journal
-| last1 = Zahnle | first1 = K.
-| display-authors = etal   | date = 2003
-| title = Cratering rates in the outer Solar System
-| url = | journal = Icarus
-| volume = 163 | issue = 2| pages = 263–289
-| doi = 10.1016/s0019-1035(03)00048-4
-|bibcode = 2003Icar..163..263Z | citeseerx = 10.1.1.520.2964
-}}
-</ref>
+Other researchers doubt the heavy bombardment, arguing for example that the apparent clustering of lunar impact-melt ages is a statistical artifact produced by sampling rocks scattered from a single large impact.<ref name="taylor1" /> A range of evidence suggests that there may instead have been a more extended period of lunar bombardment, lasting from approximately 4.2 billion years ago to 3.5 billion years ago.<ref name=Zellner>{{Cite journal |last=Zellner |first=Nicolle E. B.|author-link= Nicolle Zellner |date=September 2017 |title=Cataclysm No More: New Views on the Timing and Delivery of Lunar Impactors |journal=Origins of Life and Evolution of Biospheres |language=en |volume=47 |issue=3 |pages=261–280 |doi=10.1007/s11084-017-9536-3 |issn=0169-6149 |pmc=5602003 |pmid=28470374 |arxiv=1704.06694 |bibcode=2017OLEB...47..261Z }}</ref>
 == Evidence for a cataclysm ==
-The main piece of evidence for a lunar cataclysm comes from the [[radiometric dating|radiometric ages]] of impact melt rocks that were collected during the Apollo missions. The majority of these impact melts are believed to have formed during the collision of asteroids or comets tens of kilometres across, forming impact craters hundreds of kilometres in diameter. The [[Apollo 15]], [[Apollo 16|16]], and [[Apollo 17|17]] landing sites were chosen as a result of their proximity to the [[Mare Imbrium|Imbrium]], [[Mare Nectaris|Nectaris]], and [[Mare Serenitatis|Serenitatis]] basins, respectively.
+The main piece of evidence for a lunar cataclysm comes from the [[radiometric dating|radiometric ages]] of impact melt rocks that were collected during the Apollo missions. The majority of these impact melts are thought to have formed during the collision of asteroids or comets tens of kilometres across, forming impact craters hundreds of kilometres in diameter. The [[Apollo 15]], [[Apollo 16|16]], and [[Apollo 17|17]] landing sites were chosen as a result of their proximity to the [[Mare Imbrium|Imbrium]], [[Mare Nectaris|Nectaris]], and [[Mare Serenitatis|Serenitatis]] basins, respectively.<ref name=Crockett>{{cite journal |last=Crockett |first=Christopher |date=16 July 2019 |title=How the moon landings changed our view of the solar system |journal=[[Knowable Magazine]] |department=Annual reviews |doi=10.1146/knowable-071519-1 |doi-access=free |url=https://knowablemagazine.org/article/physical-world/2019/how-moon-landings-changed-our-view-solar-system |access-date=11 August 2022 |language=en}}</ref>
-The apparent clustering of ages of these impact melts, between about 3.8 and 4.1&nbsp;Ga, led to postulation that the ages record an intense bombardment of the [[Moon]].<ref>{{Cite journal |last=Tera |first=F. |last2=Papanastassiou |first2=D.A. |last3=Wasserburg |first3=G.J.| date=1974 |title=Isotopic evidence for a terminal lunar cataclysm |journal=Earth Planet. Sci. Lett.|volume=22 | issue=22| pages=1–21 | doi=10.1016/0012-821x(74)90059-4|bibcode=1974E&PSL..22....1T}}</ref> They called it the "lunar cataclysm" and proposed that it represented a dramatic increase in the rate of bombardment of the Moon around 3.9&nbsp;Ga. If these impact melts were derived from these three basins, then not only did these three prominent impact basins form within a short interval of time, but so did many others based on [[Stratigraphy|stratigraphic]] grounds. At the time, the conclusion was considered controversial.
+The apparent clustering of ages of these impact melts, between about 3.8 and 4.1&nbsp;Ga, led investigators to postulate that those ages record an intense bombardment of the [[Moon]].<ref>{{cite journal | last1=Tera | first1=F. | last2=Papanastassiou | first2=D.A. | last3=Wasserburg | first3=G.J. | year=1974 |title=Isotopic evidence for a terminal lunar cataclysm |journal=[[Earth and Planetary Science Letters]] | volume=22 | issue=22 | pages=1–21 | doi=10.1016/0012-821x(74)90059-4 |bibcode=1974E&PSL..22....1T}}</ref> They named it the "lunar cataclysm" and proposed that it represented a dramatic increase in the rate of bombardment of the Moon around 3.9&nbsp;Ga. If these impact melts were derived from these three basins, then not only did these three prominent impact basins form within a short interval of time, but so did many others based on [[Stratigraphy|stratigraphic]] grounds.<ref name=Crockett/>  At the time, the hypothesis was considered controversial.
-As more data has become available, particularly from [[lunar meteorite]]s, this theory, while still controversial, has gained in popularity. The lunar meteorites are believed to randomly sample the lunar surface, and at least some of these should have originated from regions far from the Apollo landing sites. Many of the [[Feldspar|feldspathic]] lunar meteorites probably originated from the lunar far side, and impact melts within these have recently been dated. Consistent with the cataclysm hypothesis, none of their ages was found to be older than about 3.9&nbsp;Ga.<ref>{{Cite journal |last=Cohen |first=B. A. |last2=Swindle |first2=T. D. |last3=Kring |first3=D. A. |date=2000 |title=Support for the Lunar Cataclysm Hypothesis from Lunar Meteorite Impact Melt Ages |journal=[[Science (journal)|Science]] |volume=290 |issue=5497 |pages=1754–1755 |doi=10.1126/science.290.5497.1754 |postscript=<!--None--> |pmid=11099411|bibcode = 2000Sci...290.1754C }}</ref> Nevertheless, the ages do not "cluster" at this date, but span between 2.5 and 3.9&nbsp;Ga.<ref>{{Cite journal |last=Hartmann |first=William K. |last2=Quantin |first2=Cathy |last3=Mangold |first3=Nicolas |date=2007 |volume=186 |issue=1 |pages=11–23 |journal=Icarus |title=Possible long-term decline in impact rates: 2. Lunar impact-melt data regarding impact history |doi=10.1016/j.icarus.2006.09.009 |postscript=<!--None--> |bibcode=2007Icar..186...11H}}</ref>
+As more data has become available, particularly from [[lunar meteorite]]s, this hypothesis, while still controversial, has become more popular. The lunar meteorites are thought to randomly sample the lunar surface, and at least some of these should have originated from regions far from the Apollo landing sites. Many of the [[Feldspar|feldspathic]] lunar meteorites probably originated from the lunar far side, and impact melts within these have recently been dated. Consistent with the cataclysm hypothesis, none of their ages was found to be older than about 3.9&nbsp;Ga.<ref>{{cite journal |last1=Cohen |first1=B.A. |last2=Swindle |first2=T.D. |last3=Kring |first3=D. A. |year=2000 |title=Support for the Lunar cataclysm hypothesis from Lunar meteorite impact melt ages |journal=[[Science (journal)|Science]] |volume=290 |issue=5497 |pages=1754–1755 |doi=10.1126/science.290.5497.1754 |pmid=11099411 |bibcode = 2000Sci...290.1754C }}</ref><ref>{{cite web | last =  Cohen  |first=Barbara | date = 24 January 2001 | title = Lunar meteorites and the Lunar cataclysm | website = Planetary science research discoveries (psrd.hawaii.edu) | publisher = [[University of Hawaii]] | url= http://www.psrd.hawaii.edu/Jan01/lunarCataclysm.html}}</ref> Nevertheless, the ages do not "cluster" at this date, but span between 2.5 and 3.9&nbsp;Ga.<ref>{{cite journal |last1=Hartmann |first1=William K. |last2=Quantin |first2=Cathy |last3=Mangold |first3=Nicolas |year=2007 |title=Possible long-term decline in impact rates: 2.&nbsp;Lunar impact-melt data regarding impact history |journal=[[Icarus (journal)|Icarus]] |volume=186 |issue=1 |pages=11–23  |doi=10.1016/j.icarus.2006.09.009 |bibcode=2007Icar..186...11H}}</ref>
-Dating of [[howardite]], [[eucrite]] and [[diogenite]] (HED) meteorites and [[H chondrite]] meteorites originating from the asteroid belt reveal numerous ages from 3.4–4.1 Ga and an earlier peak at 4.5 Ga. The 3.4–4.1&nbsp;Ga ages has been interpreted as representing an increase in impact velocities as computer simulations using hydrocode reveal that the volume of impact melt increases 100–1,000 times as the impact velocity increases from the current asteroid belt average of 5&nbsp;km/s to 10&nbsp;km/s. Impact velocities above 10&nbsp;km/s require very high inclinations or the large eccentricities of asteroids on planet crossing orbits. Such objects are rare in the current asteroid belt but the population would be significantly increased by the sweeping of resonances due to giant planet migration.<ref name=March_etal_2013>{{cite journal|last=Marchi|first=S. |author2=Bottke, W. F. |author3=Cohen, B. A. |author4=Wünnemann, K. |author5=Kring, D. A. |author6=McSween, H. Y. |author7=de Sanctis, M. C. |author8=O'Brien, D. P. |author9=Schenk, P. |author10=Raymond, C. A. |author11=Russell, C. T. |title=High-velocity collisions from the lunar cataclysm recorded in asteroidal meteorites|journal=Nature Geoscience|date=2013|volume=6|issue=4|pages=303–307|doi=10.1038/ngeo1769|bibcode = 2013NatGe...6..303M }}</ref>
+Dating of [[howardite]], [[eucrite]] and [[diogenite]] ([[HED meteorite|HED]]) meteorites and [[H chondrite]] meteorites originating from the asteroid belt reveal numerous ages from 3.4–4.1&nbsp;Ga and an earlier peak at 4.5&nbsp;Ga. The 3.4–4.1&nbsp;Ga ages has been interpreted as representing an increase in impact velocities as computer simulations using hydrocode<ref>{{cite report |last=Benson |first=David J. |year=1990 |title=Computational methods in Lagrangian and Eulerian hypotheses |url=https://csm.mech.utah.edu/content/wp-content/uploads/2011/03/GoBagComputationalMethodsInLagrangianAndEulerianHydrocodes.pdf |access-date=11 January 2021 |via=csm.mech.utah.edu}} — Compares computer software for modelling asteroid / planetessmial impacts based on deformable rigid bodies (Lagrangian) and fluid-like rubble-piles (Eulerian).</ref> reveal that the volume of impact melt increases 100–1,000&nbsp;times as the impact velocity increases from the current asteroid belt average of 5&nbsp;km/s to 10&nbsp;km/s. Impact velocities above 10&nbsp;km/s require very high inclinations or the large eccentricities of asteroids on planet-crossing orbits. Such objects are rare in the current asteroid belt but the population would be significantly increased by the sweeping of resonances due to giant planet migration.<ref name=March_etal_2013>{{cite journal |last1=Marchi |first1=S. |last2=Bottke |first2=W.F. |last3=Cohen |first3=B.A. |last4=Wünnemann |first4=K. |last5=Kring |first5=D.A. |last6=McSween |first6=H.Y. |last7=de&nbsp;Sanctis |first7=M.C. |last8=O'Brien |first8=D.P. |last9=Schenk |first9=P. |last10=Raymond |first10=C.A. |last11=Russell |first11=C.T. |display-authors=6 |year=2013 |title=High-velocity collisions from the lunar cataclysm recorded in asteroidal meteorites |journal=[[Nature Geoscience]] |volume=6 |issue=4 |pages=303–307 |doi=10.1038/ngeo1769 |bibcode = 2013NatGe...6..303M }}</ref>
-Studies of the highland crater size distributions suggest that the same family of projectiles struck Mercury and the Moon during the ''Late Heavy Bombardment''.<ref name=Strom_1979>{{cite journal|last=Strom|first=R. G.|title=Mercury – A post-Mariner 10 assessment|journal=Space Science Reviews|date=1979|volume=24|issue=1|pages=3–70|doi=10.1007/bf00221842|bibcode=1979SSRv...24....3S}}</ref> If the history of decay of late heavy bombardment on Mercury also followed the history of late heavy bombardment on the Moon, the youngest large basin discovered, [[Caloris Planitia|Caloris]], is comparable in age to the youngest large lunar basins, Orientale and Imbrium, and all of the plains units are older than 3 billion years.<ref name="NASAsp467">{{cite book | title=Planetary Geology in the 1980s | url=https://history.nasa.gov/SP-467/sp467.htm | chapter=section 3.3.1. Chronology of Planetary Surfaces: Mercury | chapter-url=https://history.nasa.gov/SP-467/ch3.htm | publisher=[[NASA]] | author=Veverka, Joseph | year=1985 | location=Washington D.C.}}</ref>
+Studies of the highland crater size distributions suggest that the same family of projectiles struck Mercury and the Moon during the Late Heavy Bombardment.<ref name=Strom_1979>{{cite journal |last=Strom |first=R.G. |year=1979 |title=Mercury – a post-Mariner&nbsp;10 assessment |journal=[[Space Science Reviews]] |volume=24 |issue=1 |pages=3–70 |doi=10.1007/bf00221842 |bibcode=1979SSRv...24....3S |s2cid=122563809}}</ref> If the history of decay of late heavy bombardment on Mercury also followed the history of late heavy bombardment on the Moon, the youngest large basin discovered, [[Caloris Planitia|Caloris]], is comparable in age to the youngest large lunar basins, Orientale and Imbrium, and all of the plains units are older than 3 billion years.<ref name=NASAsp467>{{cite report | last=Veverka | first=Joseph |author-link=Joseph Veverka | year=1985 | section=section&nbsp;3.3.1. Chronology of planetary surfaces: Mercury | title=Planetary Geology in the 1980s   | series = History | publisher=[[National Aeronautics and Space Administration]] | place=Washington, DC | url=https://history.nasa.gov/SP-467/sp467.htm | section-url=https://history.nasa.gov/SP-467/ch3.htm}}</ref>
 == Criticisms of the cataclysm hypothesis ==
-While the cataclysm hypothesis has recently gained in popularity, particularly among dynamicists who have identified possible causes for such a phenomenon, the cataclysm hypothesis is still controversial and based on debatable assumptions. Two criticisms are that (1) the "cluster" of impact ages could be an artifact of sampling a single basin's ejecta, and (2) that the lack of impact melt rocks older than about 4.1&nbsp;Ga is related to all such samples having been pulverized, or their ages being reset.
+While the cataclysm hypothesis has recently become more popular (in the last fifty years), particularly among dynamicists who have identified possible causes for such a phenomenon, it is still controversial and based on debatable assumptions. Two criticisms are that (1) the "cluster" of impact ages could be an artifact of sampling a single basin's ejecta, and (2) that the lack of impact melt rocks older than about 4.1&nbsp;Ga is related to all such samples having been pulverized, or their ages being reset.<ref name="Bottke"/><ref name="Crockett"/>
-The first criticism concerns the origin of the impact melt rocks that were sampled at the Apollo landing sites. While these impact melts have been commonly attributed to having been derived from the ''closest basin'', it has been argued that a large portion of these might instead be derived from the Imbrium basin.<ref>{{cite journal | last = L. A. Haskin, R. L. Korotev, R. L. Rockow, B. L. Jolliff | date = 1998 | title = The case for an Imbrium origin of the Apollo thorium-rich impact-melt breccias | first4 = Bradley L. | last4 = Jolliff | first3 = Kaylynn M. | last3 = Rockow | journal = Meteorit. Planet. Sci. | first2 = Randy L. | volume = 33 | last2 = Korotev | pages = 959–979 | doi = 10.1111/j.1945-5100.1998.tb01703.x | first1 = Larry A.|bibcode = 1998M&PS...33..959H | issue = 5 }}</ref> The Imbrium impact basin is the youngest and largest of the [[multi-ring basins]] found on the central nearside of the Moon, and quantitative modeling shows that significant amounts of ejecta from this event should be present at all of the Apollo landing sites. According to this alternative hypothesis, the cluster of impact melt ages near 3.9&nbsp;Ga simply reflects material being collected from a single impact event, Imbrium, and not several. Additional criticism also argues that the age spike at 3.9 Ga identified in 40Ar/39Ar dating could also be produced by an episodic early crust formation followed by partial 40Ar losses as the impact rate declined.<ref>
+The first criticism concerns the origin of the impact melt rocks that were sampled at the Apollo landing sites. While these impact melts have been commonly attributed to having been derived from the closest basin, it has been argued that a large portion of these might instead be derived from the Imbrium basin.<ref>{{cite journal | last1 = L. A. Haskin, R. L. Korotev, R. L. Rockow, B. L. Jolliff | date = 1998 | title = The case for an Imbrium origin of the Apollo thorium-rich impact-melt breccias | first4 = Bradley L. | last4 = Jolliff | first3 = Kaylynn M. | last3 = Rockow | journal = Meteorit. Planet. Sci. | first2 = Randy L. | volume = 33 | last2 = Korotev | pages = 959–979 | doi = 10.1111/j.1945-5100.1998.tb01703.x | first1 = Larry A.|bibcode = 1998M&PS...33..959H | issue = 5 | doi-access =  | s2cid = 129464985 }}</ref> The Imbrium impact basin is the youngest and largest of the [[multi-ring basins]] found on the central nearside of the Moon, and quantitative modeling shows that significant amounts of ejecta from this event should be present at all of the Apollo landing sites. According to this alternative hypothesis, the cluster of impact melt ages near 3.9&nbsp;Ga simply reflects material being collected from a single impact event, and not several. Additional criticism also argues that the age spike at 3.9 Ga identified in <sup>40</sup>Ar/<sup>39</sup>Ar dating could also be produced by an episodic early crust formation followed by partial <sup>40</sup>Ar losses as the impact rate declined.<ref>
 {{cite journal
 | last1 = Boehnke| first1 = P.
@@ Line 41: / Line 29: @@
 | volume = 113 | issue = 39 | pages = 10802–10806
 | doi = 10.1073/pnas.1611535113 | pmid=27621460 | pmc=5047187
-|bibcode = 2016PNAS..11310802B }}
+|bibcode = 2016PNAS..11310802B | doi-access = free
+}}
 </ref>
-A second criticism concerns the significance of the lack of impact melt rocks older than about 4.1&nbsp;Ga. One hypothesis for this observation that does not involve a cataclysm is that old melt rocks did exist, but that their radiometric ages have all been reset by the continuous effects of impact cratering over the past 4 billion years. Furthermore, it is possible that these putative samples could all have been pulverized to such small sizes that it is impossible to obtain age determinations using standard radiometric methods.<ref name=Hartmann_2003>{{cite journal|last=Hartmann|first=W. K.|title=Megaregolith evolution and cratering cataclysm models – Lunar cataclysm as a misconception (28 years later)|journal=Meteoritics & Planetary Science|date=2003|volume=38|issue=4|pages=579–593|doi=10.1111/j.1945-5100.2003.tb00028.x|bibcode = 2003M&PS...38..579H }}</ref>
+A second criticism concerns the significance of the lack of impact melt rocks older than about 4.1&nbsp;Ga. One hypothesis for this observation that does not involve a cataclysm is that old melt rocks did exist, but that their radiometric ages have all been reset by the continuous effects of impact cratering over the past 4 billion years. Furthermore, it is possible that these putative samples could all have been pulverized to such small sizes that it is impossible to obtain age determinations using standard radiometric methods.<ref name=Hartmann_2003>{{cite journal|last=Hartmann|first=W. K.|title=Megaregolith evolution and cratering cataclysm models – Lunar cataclysm as a misconception (28 years later)|journal=Meteoritics & Planetary Science|date=2003|volume=38|issue=4|pages=579–593|doi=10.1111/j.1945-5100.2003.tb00028.x|bibcode = 2003M&PS...38..579H |s2cid=56432789|doi-access=free}}</ref> Scientists continue to study the bombardment history of the moon in an attempt to clarify the history of the inner solar system.<ref name="Crockett"/><ref name="Bottke"/>
-Latest reinterpretation of crater statistics suggests that the flux on the Moon and on Mars may have been lower in general. Thus, the recorded crater population can be explained without any peak in the earliest bombardment of the inner Solar System.
 == Geological consequences on Earth ==
 {{Life timeline}}
-If a [[wikt:cataclysmic|cataclysmic]] cratering event truly occurred on the Moon, the Earth would have been affected as well. Extrapolating lunar cratering rates<ref>{{cite journal|last=Ryder|first=Graham|title=Mass flux in the ancient Earth-Moon system and benign implications for the origin of life on Earth|journal=Journal of Geophysical Research: Planets|date=2002|volume=107|issue=E4|pages=6–1–6–13|doi=10.1029/2001JE001583|bibcode=2002JGRE..107.5022R|hdl=2060/20030071675}}</ref> to Earth at this time suggests that the following number of craters would have formed:<ref>{{Cite journal|last=Ryder |first=G. |title=Heavy Bombardment on the Earth at ~3.85 Ga: The Search for Petrographic and Geochemical Evidence |journal=Origin of the Earth and Moon |page=475 |bibcode=2000orem.book..475R |year=2000 }}</ref>
+If a [[wikt:cataclysmic|cataclysmic]] cratering event truly occurred on the Moon, Earth would have been affected as well. Extrapolating lunar cratering rates<ref>{{cite journal|last=Ryder|first=Graham|title=Mass flux in the ancient Earth-Moon system and benign implications for the origin of life on Earth|journal=Journal of Geophysical Research: Planets|date=2002|volume=107|issue=E4|pages=6–1–6–13|doi=10.1029/2001JE001583|bibcode=2002JGRE..107.5022R|hdl=2060/20030071675|hdl-access=free}}</ref> to Earth at this time suggests that the following number of craters would have formed:<ref>{{Cite book|last=Ryder |first=G. |title=Heavy Bombardment on the Earth at ~3.85 Ga: The Search for Petrographic and Geochemical Evidence |journal=Origin of the Earth and Moon |page=475 |bibcode=2000orem.book..475R |year=2000 |doi=10.2307/j.ctv1v7zdrp.30 }}</ref>
 * 22,000 or more [[impact crater]]s with diameters >{{convert|20|km|abbr=on}},
 * about 40 impact basins with diameters about {{convert|1000|km|abbr=on}},
 * several impact basins with diameters about {{convert|5000|km|abbr=on}},
-Before the formulation of the LHB theory, geologists generally assumed that the Earth remained molten until about 3.8&nbsp;Ga. This date could be found in many of the [[Oldest dated rocks|oldest-known rocks]] from around the world, and appeared to represent a strong "cutoff point" beyond which older rocks could not be found. These dates remained fairly constant even across various dating methods, including the system considered the most accurate and least affected by environment, [[uranium–lead dating]] of [[zircon]]s. As no older rocks could be found, it was generally assumed that the Earth had remained molten until this date, which defined the boundary between the earlier [[Hadean]] and later [[Archean]] eons. Nonetheless, more recently, in 1999, the oldest known rock on Earth was dated to be 4.031 ± 0.003 billion years old, and is part of the [[Acasta Gneiss]] of the [[Slave Craton]] in northwestern Canada.<ref name="Bowring">{{cite journal | doi = 10.1007/s004100050465 | title = Priscoan (4.00–4.03 Ga) orthogneisses from northwestern Canada | year = 1999 | author = Bowring, Samuel A. | journal = Contributions to Mineralogy and Petrology | volume = 134 | issue = 1 | page = 3 | bibcode=1999CoMP..134....3B | last2 = Williams | first2 = Ian S.}}</ref>
+Before the formulation of the LHB hypothesis, geologists generally assumed that Earth remained molten until about 3.8&nbsp;Ga. This date could be found in many of the [[Oldest dated rocks|oldest-known rocks]] from around the world, and appeared to represent a strong "cutoff point" beyond which older rocks could not be found. These dates remained fairly constant even across various dating methods, including the system considered the most accurate and least affected by environment, [[uranium–lead dating]] of [[zircon]]s. As no older rocks could be found, it was generally assumed that Earth had remained molten until this date, which defined the boundary between the earlier [[Hadean]] and later [[Archean]] eons. Nonetheless, in 1999, the [[Oldest dated rocks#Oldest terrestrial material|oldest known rock on Earth]] was dated to be 4.031 ± 0.003 billion years old, and is part of the [[Acasta Gneiss]] of the [[Slave Craton]] in northwestern Canada.<ref name="Bowring">{{cite journal | doi = 10.1007/s004100050465 | title = Priscoan (4.00–4.03 Ga) orthogneisses from northwestern Canada | year = 1999 | author = Bowring, Samuel A. | journal = Contributions to Mineralogy and Petrology | volume = 134 | issue = 1 | page = 3 | bibcode=1999CoMP..134....3B | last2 = Williams | first2 = Ian S.| s2cid = 128376754 }}</ref>
-Older rocks could be found, however, in the form of [[asteroid]] fragments that fall to Earth as [[meteorite]]s. Like the rocks on Earth, asteroids also show a strong cutoff point, at about 4.6&nbsp;Ga, which is assumed to be the time when the first solids formed in the [[protoplanetary disk]] around the then-young Sun. The Hadean, then, was the period of time between the formation of these early rocks in space, and the eventual solidification of the Earth's crust, some 700 million years later. This time would include the accretion of the planets from the disk and the slow cooling of the Earth into a solid body as the gravitational potential energy of accretion was released.
+Older rocks could be found, however, in the form of [[asteroid]] fragments that fall to Earth as [[meteorite]]s. Like the rocks on Earth, asteroids also show a strong cutoff point, at about 4.6&nbsp;Ga, which is assumed to be the time when the first solids formed in the [[protoplanetary disk]] around the then-young Sun. The Hadean, then, was the period of time between the formation of these early rocks in space, and the eventual solidification of Earth's crust, some 700 million years later. This time would include the accretion of the planets from the disk and the slow cooling of Earth into a solid body as the [[gravitational energy|gravitational potential energy]] of accretion was released.
 Later calculations showed that the rate of collapse and cooling depends on the size of the rocky body. Scaling this rate to an object of Earth mass suggested very rapid cooling, requiring only 100 million years.<ref>[http://adsabs.harvard.edu/abs/2001AGUFM.U52A0009M Lithosphere-Hydrosphere Interactions on the Hadean (>4 Ga) Earth], covers many of the Hadean issues and timelines in depth</ref> The difference between measurement and theory presented a conundrum at the time.
-The LHB offers a potential explanation for this anomaly. Under this model, the rocks dating to 3.8&nbsp;Ga solidified only after much of the crust was destroyed by the LHB. Collectively, the [[Acasta Gneiss]] in the North American cratonic shield and the gneisses within the [[Jack Hills]] portion of the Narryer Gneiss Terrane in Western Australia are the oldest continental fragments on Earth, yet they appear to post-date the LHB. The oldest mineral yet dated on Earth, a 4.404 Ga zircon from Jack Hills, predates this event, but it is likely a fragment of crust left over from before the LHB, contained within a much younger (~3.8&nbsp;Ga old) rock. {{Citation needed|date=September 2008}}
+The LHB offers a potential explanation for this anomaly. Under this model, the rocks dating to 3.8&nbsp;Ga solidified only after much of the crust was destroyed by the LHB. Collectively, the [[Acasta Gneiss]] in the North American cratonic shield and the gneisses within the [[Jack Hills]] portion of the Narryer Gneiss Terrane in Western Australia are the oldest continental fragments on Earth, yet they appear to post-date the LHB. The oldest mineral yet dated on Earth, a 4.404 Ga zircon from Jack Hills, predates this event, but it is likely a fragment of crust left over from before the LHB, contained within a much younger (~3.8&nbsp;Ga old) rock.{{Citation needed|date=September 2008}}
-The Jack Hills zircon led to something of a revolution in our understanding of the Hadean eon.<ref>[https://www.sciencedaily.com/releases/2005/12/051227155953.htm Revising Earth's Early History]</ref> Older references generally show that Hadean Earth had a molten surface with prominent [[volcano]]s. The name "Hadean" itself refers to the "hellish" conditions assumed on Earth for the time, from the Greek [[Hades]]. Zircon dating suggested, albeit controversially, that the Hadean surface was solid, temperate, and covered by acidic oceans. This picture derives from the presence of particular isotopic ratios that suggest the action of water-based chemistry at some time before the formation of the oldest rocks (see [[Cool early Earth]]).<ref>{{cite web|url=http://ocean.tamu.edu/Quarterdeck/1999/01/morse.html |title=Carbonates' role in the chemical evolution of oceans on Earth & Mars |deadurl=bot: unknown |archiveurl=https://web.archive.org/web/20100613174016/http://ocean.tamu.edu/Quarterdeck/1999/01/morse.html |archivedate=2010-06-13 |df= }}</ref>
+The Jack Hills zircon led to an evolution in understanding of the Hadean eon.<ref>[https://www.sciencedaily.com/releases/2005/12/051227155953.htm Revising Earth's Early History]</ref> Older references generally show that Hadean Earth had a molten surface with prominent [[volcano]]s. The name "Hadean" itself refers to the "hellish" conditions assumed on Earth for the time, from the Greek [[Hades]]. Zircon dating suggested, albeit controversially, that the Hadean surface was solid, temperate, and covered by acidic oceans. This picture derives from the presence of particular isotopic ratios that suggest the action of water-based chemistry at some time before the formation of the oldest rocks (see [[Cool early Earth]]).<ref>{{cite web|url=http://ocean.tamu.edu/Quarterdeck/1999/01/morse.html |title=Carbonates' role in the chemical evolution of oceans on Earth & Mars |url-status=bot: unknown |archive-url=https://web.archive.org/web/20100613174016/http://ocean.tamu.edu/Quarterdeck/1999/01/morse.html |archive-date=2010-06-13 }}</ref>
-Of particular interest, Manfred Schidlowski argued in 1979 that the carbon isotopic ratios of some sedimentary rocks found in [[Greenland]] were a relic of organic matter. There was much debate over the precise dating of the rocks, with Schidlowski suggesting they were about 3.8&nbsp;Ga old, and others suggesting a more "modest" 3.6&nbsp;Ga. In either case it was a very short time for [[abiogenesis]] to have taken place, and if Schidlowski was correct, arguably too short a time. The ''Late Heavy Bombardment'' and the "re-melting" of the crust that it suggests provides a timeline under which this would be possible; [[life]] either formed immediately after the ''Late Heavy Bombardment'', or more likely survived it, having arisen earlier during the [[Hadean]]. Recent studies suggest that the rocks Schidlowski found are indeed from the older end of the possible age range at about 3.85&nbsp;Ga, suggesting the latter possibility is the most likely answer.<ref name="AB-20021014">{{cite web |last=Tenenbaum |first=David |title=When Did Life on Earth Begin? Ask a Rock |url=http://www.astrobio.net/news-exclusive/when-did-life-on-earth-begin-ask-a-rock/ |date=October 14, 2002 |work=Astrobiology Magazine |accessdate=April 13, 2014 }}</ref> More recent studies have found no evidence for the isotopically light carbon ratios that were the basis for the original claims.<ref>{{cite journal | doi=10.1016/j.precamres.2006.02.005 | volume=147 | issue=1–2 | title=Petrography and geochemistry of apatites in banded iron formation, Akilia, W. Greenland: Consequences for oldest life evidence | journal=Precambrian Research | pages=100–106|bibcode = 2006PreR..147..100N | year=2006 | last1=Nutman | first1=A.P | last2=Friend | first2=C.R.L }}</ref><ref>{{cite journal|title=Questioning the evidence for Earth's earliest life—Akilia revisited|first1=Aivo|last1=Lepland|first2=Mark A. van|last2=Zuilen|first3=Gustaf|last3=Arrhenius|first4=Martin J.|last4=Whitehouse|first5=Christopher M.|last5=Fedo|date=1 January 2005|journal=Geology|volume=33|issue=1|pages=77–79|doi=10.1130/G20890.1|bibcode = 2005Geo....33...77L }}</ref><ref>{{cite journal|url=http://jgs.lyellcollection.org/content/166/2/335|title=The Akilia Controversy: field, structural and geochronological evidence questions interpretations of >3.8 Ga life in SW Greenland|first1=Martin J.|last1=Whitehouse|first2=John S.|last2=Myers|first3=Christopher M.|last3=Fedo|date=1 March 2009|journal=Journal of the Geological Society|volume=166|issue=2|pages=335–348|via=jgs.lyellcollection.org|doi=10.1144/0016-76492008-070}}</ref>
+Of particular interest, Manfred Schidlowski argued in 1979 that the carbon isotopic ratios of some sedimentary rocks found in [[Greenland]] were a relic of organic matter: the ratio of carbon-12 to carbon-13 was unusually high, normally a sign of "processing" by life. There was much debate over the precise dating of the rocks, with Schidlowski suggesting they were about 3.8&nbsp;Ga old, and others suggesting a more "modest" 3.6&nbsp;Ga. In either case it was a very short time for [[abiogenesis]] to have taken place, and if Schidlowski was correct, arguably too short a time. The Late Heavy Bombardment and the "re-melting" of the crust that it suggests provides a timeline under which this would be possible: [[life]] either formed immediately after the Late Heavy Bombardment, or more likely survived it, having arisen earlier during the [[Hadean]]. A 2002 study suggest that the rocks Schidlowski found are indeed from the older end of the possible age range at about 3.85&nbsp;Ga, suggesting the latter possibility is the most likely answer.<ref name="AB-20021014">{{Cite web |last=Tenenbaum |first=David |date=October 14, 2002 |title=When Did Life on Earth Begin? Ask a Rock |url=http://www.astrobio.net/news-exclusive/when-did-life-on-earth-begin-ask-a-rock/ |url-status=usurped |archive-url=https://web.archive.org/web/20210628022131/https://www.astrobio.net/news-exclusive/when-did-life-on-earth-begin-ask-a-rock/ |archive-date=2021-06-28 |access-date=April 13, 2014 |website=[[Astrobiology Magazine]]}}</ref> Studies from 2005, 2006 and 2009 have found no evidence for the isotopically-light carbon ratios that were the basis for the original claims of early Hadean life.<ref>{{cite journal|title=Questioning the evidence for Earth's earliest life—Akilia revisited|first1=Aivo|last1=Lepland|first2=Mark A. van|last2=Zuilen|first3=Gustaf|last3=Arrhenius|first4=Martin J.|last4=Whitehouse|first5=Christopher M.|last5=Fedo|date=1 January 2005|journal=Geology|volume=33|issue=1|pages=77–79|doi=10.1130/G20890.1|bibcode = 2005Geo....33...77L }}</ref><ref>{{cite journal | doi=10.1016/j.precamres.2006.02.005 | volume=147 | issue=1–2 | title=Petrography and geochemistry of apatites in banded iron formation, Akilia, W. Greenland: Consequences for oldest life evidence | journal=Precambrian Research | pages=100–106|bibcode = 2006PreR..147..100N | year=2006 | last1=Nutman | first1=A.P | last2=Friend | first2=C.R.L }}</ref><ref>{{cite journal|url=http://jgs.lyellcollection.org/content/166/2/335|title=The Akilia Controversy: field, structural and geochronological evidence questions interpretations of >3.8 Ga life in SW Greenland|first1=Martin J.|last1=Whitehouse|first2=John S.|last2=Myers|first3=Christopher M.|last3=Fedo|date=1 March 2009|journal=Journal of the Geological Society|volume=166|issue=2|pages=335–348|via=jgs.lyellcollection.org|doi=10.1144/0016-76492008-070|bibcode=2009JGSoc.166..335W|s2cid=129702415}}</ref> However, a similar study of Jack Hills rocks from 2008 shows traces of the same sort of potential organic indicators. Thorsten Geisler of the Institute for Mineralogy at the [[University of Münster]] studied traces of carbon trapped in small pieces of diamond and graphite within zircons dating to 4.25&nbsp;Ga.<ref name="NS-20080702">{{cite web |last=Courtland |first=Rachel |title=Did newborn Earth harbour life? |url=https://www.newscientist.com/article/dn14245-did-newborn-earth-harbour-life.html |date=July 2, 2008 |work=[[New Scientist]] |access-date=April 13, 2014}}</ref>
-More recently, a similar study of Jack Hills rocks shows traces of the same sort of potential organic indicators. Thorsten Geisler of the Institute for Mineralogy at the [[University of Münster]] studied traces of carbon trapped in small pieces of diamond and graphite within zircons dating to 4.25&nbsp;Ga. The ratio of carbon-12 to carbon-13 was unusually high, normally a sign of "processing" by life.<ref name="NS-20080702">{{cite web |last=Courtland |first=Rachel |title=Did newborn Earth harbour life? |url=https://www.newscientist.com/article/dn14245-did-newborn-earth-harbour-life.html |date=July 2, 2008 |work=[[New Scientist]] |accessdate=April 13, 2014}}</ref>
+Three-dimensional computer models developed in May 2009 by a team at the [[University of Colorado Boulder|University of Colorado at Boulder]] postulate that much of Earth's crust, and the microbes living in it, could have survived the bombardment. Their models suggest that although the surface of Earth would have been sterilized, [[hydrothermal vent]]s below Earth's surface could have incubated life by providing a sanctuary for [[Thermophile|thermophile microbes]].<ref name="RN-20090520">{{cite news |last=Steenhuysen |first=Julie |title=Study turns back clock on origins of life on Earth |url=https://www.reuters.com/article/us-asteroids-idUSTRE54J5PX20090520 |date=May 20, 2009 |newspaper=[[Reuters]] |access-date=April 13, 2014 }}</ref>
+In April 2014, scientists reported finding evidence of the largest terrestrial meteor [[impact event]] to date near the [[Barberton Greenstone Belt#Barberton greenstone belt|Barberton Greenstone Belt]]. They estimated the impact occurred about 3.26 billion years ago and that the impactor was approximately {{convert|37|to|58|km|abbr=off}} wide. The crater from this event, if it still exists, has not yet been found.<ref>{{cite web|url=http://news.agu.org/press-release/scientists-reconstruct-ancient-impact-that-dwarfs-dinosaur-extinction-blast/|title=Scientists reconstruct ancient impact that dwarfs dinosaur-extinction blast|publisher=American Geophysical Union|date=April 9, 2014}}</ref>
-Three-dimensional computer models developed in May 2009 by a team at the [[University of Colorado Boulder|University of Colorado at Boulder]] postulate that much of Earth's crust, and the microbes living in it, could have survived the bombardment. Their models suggest that although the surface of the Earth would have been sterilized, [[hydrothermal vent]]s below the Earth's surface could have incubated life by providing a sanctuary for [[Thermophile|heat-loving microbes]].<ref name="RN-20090520">{{cite news |last=Steenhuysen |first=Julie |title=Study turns back clock on origins of life on Earth |url=https://www.reuters.com/article/2009/05/20/us-asteroids-idUSTRE54J5PX20090520 |date=May 20, 2009 |newspaper=[[Reuters]] |accessdate=April 13, 2014 }}</ref>
-In April 2014, scientists reported finding evidence of the largest terrestrial meteor [[impact event]] to date near the [[Barberton Greenstone Belt#Barberton greenstone belt|Barberton Greenstone Belt]]. They estimated the impact occurred about 3.26 billion years ago and that the impactor was approximately 37 to 58 kilometres (23 to 36 miles) wide. The crater from this event, if it still exists, has not yet been found.<ref>{{cite web|url=http://news.agu.org/press-release/scientists-reconstruct-ancient-impact-that-dwarfs-dinosaur-extinction-blast/|title=Scientists reconstruct ancient impact that dwarfs dinosaur-extinction blast|publisher=American Geophysical Union|date=April 9, 2014}}</ref>
 == Possible causes ==
@@ Line 76: / Line 61: @@
 === Giant-planet migration ===
 {{Details|Nice model}}
-[[File:Lhborbits.png|thumb|500px|Simulation showing outer planets and planetesimal belt: a) Early configuration, before Jupiter (green) and Saturn (orange) reach 2:1 resonance b) Scattering of planetesimals into the inner Solar System after the orbital shift of Neptune (dark blue) and Uranus (light blue) c) After ejection of planetesimals by planets.<ref name="Gomes05">{{cite journal
+[[File:Lhborbits.png|thumb|600px|left|Simulation showing outer planets and planetesimal belt: (a)&nbsp;Early configuration, before Jupiter (green) and Saturn (orange) reach 2:1 resonance; (b)&nbsp;Scattering of planetesimals into the inner Solar System after the orbital shift of Neptune (dark blue) and Uranus (light blue); (c)&nbsp;After ejection of planetesimals by planets.<ref name="Gomes05"/>]]
+{{clear}}
- | title=Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets
+In the Nice model, the Late Heavy Bombardment is the result of a dynamical instability in the outer Solar System. The original Nice model simulations by Gomes ''et al.'' began with the [[Solar System]]'s giant planets in a tight orbital configuration surrounded by a rich [[Trans-Neptunian object|trans-Neptunian belt]]. Objects from this belt stray into planet-crossing orbits, causing the orbits of the planets to migrate over several hundred million years. Jupiter and Saturn's orbits drift apart slowly until they cross a 2:1 [[orbital resonance]], causing the [[orbital eccentricity|eccentricities]] of their orbits to increase. The orbits of the planets become unstable and Uranus and Neptune are scattered onto wider orbits that disrupt the outer belt, causing a bombardment of comets as they enter planet-crossing orbits. Interactions between the objects and the planets also drive a faster migration of Jupiter and Saturn's orbits. This migration causes resonances to sweep through the asteroid belt, increasing the eccentricities of many asteroids until they enter the inner Solar System and impact the terrestrial planets.<ref name=taylor1>{{cite web |author=Taylor, G. Jeffrey. |url=http://www.psrd.hawaii.edu/Aug06/cataclysmDynamics.html |title=Wandering Gas Giants and Lunar Bombardment |date=August 2006 |publisher=University of Hawaii}}</ref><ref name="Gomes05">{{Cite journal |last1=Gomes |first1=R. |last2=Levison |first2=H. F. |last3=Tsiganis |first3=K. |last4=Morbidelli |first4=A. |date=2005 |title=Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets |journal=[[Nature (journal)|Nature]] |volume=435 |issue=7041 |pages=466–469 |doi=10.1038/nature03676 |pmid=15917802 |bibcode=2005Natur.435..466G|doi-access=free }}</ref>
-  |author1=R. Gomes |author2=H. F. Levison |author3=K. Tsiganis |author4=A. Morbidelli | journal=Nature
-  | date=2005
-  | volume=435
-  | pages=466–469
-  | doi=10.1038/nature03676
-  | pmid=15917802
- | issue=7041|bibcode = 2005Natur.435..466G }}</ref>]]
-In the Nice model the Late Heavy Bombardment is the result of a dynamical instability in the outer Solar System. The original Nice model simulations by Gomes ''et al.'' began with the [[Solar System]]'s giant planets in a tight orbital configuration surrounded by a rich [[Trans-Neptunian object|trans-Neptunian belt]]. Objects from this belt stray into planet crossing orbits causing the orbits of the planets to migrate over several hundred million years. Jupiter and Saturn's orbits drift apart slowly until they cross a 2:1 [[orbital resonance]] causing the [[orbital eccentricity|eccentricities]] of their orbits to increase. The orbits of the planets become unstable and Uranus and Neptune are scattered onto wider orbits that disrupt the outer belt, causing a bombardment of comets as they enter planet crossing orbits. Interactions between the objects and the planets also drive a faster migration of Jupiter and Saturn's orbits. This migration causes resonances to sweep through the asteroid belt, increasing the eccentricities of many asteroids until they enter the inner Solar System and impact the terrestrial planets.<ref name=taylor1>Taylor, G. Jeffrey. [http://www.psrd.hawaii.edu/Aug06/cataclysmDynamics.html "Wandering Gas Giants and Lunar Bombardment"]</ref><ref>{{Cite journal |last=Gomes |first=R. |last2=Levison |first2=H. F. |last3=Tsiganis |first3=K. |last4=Morbidelli |first4=A. |date=2005 |title=Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets |journal=[[Nature (journal)|Nature]] |volume=435 |issue=7041 |pages=466–469 |doi=10.1038/nature03676 |pmid=15917802 |postscript=<!--None--> |bibcode = 2005Natur.435..466G }}</ref>
-The Nice model has undergone some modification since its initial publication. The giant planets now begin in a multi-resonant configuration due an early gas-driven migration through the protoplanetary disk.<ref name=Morbidelli_etal_2007>{{cite journal|last=Morbidelli|first=Alessandro|author2=Tsiganis, Kleomenis |author3=Crida, Aurélien |author4=Levison, Harold F. |author5= Gomes, Rodney |title=Dynamics of the Giant Planets of the Solar System in the Gaseous Protoplanetary Disk and Their Relationship to the Current Orbital Architecture|journal=The Astronomical Journal|date=2007|volume=134|issue=5|pages=1790–1798|doi=10.1086/521705|arxiv=0706.1713|bibcode=2007AJ....134.1790M}}</ref> Interactions with the trans-Neptunian belt allow their escape from the resonances after several hundred million years.<ref name=Levison_2011>{{cite journal|last=Levison|first=Harold F.|author2=Morbidelli, Alessandro |author3=Tsiganis, Kleomenis |author4=Nesvorný, David |author5= Gomes, Rodney |title=Late Orbital Instabilities in the Outer Planets Induced by Interaction with a Self-gravitating Planetesimal Disk|journal=The Astronomical Journal|date=2011|volume=142|issue=5|page=152|doi=10.1088/0004-6256/142/5/152|url=http://iopscience.iop.org/1538-3881/142/5/152/ |bibcode=2011AJ....142..152L}}</ref> The encounters between planets that follow include one between an [[ice giant]] and Saturn the that propels the ice giant onto a Jupiter-crossing orbit followed by an encounter with Jupiter that drives the ice giant outward. This [[jumping-Jupiter scenario]] quickly increases the separation of Jupiter and Saturn, limiting the effects of resonance sweeping on the asteroids and the terrestrial planets.<ref name=Brasser_2009>{{cite journal|last=Brasser|first=R.|author2=Morbidelli, A. |author3=Gomes, R. |author4=Tsiganis, K. |author5= Levison, H. F. |title=Constructing the secular architecture of the solar system II: the terrestrial planets|journal=Astronomy and Astrophysics|date=2009|volume=507|issue=2|pages=1053–1065|doi=10.1051/0004-6361/200912878|arxiv=0909.1891|bibcode=2009A&A...507.1053B}}</ref><ref name=Morbidelli_etal_2010>{{cite journal|last=Morbidelli|first=Alessandro|author2=Brasser, Ramon |author3=Gomes, Rodney |author4=Levison, Harold F. |author5= Tsiganis, Kleomenis |title=Evidence from the Asteroid Belt for a Violent Past Evolution of Jupiter's Orbit|journal=The Astronomical Journal|date=2010|volume=140|issue=5|pages=1391–1401|doi=10.1088/0004-6256/140/5/1391|arxiv=1009.1521|bibcode=2010AJ....140.1391M}}</ref> While this is required to preserve the low eccentricities of the terrestrial planets and avoid leaving the asteroid belt with too many high eccentricity asteroids, it also reduces the fraction of asteroids removed from the main asteroid belt, leaving a now nearly depleted [[E-belt asteroids|inner band]] of asteroids as the primary source of the impactors of the LHB.<ref name=Bottke>Bottke, W.F., et al., 2012, An Archaean heavy bombardment from a destabilized extension of the asteroid belt, Nature, 485, pp. 78–81.</ref> The ice giant is often ejected following its encounter with Jupiter leading some to propose that the Solar System began with [[five-planet Nice model|five giant planets]].<ref name=Nesvorny_2011>{{cite journal|last=Nesvorný|first=David|title=Young Solar System's Fifth Giant Planet?|journal=The Astrophysical Journal Letters|date=2011|volume=742|issue=2|page=L22|doi=10.1088/2041-8205/742/2/L22|arxiv=1109.2949|bibcode=2011ApJ...742L..22N}}</ref> Recent works, however, have found that impacts from this inner asteroid belt would be insufficient to explain the formation of ancient impact spherule beds and the lunar basins,<ref name="Johnson_etal_2016">{{cite journal|last1=Johnson|first1=Brandon C.|last2=Collins|first2=Garath S.|last3=Minton|first3=David A.|last4=Bowling|first4=Timothy J.|last5=Simonson|first5=Bruce M.|last6=Zuber|first6=Maria T.|title=Spherule layers, crater scaling laws, and the population of ancient terrestrial impactors|journal=Icarus|date=2016|volume=271|pages=350–359|doi=10.1016/j.icarus.2016.02.023|bibcode=2016Icar..271..350J|hdl=10044/1/29965}}</ref> and that the asteroid belt was probably not the source of the Late Heavy Bombardment.<ref name="Nesvorny_2016">{{cite journal|last1=Nesvorny|first1=David|last2=Roig|first2=Fernando|last3=Bottke|first3=William F.|title=Modeling the Historical Flux of Planetary Impactors|date=2016|arxiv=1612.08771|doi=10.3847/1538-3881/153/3/103|volume=153|issue=3|journal=The Astronomical Journal|page=103|bibcode = 2017AJ....153..103N }}</ref>
+The Nice model has undergone some modification since its initial publication. The giant planets now begin in a multi-resonant configuration due to an early gas-driven migration through the protoplanetary disk.<ref name=Morbidelli_etal_2007>{{cite journal |author1=Morbidelli, Alessandro |author2=Tsiganis, Kleomenis |author3=Crida, Aurélien |author4=Levison, Harold F. |author5= Gomes, Rodney |title=Dynamics of the Giant Planets of the Solar System in the Gaseous Protoplanetary Disk and Their Relationship to the Current Orbital Architecture |journal=The Astronomical Journal |date=2007 |volume=134 |issue=5 |pages=1790–1798 |doi=10.1086/521705 |arxiv=0706.1713 |bibcode=2007AJ....134.1790M|s2cid=2800476 }}</ref> Interactions with the trans-Neptunian belt allow their escape from the resonances after several hundred million years.<ref name=Levison_2011>{{cite journal |author1=Levison, Harold F. |author2=Morbidelli, Alessandro |author3=Tsiganis, Kleomenis |author4=Nesvorný, David |author5=Gomes, Rodney |title=Late Orbital Instabilities in the Outer Planets Induced by Interaction with a Self-gravitating Planetesimal Disk |journal=The Astronomical Journal |date=2011 |volume=142 |issue=5 |page=152 |doi=10.1088/0004-6256/142/5/152 |bibcode=2011AJ....142..152L|doi-access=free }}</ref> The encounters between planets that follow include one between an [[ice giant]] and Saturn that propels the ice giant onto a Jupiter-crossing orbit followed by an encounter with Jupiter that drives the ice giant outward. This [[jumping-Jupiter scenario]] quickly increases the separation of Jupiter and Saturn, limiting the effects of resonance sweeping on the asteroids and the terrestrial planets.<ref name=Brasser_2009>{{cite journal |author1=Brasser, R. |author2=Morbidelli, A. |author3=Gomes, R. |author4=Tsiganis, K. |author5=Levison, H.F. |title=Constructing the secular architecture of the solar system II: the terrestrial planets |journal=Astronomy and Astrophysics |date=2009 |volume=507 |issue=2 |pages=1053–1065 |doi=10.1051/0004-6361/200912878 |arxiv=0909.1891 |bibcode=2009A&A...507.1053B|s2cid=2857006 }}</ref><ref name=Morbidelli_etal_2010>{{cite journal |author1=Morbidelli, Alessandro |author2=Brasser, Ramon |author3=Gomes, Rodney |author4=Levison, Harold F. |author5=Tsiganis, Kleomenis |title=Evidence from the asteroid belt for a violent past evolution of Jupiter's orbit |journal=The Astronomical Journal |date=2010 |volume=140 |issue=5 |pages=1391–1401 |doi=10.1088/0004-6256/140/5/1391 |arxiv=1009.1521 |bibcode=2010AJ....140.1391M|s2cid=8950534 }}</ref> While this is required to preserve the low eccentricities of the terrestrial planets and avoid leaving the asteroid belt with too many high-eccentricity asteroids, it also reduces the fraction of asteroids removed from the main asteroid belt, leaving a now-nearly-depleted [[E-belt asteroids|inner band]] of asteroids as the primary source of the impactors of the LHB.<ref name="Archaean">{{cite journal |author1=Bottke, W.F. |display-authors=etal |year=2012 |title=An Archaean heavy bombardment from a destabilized extension of the asteroid belt |journal=Nature |volume=485 |issue=7396 |pages=78–81|doi=10.1038/nature10967 |pmid=22535245 |bibcode=2012Natur.485...78B |s2cid=4423331 }}</ref> The ice giant is often ejected following its encounter with Jupiter, leading some to propose that the Solar System began with [[five-planet Nice model|five giant planets]].<ref name=Nesvorny_2011>{{cite journal |last=Nesvorný |first=David |title=Young Solar System's Fifth Giant Planet? |journal=The Astrophysical Journal Letters |date=2011 |volume=742 |issue=2 |page=L22 |doi=10.1088/2041-8205/742/2/L22 |arxiv=1109.2949 |bibcode=2011ApJ...742L..22N|s2cid=118626056 }}</ref> Recent{{When|date=August 2021}} works, however, have found that impacts from this inner asteroid belt would be insufficient to explain the formation of ancient impact [[spherule]] beds and the lunar basins,<ref name="Johnson_etal_2016">{{cite journal |last1=Johnson |first1=Brandon C. |last2=Collins |first2=Garath S. |last3=Minton |first3=David A. |last4=Bowling |first4=Timothy J. |last5=Simonson |first5=Bruce M. |last6=Zuber |first6=Maria T. |title=Spherule layers, crater scaling laws, and the population of ancient terrestrial impactors |journal=Icarus |date=2016 |volume=271 |pages=350–359 |doi=10.1016/j.icarus.2016.02.023 |bibcode=2016Icar..271..350J |hdl=10044/1/29965|hdl-access=free }}</ref> and that the asteroid belt was probably not the source of the Late Heavy Bombardment.<ref name="Nesvorny_2016">{{cite journal |last1=Nesvorný |first1=David |last2=Roig |first2=Fernando |last3=Bottke |first3=William F. |title=Modeling the historical flux of planetary impactors |year=2016 |arxiv=1612.08771 |doi=10.3847/1538-3881/153/3/103 |volume=153 |issue=3 |journal=The Astronomical Journal |page=103 |bibcode=2017AJ....153..103N|s2cid=119028988 |doi-access=free }}</ref>
-=== Late Uranus/Neptune formation ===
+=== Late formation of Uranus and Neptune ===
-According to one [[planetesimal]] simulation of the establishment of the planetary system, the outermost planets Uranus and Neptune formed very slowly, over a period of several billion years.<ref>{{cite journal|title=Formation of planets around stars of various masses. I – Formulation and a star of one solar mass|first=T.|last=Nakano|date=1 January 1987|journal=Monthly Notices of the Royal Astronomical Society|volume=224|pages=107–130|via=NASA ADS|doi=10.1093/mnras/224.1.107|bibcode=1987MNRAS.224..107N}}</ref> Harold Levison and his team have also suggested that the relatively low density of material in the outer Solar System during planet formation would have greatly slowed their accretion.<ref>{{cite web | url = http://www.psrd.hawaii.edu/Aug01/bombardment.html | title = Uranus, Neptune, and the Mountains of the Moon | publisher = Planetary Science Research Discoveries | last = G. J. Taylor | date = August 21, 2001}}</ref>
+According to one [[planetesimal]] simulation of the establishment of the planetary system, the outermost planets Uranus and Neptune formed very slowly, over a period of several billion years.<ref>{{cite journal|title=Formation of planets around stars of various masses. I – Formulation and a star of one solar mass |first=Takenori |last=Nakano |date=1 January 1987 |journal=Monthly Notices of the Royal Astronomical Society |volume=224 |pages=107–130 |via=NASA ADS |doi=10.1093/mnras/224.1.107 |bibcode=1987MNRAS.224..107N |doi-access=free }}</ref> Harold Levison and his team have also suggested that the relatively low density of material in the outer Solar System during planet formation would have greatly slowed their accretion.<ref>{{cite web | url = http://www.psrd.hawaii.edu/Aug01/bombardment.html | title = Uranus, Neptune, and the Mountains of the Moon | publisher = Planetary Science Research Discoveries | last = G. J. Taylor | date = August 21, 2001}}</ref>
-This "late appearance" of these planets has therefore been suggested as a different reason for the LHB. However, recent calculations of gas-flows combined with planetesimal runaway growth in the outer Solar System imply that [[Giant planet|Jovian planets]] formed extremely rapidly, on the order of 10 My, which does not support this explanation for the LHB.
+The late formation of these planets has therefore been suggested as a different reason for the LHB. However, recent{{When|date=August 2021}} calculations of gas-flows combined with planetesimal runaway growth in the outer Solar System imply that [[Giant planet|Jovian planets]] formed extremely rapidly, on the order of 10 My, which does not support this explanation for the LHB.
 === Planet V hypothesis ===
 {{Main|Planet V}}
-The Planet V hypothesis posits that a fifth [[terrestrial planet]] created the ''Late Heavy Bombardment'' when its meta-stable orbit entered the inner asteroid belt. The hypothetical fifth terrestrial planet, Planet V, had a mass less than half of Mars and originally orbited between Mars and the asteroid belt. Planet V's orbit became unstable due to perturbations from the other inner planets causing it to intersect the inner asteroid belt. After close encounters with Planet V, many asteroids entered Earth-crossing orbits producing the ''Late Heavy Bombardment''. Planet V was ultimately lost, likely plunging into the Sun. In numerical simulations, an uneven distribution of asteroids, with the asteroids heavily concentrated toward the inner asteroid belt, has been shown to be necessary to produce the LHB via this mechanism.<ref name=Brasser_2011>{{cite journal|last=Brasser|first=R|author2=Morbidelli, A. |title=The terrestrial Planet V hypothesis as the mechanism for the origin of the late heavy bombardment|journal=Astronomy & Astrophysics|date=2011|volume=535|page=A41|doi=10.1051/0004-6361/201117336|bibcode=2011A&A...535A..41B}}</ref> An alternate version of this hypothesis in which the lunar impactors are debris resulting from Planet V impacting Mars, forming the [[North Polar Basin (Mars)|Borealis Basin]], has been proposed to explain a low number of giant lunar basins relative to craters and a lack of evidence of cometary impactors.<ref name=Minton_etal_2015>{{cite journal|last1=Minton|first1=D. A.|last2=Jackson|first2=A. P.|last3=Asphaug|first3=E.|last4=Fassett|first4=C. I.|last5=Richardson|first5=J. E.|title=Debris from Borealis Basin Formation as the Primary Impactor Population of Late Heavy Bombardment|journal=Workshop on Early Solar System Impact Bombardment III|date=2015|page=No. 1826, 3033|url=http://www.hou.usra.edu/meetings/bombardment2015/pdf/3033.pdf}}</ref><ref name=Minton_Richardson_Fassett_2015>{{cite journal|last1=Minton|first1=David A.|last2=Richard|first2=James E.|last3=Fassett|first3=Caleb I.|title=Re-examining the main asteroid belt as the primary source of ancient lunar craters|journal=Icarus|date=2015|volume=247|pages=172–190|arxiv=1408.5304|doi=10.1016/j.icarus.2014.10.018|bibcode=2015Icar..247..172M}}</ref>
+The Planet V hypothesis posits that a fifth [[terrestrial planet]] caused the Late Heavy Bombardment when its meta-stable orbit entered the inner asteroid belt. The hypothetical fifth terrestrial planet, Planet V, had a mass less than half of Mars and originally orbited between Mars and the asteroid belt. Planet V's orbit became unstable due to perturbations from the other inner planets causing it to intersect the inner asteroid belt. After close encounters with Planet V, many asteroids entered Earth-crossing orbits, causing the Late Heavy Bombardment. Planet V was ultimately lost, likely plunging into the Sun. In numerical simulations, an uneven distribution of asteroids, with the asteroids heavily concentrated toward the inner asteroid belt, has been shown to be necessary to produce the LHB via this mechanism.<ref name=Brasser_2011>{{cite journal|last=Brasser|first=R|author2=Morbidelli, A. |title=The terrestrial Planet V hypothesis as the mechanism for the origin of the late heavy bombardment|journal=Astronomy & Astrophysics|date=2011|volume=535|page=A41|doi=10.1051/0004-6361/201117336|bibcode=2011A&A...535A..41B|doi-access=free}}</ref> An alternate version of this hypothesis in which the lunar impactors are debris resulting from Planet V impacting Mars, forming the [[North Polar Basin (Mars)|Borealis Basin]], has been proposed to explain a low number of giant lunar basins relative to craters and a lack of evidence of cometary impactors.<ref name=Minton_etal_2015>{{cite journal|last1=Minton|first1=D. A.|last2=Jackson|first2=A. P.|last3=Asphaug|first3=E.|last4=Fassett|first4=C. I.|last5=Richardson|first5=J. E.|title=Debris from Borealis Basin Formation as the Primary Impactor Population of Late Heavy Bombardment|journal=Workshop on Early Solar System Impact Bombardment III|date=2015|volume=1826|page=3033|bibcode=2015LPICo1826.3033M|url=http://www.hou.usra.edu/meetings/bombardment2015/pdf/3033.pdf}}</ref><ref name=Minton_Richardson_Fassett_2015>{{cite journal|last1=Minton|first1=David A.|last2=Richard|first2=James E.|last3=Fassett|first3=Caleb I.|title=Re-examining the main asteroid belt as the primary source of ancient lunar craters|journal=Icarus|date=2015|volume=247|pages=172–190|arxiv=1408.5304|doi=10.1016/j.icarus.2014.10.018|bibcode=2015Icar..247..172M|s2cid=55230320}}</ref>
 === Disruption of Mars-crossing asteroid ===
-A hypothesis proposed by Matija Ćuk posits that the last few basin-forming impacts were the result of the collisional disruption of a large Mars-crossing asteroid. This [[4 Vesta|Vesta]]-sized asteroid was a remnant of a population which initially was much larger than the current main asteroid belt. Most of the pre-Imbrium impacts would have been due to these Mars-crossing objects, with the early bombardment extending until 4.1 billion years ago. A lull in basin-forming impacts then followed during which the lunar magnetic field decayed. Then roughly 3.9 billion years ago a catastrophic impact disrupted the Vesta-sized asteroid radically increasing the population of Mars-crossing objects. Many of these objects then evolved onto Earth-crossing orbits producing a spike in the lunar impact rate during which the last few lunar impact basins are formed. Ćuk points to the weak or absent residual magnetism of the last few basins and a change in the size-frequency distribution of craters which formed during this late bombardment as evidence supporting this hypothesis.<ref name=Cuk_1012>{{cite journal|last=Ćuk|first=Matija|title=Chronology and sources of lunar impact bombardment|journal=Icarus|date=2012|volume=218|issue=1|pages=69–79|doi=10.1016/j.icarus.2011.11.031|arxiv=1112.0046|bibcode=2012Icar..218...69C}}</ref> The timing<ref name=Cuk_etal_2010>{{cite journal|last=Ćuk|first=Matija|author2=Gladman, Brett J. |author3=Stewart, Sarah T. |title=Constraints on the source of lunar cataclysm impactors|journal=Icarus|date=2010|volume=207|issue=2|pages=590–594|doi=10.1016/j.icarus.2009.12.013|arxiv=0912.1847|bibcode=2010Icar..207..590C}}</ref><ref name=Malhatra_2011>{{cite journal|last=Malhotra|first=Renu|author2=Strom, Robert G. |title=Comment on "Constraints on the source of lunar cataclysm impactors"|journal=Icarus|date=2011|volume=216|issue=1|pages=359–362|doi=10.1016/j.icarus.2010.11.037|bibcode=2011Icar..216..359M}}</ref><ref name=Cuk_etal_2011>{{cite journal|last=Ćuk|first=Matija|author2=Gladman, Brett J. |author3=Stewart, Sarah T. |title=Rebuttal to the comment by Malhotra and Strom on "Constraints on the source of lunar cataclysm impactors"|journal=Icarus|volume=216|issue=1|pages=363–365|doi=10.1016/j.icarus.2011.08.011|bibcode=2011Icar..216..363C|year=2011}}</ref><ref name=Fasset_2012>{{cite journal|last=Fassett|first=C. I.|author2=Head, J. W. |author3=Kadish, S. J. |author4=Mazarico, E. |author5=Neumann, G. A. |author6=Smith, D. E. |author7= Zuber, M. T. |title=Lunar impact basins: Stratigraphy, sequence and ages from superposed impact crater populations measured from Lunar Orbiter Laser Altimeter (LOLA) data|journal=Journal of Geophysical Research|date=2012|volume=117|issue=E12|pages=n/a|doi=10.1029/2011JE003951|bibcode=2012JGRE..117.0H06F}}</ref> and the cause<ref name=Marchi_2012>{{cite journal|last=Marchi|first=Simone|author2=Bottke, William F. |author3=Kring, David A. |author4= Morbidelli, Alessandro |title=The onset of the lunar cataclysm as recorded in its ancient crater populations|journal=Earth and Planetary Science Letters|date=2012|volume=325|pages=27–38|doi=10.1016/j.epsl.2012.01.021|bibcode=2012E&PSL.325...27M}}</ref> of the change in the size-frequency distribution of craters is controversial.
+A hypothesis proposed by Matija Ćuk posits that the last few basin-forming impacts were the result of the collisional disruption of a large Mars-crossing asteroid. This [[4 Vesta|Vesta]]-sized asteroid was a remnant of a population which initially was much larger than the current main asteroid belt. Most of the pre-Imbrium impacts would have been due to these Mars-crossing objects, with the early bombardment extending until 4.1 billion years ago. A period without many basin-forming impacts then followed, during which the lunar magnetic field decayed. Then, roughly 3.9 billion years ago, a catastrophic impact disrupted the Vesta-sized asteroid, significantly increasing the population of Mars-crossing objects. Many of these objects then evolved onto Earth-crossing orbits, producing a spike in the lunar impact rate during which the last few lunar impact basins are formed. Ćuk points to the weak or absent residual magnetism of the last few basins and a change in the size–frequency distribution of craters which formed during this late bombardment as evidence supporting this hypothesis.<ref name=Cuk_1012>{{cite journal|last=Ćuk|first=Matija|title=Chronology and sources of lunar impact bombardment|journal=Icarus|date=2012|volume=218|issue=1|pages=69–79|doi=10.1016/j.icarus.2011.11.031|arxiv=1112.0046|bibcode=2012Icar..218...69C|s2cid=119267171}}</ref> The timing<ref name=Cuk_etal_2010>{{cite journal|last=Ćuk|first=Matija|author2=Gladman, Brett J. |author3=Stewart, Sarah T. |title=Constraints on the source of lunar cataclysm impactors|journal=Icarus|date=2010|volume=207|issue=2|pages=590–594|doi=10.1016/j.icarus.2009.12.013|arxiv=0912.1847|bibcode=2010Icar..207..590C}}</ref><ref name=Malhatra_2011>{{cite journal|last=Malhotra|first=Renu|author2=Strom, Robert G. |title=Comment on "Constraints on the source of lunar cataclysm impactors"|journal=Icarus|date=2011|volume=216|issue=1|pages=359–362|doi=10.1016/j.icarus.2010.11.037|arxiv=0912.1847|bibcode=2011Icar..216..359M}}</ref><ref name=Cuk_etal_2011>{{cite journal|last=Ćuk|first=Matija|author2=Gladman, Brett J. |author3=Stewart, Sarah T. |title=Rebuttal to the comment by Malhotra and Strom on "Constraints on the source of lunar cataclysm impactors"|journal=Icarus|volume=216|issue=1|pages=363–365|doi=10.1016/j.icarus.2011.08.011|bibcode=2011Icar..216..363C|year=2011}}</ref><ref name=Fasset_2012>{{cite journal|last=Fassett|first=C. I.|author2=Head, J. W. |author3=Kadish, S. J. |author4=Mazarico, E. |author5=Neumann, G. A. |author6=Smith, D. E. |author7= Zuber, M. T. |title=Lunar impact basins: Stratigraphy, sequence and ages from superposed impact crater populations measured from Lunar Orbiter Laser Altimeter (LOLA) data|journal=Journal of Geophysical Research|date=2012|volume=117|issue=E12|pages=n/a|doi=10.1029/2011JE003951|bibcode=2012JGRE..117.0H06F|hdl=1721.1/85892|s2cid=17500741 |hdl-access=free}}</ref> and the cause<ref name=Marchi_2012>{{cite journal|last=Marchi|first=Simone|author2=Bottke, William F. |author3=Kring, David A. |author4= Morbidelli, Alessandro |title=The onset of the lunar cataclysm as recorded in its ancient crater populations|journal=Earth and Planetary Science Letters|date=2012|volume=325|pages=27–38|doi=10.1016/j.epsl.2012.01.021|bibcode=2012E&PSL.325...27M}}</ref> of the change in the size–frequency distribution of craters is controversial.
 === Other potential sources ===
-A number of other possible sources of the ''Late Heavy Bombardment'' have been investigated. Among these are additional Earth satellites orbiting independently or as lunar trojans, planetesimals left over from the formations of the terrestrial planets, Earth or Venus co-orbitals, and the breakup of a large main belt asteroid. Additional Earth satellites on independent orbits were shown to be quickly captured into resonances during the Moon's early tidally-driven orbital expansion and were lost or destroyed within in a few million years<ref name=Cuk_2008>{{cite journal|last=Cuk|first=M.|title=Orbital Evolution of the Moon and the Lunar Cataclysm|journal=Workshop on the Early Solar System Impact Bombardment|date=2008|page=LPI Contribution No. 1439 p. 29|url=http://www.lpi.usra.edu/meetings/bombardment2008/pdf/3006.pdf}}</ref> Lunar trojans were found to be destabilized within 100 million years by a solar resonance when the Moon reached 27 Earth radii.<ref name=Cuk_Gladman_2009>{{cite journal|last=Ćuk|first=Matija|author2=Gladman, Brett J. |title=The fate of primordial lunar Trojans|journal=Icarus|date=2009|volume=199|issue=2|pages=237–244|doi=10.1016/j.icarus.2008.10.022|bibcode=2009Icar..199..237C}}</ref> Planetesimals left over from the formation of the terrestrial planets were shown to be depleted too rapidly due to collisions and ejections to form the last lunar basins.<ref name=Bottke_etal_2007>{{cite journal|last=Bottke|first=Wiliam F.|author2=Levison, Harold F. |author3=Nesvorný, David |author4= Dones, Luke |title=Can planetesimals left over from terrestrial planet formation produce the lunar Late Heavy Bombardment?|journal=Icarus|date=2007|volume=190|issue=1|pages=203–223|doi=10.1016/j.icarus.2007.02.010|bibcode=2007Icar..190..203B}}</ref> The long-term stability of primordial Earth or Venus co-orbitals (trojans or objects with horseshoe orbits) in conjunction with the lack of current observations indicate that they were unlikely to have been common enough to contribute to the LHB.<ref name=Cuk_etal_2012>{{cite journal|last=Cuk, M.|author2=Hamilton, D. P. |author3=Holman, M. J. |title=Long-term stability of horseshoe orbits|journal=[[Monthly Notices of the Royal Astronomical Society]]|date=2012|volume=426|issue=4|pages=3051–3056|doi=10.1111/j.1365-2966.2012.21964.x|url=http://mnras.oxfordjournals.org/content/426/4/3051 |bibcode=2012MNRAS.426.3051C|arxiv = 1206.1888 }}</ref> Producing the LHB from the collisional disruption of a main belt asteroid was found to require at minimum a 1,000–1,500&nbsp;km parent body with the most favorable initial conditions.<ref name=Ito_Malhotra_2006>{{cite journal|last=Ito|first=Takashi|author2=Malhotra, Renu |title=Dynamical transport of asteroid fragments from the ν6 resonance|journal=Advances in Space Research|date=2006|volume=38|issue=4|pages=817–825|doi=10.1016/j.asr.2006.06.007|arxiv=astro-ph/0611548|bibcode=2006AdSpR..38..817I}}</ref> Debris produced by collisions among inner planets, now lost, has also been proposed as a source of the LHB.<ref name=Volk_Gladman_2015>{{cite journal|last1=Volk|first1=Kathryn|last2=Gladman|first2=Brett|title=Consolidating and Crushing Exoplanets: Did It Happen Here?|journal=The Astrophysical Journal Letters|volume=806|issue=2|page=L26|doi=10.1088/2041-8205/806/2/L26|arxiv=1502.06558|bibcode = 2015ApJ...806L..26V |year=2015}}</ref>
+A number of other possible sources of the Late Heavy Bombardment have been investigated. Among these are additional Earth satellites orbiting independently or as lunar trojans, planetesimals left over from the formations of the terrestrial planets, Earth or Venus co-orbitals, and the breakup of a large main belt asteroid. Additional Earth satellites on independent orbits were shown to be quickly captured into resonances during the Moon's early tidally-driven orbital expansion and were lost or destroyed within a few million years.<ref name=Cuk_2008>{{cite journal|last=Cuk|first=M.|title=Orbital Evolution of the Moon and the Lunar Cataclysm|journal=Workshop on the Early Solar System Impact Bombardment|date=2008|volume=1439|page=29|bibcode=2008LPICo1439...29C|url=http://www.lpi.usra.edu/meetings/bombardment2008/pdf/3006.pdf}}</ref> Lunar trojans were found to be destabilized within 100 million years by a solar resonance when the Moon reached 27 Earth radii.<ref name=Cuk_Gladman_2009>{{cite journal|last=Ćuk|first=Matija|author2=Gladman, Brett J. |title=The fate of primordial lunar Trojans|journal=Icarus|date=2009|volume=199|issue=2|pages=237–244|doi=10.1016/j.icarus.2008.10.022|bibcode=2009Icar..199..237C}}</ref> Planetesimals left over from the formation of the terrestrial planets were shown to be depleted too rapidly due to collisions and ejections to form the last lunar basins.<ref name=Bottke_etal_2007>{{cite journal|last=Bottke|first=William F.|author2=Levison, Harold F. |author3=Nesvorný, David |author4= Dones, Luke |title=Can planetesimals left over from terrestrial planet formation produce the lunar Late Heavy Bombardment?|journal=Icarus|date=2007|volume=190|issue=1|pages=203–223|doi=10.1016/j.icarus.2007.02.010|bibcode=2007Icar..190..203B}}</ref> The long-term stability of primordial Earth or Venus co-orbitals (trojans or objects with horseshoe orbits) in conjunction with the lack of current observations indicate that they were unlikely to have been common enough to contribute to the LHB.<ref name=Cuk_etal_2012>{{cite journal|last=Cuk, M.|author2=Hamilton, D. P. |author3=Holman, M. J. |title=Long-term stability of horseshoe orbits|journal=[[Monthly Notices of the Royal Astronomical Society]]|date=2012|volume=426|issue=4|pages=3051–3056|doi=10.1111/j.1365-2966.2012.21964.x|doi-access=free |bibcode=2012MNRAS.426.3051C|arxiv = 1206.1888 |s2cid=2614886 }}</ref> Producing the LHB from the collisional disruption of a main belt asteroid was found to require at minimum a 1,000–1,500&nbsp;km parent body with the most favorable initial conditions.<ref name=Ito_Malhotra_2006>{{cite journal|last=Ito|first=Takashi|author2=Malhotra, Renu |title=Dynamical transport of asteroid fragments from the ν6 resonance|journal=Advances in Space Research|date=2006|volume=38|issue=4|pages=817–825|doi=10.1016/j.asr.2006.06.007|arxiv=astro-ph/0611548|bibcode=2006AdSpR..38..817I|s2cid=17843014}}</ref> Debris produced by collisions among inner planets, now lost, has also been proposed as a source of the LHB.<ref name=Volk_Gladman_2015>{{cite journal|last1=Volk|first1=Kathryn|last2=Gladman|first2=Brett|title=Consolidating and Crushing Exoplanets: Did It Happen Here?|journal=The Astrophysical Journal Letters|volume=806|issue=2|page=L26|doi=10.1088/2041-8205/806/2/L26|arxiv=1502.06558|bibcode = 2015ApJ...806L..26V |year=2015|s2cid=118052299}}</ref>
 == Exosystem with possible Late Heavy Bombardment ==
 {{Main|Eta Corvi}}
-Evidence<ref>Spitzer Observations of η Corvi :  Evidence at ~1 Gyr for an LHB-Like Delivery of Organics & Water-Rich Material to the THZ of a Sun-Like Star. C.M. Lisse, C. H. Chen, M. C. Wyatt, A. Morlok, P. Thebault, G. Bryden, D.M. Watson, P. Manoj, P. Sheehan, G. Sloan, T.M. Currie, ''Lunar and Planetary Institute Science Conference Abstracts' '''42''', (March 20, 2011), p. 2438, {{bibcode|2011LPI....42.2438L}}.</ref> has been found for Late Heavy Bombardment-like conditions around the star [[Eta Corvi]].
+Evidence has been found for Late Heavy Bombardment-like conditions around the star [[Eta Corvi]].<ref>“Spitzer Observations of η Corvi :  Evidence at ~1 Gyr for an LHB-Like Delivery of Organics & Water-Rich Material to the THZ of a Sun-Like Star.” C.M. Lisse, C.H. Chen, M.C. Wyatt, A. Morlok, P. Thebault, G. Bryden, D.M. Watson, P. Manoj, P. Sheehan, G. Sloan, T.M. Currie, ''Linar and Planetary Institute Science Conference Abstracts'' '''42''', (March 20, 2011), p. 2438, {{bibcode|2011LPI....42.2438L}}.</ref>
 ==See also==
@@ Line 115: / Line 93: @@
 {{Reflist|30em}}
-==External links==
-* {{cite web | url =http://www.psrd.hawaii.edu/Aug06/cataclysmDynamics.html | title = Wandering Gas Giants and Lunar Bombardment | publisher = Planetary Science Research Discoveries | last = G. Jeffrey Taylor | date = August 24, 2006}}
-* {{ cite web | url= http://www.psrd.hawaii.edu/Jan01/lunarCataclysm.html | title = Lunar Meteorites and the Lunar Cataclysm | publisher = Planetary Science Research Discoveries | last = Barbara Cohen | date =  January 24, 2001}}
-* Ker Than, "[http://www.space.com/scienceastronomy/060417_mm_lunar_meteor.html New Insight into Earth’s Early Bombardment]" – Space.com, April 17, 2006.
-* [https://web.archive.org/web/20050503131433/http://www.geolsoc.org.uk/template.cfm?name=LHB Late Heavy Bombardment was asteroidal, not cometary], The Geological Society, March 4, 2002.
-* Robert Roy Britt, "[http://www.space.com/scienceastronomy/planetearth/earth_bombarded_020724.html Evidence for Ancient Bombardment of Earth,]" – Space.com, July 24, 2002.
 {{The Moon}}
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 {{Portal bar|Astronomy|Solar System|Space}}
-[[Category:Archean events]]
 [[Category:Astronomical events of the Solar System]]
 [[Category:Events in the geological history of Earth]]
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 [[Category:Lunar science]]
 [[Category:Solar System dynamic theories]]
+[[Category:Pre-Nectarian]]
+[[Category:Nectarian]]
+[[Category:Hadean events]]

v t e Moon
Outline
Physical properties	Internal structure Topography Atmosphere Gravity field Hill sphere Magnetic field Sodium tail Moonlight Earthshine
Orbit	Lunar distance Orbital elements Distance Perigee and apogee Libration Nodes Nodal period Precession Syzygy New moon Full moon Eclipses Lunar eclipse Total penumbral lunar eclipse Tetrad Solar eclipse Solar eclipses on the Moon Eclipse cycle Supermoon Tide Tidal force Tidal locking Tidal acceleration Tidal range Lunar station
Surface and features	Selenography Terminator Limb Hemispheres Near side Far side Poles North pole South pole Face Maria List Mountains Peak of eternal light Valleys Volcanic features Domes Calderas Lava tubes Craters List Ray systems Permanently shadowed craters South Pole–Aitken basin Soil swirls Rilles Wrinkle ridges Rocks Lunar basalt 70017 Changesite-(Y) Water Space weathering Micrometeorite Sputtering Quakes Transient lunar phenomenon Selenographic coordinate system
Science	Observation Libration Lunar theory Origin Giant-impact hypothesis Theia Lunar magma ocean Geology Timescale Late Heavy Bombardment Lunar meteorites KREEP Volcanism Experiments Lunar laser ranging ALSEP Lunar sample displays Apollo 11 Apollo 17 Lunar seismology
Exploration	Missions Apollo program Explorers Probes Landing Colonization Moonbase Tourism Lunar resources
Time-telling and navigation	Lunar calendar Lunisolar calendar Month Lunar month Nodal period Fortnight Sennight Lunar station Lunar distance
Phases and names	New Full Names Crescent Super and micro Blood Blue Black Dark Wet Tetrad
Daily phenomena	Moonrise Meridian passage Moonset
Related	Lunar deities Lunar effect Earth phase Moon illusion Pareidolia Man in the Moon Moon rabbit Craters named after people Artificial objects on the Moon Memorials on the Moon Moon in science fiction list Apollo era futuristic exploration Hollow Moon Moon landing conspiracy theories Moon Treaty "Moon is made of green cheese" Natural satellite Double planet Lilith (hypothetical second moon) Splitting of the Moon
Category

v t e Impact cratering on Earth
Impact crater Impact event
Lists	Worldwide Africa Antarctica Asia and Russia Australia Europe North America South America By country Possible
Confirmed ≥20 km diameter	Acraman Amelia Creek Araguainha Beaverhead Boltysh Carswell Charlevoix Chesapeake Bay Chicxulub Clearwater East and West Gosses Bluff Haughton Kamensk Kara Karakul Keurusselkä Lappajärvi Logancha Manicouagan Manson Mistastin Mjølnir Montagnais Morokweng Nördlinger Ries Obolon' Popigai Presqu'île Puchezh-Katunki Rochechouart Saint Martin Shoemaker Siljan Ring Slate Islands Steen River Strangways Sudbury Tookoonooka Tunnunik Vredefort Woodleigh Yarrabubba
Topics	Alvarez hypothesis Australite Breccia Coesite Complex crater Cretaceous–Paleogene boundary Cryptoexplosion Ejecta blanket Impact crater Impact structure Impactite Late Heavy Bombardment Lechatelierite Meteorite Moldavite Ordovician meteor event Philippinite Planar deformation features Shatter cone Shock metamorphism Shocked quartz Stishovite Suevite Tektite
Research	Ralph Belknap Baldwin Daniel Barringer Barringer Medal Edward C. T. Chao Robert S. Dietz William Kenneth Hartmann H. Jay Melosh Graham Ryder Peter H. Schultz Eugene Merle Shoemaker Earth Impact Database Impact Field Studies Group Lunar and Planetary Institute Traces of Catastrophe