Citizendia

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. In Astronomy, Geology, and Paleontology, mya or " mya " is an abbreviation for "million years ago". EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 The VENUS ( V ictoria E xperimental N etwork U nder the S ea project is a cabled sea floor observatory operated by the University 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. The asteroid belt is the region of the Solar System located roughly between the orbits of the Planets Mars and Jupiter. The Kuiper belt (ˈkaɪpɚ to rhyme with "viper" sometimes called the Edgeworth-Kuiper belt, is a region of the Solar System beyond the planets extending 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.

Contents

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. Radiometric dating (often called radioactive dating) is a technique used to date materials usually based on a comparison between the observed abundance of a naturally occurring 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. Mare Imbrium, Latin for "Sea of Showers" or "Sea of Rains" is a vast Lunar mare (mahr'-ay filling a basin on Earth 's Moon The Sea of Nectar ( Mare Nectaris) is a small Lunar mare or sea (a volcanic lava plain noticeably darker than the rest of the Moon 's surface located Mare Serenitatis (the "sea of serenity" is a Lunar mare that sits just to the east of Mare Imbrium on Earth's moon.

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. Stratigraphy, a branch of Geology, studies rock layers and layering ( stratification) 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. A Lunar meteorite is a Meteorite that is known to have originated on the Moon. 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. [1] Nevertheless, the ages do not "cluster" at this date, but span between 2500 and 3900 Mya. [2]

Studies of the highland crater size distributions suggest that the same family of projectiles struck Mercury and the Moon during late heavy bombardment. [3] 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. [4]

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. [5] 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:

Serious environmental damage would occur about every 100 years. In the broadest sense the term impact crater can be applied to any depression natural or manmade resulting from the high velocity impact of a projectile with larger body In the broadest sense the term impact crater can be applied to any depression natural or manmade resulting from the high velocity impact of a projectile with larger body

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. Uranium-lead is one of the oldest and most refined Radiometric dating schemes with a routine age range of about 1 million years to over 4 Zircon is a Mineral belonging to the group of nesosilicates. Its chemical name is Zirconium silicate and its corresponding chemical formula is 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. The Hadean (ˈheɪdiən is the geologic eon before the Archean.

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. Asteroids, sometimes called Minor planets or planetoids', are bodies—primarily of the inner Solar System —that are smaller than planets but 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. A protoplanetary disk (or proplyd) is a rotating Circumstellar disk of dense gas surrounding a young newly formed star a T Tauri star or Herbig star 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. [6] 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 Acasta Gneiss is a rock Outcrop of Archaean Tonalite Gneiss in the Slave craton in Northwest Territories The Jack Hills are located in the Narryer Gneiss Terrane of the Yilgarn Craton, Western Australia, and comprise an 80 km long northeast-trending belt of 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. [7]

This has led to something of a revolution in the understanding of the earliest stages of Earth's history during the Hadean. [8] Older references generally show the Hadean Earth having a molten surface with prominent volcanos, for instance Hadean time. Plate tectonics and hotspots Divergent plate boundaries At the 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. [9]

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. Greenland (Kalaallit Nunaat meaning "Land of the Greenlanders" Grønland is a self-governing Danish Province located between the 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. In the Natural sciences, Abiogenesis, or origin of life, is the study of how Life on Earth emerged from Inanimate Organic 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. [10] It should be pointed out, however, that Schidlowski's argument remains a topic of heated debate.

Possible causes

Gas giant migration

A series of simulations [11] 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. A trans-Neptunian object (TNO is any object in the Solar system that Orbits the sun at a greater distance on average than Neptune. 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. Planetesimals are solid objects thought to exist in Protoplanetary disks and in Debris disks A widely accepted theory of planet formation the so-called planetesimal [12] If this is the case, this would make them a candidate for causing the LHB. [13]

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. A gas giant (sometimes also known as a Jovian planet after the planet Jupiter, or giant planet) is a large Planet that is not primarily In that case some other mechanism must be responsible for the LHB catastrophe.

Planet V theory

Main article: Planet V

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. Planet V is a hypothetical fifth planet hypothesized by NASA scientists John Chambers and Jack Lissauer to have once existed between The asteroid belt is the region of the Solar System located roughly between the orbits of the Planets Mars and Jupiter. The orbit of this planet was pretty circular but meta-stable, and it was disrupted at the time of LHB, becoming eccentric, starting to sling about asteroids to collide with the inner planets.

References

  1. ^ B. A. Cohen, T. D. Swindle, D. A. Kring (2000). "Support for the Lunar Cataclysm Hypothesis from Lunar Meteorite Impact Melt Ages". Science 290 (5497): 1754–1755.  
  2. ^ William K. Hartmann, Cathy Quantin, and Nicolas Mangold (2007). "Possible long-term decline in impact rates: 2. Lunar impact-melt data regarding impact history". Icarus 186: 11–23.  
  3. ^ Strom, 1979
  4. ^ Chronology of Planetary Surfaces, section 3. 3. 1
  5. ^ L. A. Haskin, R. L. Korotev, R. L. Rockow, B. L. Jolliff (1998). "The case for an Imbrium origin of the Apollo thorium-rich impact-melt breccias". Meteorit. Planet. Sci. 33: 959–979.  
  6. ^ Lithosphere-Hydrosphere Interactions on the Hadean (>4 mya) Earth, covers many of the Hadean issues and timelines in depth
  7. ^ reference needed
  8. ^ Revising Earth's Early History
  9. ^ Carbonates' role in the chemical evolution of oceans on Earth & Mars
  10. ^ When Did Life on Earth Begin? Ask a Rock
  11. ^ Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets, Gomes, Levison, Tsiganis, Morbidelli - Nature 26, May 2005.
  12. ^ Formation of planets around stars of various masses
  13. ^ Uranus, Neptune, and the Mountains of the Moon

External links

© 2009 citizendia.org; parts available under the terms of GNU Free Documentation License, from http://en.wikipedia.org
 Dapyx Software network: MP3 Explorer | Ebook Manager | Zenithic