Late Heavy Bombardment

The Late Heavy Bombardment (commonly referred to as the lunar cataclysm, or LHB) is a period of time approximately 3,800 to 4,100 million years ago (mya) during which a large number of impact craters are believed to have formed on the Moon, and by inference on Earth, Mercury, Venus, and Mars as well. The evidence for this event comes primarily from the dating of lunar samples, which indicates that most impact melt rocks formed in this very narrow interval of time. While many hypotheses have been put forth to explain a "spike" in the flux of either asteroidal or cometary materials to the inner solar system, no consensus yet exists as to its cause. One popular theory postulates that the gas giant planets migrated in orbit at this time, causing objects in either the asteroid belt or Kuiper belt to be put onto eccentric orbits that reached the terrestrial planets. Nevertheless, some argue that the lunar sample data do not require a cataclysmic cratering event near 3900 mya, and that the apparent clustering of impact melt ages near this time is an artifact of sampling material affected by a single large impact basin.

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 believed to have formed during the collision of asteroids or comets tens of kilometers across, forming impact craters hundreds of kilometers 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.

Under study on Earth, the ages of impact melts collected at these sites clustered between about 3800 and 4100 mya. The apparent clustering of ages of these was first noticed in the mid-1970s by Fouad Tera, Dimitri Papanastassiou, and Gerald Wasserburg who postulated that the ages record an intense bombardment of the Moon. They called it the "lunar cataclysm" and proposed that it represented a dramatic increase in the rate of bombardment of the Moon around 3900 mya. 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. At the time, the conclusion was considered controversial.

As more data became available, particularly from lunar meteorites, 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 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 3900 mya. [cite journal | last = B. A. Cohen, T. D. Swindle, D. A. Kring | date = 2000 | title = Support for the Lunar Cataclysm Hypothesis from Lunar Meteorite Impact Melt Ages | journal = Science | volume = 290 (5497) | pages = 1754–1755] Nevertheless, the ages do not "cluster" at this date, but span between 2500 and 3900 mya. [cite journal | last = William K. Hartmann, Cathy Quantin, and Nicolas Mangold | date = 2007 | volume = 186 | 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]

Studies of the highland crater size distributions suggest that the same family of projectiles struck Mercury and the Moon during late heavy bombardment. [Strom, 1979] 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. [ [http://history.nasa.gov/SP-467/ch3.htm Chronology of Planetary Surfaces] , section 3.3.1]

Criticisms of the cataclysm hypothesis

While the cataclysm hypothesis has recently gained in popularity, particularly among dynamicists who are trying to find possible causes for such a phenomenon, it is important to realize that the cataclysm hypothesis is still a controversial theory that is 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 4100 mya is related to such samples having been pulverized, or their ages being reset.

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. [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 | journal = Meteorit. Planet. Sci. | volume = 33 | pages = 959–979] 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 3900 Mya simply reflects material being collected from a single impact event, Imbrium, and not several.

A second criticism concerns the significance of the lack of impact melt rocks older than about 4100 mya. One hypothesis for this observation that does not involve a cataclysm is that old melt rocks did exist, but that their ages have been reset by the continuous effects of impact cratering over the past 4 billion years. Furthermore, it is possible that these putative samples could have been pulverized to such small sizes that it is impossible to obtain age determinations using standard radiometric methods.

Geological consequences on Earth

If a lunar cataclysmic cratering event were truly to have occurred on the Moon, the Earth would have been affected as well. Extrapolating lunar cratering rates to Earth at this time suggest that the following number of craters would have formed:Fact|date=July 2008
* 22,000 or more impact craters with diameters > 20 km,
* about 40 impact basins with diameters about 1000 km,
* several impact basins with diameter about 5,000 km,Serious environmental damage would occur about every 100 years.

Prior to the introduction of the Late Heavy Bombardment theory, it was generally assumed that the Earth had remained molten until about 3800 mya. This date could be found in all 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 the Earth had remained molten until this point in time, which defined the boundary between the earlier Hadean and later Archean epochs.

Older rocks could be found, however, in the form of asteroids that fall to Earth and can be found in Antarctica when the glaciers carry them to the edges of the continental plate. Like the rocks on Earth, asteroids also show a strong cutoff point, at about 4600 mya, 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 Myr later. This time would include the accretion of the planets from the disk and its slow cooling into a solid as the gravitational potential energy of this collapse was released.

Later calculations showed that the rate of collapse and cooling was dependent on the size of the body, and applying this to an Earth-sized mass suggested this should have happened quite quickly, as quickly as 100 Myr. [ [http://adsabs.harvard.edu/abs/2001AGUFM.U52A0009M Lithosphere-Hydrosphere Interactions on the Hadean (>4 mya) Earth] , covers many of the Hadean issues and timelines in depth] The difference between measurement and theory was something of a mystery at the time.

The Late Heavy Bombardment is now offered as an explanation of this oddity. Under this model, the rocks dating to 3800 mya represent those that were solidifying after much of the crust was destroyed by the Bombardment. The Acasta Gneiss in the North American cratonic shield and gneisses within the Jack Hills portion of the Narryer Gneiss Terrane in Western Australia are, collectively, the oldest continental fragments on Earth and do not predate the late heavy bombardment. The oldest mineral yet dated on Earth, a zircon from Jack Hills, predates this event but may simply be a fragment of crust left over from this event, contained within a much younger (~3800 Myr old) rock. Fact|date=September 2008 Some geologists believe they have found 4.28 billion year old rock in Quebec, Canada, though [http://news.nationalgeographic.com/news/2008/09/080925-oldest-rocks.html] .

This has led to something of a revolution in the understanding of the earliest stages of Earth's history during the Hadean. [ [http://www.sciencedaily.com/releases/2005/12/051227155953.htm Revising Earth's Early History] ] Older references generally show the Hadean Earth having a molten surface with prominent volcanos, for instance [http://www.ucmp.berkeley.edu/precambrian/hadean.html Hadean time] . The name referred to the "hellish" conditions assumed on Earth for the time. It is now believed that the Hadean surface was solid, temperate, and water covered (albeit acidic). This is due to the presence of several particular isotopic ratios which suggest water-based chemistry took place at some point prior to the formation of the oldest rocks. [ [http://ocean.tamu.edu/Quarterdeck/1999/01/morse.html Carbonates' role in the chemical evolution of oceans on Earth & Mars] ]

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 3800 Myr old, and others suggesting a more "modest" 3600 Myr. 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 3850 Myr, suggesting the latter possibility is the most likely answer. [ [http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=293 When Did Life on Earth Begin? Ask a Rock] ] It should be pointed out, however, that Schidlowski's argument remains a topic of heated debate.

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 4250 Myr. The ratio of carbon-12 to carbon-13 was unusually high, normally a sign of "processing" by life. [ Rachel Courtland, " [http://space.newscientist.com/article/dn14245-did-newborn-earth-harbour-life.html?feedId=online-news_rss20 Did newborn Earth harbour life?] ", "New Scientist", July 2, 2008]

Possible causes

Gas giant migration

A series of simulations [ [http://www.nature.com/nature/journal/v435/n7041/pdf/nature03676.pdf Origin of the cataclysmic "Late Heavy Bombardment" period of the terrestrial planets] , Gomes, Levison, Tsiganis, Morbidelli - Nature 26, May 2005.] by Gomes et al. starts with a solar system where the gas giant planets are in a tight configuration. This configuration is in itself stable, but assuming a rich ur-trans-Neptunian Belt, stray transneptunians interacted with these planets causing them to migrate slowly during a time of several hundred million years. Jupiter is predicted to migrate inward, whereas the other planets go outwards. By this migration, the solar system became catastrophically unstable when Jupiter and Saturn reached an 1:2 orbital resonance, causing the outer solar system to reconfigure rapidly to a wide jovian system. As these planets migrated, resonances would be "swept" through the asteroid belt and Kuiper belt. These resonances would increase the eccentricity for the objects, allowing them to enter the inner solar system and impact with the terrestrial planets.

Late Uranus/Neptune Formation

Harold Levison and his team have suggested that the relatively low density of material in the outer Solar System during planet formation would have greatly slowed their accretion. If this is the case, the "late appearance" of these planets suggests a reason for the LHB. According to earlier planetesimal simulations for the establishing of the planetary system, the outermost planets Uranus and Neptune formed very slowly over a period of several billion years. [ [http://adsabs.harvard.edu/abs/1987MNRAS.224..107N Formation of planets around stars of various masses] ] If this is the case, this would make them a candidate for causing the LHB. [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]

Recent calculations of gas-flows combined with planetesimal runaway growth in the outer solar system show the possibility that all Jovian planets formed extremely rapidly, on the order of 10 mya. In that case some other mechanism must be responsible for the LHB catastrophe.

Planet V theory

One such mechanism is presented by the Planet V simulations, that posits the existence of a fifth planet, smaller than Mars, in the inner solar system, outside the orbit of Mars but inside the asteroid belt. The orbit of this planet was theorized to be nearly circular but meta-stable, and was disrupted at the time of LHB, becoming eccentric, starting to sling about asteroids to collide with the inner planets.Fact|date=June 2008

References

External links

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* Ker Than, " [http://www.space.com/scienceastronomy/060417_mm_lunar_meteor.html New Insight into Earth’s Early Bombardment] " - Space.com, April 17, 2006.
* [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.