Name: 火星決してなかった世界 (Mars: World That Never Was)
Uploaded: 2021-01-14T04:42:47.000Z
Duration: 25 min 18 s
Description: VoiceTubeの動画で発音を聞きながら英語表現を覚えよう！学べる英語：

There is a region on Mars, roughly the size of Australia, that rises high above the surface

of the planet. Three of the largest volcanoes in the solar system line up to guard its western

To the East, a vast canyon, six to seven times deeper than the Grand Canyon, cuts into the

barren Martian plain. This strange region, once so baffling to scientists, recalls the

planet's violent past, a time long ago when the planet's core erupted, pushing molten

It's part of a larger story, a planetary tragedy, in which Mars began its descent into the cold,

dry, and lifeless state that we see today. We are now scouring its surprisingly complex

surface for clues to the events that long ago doomed the Red Planet, Mars.

Since the early 1960s, we've tried 46 times to send spacecraft to Mars across the 55 million

kilometers of its closest approach to Earth.

Over half failed at launch or upon arrival. The rest flew around the planet, snapping

pictures, recording data. Or they landed to test its soil and rocks, and crawl around

These probes may one day pave the way for human explorers, who will dig deeper still,

in search of answers to our most pressing question:

Did Mars, at some point long ago develop far enough for life to arise? If so, does anything

still live within Mars' dusty plains, beneath its ice caps, or somewhere underground?

Mars does not give up its secrets easily. Over the years, that has led observers on

this planet to jump to all sorts of conclusions.

In the year 1877, the Italian astronomer Giovanni Schiaparelli noted markings on Mars' surface,

a latticework of lines. He called them "canali" in Italian, meaning "channels" in English.

A careful and thorough observer, Schiaparelli began to sketch them and name them, connecting

Over a 15-year period, beginning in 1894, the American astronomer, Percival Lowell,

closely examined these features. He saw a remarkable drama unfolding on our neighboring

In his view, Schiaparelli's channels were artificial canals, designed perhaps to carry

melting snow from the poles to the dry interior. After all, on Earth, the Suez Canal had been

open since 1869. Construction on the Panama Canal had just gotten underway.

The Martian canals, Lowell surmised, had been built by a sophisticated society confronting

an environmental catastrophe on the grandest of scales. Its inhabitants faced an urgent

choice: move water across vast arid regions, or perish on an increasingly dry planet.

In a series of three best-selling books, Lowell took his case to the public. The public responded

With the means to remake an entire planet, perhaps these Martians were more advanced

than humans. Some of us began offering schemes for making contact. Giant mirrors to flash

greetings. Light beams. Mental telepathy.

Lowell vision fell by the wayside in 1964. The Mariner Four spacecraft flew by Mars and

got a good look. What it saw looked more like the Moon than the Earth. Three more Mariners

followed, culminating in the arrival of Mariner Nine in 1971, the first spacecraft to go into

These missions documented a heavily cratered landscape, pocked with huge dormant volcanoes...

and cut with the deepest and longest canyon in the solar system. They saw no traces of

Then, in the mid-1970's, two lander-orbiter robot teams, named Viking, went in for an

even closer look. The landers tested the soil for the chemical residues of life. All the

evidence from Viking told us: Mars is dead. And extremely harsh.

The mission recorded Martian surface temperatures from -17 degrees Celsius down to -107. We

now know it can get even colder than that at the poles. The atmosphere is 95% carbon

dioxide, with only traces of oxygen. And it's extremely thin, with less than one percent

the surface pressure of Earth's atmosphere.

And it's bone dry. In fact, the Sahara Desert is a rainforest compared to Mars, where water

vapor is a trace gas in the atmosphere. On Earth, impact craters erode over time from

wind and water... and even volcanic activity. On Mars, they can linger for billions of years.

Earth's surface is shaped and reshaped by the horizontal movement of plates that make

up its crust driven by heat welling up from the planet's hot interior. At half the width

and only 11% the mass of Earth, Mars doesn't generate enough heat to support wide-scale

Nor does it have the gravity to hold a thick atmosphere needed to store enough heat at

the surface to allow liquid water to flow. Nonetheless, some areas that looked to Viking-era

scientists like craters and volcanic areas, were later shown to be riverbeds, lake bottoms,

If water once flowed on Mars' surface, where did it all go?

This was the scene at NASA's Jet Propulsion Lab in 2004. The twin rovers Spirit and Opportunity

had just bounced down on the Red Planet. When the excitement died down, the rovers were

set off on one of the most remarkable journeys in the history of planetary exploration.

Opportunity had come to rest in a small crater near the equator, at a spot called Meridiani

Planum. Here, in plain view, on a nearby crater wall, its camera revealed exposed bedrock,

the first ever seen on Mars. Not far away, the rover found layered rocks on the face

of a cliff. On Earth, they typically form as sedimentary layers at the bottom of oceans.

And at every turn, Opportunity rolled across tiny, smooth, round pellets. They became known

as "blueberries" because they appeared purplish-brown against Mars' rust-colored surface. Initially

thought to be volcanic in origin, they turned out to be iron-rich spherules of the type

that form within cavities in the mud at the bottom of an ocean.

Drilling into rocks, the rover inserted a spectrometer to read the mineral content.

The readings showed significant amounts of sulfate salt, a tracer for standing water.

That wasn't all. Spirit's broken wheel, dragging behind it, exposed soils saturated in salt.

Clearly there once was water on Mars' surface, but how long ago? And, if there is anything

left, where would you find it? One possible answer: the North Pole. From orbit, this region

seemed to be covered in frozen CO2 - what we call dry ice. But was there water ice below

Enter Phoenix, a lander that touched down near the North Pole in early 2008. Radar readings

from orbit, taken by the Mars Express mission, hinted at the presence of ice just below the

The Phoenix lander's descent thrusters blew away the top layer of soil, allowing its camera

to snap pictures of what looked like ice. Scientists instructed the robot to conduct

a simple experiment: reach out and 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 CO2, 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.

There are indications that the North Pole was actually warm enough in the recent past

for water ice to become liquid. The Mars Reconaissance Orbiter, or MRO, used radar pulses to peer

beneath the surface of the ice cap. These data reveal that the ice, just over a mile

thick, formed in a succession of layers as the climate alternated between warm and cold.

Our planet avoids mood swings like this in part because its spin is stabilized by a massive

moon. Mars' spin is not, so it can really wobble, with the pole tilting toward the sun

for long periods. New observations by the MRO spacecraft show that these wobbles can

lead to dramatic releases of CO2, and warming periods due to an increase in the greenhouse