| LOS ANGELES
LOS ANGELES Aug 3 A primordial collision of
two moons that once orbited Earth explains why the present-day
moon is a bit lopsided and its far side much rockier than the
lunar surface facing our planet, scientists said on Wednesday.
Research published in the journal Nature suggests the early
moon was shadowed by a smaller companion satellite, about
one-thirtieth of its own mass, as the two bodies circled the
infant Earth in tandem more than 4 billion years ago.
But as the moons evolved further from the Earth, coming
under greater influence of the Sun's gravitational force, the
stability of their co-orbit was upset, putting them on a
collision course, said Erik Asphaug, a planetary scientist at
the University of California, Santa Cruz.
After about 100 million years of cohabitation, the smaller
moon finally crashed into the larger moon in an impact that
unfolded over several hours and resulted in a merger of the two
celestial bodies, Asphaug said.
All this occurred just 80,000 miles from Earth, about a
third of its current 250,000-mile distance from the moon.
The violent but slow-motion celestial union explains much
about the striking difference between the mountainous far side
of the moon and the smoother lunar surface of vast plains
visible to Earth in the night sky, the scientists concluded.
Rather than form a giant crater, a low-velocity collision
like the one theorized in the study would have piled material
up into a thick, jagged layer of solid crust. In fact, the
moon's crust is about 30 miles (50 kilometers) thicker on its
far side than on its near side.
The lunar crash also accounts for an overall asymmetry of
the moon -- bulging out more on its far side -- not easily
explained by tidal forces exerted by the Earth.
Evidence of a collision exists further in variations in the
composition of the moon's crust, the near side of which is far
richer in potassium, rare-earth elements and phosphorus.
Those elements would have been concentrated in molten
material still cooling beneath the moon's surface and
splattered away to the opposite side by the force of the
impact, Asphaug said.
The research builds on the widely accepted "giant impact"
model for the origin of the moon. According to that theory, the
moon was created from debris ejected by the last of a series of
collisions between the primordial Earth and about 10 Mars-sized
proto-planets during the early formation of the solar system.
The study suggests that moon-forming giant impact also
created a smaller, companion body that eventually coalesced
with the larger satellite to form today's single moon.
(Editing by Todd Eastham)