New Research Opportunities in the Earth Sciences - New Research Opportunities in the Earth Sciences - page 14

Earth was obliterated and a substantial portion of the impacting material was thrown
back into orbit, creating a post-impact accretion disk surrounding the proto-Earth,
complete with its own silicate vapor atmosphere. These simulations also predict that
the Moon consists primarily of material from the impacting object, and not material
from proto-Earth. This computational model is challenged by remarkable similarity in
oxygen isotopes found between lunar and Earth rocks, raising questions about the
partitioning of material during impact.
Despite its widespread acceptance, direct evidence of a Moon-forming giant
impact—the smoking gun in Earth's early history—remains elusive. Similarly, our
understanding of the events accompanying giant impacts and their consequences for
the chemical and physical modification of the early Earth remain sketchy. Further
delineation of the Moon-forming event and its consequences for Earth are high
priorities for the coming decade.
Figure 2.2 Two time slices in the animation of a glancing impact of a Mars-sized planetary
embryo into the proto-Earth. The silicate mantles of both objects are shown in yellow,
whereas their metallic cores are shown in red. The first image is slightly before the impact;
the second is about two orbital rotations of the proto-Earth following impact. SOURCE:
Reprinted from Canup (2004), with permission from Elsevier.
Terrestrial Magma Oceans
Magma oceans, an almost inevitable consequence of large planetary impacts
given the energies involved, were first proposed to explain the plagioclase-dominated
crust of the Moon (Warren, 1985), and differentiation in an early magma ocean on
Mars is thought to be responsible for the range in source compositions of Martian
meteorites (Borg and Draper, 2003). As is the case for a moon-forming impact,
indisputable evidence for magma oceans and their associated early atmosphere on
Earth remains elusive, although there is indirect evidence from abundance patterns of
the elements affected by core formation (Kleine et al., 2004), plus some isotopic