An Impact Origin of the Earth-Moon System

Robin Canup 

Southwest Research Institute

The currently-favored theory for the origin of the Earth-Moon system
is the giant impact hypothesis.  Models of the formation of
the Moon from an impact-generated debris cloud have shown that the
efficiency of incorporation of disk material into a moon or moons is less
than 50 percent.  While stability analyses indicate that a single moon is
the likely end state of accretion in an impact-generated disk, forming a
single Moon with a lunar mass has proved more difficult.
Hydrodynamic simulations of the impact event with greatly-improved
numerical resolution predict sufficiently massive debris disks for two
impact scenarios: 1) an impact with twice the angular momentum of the
current Earth-Moon system, or 2) an impact that occurred when Earth had
only about 65 percent of its current mass.  Terrestrial accretion
simulations suggest both scenarios may be plausible, and a newly-
developed scaling relationship implies that impacts intermediate between
these two cases may also yield the Moon.  Recent work also suggests that a
resonant interaction between the newly formed Moon and the
inner protolunar disk could have provided the Moon with its initial
~ 10 degree orbital inclination, allowing an impact origin to be reconciled
with the current lunar orbit