Discover – June 2019

JUNE 2019. DISCOVER 47

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Before Apollo, humans could only learn so much about the

moon. All they had were images of its near side, and almost no

information at all about its far side. After physically going there,

here are a few of the questions we’ve now answered.

WHAT CAUSED THE LUNAR FEATURES?

Scientists once debated whether the features we can see on the

moon’s face were impact craters from meteors, or signs of ancient

volcanism. Then Apollo showed

in stunning detail the difference

between the moon’s familiar near

side and its highly cratered far

side — along with close-up views

of the craters in question — quash-

ing the idea of conical volcanoes

on the moon. But the dark lunar

splotches did turn out to be lava-

based basalts, proof the moon once

had a more active molten interior.

WHERE DID THE MOON

COME FROM?

Our current understanding is

that an ancient Mars-sized world

slammed into early Earth billions

of years ago, and that the detritus

from this encounter eventually

coalesced into the moon. It was

Apollo’s recovered moon rocks

that pointed scientists toward

this dramatic and unlikely story.

Analyses revealed how eerily

similar Earth’s and the moon’s chemical distributions are, a

sign they share a common source. Studies also show that moon

rocks are stripped of nearly all volatiles (materials that boil

away easily), providing evidence of some long-ago disaster that

eliminated them.

WHAT WERE CONDITIONS LIKE IN THE EARLY SOLAR SYSTEM?

All the rocky bodies we’ve studied bear scars from meteor

impacts, some new and many very old. But Earth recycles its

surface regularly, while the lunar surface keeps

a pristine historical record. And the moon is one

of the only places we’ve been able to gather such

ancient materials to measure their absolute ages,

indicating, for instance, that one of its many

craters formed about 3.9 billion years ago. The

lunar rocks ultimately tell us of a violent chapter

in solar system history, when a period known

as the Late Heavy Bombardment rained down

space rocks, heavily scarring the inner planets.

WHAT’S INSIDE THE MOON?

While astronauts didn’t do any deep mining, they did place seis-

mometers at multiple sites to analyze moonquakes. These can

come from the moon itself, from meteor impacts that still occur

and from astronaut-made explosions set off to watch how the

reverberations travel through the lunar material. The readings

revealed a crust around 40 miles thick, and a small core some 400

miles across.^ D

THE SCIENCE

OF APOLLO

The mission data gathered remain

the most valuable information we

have about the history of the moon

— and the solar system.

BY KOREY HAYNES

The Apollo program

ran from 1961 to

1972 and included

11 crewed missions.

The moon is about

a billion times less

seismically active

than Earth.

The oldest

lunar samples are

4.5 billion years old.

All told, the Apollo

missions returned

over 800 pounds

of moon rocks.

Growing Influence

Over 2,500 academic papers use data gathered by the

Apollo missions, and the number is still growing.

JUNE 2019. DISCOVER 47

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Moon’s far side

Astronaut and geologist Harrison Schmitt stands beside a boulder.

Buzz Aldrin sets up a seismometer.

Source: Astronomy and Geophysics, Vol. 53, 2012