How the Earth Was Made

>> Earth. A 4.5-billion-year-

old planet, still evolving.

As continents shift and clash,

volcanoes erupt, glaciers grow

and recede, the Earth's crust

is carved in numerous and

fascinating ways, leaving a

trail of geological mysteries

behind. In this episode,

investigators are exploring

the driest place on Earth--

the Atacama Desert in Chile.

This barren landscape is 50

times drier than Death Valley.

Now scientists are piecing

together the puzzle of how

this desert was made.

From raging volcanoes to

colossal mountains, oceans,

the clues they uncover also

provide a window into the

formation of the Earth itself.

S01x06 Driest Place on Earth Original Air Date on March 17, 2009

== sync, corrected by elderman ==

Earth is a blue planet,

engulfed by water. But in this

desolate chunk of Northern

Chile, you won't find a single

drop. Wedged between the

Pacific Ocean and coastal

volcanoes to the west, and the

Andes to the east, is Atacama,

the driest desert in the

world.

600 miles long, and narrow--on

average just 100 miles wide--

it's the same size as Iowa.

Now scientists are on a mission

to find out how it was made.

The investigation begins in

the sleepy town of Quillagua.

A rare green oasis, its only

lifeline is a stream trickling

300 miles from the Andes to

the Pacific.

It is home to the official

government rain gauge, so

geologist John Houston has

come here to find out how dry

the driest place on Earth

really is.

>> This is a pluviometer. It

measures the rainfall, uh,

every day.

>> Ah, ok.

>> For Marisa Vera, a government

scientist, it's a job with few

surprises.

>> How much rainfall has this

instrument recorded?

>> In the last 15 years, less

than one millimeter per year.

>> Less than one millimeter

a year?

>> Yes.

>> But was it every year?

>> It rains only three years.

>> That's incredible.

>> Exactly.

>> So less than one millimeter

a year.

>> On average, it rains three

one-hundredths of an inch a

year.

It would take a century for

Atacama's rainfall to fill a

coffee cup. How does this

compare with other deserts?

>> Here we have a cylinder,

and I'm going to show you the

difference between the amount

of rainfall per annum here and

the amount of rainfall in

other deserts. So if I fill

this jar up, right up to about

there, that is roughly the

rainfall that you get in the

Sahara. Now if I pour most of

that away, we get to that

level, that represents what we

have in the Mojave Desert,

five inches per annum. If I

pour all that away, except for

that little drop in the bottom

there, and that's the

equivalent of what we have

here in the heart of the

Atacama Desert. That is such a

small amount of rainfall that

it means it's the driest place

on Earth.

>> In his quest to find out

why Atacama gets so little

rainfall, Houston leaves the

oasis behind and heads into

the desert.

By the side of the

Pan-American Highway, a road

which runs the length of the

continent, he discovers the

first clue.

>> Well, here we are at the

Tropic of Capricorn. This is

one of the most important

latitudes in the world and

it is absolutely critical in

explaining why the Atacama

desert is in this location

here.

>> Most of the world's deserts

straddle one of two special

latitudes.

In the Southern Hemisphere,

the Tropic of Capricorn runs

through Atacama and Africa's

Namib and Kalahari deserts.

In the north, the Tropic of

Cancer runs right through the

vast Sahara. At these

particular positions on the

planet, the air is extremely

dry.

>> This instrument is called a

whirling hygrometer. What this

does is to measure the

relative humidity of the air.

And the reading on here gives

us a relative humidity of ten

per cent. That's really low,

really low. Um, there aren't

many places in the world where

you'd get a relative humidity

as low as that.

>> Back in the early 1700s,

scientists discovered why

tropical air is so dry.

European ships sailing to

America relied upon the trade

winds to power their

crossings, but English

meteorologist George Hadley

was mystified why they blew

westward when they should blow

directly north.

His studies would lead

scientists to understand how

air circulates around the

Earth.

At the equator, moisture-rich

air gets heated by the sun and

rises. As this hot, wet air

flows away from the Equator,

it quickly sheds its water as

rain. By the time it reaches

the two tropic latitudes, the

air has lost nearly all of its

moisture, resulting in no rain

on the land below.

The mystery, though, is why

Atacama gets so much less rain

than anyplace else.

Scientists hope to crack the

case by figuring out how

Atacama first formed.

On the hunt for clues, Houston

travels deep into the true

desert. This closely guarded

location was discovered during

routine mapping by geologists

back in the '70s, but the huge

significance of their find

wasn't realized until 1998.

This band of boulders is the

single most important clue to

Atacama's beginnings.

It's a delicate rock called

gypsum.

A simple test shows how

fragile it is.

>> If I pour a little bit of

water on top of that, you will

see that it very rapidly falls

apart. What's happening here,

of course, is that when I'm

putting water on this you see

it dissolve, I mean, it's just

going to fall apart.

>> The survival of gypsum as a

solid rock tells scientists

there hasn't been any heavy