The Earth, over its 4.5 billion year history,
has been pummeled by asteroids.
Eroded by wind and rain.
Covered over with flowing lava.
Wrinkled and gouged by shifts in its crust.
Most traces of its distant past have long
since been destroyed.
But there is a place where clues to the early
history of our planet are still largely intact.
The moon.
Scientists have been reconstructing its history
by scouring its surface, mapping its mountains
and craters, and probing its interior regions.
What are they learning about our own planet's
beginnings, by going back in time, to the
mysterious Birth of the Moon.
The space age made possible rockets with enough
power to blast humans and their life support
systems into orbit.
More than three hundred manned space flights
have shown that humans can live and explore
beyond our Earth.
None of these missions have done more to shape
our connection to the cosmos, and define who
were are, than the fabled flights of Apollo.
But not for reasons you might think.
We sent astronauts to the moon as a symbol
of confidence in the face of the great cold
war struggle.
Many hoped that would lead to further journeys
to Mars, and to an age of living in space.
Those goals proved too grand in the face of
all the preoccupations, turbulence, and change
that would crash onto our own planetary shores.
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 - 382 kilograms worth -- 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 apollo rocks have brought us closer to
the answer, but basic mysteries still remain.
The moon orbits Earth at an average distance
of 384,400 kilometers.
It's relatively small, with less than one
percent the surface area, two percent the
volume, and one percent the mass of Earth.
With no atmosphere, temperatures range, in
Celsius, from -233 degrees at night to 123
degrees during the day.
As the brightest object in the night sky,
the moon has guided people for millennia...
by defining the rhythms of life and animating
our myths.
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.
Flash forward to the modern age of lunar studies.
1959 saw the first in a fleet of probes launched
by the Soviet Union and the United States
to shoot closeup pictures, take readings,
and crash onto its surface.
We learned then just how different the moon
is from Earth, with its cratered and dessicated
landscapes, and lack of a magnetic field.
That intensified a debate about the moon's
origin that went back centuries.
The so-called fission theory, championed by
George Howard Darwin, son of Charles Darwin,
held that the moon was once part of the Earth,
cast off by the rapid spin of its young parent.
For proof, look no farther than the Pacific
Ocean, a giant hole in the Earth's surface.
Then there's the capture theory, which holds
that the moon was a wayward object that floated
through our solar system and was pulled into
orbit by Earth's gravity.
A third idea came from the American astronomer
Thomas Jefferson Jackson See, also known for
his attacks on Einstein's theories and for
charges of plagerism that were leveled at
him.
He suggested that the Moon formed near Earth
and gradually fell under its gravitational
spell.
In that case, the moon should be a mini-Earth,
which we now know it's not.
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.
Not long after the last Apollo mission went
into the history books, this initial evidence
coalesced into a radical new idea first presented
in 1974 by the scientist and artist, William
Hartmann.
His theory of the moon's formation is played
out in this contemporary scenario.
Sharing an orbit with Earth was a Mars-sized
body called Theia, named for a Titan in Greek
myth who gave birth to the moon goddess, Selene.
Its orbit became unstable and it headed in
Earth's direction.
Theia hit at an oblique angle, causing the
Earth to spin faster and debris from both
Theia and the Earth to fly into orbit.
When the dust settled, the debris began to
coalesced in Earth orbit, forming the Moon.
By then, volatile compounds like water had
evaporated.
The moon, then, comes primarily from the mantles
of the Earth and Theia.
That's why overall the moon is lighter, or
less dense, than Earth.
It has a core of solid iron like Earth, but
this core is relatively small compared to
ours.
From this violent beginning, the moon gradually
coole, and the magma that lined its surface
hardened into a crust.
Yet it was still subject to intense bombardment
in the chaotic environment of the early solar
system.
You can see what a battering the moon has
taken in this global elevation map compiled
by the Lunar Reconnaissance Orbiter.
Its surface has been hammered by large impacts
at every stage in its history.
The Apollo astronauts encountered a secondary
effect of all the impacts, rocks formed in
the crucible of widespread volcanism.
On Earth, volcanic eruptions are often powered
by water, heated and pressurized by friction
from the movement of immense tectonic plates.
Neither factor exists on the Moon.
Instead, lunar volcanoes are tied to impacts
forceful enough to melt the surface and release
material from the hot interior.
In the lower gravity of the Moon, volcanic
eruptions would have splattered high above
the landscape.
Molten rock then flowed out in thin smooth
layers that reached far from its source.
That's how the mare regions that cover about
one-sixth of the entire lunar surface were
formed.
These immense volcanic basins are thought
to date back to period of large and violent
impacts from three to four billion years ago.
The materials brought up by these impacts
are made up of heavier compounds that would
have initially sunk into the magma ocean.
Scientists have also detected volcanic residues
in recent impacts that have been imaged by
LRO cameras.
This is the Aristarchus Plateau with a mysterious
channel cut by flowing lava and punctuated
by two immense craters.
One was created by an impact just 450 million
years ago.
Along its walls are blocks of rock from the
lunar crust that fragmented in the impact.
Scientists detected volcanic glass-like deposits
that are the legacy of ancient eruptions.
Then there's the strange landscape of the
Tycho crater, from an impact about 100 million
years ago.
Its central peak, rising two kilometers above
the crater floor, may be material blasted
out by the impact.
The same is true for a 120-meter wide boulder
that sits on the summit.
Around it, and on it, scientists see evidence
of rock that was melted in the heat of the
impact itself.
Since its birth, the moon has exerted a constant
and powerful influence on Earth.
Gravitational drag from the moon slows Earth's
rotation by 2.3 milliseconds per century...
while causing the moon to drift away at a
rate of 3.8 centimeters per year.
The moon's tidal pull acts to stabilize Earth's
rotation about its axis, and the tides help
promote the formation of ocean currents that
shape our climate.
To think that the root of this intimate relationship
between Earth and the moon stems from a violent
collision so long ago.
However, the giant impact theory is not universally
accepted.
For one thing, there's the issue of water.
The dryness of the moon is consistent with
the giant impact theory, which predicts that
water and other volatile compounds would have
evaporated out of the lunar debris cloud.
But the moon may not be so dry after all.
In 1998, the unmanned Lunar Prospector began
an ambitious mission to map the lunar surface
with an instrument called a gamma-ray spectrometer.
The data showed that hydrogen is spread widely
across the poles of the moon.
If it's from ice, millions of tons of water
could well be embedded in the soil there.
It turns out there are places where water
can exist on the moon, where the sun never
shines.
Polar craters, forever in shadow, could shelter
significant quantities of ice against solar
radiation.
Prospecting for ice, the LCross probe arrived
at the moon in 2009 aboard the Lunar Reconnaissance
Orbiter, on a collision course with the 97
kilometer wide Cabeus Crater at the south
pole.
Seconds after it slammed into the crater,
a trailing probe detected a tenuous cloud
rising up from the crater.
In the cloud was a mix of carbon monoxide,
ammonia, methane, and more.
5.6% of the crater floor is water ice, a higher
concentration than some areas on Earth.
The presence of water is good news for explorers
who dream of one day spending quality time
on the moon.
But how did it get there?
It might have been delivered by comets striking
the moon over the eons.
There's another explanation, one that emerged
from rocks gathered by the Apollo astronauts.
The samples are graced with tiny glassy globules,
minerals melted in the crucible of lunar volcanism.
Analysing these Apollo rocks with new techniques,
several teams of scientists have found a compound
called hydroxyl.
It's H2O, with only one H, and could be turned
into water, say, for a moon base.
Much of what the moon had was likely lost
to space from impacts.
Tiny residues might remain in polar craters.
The rest, still locked up inside the moon,
might be enough to cover its surface in a
layer of water one meter deep.
Does the presence of water inside the moon
send us back to square one on its formation?
It might not, if it had been carried in by
comets in the moon's early days.
One new study suggests that the giant impact
scenario is more complex than we thought.
It holds that another, much smaller body formed
alongside the moon and eventually merged with
it.
The impact literally rearranged the moon's
interior, pushing molten magma onto the near
side, while adding an extra layer of crust
to the far side.
That explains the lopsided topography of the
moon, the dark lava basins on the near side,
and the highlands of the far side.
Human geologists with a network of seismographs
could confirm this idea.
The next best thing is a mission called Grail,
a pair of spacecraft orbiting the moon in
unison.
Such a mission has already performed a kind
of planetary CAT-scan of Earth.
By measuring subtle changes in the distance
between the craft, scientists discerned variations
in Earth's gravity.
The data resulted in a map of ground water
all around the planet.
Flying around the moon, the Grail pair could
provide new details about the size and composition
of the Moon's core, adding support for the
giant impact theory, or sending science in
a whole new direction.
The intimate connection between the Earth
and the moon appears to trace back to their
earliest times.
If that's true, then the formation of this
desolate world can tell us about our own...
A blue green world with a companion reflective
enough to light up the night.
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