Impacts Moon-forming

There is now consensus that the Moon formed following a colhsion of the early Earth with a Mars-sized impactor (as summarized in the 1986 book Origin oftheMoon). The timing of the impact and subsequent assembly of the Moon are not tightly constrained, but models of radiogenic isotopes suggest an age of 40 to 50 Myr after solar system formation (Halliday,... 

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). 

The early history of Earth is greatly influenced by the probable impact of a Marssized body to form the Moon. Core-formation models suggest both Earth and the impactor were already differentiated by the time of the impact (Tonks Melosh 1992). The lack of a clear182W excess in uncontaminated lunar samples implies that the Moon-forming impact took place >50 Myr after the start of the Solar System (Touboul et al. 2007). The oldest known lunar samples are 150 Myr younger than CAIs, based on Sm-Nd dating (Touboul etal. 2007), which provides a lower limit on the Moon s age. 

The low Fe abundance in the lunar mantle suggests the Moon-forming impact happened late in Earth s accretion (Canup Asphaug 2001). It may have been the last collision with another embryo. Simulations of terrestrial-planet formation find that low-velocity, oblique impacts are common (Agnor et al. 1999), so that planets like Earth and Venus are likely to experience at least one such impact during their formation. This suggests large satellites may be a common outcome of terrestrial-planet formation. 

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. 

Y244 that represents the number of Xe atoms produced per decay of " " Pu, a similar closure age of 96 Ma is obtained. For example, note that if atmospheric xenon loss occurred during a massive Moon-forming impact, then the closure period corresponds to the time after an instantaneous catastrophic loss event. 

A Moon-forming collision of an approximately Mars-sized body with Earth (Hartmann and Davis, 1975 Cameron and Ward, 1976) would clearly result in catastrophic loss of volatiles from the pre-existing atmosphere and may have caused substantial loss of deep-Earth noble gases as well. Ahrens (1990, 1993) argued that virtually complete expulsion might have occurred by direct ejection from the impacted hemisphere and by shock-induced outward ramming of the antipodal planetary surface. However, losses may have been incomplete, and this event could have been followed by additional, isotopically fractionating losses driven by thermal processes (see Section 4.12.5). 

The sedimentary evidence implies the existence of oceans. Although the initial deep volatile inventory of the planet would have been removed by the late great impact that formed the Moon, much of the water presently in Earth s oceans would have degassed from the hot mantle or infallen as comets soon after that great impact, and the ongoing volcanism would have added more. 

Giant impacts and Moon-forming event, magma ocean... 

The Moon formed through a giant impact. It is now thought likely that the Moon formed as a result of a giant impact event. The timing of this event, its contribution to terrestrial melting, and the bulk composition of the Earth are important subjects of current research. 

To what extent did the Moon-forming impact drive the process of core formation ... 

Several episodes of volatile loss then followed in which the overall concentrations of all volatiles were reduced by one or two orders of magnitude. It is likely that this volatile loss took place through impact degassing and Porcelli et al. (2001) have proposed that this was the Moon-forming impact event, at ca. 30 Ma after t0. Impacting on this scale would have led to extensive melting and volatile loss from all the outer... 

Large impactors bombarded early Earth at the same time that large impact basins formed on the Moon [66,73]. Impact-induced shock waves associated with the projectile bombardments would have played an important chemical role on early Earth. The role has been discussed particularly in relation to the origin of life. 

Figure 5. Model ages for instantaneous core formation in planetary bodies as a function ofparent body size. Given the small size of the Moon, the W model age of the Moon is relatively young. This suggests that the Moon formed by a unique process, consistent with the giant impact hypothesis for the origin of the Moon.

A number of arguments can be made in favor of cometary carriers for inner planet volatiles. As noted above, reduction of the Ne/Ar ratio relative to the solar ratio, resembling the elemental pattern on Venus, is likely in such ices. Modeling discussed below indicates that a source of this nature could have supplied essentially identical primary atmospheres to both Venus and Earth if an initially Venus-like atmosphere on Earth were later elementally fractionated in hydrodynamic escape powered by a giant Moon-forming impact. 

Y244 = 6.5 X 10 for production of Xe, a similar closure age of 82 Ma is obtained (see Fig. 2c). If atmospheric Xe loss occurred during a massive Moon-forming impact, then the closure period corresponds to the time after an instantaneous catastrophic loss event. Somewhat earlier ages of 50-70 Ma are obtained with further assumptions about the amount of Xe loss (Ozima and Podosek 1999 Porcelli et al. 2001). If radiogenic Xe was lost over about one half-life of Pu (80 Ma), then 40 /o of the Xe remaining in the Earth is in the atmosphere, compatible with the fraction of " Ar in the atmosphere (using a BSE value of 270 ppm K). Note that in the first 100 Ma, only 6% of... 

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