Name: _______________________________________
Period: ________ Date: ____________
Formation of the Moon
Directions: Use Glencoe Blue Hardcover p. 203, 204, 206, 207 AND the attached articles to complete the
boxes below. Then, complete the back side of this sheet to show how the Moon was formed.
Capture Theory
Simultaneous Formation Theory
Problems with Capture Theory
Problems with Simultaneous Formation
Impact Theory
Problems SOLVED by Impact Theory
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Name: _______________________________________
Period: ________ Date: ____________
26
Re-Tell the Story of the Moon’s Formation:
In the space below, either write a paragraph (or paragraphs), or draw a series of LABELED
pictures to show how the moon was formed. Include at least THREE PIECES OF EVIDENCE
that support this theory. Use the readings and your notes on the front of this page to help you.
Name: _______________________________________
Period: ________ Date: ____________
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Theories of the Moon’s Formation
Credit: Imelda B. Joson and Edwin L. Aguirre
Adapted from: http://www.space.com/25322-moon-formation-wild-theories.html
Introduction
The moon has fascinated humanity for millenia, exerting a powerful pull on our imagination just
as it tugs on the oceans to create the tides.
After gazing up at the moon for all those years, however, we're still not exactly sure how it came
to be. Here's a brief rundown of the most prominent theories scientists have come up with to
explain the moon's origin, including a few relatively wild ones with little observational evidence
to back them up.
Capture Theory
Some researchers suggest that the moon may have originally formed elsewhere — perhaps even
around another planet, such as Venus — before being grabbed by Earth's gravitational pull.
Other worlds have gained moons in this manner. For example, Phobos and Deimos, the two tiny
satellites of Mars, are thought to be captured asteroids.
The capture idea isn't really an origin theory, of course; it just concerns how the moon came to
orbit Earth. And it has some major problems, the most serious of which is the geochemical
similarity of the Earth and moon. The two bodies have nearly identical oxygen isotope ratios,
suggesting that they formed from the same pool of raw material. The second problem with this
theory is that the Moon is so large compared to Earth, that Earth’s gravity wouldn’t have been
strong enough to capture the Moon and drag it in to orbit.
Fission Theory
Another idea — apparently thought up by Charles Darwin's son George in the 19th century —
posits that the material that formed the moon was ejected into space by a molten, fast-spinning
Earth in the very early days of the solar system.
Most scientists discount the fission hypothesis, saying that Earth could not have been spinning
fast enough to expel a huge blob of rock. But one 2010 study suggested that a natural nuclear
explosion, set up by the superconcentration of radioactive elements, may have provided the kick
Name: _______________________________________
Period: ________ Date: ____________
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to dislodge a moon-size piece of the early Earth into orbit. The problem with this theory is that
the Earth’s crust and the Moon’s crust aren’t made of exactly the same materials, which is what
would be expected if a blob of Earth suddenly got thrown out in to space.
Co-formation (Or, Simultaneous Formation Theory)
It's also possible that the moon formed alongside Earth 4.5 billion years ago, coalescing from gas
and dust in the same part of our solar system's protoplanetary disk.
While this hypothesis can account for the isotopic similarities between the Earth and moon, it
falls short in other ways. It cannot explain the high angular momentum (shape of the Moon’s
orbit around the Earth) of the Earth-moon system, for example, or why the moon has such a
small iron core compared to that of our planet. Additionally, Earth’s crust and the Moon’s crust
are not similar enough for both bodies to have formed from the same materials at the same time.
Colliding Planetesimals (Or, Binary Accretion Theory)
Some scientists have suggested that the moon condensed from the debris produced when
planetesimals — the building blocks that grew into Earth, Mars and other full-fledged planets —
slammed into each other shortly after the solar system formed.
Little evidence supports this theory, which also cannot explain the geochemical similarities
between the Earth and its natural satellite. Again, Earth’s composition and the Moon’s
composition are too similar if lots of little planetesimals with different compositions collided and
stuck together.
Giant Impact Theory
The leading theory of the moon's formation posits that it coalesced from material blasted into
space when a planet-size body slammed into the newly formed Earth about 4.4 billion years ago.
One variant of this idea holds that the impactor, dubbed "Theia," was about the size of Mars.
Another version, introduced in 2012, suggests that both the impactor and the target — the protoEarth — were about 50 percent as massive as Earth is today.
While the giant-impact hypothesis continues to be tweaked and refined, it does the best job of
explaining the moon's composition and orbit, most scientists say. For example, the theory
predicts a small iron core for the moon, since it would have formed primarily from the mantles of
the impactor and early Earth (both of which lacked iron, which had already been concentrated
deep in the core). It also explains why the Moon rotates at roughly the same speed that it
revolves around the Earth. Finally, it explains why the Earth and Moon are similar but not
exactly alike in composition.