Giant impact

Krot, A.N., Amelin, Y., Cassen, P. and Meibom, A. (2005) Young chondmles in CB chondrites from a giant impact in the early solar system. Nature 436, 989-992. 

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. 

Ringwood, A. E., Kato, T., Hibbeson, W. Ware, N. (1991) Partitioning of Cr, V, and Mn between mantles and cores of differentiated planetesimals Implications for giant impact hypothesis of lunar origin. Icarus, 89,122-8. 

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. 

Tonks W. B. and Melosh H. J. (1992) Core formation by giant impacts. Icarus 100, 326-346. 

EVIDENCE FOR LATE ACCRETION, CORE FORMATION, AND CHANGES IN VOLATILES AFTER THE GIANT IMPACT... 

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 origin of the Moon has been the subject of intense scientific interest for over a century but particularly since the Apollo missions provided samples to study. The most widely accepted current theory is the giant impact theory but this idea has evolved from others and alternative hypotheses have been variously considered. Wood (1984) provides a very useful review. The main theories that have been considered are as follows ... 

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... 

Certain features of the Moon may be a consequence of the giant impact itself. The volatile-depleted composition of the Moon, in particular, has been explained as a consequence of... 

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