후기 대충돌

후기 대충돌을 겪는 달(위)과 현재의 달(아래)의 상상도.

후기 대충돌(Late Heavy Bombardment, LHB) 또는 후기 대폭격, 달의 대재앙(Lunar cataclysm 루나 캐터클리즘^[*])은 명왕누대와 초시생대 사이, 41억~38억 년 전에 일어났으리라 여겨지는 가설 상의 사건으로, 이 기간 동안 비정상적으로 많은 소행성이 내태양계의 지구형 행성과 충돌했으리라 추정되고 있다. 최근 후기 대충돌의 존재 여부 자체에 의문이 제기되기도 한다.

후기 대충돌의 증거는 아폴로 탐사 당시 수집한 표본의 방사능 연대 측정 결과 충돌로 인한 암석의 용융 간 시간 간격이 비교적 짧다는 점에서 유래하였다. 내태양계 지역에 소행성이나 혜성 등 충돌체가 급증한 이유를 설명하려 많은 가설이 제기되었지만 현재까지 결론은 나지 못하였다. 일부에서는 표본 사이의 시간 간격이 짧은 것은 거대한 충돌 지역 한 곳에서만 표본을 채취하였기 때문에 생기는 겉보기 문제이며, 이 현상을 설명하기 위해 과거의 대충돌을 가정할 필요가 없다고 주장한다.

후기 대충돌의 증거

후기 대충돌의 가장 큰 증거는 아폴로 15호, 16호, 17호가 각각 비의 바다, 감로주의 바다, 평온의 바다에 위치한 충돌 분지에서 채취한, 충돌로 인해 용융된 암석의 방사능 연대 측정 결과로, 암석의 나이가 38억 년과 41억 년 사이에 집중되어 있다는 점에서 당시 달에 극심한 소행성 충돌이 일어나고 있었다는 가정이 제기되었다. 이 가설을 최초에는 달의 대재앙(Lunar cataclysm)으로 칭했으며, 암석의 나이를 39억 년 전 달에 충돌하는 소행성의 비율이 급증한 증거로 제시하였다. 당시에는 이 가설에 논란의 여지가 많다고 보고, 크게 주목하지 않았다.

월운석 자료가 축적되며, 논란은 계속되었지만, 대충돌 가설은 점점 지지를 얻었다. 월운석은 달의 여러 지역에서 고르게 방출되었다고 간주되며, 장석질 월운석은 달의 뒷면에서 유래했으리라 추정될 정도이다. 월운석의 연대 측정 결과, 39억 년 이전의 운석은 단 하나도 발견되지 않았으며, 이는 대충돌 가설과 일치하는 결과이다. 하지만 운석의 연대는 25억 ~ 39억 년 정도로 분산되어 있으며, 어느 한 시점에 집중되는 양상은 보이지 않았다.

소행성대에서 유래한 하워다이트, 유크라이트, 디오제나이트, H 콘드라이트 운석의 연대는 주로 34억~41억 년 사이였으며, 충돌이 집중된 가장 이른 시기는 45억 년 전이다.

The 3.4–4.1 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 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.

Studies of the highland crater size distributions suggest that the same family of projectiles struck Mercury and the Moon during the Late Heavy Bombardment. 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.

대충돌 가설에 대한 반론

대충돌 가설에 대한 반론은 크게 두 가지로, 첫째는 암석의 연대가 집중된 것은 한 충돌 분지에서만 표본을 채취한 결과로 인한 것일 수 있다는 것이며, 둘째는 41억 년 이상 된 암석은 이미 가루가 되었거나 중간에 연대가 초기화되었을 수 있다는 것이다.

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. 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, 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.

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. 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.

지구의 영향

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

• 22,000 or more impact craters with diameters >,
• about 40 impact basins with diameters about ,
• several impact basins with diameters about ,

Before the formulation of the LHB theory, geologists generally assumed that the 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 the 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.

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 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.

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. 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.

The Jack Hills zircon led to something of a revolution in our understanding of the Hadean eon. 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).

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 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. Recent studies 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. More recent studies have found no evidence for the isotopically light carbon ratios that were the basis for the original claims.

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 Ga. The ratio of carbon-12 to carbon-13 was unusually high, normally a sign of "processing" by life.

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 the Earth would have been sterilized, hydrothermal vents below the Earth's surface could have incubated life by providing a sanctuary for heat-loving microbes.

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 wide. The crater from this event, if it still exists, has not yet been found.

원인

거대 행성의 전이

이 주제의 자세한 내용은 니스 모형 문서를 참고하십시오.

외행성과 미행성대를 보여주는 시뮬레이션으로, a) 목성과 토성이 2:1 공명에 이르기 전의 설정, b) 해왕성(짙은 파랑)과 천왕성(옅은 파랑)의 궤도 이동 이후 내태양계에서의 미행성 산란, c) 행성에 의한 미행성의 방출 이후를 보여주고 있다.

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.

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. Interactions with the trans-Neptunian belt allow their escape from the resonances after several hundred million years. 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. 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. The ice giant is often ejected following its encounter with Jupiter leading some to propose that the Solar System began with five giant planets. 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, and that the asteroid belt was probably not the source of the Late Heavy Bombardment.

천왕성 및 해왕성의 늦은 형성

한 미행성 시뮬레이션에서는 최외곽 행성인 천왕성과 해왕성이 몇십억 년에 걸쳐 느리게 형성되었으며, 실제 일부에서는 태양계 형성 당시 외태양계의 물질 밀도가 낮아 강착 속도가 감소하였을 것이라고 주장한다. 최외곽 행성의 늦은 형성 또한 후기 대충돌의 이유로서 제안되었다. 하지만 미행성의 폭주 효과와 결합하여 계산하면 외태양계의 기체 행성은 1000만 년 정도의 시간 동안 매우 빠르게 형성되는 것으로 밝혀져, 후기 대충돌의 발생과는 거리가 있다.

행성 V 가설

 이 부분의 본문은 행성 V입니다.

행성 V 가설은 화성 질량의 절반 정도 되는 다섯 번째 지구형 행성이 화성과 소행성대 사이에 존재했으며, 내행성 간의 섭동으로 행성 V가 소행성대에 진입해 소행성 다수가 지구 근방으로 섭동되어 후기 대충돌이 일어났다는 가설이다. 행성 V는 태양과 충돌하는 등 결과적으로 소멸되었을 것으로 여겨진다. 수학적인 계산 결과 소행성대 안쪽에 소행성이 몰려 있는 불균형 분포가 이루어져 있어야 이 가설을 통해 후기 대충돌을 설명할 수 있음이 드러났다. 달에 충돌구에 비해 상대적으로 분지가 없다는 점과 혜성의 충돌 증거가 부족하다는 점을 들어, 후기 대충돌은 행성 V가 화성과 충돌하여 생긴 파편으로 인해 발생했다는 가설 또한 존재한다.

화성 횡단 소행성의 교란

한 가설에서는 충돌 분지의 형성 중 일부는 대형 화성 횡단 소행성의 충돌로 인해 형성되었다고 제안한다. 소행성의 크기는 베스타 정도이며, 현재의 소행성대보다 컸던 소행성군의 잔재였을 것으로 추정된다. 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. The timing and the cause 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 Lunar trojans were found to be destabilized within 100 million years by a solar resonance when the Moon reached 27 Earth radii. 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. 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. 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. Debris produced by collisions among inner planets, now lost, has also been proposed as a source of the LHB.

후기 대충돌의 가능성이 있는 외계 행성

 이 부분의 본문은 까마귀자리 에타입니다.

항성 까마귀자리 에타 주변에서 후기 대충돌과 유사한 현상이 발견되었다.

같이 보기

• 태양계의 형성과 진화
• 니스 모형

외부 링크

• Ker Than, "New Insight into Earth’s Early Bombardment" – Space.com, April 17, 2006.
• Late Heavy Bombardment was asteroidal, not cometary, The Geological Society, March 4, 2002.
• Robert Roy Britt, "Evidence for Ancient Bombardment of Earth," – Space.com, July 24, 2002.

This article "후기 대충돌" is from Wikipedia. The list of its authors can be seen in its historical and/or the page Edithistory:후기 대충돌. Articles copied from Draft Namespace on Wikipedia could be seen on the Draft Namespace of Wikipedia and not main one.

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