The Astronomy Book

187

See also: The discovery of Ceres 94–99 ■ The composition of comets 207 ■ Investigating craters 212 ■
The Space Race 242–49

ATOMS, STARS, AND GALAXIES

An alternative theory, championed
in the 1940s by American chemist
Harold Urey, was the “capture”
model, in which the moon formed
elsewhere in the solar system and
fell under the control of Earth’s
gravity. However, the moon is so
big compared to Earth that most
thought such an event unlikely.
In 1946, Canadian geologist
Reginald Daly presented a third
idea. While Daly agreed with Darwin
that the moon and Earth were
formed from the same material,
he hypothesized that the driving
force was an impact between
Earth and another body, which
had thrown material into orbit.

Matching rock
Daly’s idea was more or less
ignored until the 1970s, when the
analysis of moon rocks showed that
their mineral content is a very close
match to that of Earth’s mantle (the
layer between the crust and the
core). Both are high in silicates but
have low levels of metals. If the moon
had formed elsewhere, its rocks
would be very different from Earth’s.
If it had formed from the same molten
raw materials as Earth, it would be
expected to be a mini version of
Earth, and have a larger metallic

core. However, the rock evidence

points to the moon being made from

material gouged from Earth’s surface

after the planet had solidified.

Big Splash

In the last decade, computer

modeling of possible impacts has

suggested an event now dubbed

the Big Splash. In this scenario, a

Mars-sized planet (named Theia,

after the mother of the moon in

Greek mythology) hit Earth 4.3

billion years ago—about 200 million

years after Earth’s formation. The

impact turned both bodies into

At bottom each “exact”

science is, and must

be, speculative. Its chief

tool of research, too rarely

used with both courage

and judgment, is the

regulated imagination.

Reginald Daly

Reginald Daly

Geologist Reginald Daly’s

contributions to the theories of

continental drift, plate tectonics,

and the rock cycle have proved

invaluable in understanding

the similarities and differences

between Earth and other rocky

bodies in the solar system.

Daly’s abilities as a geologist

became clear when he was

surveying the southern border of

Canada, from the Pacific Coast,

through the Rockies to the Great

Plains. The rock samples he

collected during this survey led

him to become a leading voice

on the origins of different rock

types. As early as the 1920s,

Daly proposed that material

ejected from Earth to form the

moon was a primary cause for

the dynamic character of Earth’s

crust. The impact theory was

a late addition to Daly’s work,

coming after he had retired as

the head of Harvard University’s

geology department.

Key work

1946 Origin of the Moon and

its Topography

Computer modeling suggests that a smaller

planet hit Earth 4.3 billion years ago,

and that the collision created the moon.

The moon may have

formed in a giant

impact that knocked

Earth’s magma

into orbit.

Lunar rocks match

the material in

Earth’s mantle.

seething balls of magma. Most of

Theia merged with Earth (which

explains why the planet has an

oversized metallic core) and the

“splash” hurled magma into orbit,

mostly from the rocky outer region

of the planet. This material formed

the moon. Although the Big Splash

idea is very much a hypothesis at

present, it remains the best guess

as to the origins of the moon. ■