Earth giant impacts

One more important property of Jupiter must be mentioned the Earth owes its relatively quiet periods (in geological terms) to the huge gravitational force of the giant planet. Jupiter attracts most of the comets and asteroids orbiting in its vicinity, thus protecting the Earth from impact catastrophes ... 

Trace element measurements in lunar basalts also indicate that the Moon is depleted in highly volatile elements (Taylor et al., 2006a). Estimates of some of the Moon s volatile element concentrations are compared with the Earth in Figure 13.11 a. The absence of water in lunar basalts suggests that the mantle is dry. The Moon may also be enriched in refractory elements (Fig. 13.11b). Volatile element depletion and refractory element enrichment are expected consequences of the giant impact origin and subsequent high-temperature accretion of the Moon. 

Pahlevan, K. and Stevenson, D. J. (2007) Equilibrium in the aftermath of the lumr-forming giant impact. Earth and Planetary Science Letters, 262, 438—449. 

The time scale for Earth s differentiation was discussed in Chapter 9. We learned that tungsten isotopes constrain the time interval between nebula formation and core separation as less than 30 Myr (Yin et al., 2002). Differentiation must have been complete before the giant impact that formed the Moon, currently constrained to have occurred 40—45 Myr after the solar system formed (Halliday, 2004 and references therein). 

Pepin, R. O. (1997) Evolution of earth s noble gases Consequence of assuming hydrodynamic loss driven by giant impact. Icarus, 126, 148-56. 

Canup R. M. and Asphaug E. (2001) Origin of the Moon in a giant impact near the end of the Earth s formation. Nature 412, 708-712. 

It is possible that the atmosphere was blown off by a major impact like the Moon-forming giant impact, but this is far from clear at this stage. Another mechanism that often is considered is the effect of strong ultraviolet wavelength radiation from the early Sun (Zahnle and Walker, 1982). This might affect Xe preferentially because of the lower ionization potential. It is of course possible that the Earth simply acquired an atmosphere, with xenon, like today s (Marty, 1989 Cafifee et al, 1999). However, then it is not clear how to explain the strong isotopic fractionation relative to solar and meteorite compositions. 

The giant-impact theory has been confirmed by a number of important observations. Perhaps most importantly, we know now that the Moon must have formed tens of millions of years after the start of the solar system (Lee et al, 1997 Halliday, 2000). This is consistent with a collision between already formed planets. The masses of the Earth and the impactor at the time of the giant impact have been the subject of major uncertainty. Two main classes of models are usually considered. In the first, the Earth was largely (90%) formed at the time of the impact and the impacting planet Theia was roughly Mars-sized (Cameron and Benz, 1991). A recent class of models considers the Earth to be only half-formed at the... 

Xenon isotope data have also been used to argue specifically that the Earth lost its inventory of noble gases as a consequence of the giant impact (Pepin, 1997 Porcelli and Pepin, 2000). The timing of the xenon loss event looks more... 

There are a number of lines of evidence that the Earth may have been affected by additions of further material subsequent to the giant impact. Similarly, there is limited evidence that there was additional core formation. Alternatively, there also are geochemical and dynamic constraints that strongly limit the amount of core formation and accretion since the giant impact. This is a very interesting area of research that is ripe for further development. Here are some of the key observations ... 

Benz W. and Cameron A. G. W. (1990) Terrestrial effects of the giant impact. In Origin ofthe Earth (eds. H. E. Newsom and J. H. Jones). Oxford University Press, Oxford, pp. 61-67. 

Cameron A. G. W. (2000) Higher-resolution simulations of the Giant Impact. In Origin of the Earth and Moon (eds. 

Ringwood A. E. (1989b) Flaws in the giant impact hypothesis of lunar origin. Earth Planet. Set Lett. 95, 208-214. 

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