The Formation of the Moon: The Giant Impact Hypothesis

Since the time that scientists began to understand how our solar system formed they have
been attempting to understand how our moon came to orbit around the earth.  The formation
of the moon, earth’s primary satellite, has been a subject of great controversy surrounded by a
long history of theoretical guesswork and disproved hypotheses.  Finally astronomy has
come to embrace a single theory of the origin of our moon, the “giant impact hypothesis.”

In our solar system and beyond, planetary satellites like our moon, are somewhat prevalent.  
Some planets like Jupiter have many moons, while other planets like the inner planets of our
solar system have none.  There is probably more than one way of creating a planetary satellite
and we cannot expect that all of them formed in the same way.  However, after a detailed
analysis of the moon’s physical and chemical properties we have been able to refute certain
hypotheses and formulate a rather precise one.

Long ago many astronomers thought that the moon might be a stray asteroid or meteor that
was captured by the earth’s gravitational field.  After extensive chemical tests though,
scientists ruled this possibility out because despite the fact that the composition of the moon
is different from that of the earth, it is too similar to it to have been an asteroid.  

The large objects in our solar system came out of our early solar nebulae, a large disk that
revolved around the sun and coalesced into our current planetary system.  The “nebular
hypothesis” of the origin of the moon, leads us to expect that any two objects that formed
from the same region of this disk will be very similar in chemical composition.  But the
composition of the earth and moon are more dissimilar than we might expect for the nebular
hypothesis to be correct.

The “giant impact hypothesis,” the currently accepted model, predicts that the earth was
struck by a large meteor, close to the size of the planet mars.  The tremendous impact lifted
matter from the earth’s crust and mantle, yet not the core, into orbit.  This explains why they
both have similar compositions except the earth has an iron core and the moon does not.  The
Apollo space missions brought back conclusive evidence that vindicated this hypothesis.

For more info:
Hartmann, W. K. 1997. A Brief History of the Moon. The Planetary Report. 17, 4-11.
Hartmann, W. K., R.J. Phillips, and G.J. Taylor, eds. 1986. Origin of the Moon. (Houston:
Lunar and Planetary Institute.)