Late heavy bombardment

The noble gas elements act as a record of the deposited material because they are essentially chemically inert and are also trapped within the ice of comets and meteorites. The late-heavy bombardment era must have affected both the Earth and the Moon similarly so an estimate of the collision frequency may be obtained by using the record of impacts on the Moon s surface. The collision rate calculated... 

A peak in the shock ages for HED meteorites also corresponds to the period of late heavy bombardment on the Moon. This is unlikely to be a coincidence, and provides further support for this important event in solar system history. 

The so-called late heavy bombardment, discussed more fully in Chapter 14, was a period of intense impacts by large planetesimals, concentrated in the Nectarian and Early hnbrian periods. The Procellarum and Imbrium impacts exposed subsurface rocks in the PKT. The SPA terrane formed during this time and represents not only the largest impact basin on the Moon, but also the biggest basin in the solar system. It exposes lower crust and perhaps mantle materials. 

Gomes, R., Levison, H. F., Tsiganis, K. and Morbidelli, A. (2005) Origin of the cataclysmic late heavy bombardment period of the terrestrial planets. Nature, 435, 466-469. 

Wetherill G. W. (1975b) Late heavy bombardment of the moon and terrestrial planets. Proc. 6th Lunar Sci. Conf. 1539-1561. 

Chapman C. R., Cohen B. A., and Grinspoon D. H. (2002) What are the real constraints on commencement of the late heavy bombardment In Lunar Planet Sci. XXXIII, 1627. The Lunar and Planetary Institute, Houston (CD-ROM). 

Comet-like materials are presumed to be the budding blocks of Uranus and Neptune (the ice giants) they may have played a role in the formation of Jupiter and Saturn (the gas giants) and they also played some role in transporting outer solar system volatile materials to inner planets (Delsemme, 2000). The inner solar system flux of comets may have been much higher in the past and comets may have played a role in producing the late heavy bombardment on terrestrial planets (Levison et al., 2001). Comets also exist outside the solar system and there is good evidence that they orbit a major fraction of... 

Levison H. F., Dones L., Chapman C. R., Stern S. A., Duncan M. J., and Zahnle K. (2001) Could the lunar late heavy bombardment have been triggered by the formation of Uranus and Neptune Icarus 151, 286 - 306. 

Archean era begins, oldest rocks Late heavy bombardment ... 

Fig. 1. Relative probability histograms of Slave craton detrital zircons (continuous curve with black infill below based on data from Sircombe et al. 2001), Ar/ Ar ages of impact spherules in lunar soil samples (dash-dot curve after Culler et al. 2000), and Ar/ Ar ages of impact glasses in lunar meteorites (dashed curve after Cohen et al. 2000). Time interval spans from 4500 Ma, the approximate age of formation of the Moon, to 2500 Ma, the defined Archaean-Proterozoic boundary. Vertical scales of the three curves are independent. Shaded age bars with roman numerals represent main events in basement of the Slave craton that were initially defined on the basis of individual rock age and their inheritance (see Bleeker Davis 1999). The detrital zircon data represent c. 300 zircon grains from five widely distributed samples of a c. 2800 Ma quartzite unit overlying the Mesoarchaean to Hadean-age basement complex of the Slave craton. These data represent a least-biased record of pre-2.8 Ga components of the Slave craton. The broad complementarity in the datasets should be noted. With the first major peak in Slave crustal ages (event V 3100-3200 Ma) immediately following the last major peak in the lunar spherule data. Both lunar soil and meteorite data sets support a lunar cataclysm or late heavy bombardment that appears to have erased or swamped out the pre-4.0Ga lunar record.

Late Heavy Bombardment (see Chapter 6, Section 6.3.1) 3,800-3,900 770-670 Kring and Cohen [2002)... 

At the present time the balance of evidence is against a cometary origin for prebiotic carbon on Earth, for the same comets would also have delivered water to the Earth and yet the D/H ratio of the terrestrial oceans is different from that in comets. The more likely extraterrestrial input is from asteroids and meteorites, for there is evidence from both lunar and terrestrial samples that the late heavy bombardment event at 3.9 Ga (Section 6.4.1) contributed meteoritic material to the Earth at this time. 

FIGURE 6.3 The impacting record for the Earth and Moon. The grey field for the Earth is calculated from the observed record on the Moon and expressed as impact energy (left) and depth of water evaporated (right). Superimposed upon the cratering record is the data from impact melts in lunar meteorites showing the time of the inner solar system late heavy bombardment (after Sleep et al. (1989) and Cohen et al. (2000)). 

A number of explanations have been offered for the origin of the impactors in the late heavy bombardment event. These can be categorized into either cometary or asteroid models. At the present time, isotopic and trace element data support an asteroidal rather than cometary origin (Kring Cohen, 2002). 

The implications of the late heavy bombardment event for the evolution of life on Earth are substantial. It has not gone unnoticed that there is a very short time interval between the end of the late heavy bombardment and the formation of sediments at Isua. Indeed the evidence for impacting at Isua recorded by Schoenberg et al. (2002) could signify that the two events overlapped. This means that either life did not start until after ca. 3.9 Ga, or if it had developed earlier, it had to survive a "high temperature stage" during which the oceans boiled. 

Frei, R. and Rosing, M.T., 2005. Search for traces of the late heavy bombardment on Earth - results from high precision chromium isotopes. Earth Planet. Sci. Lett., 236, 28—40. 

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