Scientists have been reconstructing the history of the moon by scouring
its surface, mapping its mountains and craters, and probing its
interior. What are they learning about our own planet's beginnings?
Decades
ago, we sent astronauts to the moon as a symbol of confidence in the
face of the great cold war struggle. Landing on the moon was a giant
leap for mankind. But it's what the astronauts picked up from the lunar
surface that may turn out to be Apollo's greatest legacy.
When
the astronauts of Apollo stepped out of their landing craft, they
entered a world draped in fine sticky dust, strewn with rocks, and
pocked with craters. They walked and rambled about, picking up rocks
that they packed for the return flight.
Back in earth-bound
labs, scientists went to work probing the rocks for clues to one of the
most vexing questions in all of science. Where did the moon come from?
The answer promised to shed light on an even grander question. Where did
Earth come from? And how did it evolve into the planet we know today?
The
nature of the moon began to come into focus four centuries ago. Galileo
Galilei had heard of an instrument built by Dutch opticians capable of
"seeing faraway things as though nearby." Galileo, in many ways the
first modern scientist, saw this new instrument as a tool to help settle
a long standing question.
What was the nature of the heavens, and how did the world of men fit within it?
To
some philosophers, the moon was a perfect, crystalline sphere of divine
substance, free of Earth's imperfections. Galileo, with his telescope,
saw a more familiar reality. He noted mountains and valleys on the moon,
features like those of Earth.
The astronauts of Apollo lifted
off on a series of missions to get a close up look at the moon and
perhaps settle the debate. Because there's no atmosphere there, the
astronauts entered landscapes that are nearly frozen in time. They could
scour the lunar surface for evidence of events going back almost to the
time of its birth.
Indeed, eons of impacts had opened up the
Moon's interior, leaving a wealth of information strewn about their
landing sites. Scientists had already noticed that some large old
craters were surrounded by concentric rings. You can see one of the most
pronounced examples in this image of the Mare Orientale, captured
recently by NASA's Lunar Reconnaissance Orbiter, or LRO. The colors show
differences in elevation.
The old view was that the impact had
melted the rock below. A newer view held that the impactor had actually
splashed down on a molten surface. That gave rise to the radical notion
that, early in its history, the moon's surface was covered in a vast
ocean of magma.
When the astronauts arrived, they found
relatively light rocks known as anorthosites. Their presence suggested
that heavier material had sunk toward the moon's interior, forcing
lighter material to the surface.
The rocks they brought back
were found to be strikingly similar to those on Earth, in part because
they share forms of oxygen, called isotopes, that scientists regard as
"blood types" for solar system bodies. Then there was this. The moon
appeared to be completely, utterly, dry, with no evidence that water was
ever present on its surface.
