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“Big Thwack” Origin of the Moon Study Guide: Four Theories
1. What are the two ways in which planets and planetary systems Change?
Through evolutionary, or gradual, changes over long periods of time. The gradual changes were
punctuated by catastrophic events such as the one that led to the formation of the Moon.
2. Why is Earth’s moon special when compared to the other moons in our solar system? Why is the
origin of the Moon such a difficult and important question to answer? Please explain.
The Moon, relative to Earth, is huge. While there are bigger moons in the solar system, they orbit
gas giants that are much larger than they are – the largest of the gas planet moons is a thousandth
of the size of their planetary host.
The reason that the moon is such a difficult question to answer is because of its very large size (an
outlier among the solar system’s moons, as explained above). It is such an important question
because it goes to the root of the origin and early history of the Earth. Knowing about the Moon’s
origins (how it formed, its age, and the ruthless bombardment that it weathered early in its history)
tells us a great deal about the formation and evolution of Earth.
3. Briefly summarize the “sister (also called co-accretion theory), capture, and daughter (also called
fission theory)” theories of the origin of the Moon (use “Big Thwack” as a reference).
a. Sister theory – that the Moon formed in its present location from a large cloud of left over
debris from Earth’s formation that remained in orbit around Earth.
b. Capture theory – the Moon was a smaller planetesimal that formed in the same general area
as Earth. At some point Earth and the Moon passed close enough to one another that the
larger Earth was able to capture the Moon in its gravitational field and swing it into orbit.
c. Fission Theory – said that the primordial Earth was spinning on its axis so fast that it
“stretched and elongated until it flung out a glob of magma” from its surface into orbit
around itself. The glob congealed into the Moon.