Impacts 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... 

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. 

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.

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)). 

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. 

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