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  • Just about every two years, the planet Mars makes its closest approach to Earth... around

  • 36 million miles.

  • That's when we pack our robotic emissaries off to the Red Planet, timing their launches

  • to spend the least effort to get there.

  • Some fly around it... snapping pictures...

  • Others land ... to sample its surface....

  • ...a few to crawl around its canyons and craters.

  • These probes may pave the way for human explorers... and, perhaps permanent settlers... who'll

  • dig deeper still... in search of answers to our most pressing question:

  • Did Mars develop far enough - and stay that way long enough - for life to arise?

  • And, if so, does anything live now within Mars' dusty plains... beneath its ice caps...

  • or maybe somewhere underground?

  • Mars does not give up its secrets easily ... it's almost as if the little planet is

  • embarrassed.

  • Over a century ago, a few observers thought they saw clues that Mars is alive.

  • In 1877, the Italian astronomer Giovanni Schiaparelli noted markings... which he saw as a latticework

  • of lines. He called them "canali" in Italian... meaning nothing more than "shallow channels"

  • in English.

  • American astronomer, Percival Lowell, found the lure of these features irresistible.

  • He saw Schiaparelli's channels as artificial canals. He speculated that they carried melting

  • snow from the poles to the dry interior.

  • After all, on Earth, the Suez Canal had recently opened to ship traffic. The Panama Canal was

  • beginning to be dug.

  • The Martian canals, Lowell said, were built by a sophisticated society confronting an

  • environmental catastrophe on the grandest of scales.

  • Those Martians, he thought, must face urgent choice: move water across vast arid regions,

  • or perish on an increasingly dry planet.

  • As the 19th Century gave way to the 20th, Lowell took his case to the public, in a series

  • of three best-selling books.

  • And the public responded with... questions.

  • Who were these Martians, who had the means to remake an entire planet?

  • Some offered schemes for making contact. Giant mirrors would flash greetings... Light beams...

  • Mental telepathy.

  • Many astronomers grew deeply skeptical... but Lowell's vision of a harsh, yet Earth-like

  • planet endured in the public's imagination..

  • That vision was dealt harsh blow in 1964. The Mariner Four spacecraft ventured in for

  • a closer look... And what it saw looked like the Moon.

  • Three more Mariners followed.

  • They found huge dormant volcanoes... the deepest and longest canyon in the solar system...but

  • not a trace of life, present or past.

  • In the mid-1970's, two lander-orbiter robot teams, named Viking, took up residence at

  • Mars.

  • Maybe the Martians were just hiding, so theVikings tested the soil for signs of life.

  • But all the evidence from Viking told us... Mars is not only barren... but in fact hostile

  • to life.

  • It's no wonder. Martian air temperatures range from -20 degrees Fahrenheit to down below

  • -200.

  • It's also very, very dry. The Sahara Desert on Earth is a rainforest, by comparison. If

  • all of the water vapor in Mars' thin atmosphere fell as snow, it would make a layer of frost

  • not thicker than your fingernail.

  • On Earth, impact craters erode over time from wind and water... and even volcanic activity.

  • On Mars, they can linger for billions of years.

  • But so can the imprint of riverbeds, lake bottoms and ocean shorelines... And the Viking

  • orbiters saw a lot of them.

  • It's not hard to believe that a great deal of water once flowed here.

  • But where did all the water go?

  • To find out, scientists needed to do real field-geology on Mars. They needed rovers...

  • travelling robots with tools and instruments.

  • In 2004, the heroic twin rovers Spirit and Opportunity bounced down... stood up... and

  • drove off, ready to roam the Red Planet.

  • Opportunity had come to rest in a small crater at a spot called Meridiani Planum... and it

  • didn't have far to travel.

  • For here, in plain view, on the crater wall before it was exposed bedrock... the first

  • ever seen on Mars.

  • Not far away, Opportunity found layered rocks on the face of a cliff. On Earth, rocks like

  • these typically form in sedimentary layers at the bottom of ancient oceans.

  • And at every turn, Opportunity rolled across tiny, smooth, almost perfectly round pellets...

  • called "blueberries" because they appeared dark bluish-brown against Mars' rust-colored

  • surface.

  • Initially thought to be volcanic in origin, these blueberries proved to be iron-rich spherules

  • of exactly the type formed within cavities at the bottom of an ocean.

  • And the rovers has another trick up their mechanical sleeves; they could drill into

  • the rock itself... brush it clean ... and nuzzle a spectrometer right up to the virgin

  • minerals.

  • They found that up to 40% of the rocks were sulfate salt... a tracer for standing water.

  • That wasn't the only evidence. Spirit's broken wheel, dragging behind it, exposed soils saturated

  • in salt.

  • Clearly these rocks formed in water. But how long ago? And, if you needed to find water

  • on Mars right now, where would you look?

  • One answer... the North Pole. From orbit, it seemed to be covered in frozen carbon dioxide

  • - what we call dry ice. But might water ice exist below the surface?

  • Enter Mars Phoenix... a lander that arrived at the North Pole in early 2008.

  • Phoenix used rockets, not airbags, to touch down in what appears to be the floor of an

  • ancient ocean.

  • The lander's descent thrusters blew away the top layer of soil, and Phoenix snapped some

  • pictures. It sure looked like ice.

  • Mission managers instructed the robot to conduct a simple experiment... reach out, dig a trench...

  • then watch what happens.

  • As expected, clumps of white stuff appeared. A couple of days later, it was gone... vaporized.

  • That means it can't be salt. Or frozen carbon dioxide, which is stable in the cold dry temperatures

  • of the Martian pole.

  • So it had to be water... the first ever directly seen on Mars.

  • In fact, Phoenix probably saw droplets of liquid salty water form and merge on its legs.

  • And there are other indications that the North Pole was actually warm enough in the recent

  • past for water to become liquid.

  • Beginning in November of 2006, Mars Reconnaissance Orbiter flew over the pole, sending out radar

  • pulses to peer beneath the surface.

  • These data have revealed that the ice cap, just over a mile thick, formed in a succession

  • of layers as the climate alternated between warm and cold.

  • Martian mood swings like this happen because the planet's spin is not stabilized by a massive

  • moon, like that of Earth. Mars can really wobble, with one pole tilting toward the sun

  • for long periods.

  • Whatever water's here now, is tiny compared to what once was. There's a growing body of

  • evidence that early in its history, Mars was on track to become positively Earth-like.

  • But something went horribly wrong.

  • Not long after its birth, the planet's molten interior welled up to spawn volcanoes and

  • hot springs... spewing gas that formed an early atmosphere.

  • As the atmosphere thickened, carbon dioxide began to trap heat from sunlight; in other

  • words, the classic greenhouse effect.

  • Temperatures rose high enough to allow liquid water to flow on the surface.

  • What then caused these waterways to evaporate... and the planet to grow cold and dry?

  • In search of the answer, the Mars Global Surveyor spacecraft was brought down to a low altitude...

  • and it made an intriguing detection.

  • MGS documented the presence of a small magnetic field emanating from the planet.

  • It's incredibly weak compared to that of Earth.

  • Our magnetic field is generated by molten rock deep in Earth's core that rises and falls

  • into a huge region below the outer curst, called the mantle.

  • That has turned our planet into an electric dynamo... the rising and sinking motion within,

  • combined with the spinning motion of the planet, generates a strong magnetic field.

  • This field is strong enough, and extends far enough into space, to deflect the wind of

  • high-energy solar particles... protecting Earth and its atmosphere.

  • In its early years, Mars too appears to have had a robust magnetic field.

  • Rocks in some of the older craters bear a strong imprint of this field.... While newer

  • ones indicate a much weaker field.

  • So what caused Mars' magnetic field to drain away... exposing the planet to solar winds?

  • The answer lies in its violent past... a past that today is still written on the landscape.

  • This is a simple elevation map of Mars' surface, from data gathered by the Mars Odyssey spacecraft.

  • The south pole, colored in orange and red, is piled high with ice that comes and goes

  • with the seasons.

  • Moving off these southern highlands, we make our way north.

  • The landscape is pocked with craters. The largest and oldest ones have slowly faded,

  • softened by windblown dust.

  • We now pass into the Tharsis region, along the equator. Here, on a vast high altitude

  • plateau, are a series of enormous volcanoes...

  • ...including the largest one in our solar system, Olympus Mons.

  • Just to the East is the great Valles Marineris. It's named for the Mariner Nine mission that

  • found this vast gash in the Martian landscape. It's about 2500 miles long and up to 400 miles

  • wide.

  • Dust devils sweep along the plains above it...

  • And dust blows up the valley slopes. We can see this incredible landform in a realistic

  • rendering of the data.

  • Giant landslides have caused the walls to slump off and pile onto the valley floor.

  • Feeding into this valley: a maze of side channels.

  • Scientists think these and other tributary features were formed when water underground

  • flowed into the main basin, and the land above collapsed.

  • Wider parts of the canyon are regarded as possible landing sites for human missions.

  • They offer flat surfaces... and possibly access to liquid water that may remain below the

  • surface even today.

  • From the layers of rock that line the canyon walls... and layers of dust on the canyon

  • floor... future science prospectors can fill in details of the planet's history.

  • The theory is that the great Valles Marineris formed early in Mars' history, as the planet

  • cooled. Its huge sides were pulled apart as the Tharsis region, just to the west, began

  • to rise up.

  • But there seems to have been another, much larger event that changed Mars' course forever.

  • Travel north, down the slopes of Mars's great volcanoes. The elevation drops as we dive

  • into what appears to have been an immense ocean.

  • With this vast basin in the north, and the very high elevations of the South, Mars is

  • a lopsided planet.

  • And here's why:

  • Early on, when the Solar System was young, Mars was apparently hit by at least 15 large

  • asteroids.

  • One of them was large enough to turn most of Mars' northern hemisphere into one huge

  • impact feature.

  • Earth, too, was walloped by objects this big.

  • But because Mars is small, only about half the size of Earth - and has only about a third

  • Earth's surface area - ... the effect would have been much more profound... and long-lasting.

  • The thinking is that these impacts heated the outer subsurface layer of the planet,

  • the "mantle." That prevented molten rock in its core from rising up... and switched off

  • Mars' magnetic field...probably forever.

  • Without its electromagnetic "deflector shield," solar radiation began to lash the planet...

  • gradually stripping it of its atmosphere... and thus it's blanket of warm air. What water

  • Mars had would have vaporized into space... or frozen underground.

  • Proof that Mars once had a thicker atmosphere may have landed, literally, in Opportunity's

  • backyard.

  • Just recently, it spied a strange bluish rock... a nickel iron meteorite, sitting out on the

  • surface.

  • Streaking through a thin atmosphere, this massive a chunk of metal should have made

  • a much bigger dent. But a much more dense atmosphere would have slowed its fall and

  • melted it in just the way that Opportunity now sees it.

  • Now there are a few competing theories that explain the disappearance of Mars magnetic

  • field and its atmosphere.

  • What's certain is that at some point early in its history, the Red Planet grew increasingly

  • desolate... and that the tragedy happened fairly quickly.

  • The Martian landscape we see today is replete with coded signals from those early times...

  • ancient riverbeds and lake bottoms, flood plains, and volcanic cones... as well as the

  • clear battering it received from impacts.

  • But is Mars a dead world?

  • An infrared telescope on Hawaii's Mauna Kea volcano was trained on Mars over several years.

  • Astronomers used a spectrometer to split the light into its individual wavelengths... to

  • identify chemical fingerprints in Mars' atmosphere.

  • They found the signature of methane gas, in amounts that change from place to place.

  • Methane wouldn't last very long under Mars' conditions... there must be some source that

  • replenishes it.

  • That source could be biological... perhaps microbes alive and well somewhere underground.

  • Or the methane could be coming from volcanic activity... or maybe chemical reactions in

  • Mars' crust.

  • So far, neither the satellites flying over Mars... nor the robots on the surface... have

  • turned up anything close to clear proof of living organisms.

  • It may take the searching eyes, flexible minds, and nimble fingers of human explorers to find

  • that buried treasure, if it exists.

  • In the meantime, we are finding that even if Mars appears dead, it's certainly not dull.

  • Though Mars has nowhere near the dynamism of Earth, with our oceans... turbulent atmosphere...

  • volcanism... and shifting continents... Mars does do some fascinating things:

  • If you take an atmosphere, however tenuous... add heat from the sun so it blows over the

  • dry plains... you get the renowned dust devils of Mars.

  • With no trees to hold down the soil ... landslides are common.

  • The Mars Reconnaissance Orbiter has returned images showing that Mars is actively remaking

  • its surface...

  • Not in canals built by the Martian engineers of Percival Lowell's imagination...

  • But in picturesque sand dunes shaped by the wind...

  • And in intricate landscapes molded by a gradually changing climate...

  • Whether or not we ever find life-forms on Mars, we can still marvel at the beauty of

  • its landforms... subtly sculpted over eons of time...

  • on a world that never was.

  • 8

Just about every two years, the planet Mars makes its closest approach to Earth... around

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火星を平らにした小惑星 (The Asteroid that Flattened Mars)

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    Wonderful に公開 2021 年 01 月 14 日
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