New Research Opportunities in the Earth Sciences - New Research Opportunities in the Earth Sciences - page 13

Geochemical and cosmochemical observations provide important constraints
on the timing and the mechanisms of accretion and segregation of the core, although
several interpretations are possible. For example, in the Hafnium-Tungsten ((Hf-W)
system, the excess radiogenic 180W in the silicate Earth relative to chondritic
meteorites has been interpreted as rapid accretion or alternatively as incomplete
mixing of the impactor with the growing Earth (Halliday, 2008; Rudge et al., 2010).
Similar interpretations have come from other short-lived isotope systems, such as
146Sm - 142 Nd (O'Neil et al., 2008), which also have implications for the earliest
crust. The fusion of geochemistry and geophysics offers many promising avenues for
better understanding formative processes that governed the early history of the Earth.
Based on isotopic evidence from meteorites, what originated as occasional
planetesimal collisions soon began to run away, leaving a small number of rapidly
growing planetary embryos. Improved chronological methods reveal that melting and
differentiation occurred within a few million years of the formation of the first solids,
probably driven by collisions and assisted by now-extinct radioactive heat producers
such as 26 Al and 60 Fe. Accordingly, the assumption that Earth formed by a continuous
influx of small particles made of pristine solar system condensates has given way to a
much more dramatic model, in which Earth was assembled by a relatively small
number of traumatic collisions involving larger objects, some of these already having
differentiated interiors and well as their own internal dynamics (Canup and Asphaug,
2001). Future progress on the processes and timing of Earth's growth in the coming
decade will rely on a diversity of approaches, including:

• Application of new isotope techniques for dating methods

• Closer integration of isotope geochemistry with astrophysical approaches to

planetary formation

• More comprehensive and more realistic dynamical models of the accretion

process

• Evolutionary studies of the chemistry and physics of planetesimal-sized

objects and planetary embryos

Response to the Moon-Forming Impact

Although conclusive evidence is still lacking that ever-larger impacts
dominated the later stages of Earth's growth, the global dynamical and thermal
implications of this process are not in doubt. Once Earth reached an appreciable mass,
the enormous amounts of kinetic and gravitational potential energy released by large
impacts dictate widespread melting, with regional and possibly global magma oceans
extending to considerable depths (Tonks and Melosh, 1993).
The compositional similarity of Earth's mantle and the Moon and realization
of the importance of large impacts in the early Solar System, together with the large
angular momentum present in the Earth-Moon system, have led to the theory that the
Moon formed as the result of a late cataclysmic impact of a Mars-sized object with
the growing Earth (Wetherill, 1990). Particle-based simulations of this giant collision
(Canup, 2004; see Figure 2.2) predict that much of the preexisting layered structure of