View source for Astronomy:Late Heavy Bombardment

{{Short description|Hypothesized astronomical event}}
[[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)]]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 asteroids and comets collided into the terrestrial planets and their natural satellites of the inner Solar System, including 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 post-accretion and planetary instability-driven populations of [[Astronomy: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 used to be widely accepted,<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> it remained difficult to provide an overwhelming amount of evidence for the hypothesis.<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> However, recent re-appraisal of the cosmo-chemical constraints indicates that there was likely no late spike ("terminal cataclysm") in the bombardment rate.<ref>{{Cite journal |last=Zellner |first=Nicolle E. B.|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 the LHB derives from [[Astronomy:Moon rock|moon rock]] samples of [[Astronomy:Lunar craters|Lunar craters]] brought back by the [[Astronomy:Apollo program|Apollo program]] [[Astronaut|astronaut]]s. [[Physics: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 [[Astronomy:Nice model|Nice model]], popular among [[Astronomy:Planetary science|planetary scientists]], postulates that the [[Astronomy:Giant planet|giant planet]]s underwent [[Astronomy:Planetary migration|orbital migration]], scattering objects from the [[Astronomy:Asteroid belt|asteroid belt]], [[Astronomy:Kuiper belt|Kuiper belt]], or both, into eccentric orbits, and into the path of the terrestrial planets.<ref name="Bottke"/>  

Other researchers doubt the heavy bombardment, arguing 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" /> 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
| 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>

== Evidence for a cataclysm ==
The main piece of evidence for a lunar cataclysm comes from the [[Physics: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 [[Engineering:Apollo 15|Apollo 15]], [[Engineering:Apollo 16|16]], and [[Engineering:Apollo 17|17]] landing sites were chosen as a result of their proximity to the [[Astronomy:Mare Imbrium|Imbrium]], [[Astronomy:Mare Nectaris|Nectaris]], and [[Astronomy: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 investigators to postulate that those ages record an intense bombardment of the [[Astronomy:Moon|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:Earth and Planetary Science Letters|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 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 [[Earth:Stratigraphy|stratigraphic]] grounds.<ref name=Crockett/>  At the time, the hypothesis was considered controversial.

As more data has become available, particularly from [[Astronomy:Lunar meteorite|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 [[Chemistry: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 |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 = [[Organization:University of Hawaii|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 |volume=186 |issue=1 |pages=11–23  |doi=10.1016/j.icarus.2006.09.009 |bibcode=2007Icar..186...11H}}</ref>

Dating of [[Astronomy:Howardite|howardite]], [[Astronomy:Eucrite|eucrite]] and [[Astronomy:Diogenite|diogenite]] ([[Astronomy: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=[[Earth:Nature Geoscience|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. |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, 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 | year=1985 | section=section&nbsp;3.3.1. Chronology of planetary surfaces: Mercury | title=Planetary Geology in the 1980s   | series = History | publisher=[[Organization:National Aeronautics and Space Administration|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 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 | 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.
| last2 = Harrison| first2 = T.M.
| date = 2016
| title = Illusory Late Heavy Bombardments
| journal = PNAS
| volume = 113 | issue = 39 | pages = 10802–10806
| doi = 10.1073/pnas.1611535113 | pmid=27621460 | pmc=5047187
|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 |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"/>

== Geological consequences on Earth ==
{{Life timeline}}
If a 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 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 |doi=10.2307/j.ctv1v7zdrp.30 }}</ref>
* 22,000 or more [[Astronomy:Impact crater|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 hypothesis, geologists generally assumed that Earth remained molten until about 3.8&nbsp;Ga. This date could be found in many of the [[Physics: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, [[Physics:Uranium–lead dating|uranium–lead dating]] of [[Chemistry:Zircon|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 [[Earth:Hadean|Hadean]] and later [[Earth:Archean|Archean]] eons. Nonetheless, in 1999, the [[Physics: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 [[Earth:Acasta Gneiss|Acasta Gneiss]] of the [[Earth:Slave Craton|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 [[Astronomy:Asteroid|asteroid]] fragments that fall to Earth as [[Astronomy:Meteorite|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 [[Astronomy:Protoplanetary disk|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 [[Physics: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 [[Earth:Acasta Gneiss|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 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 [[Earth:Volcano|volcano]]s. The name "Hadean" itself refers to the "hellish" conditions assumed on Earth for the time, from the Greek [[Religion:Hades|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 [[Earth:Cool early Earth|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 [[Place:Greenland|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 [[Biology:Abiogenesis|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 [[Earth:Hadean|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=[[Astronomy:Astrobiology Magazine|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.<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>

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 [[Organization:University of Münster|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 |access-date=April 13, 2014}}</ref>

Three-dimensional computer models developed in May 2009 by a team at the [[Organization: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, [[Earth:Hydrothermal vent|hydrothermal vent]]s below Earth's surface could have incubated life by providing a sanctuary for [[Earth: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={{wipe|Reuters}} |access-date=April 13, 2014 }}</ref>
In April 2014, scientists reported finding evidence of the largest terrestrial meteor [[Astronomy:Impact event|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 {{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>

== Possible causes ==

=== Giant-planet migration ===
{{Details|Nice model}}
[[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">{{cite journal |title=Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets |author1=Gomes, R. |author2=Levison, H.F. |author3=Tsiganis, K. |author4=Morbidelli, A. |journal=Nature |date=2005 |volume=435 |pages=466–469 |doi=10.1038/nature03676 |pmid=15917802 |issue=7041 |bibcode=2005Natur.435..466G|doi-access=free }}</ref>]]
{{clear}}
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 [[Astronomy:Solar System|Solar System]]'s giant planets in a tight orbital configuration surrounded by a rich [[Astronomy: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 [[Astronomy: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>{{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 |volume=435 |issue=7041 |pages=466–469 |doi=10.1038/nature03676 |pmid=15917802 |bibcode=2005Natur.435..466G|doi-access=free }}</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 [[Astronomy:Ice giant|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 [[Astronomy:Jumping-Jupiter scenario|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 [[Astronomy: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=0 |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 [[Astronomy: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 formation of Uranus and Neptune ===
According to one [[Astronomy:Planetesimal|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>
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 [[Astronomy: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|Astronomy:Planet V}}
The Planet V hypothesis posits that a fifth [[Astronomy:Terrestrial planet|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 [[Astronomy: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 [[Astronomy: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 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=[[Organization:Monthly Notices of the Royal Astronomical Society|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|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|Astronomy:Eta Corvi}}
Evidence has been found for Late Heavy Bombardment-like conditions around the star [[Astronomy:Eta Corvi|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==
* {{annotated link|Earth:Cool early Earth}}
* {{annotated link|Astronomy:Formation and evolution of the Solar System}}
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==References==
{{Reflist|30em}}


{{The Moon}}
{{Impact cratering on Earth}}

[[Category:Astronomical events of the Solar System]]
[[Category:Hadean]]
[[Category:Hypothetical impact events]]
[[Category:Lunar science]]
[[Category:Solar System dynamic theories]]

{{Sourceattribution|Late Heavy Bombardment}}