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  • 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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  • 7

The Earth, over its 4.5 billion year history, has been pummeled by asteroids.

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月の誕生 (Birth of the Moon)

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    Wonderful に公開 2021 年 01 月 14 日
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