Scientific American 201907

July 2019, ScientificAmerican.com 77

Erica Jawin is

a postdoctoral

research geologist

at the Smithsonian

National Museum

of Natural History.

even other planetary bodies) without visiting them.
At about 4.5  billion years old, the oldest sample is
essentially the same age as the moon itself. Most rocks
on Earth are much younger than four billion years
because of the constant recycling of the crust by plate
tectonics—a process that does not occur on the moon.
Thus, lunar samples provide an important glimpse of
ancient rocks from the early days of the solar system.
They could even tell us about the young Earth. This
March, researchers analyzing an Apollo 14 breccia (a
rock type composed of other rock fragments welded

together) determined that one of those pieces might

not be a moon rock at all. Instead it may represent the

first terrestrial meteorite—a rock that was ejected

from Earth four billion years ago and then landed on

the moon. After billions of years astronaut Alan Shep-

ard picked it up and brought it home.

LUNAR ORIGINS

Before apollo, scientists had several competing ideas

for how the moon and other planetary satellites formed.

Perhaps Earth captured another body that passed too

close. Maybe in its early days our planet spun so fast

that a blob separated from the main body. Or maybe

Earth and the moon may have formed at the same time

from the original “protoplanetary disk” that gave rise to

all the planets in our solar system. After the Apollo mis-

sions, however, we gained an entirely different picture.

Today the favored theory of the moon’s origin is

called the giant impact hypothesis. This idea, based on

FIVE SAMPLES collected during the Apollo 15, 16 and 17

missions ( 1 ), as well as the Apollo 15 sample 15415 ( 2 ),

known as the Genesis rock, which helped scientists

develop the leading theory of how the moon formed.

1

2