Late bombardment

Even today, meteorites and IDPs deliver organic materials to the Earth s surface at a rate of 1 x 106kg/year [35]. During the late bombardment period, which lasted until about 4 billion years ago, the amount of extraterrestrial organic material brought to the prebiotic Earth was likely to have been orders of magnitude greater [21]. Thus, the early Earth must have been... 

Thus core-mantle equilibration can be excluded as the source of the HSEs in the Earth s mantle. It is more likely that a late accretionary component has delivered the HSEs to the Earth s mantle, either as single Moon-sized body which impacted the Earth after the end of core formation or several late arriving planetesimals. The impac-tors must have been free of metallic iron, or the metallic iron of the projectiles must have been oxidized after the collision(s) to prevent the formation of liquid metal or sulfide that would extract HSEs into the core of the Earth. The relative abundances of the HSEs in the Earth s mantle are thus the same as in the accretionary component, but may be different from those in the bulk Earth. The late addition of PGE with chondritic matter is often designated as the late veneer hypothesis (Kimura et al., 1974 Chou, 1978 Jagoutz et al., 1979 Morgan et al., 1981 O Neill, 1991). This model requires that the mantle was free of PGE before the late bombardment established the present level of HSEs in the Earth s mantle. 

Fig. 5. The mass (M) of asteroids and comets that fell on Earth is reported as a function of the mass fraction (/) of comets among impacting bodies. Lunar cratering record (Chyba 1990), highly siderophile elements (Chyba 1991) and water deuterium to protium ratios (Dauphas et al. 2000) are shown. If these three approaches record the late bombardment over the same period of time, then M is c. 2 x 10 kg and /< 0.01. Actually, the mass of asteroids and comets evaluated by the lunar cratering record and highly siderophile elements depends on the mass fraction of comets. Given the level of uncertainty on all these estimates (M and /are reported in a log-log plot), taking this effect into account would not affect our conclusions.

It is worth while to note that the estimates based on the lunar cratering record, highly siderophile elements and water deuterium to protium (D/H) ratios integrate the late bombardment of Earth over time scales that are not necessarily... 

Nuclear fission is a process in which a heavy nucleus—usually one with a nucleon number of two hundred or more—separates into two nuclei. Usually the division liberates neutrons and electromagnetic radiation and releases a substantial amount of energy. The discoveiyi of nuclear fission is credited to Otto I lahn and Fritz Strassman. In the process of bombarding uranium with neutrons in the late 1930s, they detected several nuclear products of significantly smaller mass than uranium, one of which was identified as Ba. The theorectical underpinnings that exist to this day for nuclear fission were proposed by Lise Meitner and Otto Frisch. Shortly after Hahn and Strassman s discovery. 

In the technique developed by Willard Libby in Chicago in the late 1940s, the proportion of carbon-14 in a sample is determined by monitoring the (1 radiation from C02 obtained by burning the sample. This procedure is illustrated in Example 17.4. In the modern version of the technique, which requires only a few milligrams of sample, the carbon atoms are converted into C ions by bombardment of the sample with cesium atoms. The C ions are then accelerated with electric fields, and the carbon isotopes are separated and counted with a mass spectrometer (Fig. 17.19). 

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... 

While fast atom bombardment (FAB) [66] and TSI [25] built up the basis for a substance-specific analysis of the low-volatile surfactants within the late 1980s and early 1990s, these techniques nowadays have been replaced successfully by the API methods [22], ESI and APCI, and matrix assisted laser desorption ionisation (MALDI). In the analyses of anionic surfactants, the negative ionisation mode can be applied in FIA-MS and LC-MS providing a more selective determination for these types of compounds than other analytical approaches. Application of positive ionisation to anionics of ethoxylate type compounds led to the abstraction of the anionic moiety in the molecule while the alkyl or alkylaryl ethoxylate moiety is ionised in the form of AE or APEO ions. Identification of most anionic surfactants by MS-MS was observed to be more complicated than the identification of non-ionic surfactants. Product ion spectra often suffer from a reduced number of negative product ions and, in addition, product ions that are observed are less characteristic than positively generated product ions of non-ionics. The most important obstacle in the identification and quantification of surfactants and their metabolites, however, is the lack of commercially available standards. The problems with identification will be aggravated by an absence of universally applicable product ion libraries. 

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. 

Dauphas, N., Robert, F. and Marty, B. (2000) The late asteroidal and cometary bombardment in water deuterium to protium ratio. Icarus, 148, 508-512. 

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. 

The identification of an isotope of element 95, by Seaborg, Ghiorso, James, and Leon Morgan in late 1944 and early 1945, followed the identification of this isotope of element 96 (242Cm) as a result of the bombardment of 7j Pu with neutrons in a nuclear reactor. The production reactions, involving multiple neutron capture by plutonium, are... 

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