Original subtitles

>> 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.

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

rain since the rock formed.

So the next step was to date

it and figure out when this

place became dry.

Gypsum can't be directly

dated, but, by analyzing

fossils in the surrounding

rocks, the awesome age of the

desert was revealed.

Atacama is a staggering 150

million years old.

>> This gypsum here is an

extremely special gypsum.

If there had been any rainfall

greater than two inches in any

one year, this would have

dissolved and have been washed

away. What that means is

essentially that the Atacama

desert is the oldest desert in

the world.

>> For more than 150 million

years, while dinosaurs thrived

and became extinct, the

Himalayas formed and humans

evolved, Atacama has been a

desert. Gypsum also holds the

key to how this desert was

made. It's a chalky mineral

which forms not in deserts,

but in water. Gypsum exists in

a dissolved state in shallow,

warm, tropical seas. As the

water is evaporated away

by heat, it solidifies.

The existence of this one

little rock is a key piece of

evidence which reveals that

before Atacama became a

desert...

...it was a sea bed.

>> This really

insignificant-looking piece of

rock indicates that all this

desert was once underwater.

So this gypsum in this

location in the Atacama Desert

is absolutely critical to

understanding the whole

history of the Atacama Desert.

>> In the investigation so

far, scientists have pieced

together evidence of how and

when the desert first formed.

Atacama's location near the

Tropic of Capricorn means air

is dry and no rain falls.

Fossils found in the

surrounding gypsum rock

reveal the age of the desert.

Gypsum, a rock that forms only

in water, reveals Atacama was

once underwater.

Now, as scientists explore the

mystery of how Atacama evolved

from ocean floor to pure

desert, they unearth explosive

evidence in the investigation

of how the driest place on

Earth was made.

150 million years ago, the

Atacama Desert was a sea bed,

covered by ocean waters. But

today, some areas in the

desert are two miles above sea

level.

In the journey to find out how

this happened, scientists take

the investigation to the

eastern edge of the desert.

This strange landscape is the

largest geyser field in the

Southern Hemisphere.

>> We're up at the El Tatio

geyser field. You can see

around us that there's plenty

of hot springs and geysers,

there's plenty of steam around

and this is 'cause the air is

cool and the water is hot, and

so you have a lot of steam

and bubbling springs.

>> The boiling water is being

heated deep underground.

>> The geysers and the hot

water that you find up at El

Tatio are indications that you

have a body of hot rock

underneath us and another

indication is that you have a

bunch of volcanoes surrounding

this basin.

>> The Earth here is violently

alive.

Molten rock erupts onto the

surface, forming volcanoes.

The fiery volcanoes and the

boiling geysers are evidence

of a turbulent process

happening deep beneath the

desert.

Here, the Pacific Ocean crust

is being forced underneath

South America, much like a

spatula going underneath a

pizza. This geological process

is called subduction.

>> You have the Pacific plate

colliding with the continental

crust and the Pacific plate is

actually heavier and it slides

underneath the continental

crust. And as it does so, it

heats because it gets to a

depth of about 60 miles, and

it becomes molten.

>> This crucial depth is

called a melting zone. Hot

molten rock then thrusts

upward to form the active

volcanoes that ring the El

Tatio geyser field.

This process gives scientists

a hint to what lifted the

desert out of the ocean.

More clues are found on the

opposite side of the desert.

Geologists know that these

coastal hills were also once

volcanoes. Today, they're

completely dead, but modern

dating techniques show that

they first erupted over 195

million years ago. It's a

crucial piece of evidence

which reveals when the Pacific

plate first began to force its

way beneath South America.

At that time the desert,

indeed all of Chile, was

underwater. Over time, the

melting zone was pushed

further and further inland,

first igniting the coastal

volcanoes.

As the melting zone passed

beneath the desert, it formed

new crust, thickening and

raising the land. The Atacama

Desert slowly emerged.

50 million years ago, this

same process began to raise

the Andes.

Today, the melting zone is 140

miles inland and the molten

rock it produces ignites

volcanoes...

...and fuels El Tatio's

geysers.

But, as it passed under the

Atacama Desert, it also left

behind this.

Chuquicamata, the largest

open-pit copper mine in the

world.

Volcanic processes concentrated

the copper ore here, but it was

the desert's unique climate

that locked it in place.

>> This area of Northern Chile

produces some of the largest

and most important copper

deposits in the world. And

this is largely due to the

very dry climate.

Most of the erosion on the

Earth's surface is caused by

water. So here. Where there's

so little rainfall, and

there's very little surface

water, there's not very much

erosion and so the copper

deposit has actually

remained intact.

>> As a result, this barren

wilderness is one of the most

valuable pieces of land on the

planet.

The mystery of how a desert

can rise from the sea can be

solved.

Geysers provide evidence that

molten rock exists deep

underground. The existence of

active volcanoes shows the

movement of one continental

plate under another. Extinct

volcanoes show this process

began at the coast and

pushed inland, raising the

desert above the ocean.

The next step is to try and

figure out what turned this

ancient sea floor into the

driest place on Earth. A quest

that spans 200 years of

history and solves the riddle

of what brought these penguins

to the edge of the desert.

The Atacama Desert is

intriguing because it is the

driest place on Earth.

Deserts by their very nature

are dry, but Atacama is

unique. It's 50 times drier

than Death Valley in

California. And it's not

because it's hotter. Atacama

averages around 80 degrees

Fahrenheit during the day,

whereas temperatures in Death

Valley regularly soar above

110.

The search for what turned

this strip of land from a

regular desert into the

world's driest place begins

out on the open sea.

>> One of the curious things

about the Atacama is that we

actually see here penguins.

Penguins obviously like cold

water and that's really

confusing when you think of

on shore we have really hot

conditions. In fact, the

temperature of the water here

is about 55 degrees

Fahrenheit, whereas on land

the temperature is something

like 80 degrees Fahrenheit.

>> These penguins were first

described by explorer

Alexander Von Humboldt, over

200 years ago. While traveling

along this coast, he was

puzzled by the huge variety of

marine life. Measuring the

temperature of the water gave

him an explanation. It was 20

degrees colder than expected--

perfect for sea life like

penguins.

Centuries later,

meteorologists began to wonder

if this chilly belt of water,

called the Humboldt Current

after the explorer, was the

reason Atacama became the

driest place on Earth.

>> The Humboldt Current comes

all the way up from

Antarctica, bringing with it

cold water, and it is this

cold water which creates this

dull grey day that we see

here, with a fog overlying us.

>> It causes the air above it

to cool, forming a thick bank

of cold cloud and fog which

clings to the shore. Hot, dry

air descends at the tropics.

Here, that hot air sits on top

of the cold, heavy rainclouds,

holding them down.

Meteorologists call this an

inversion layer. Trapped at

3,000 feet, the clouds can't

rise up and shed their rain on

the high-altitude desert.

>> The inversion layer

prevents any moisture that may

accumulate close to the sea

from moving inland. So that is

one of the reasons why this

Humboldt Current actually

contributes to the dryness of

the Atacama Desert that we see

just over there.

>> But is it this inversion

layer, created by the

Humboldt Current, that has

turned Atacama into the

driest desert in the world?

In the desert's northern tip,

in a desolate place called

Quebrada Aroma, geologist

Laura Evenstar is looking for

clues to solve this riddle.

She's trying to put a date on

when the desert became so very

dry.

Other deserts, like the

Mojave, don't get much rain,

but when they do, it's

dramatic. Storms bring heavy

rains and flash floods.

But not here in the Quebrada

Aroma, which is now totally

dry. One way to date the last

time there would have been

enough rainfall to cause a

flash flood is to try to find

out how long the rocks have

been lying there undisturbed.

>> What we have here is a

miniature demonstration of

what goes on if you start

having large amounts of

rainfall. So, this is our

rainfall here...

...and what we can see is that

when we start raining on our

desert surface,

it'll pick up the boulders and

move them around and then when

there's no water here, the

boulders just sit still and

don't move.

>> The surface of Quebrada

Aroma is strewn with rocks, so

she's cracking them open to

reveal evidence of exactly

when water last flooded the

landscape.

[Clanging]

>> What we do is we have to

knock a bit off, and then we

examine it and have a look at

whether it's got a--a very

dark color, and hopefully we

can be able to see some of the

black minerals, which is what

we're looking for.

>> The tiny black crystalline

minerals are pyroxenes.

They're crucial evidence

because, like microscopic

geologic clocks, their

chemistry changes when

exposed to cosmic radiation

over time.

>> The sun is only producing a

tiny bit of the radiation

which will hit this rock, the

majority of it is coming from

all the stars you see in the

night sky. What it does to the

rock is basically, uh, just

bakes it, a bit like a really

bad suntan, so it just comes

down, hits it and cooks it.

>> As the rock gets cooked by

cosmic rays, the pyroxenes

break down and produce a gas

called helium-3.

>> We can record how much

helium-3 is within this rock,

and the more we have, the

longer that it has been

exposed to cosmic or, uh,

solar radiation.

>> Helium-3 gas is only

produced in microscopic

quantities, so Evenstar takes

her samples to a lab 7,000

miles away in Glasgow,

Scotland.

>> So what we do, uh, using a

laser, is we shoot the laser

into one of the wells, and

vaporize our crystals.

And that's releasing the

helium-3, then the helium-3 is

going to go through all this

complicated machinery,

eventually run through the

mass spectrometer.

>> By analyzing this data, she

can figure out the last time

the boulders were moved.

>> The oldest age sample we've

actually recorded has been 23

million years. So what this

means is that, within certain

areas of the Atacama Desert,

these boulders have been

sitting there and not moved by

water for 23 million years.

So the Atacama Desert is one of

the oldest undisturbed

surfaces in the world. These

boulders were there before

humans even started to exist,

they are incredibly old.

>> Evenstar has discovered that

there are places in the desert

which have been bone dry for

23 million years.

This date is a crucial clue in

the investigation, because it

coincides with the birth of

the Humboldt Current. South

America was once joined to

Antarctica. But, roughly 25

million years ago, these

continents split. A channel

opened. Freezing water began

to circulate round the pole,

and thundered north along the

coast.

This cold current

formed an inversion layer,

trapping coastal rainclouds

and starting Atacama's slow

transformation into the driest

place in the world.

But the Humboldt Current is

not the only culprit.

Ironically, the quest to find

out how the desert became so

dry comes up against one of

the wettest places on Earth.

On the other side of Atacama

is the Amazon, but the heavy

rainfall from the rainforest

doesn't get anywhere near the

desert. The reason why is in

plain sight. Between the

Amazon and the Atacama Desert

lies the vast Andes Mountain

range. Geologic evidence

suggests the Andes finally

grew high enough, some ten

million years ago, to prevent

any rain from reaching the

desert. It's called a

rainshadow effect, and it's

the final factor which drove

Atacama to become the driest

place on Earth.

The evidence for what turned

Atacama so incredibly dry is

mounting. The Humboldt Current

creates a weather system that

allows no rainfall. Helium-3

in rocks shows that the

process of desiccation

began 23 million years ago.

The rising Andes, ten million

years ago, made it drier

still. The investigation would

seem to be conclusive.

Atacama has been a barren,

essentially rainless landscape

for millions of years.

But then something happened to

blow that conclusion wide

open. Tiny shards of stone

revealed that an ancient

civilization once lived here.

But how could people live in

the world's driest desert?

The Atacama Desert is by far

the driest place on Earth, and

by piecing together the

evidence, scientists believed

it had been so for millions of

years.

Yet, at a remote site called

Guanaqueros, paleoecologist

Claudio LaTorre made an

intriguing discovery which

paints a more complex picture.

>> This is, uh, an

extraordinary find, and this

was probably a little knife or

a scraper that's been broken

off and discarded. That could

probably still cut.

>> To the untrained eye, it

looks like a simple rock

shard, but LaTorre can see

it's been worked into a tool.

And he's found hundreds of

them. They're clues that

reveal ancient humans once

lived here.

>> This was not just a

temporary residence, this was

something where people were

living and working and banging

away at rocks and making

artifacts and living off this

landscape, using the resources

at hand.

>> As water is essential for

life, it seems impossible that

any kind of plant, animal or

human life could survive here.

LaTorre suspects that some

regions of this 57,000-square-

mile desert were once much

wetter. Not millions of years

ago, but during the time

humans walked the Earth.

In 1997, he set off on a

mission to hunt for evidence.

Today, he's retracing that

journey. Changes in the

climate can be seen in the

rocks, so LaTorre examines the

cliff layer by layer. He finds

a crucial piece of evidence.

>> This is actually where the

interesting part of the story

comes in.

>> This chalky rock is called

diatomite.

It's made from the crushed

remains of fossilized algae,

microscopic life forms which

only live in freshwater.

>> What this rock is telling us

is that we had basically a

wetland.

Whereas you look at the

landscape across today and we

see that it's basically about

as dry as you can get.

>> Sometime in the past there

was water on the surface of

the desert. Latorre's next

task was to find out when.

Radiocarbon dating is one of

the most accurate methods of

dating, but using this method

means sampling something

organic. So LaTorre combed the

desert for clues.

>> The way we work

is basically poking our heads

into every little hole and

crevice that we can find.

When we found this place, we

couldn't believe our eyes.

>> He accidentally and luckily

stumbled upon the most

important piece of evidence in

this investigation. At the

back of the cave was a vast

nest. It's made from the feces

of thousands of generations of

tiny mammals. The size and

shape of the pellets told

LaTorre those animals were

Chinchilla rats.

[Squeaking]

And it also contained the

critical clue he was searching

for--

organic material.

>> When we found this site,

one of the most exciting

discoveries that we made was

the fact that it's full of

grasses. Now, look across the

landscape today and tell me

where those grasses are, and

we immediately knew that we

were talking about some major

vegetation change.

>> This grass looks as fresh

and crisp as if it was

collected yesterday. But when

LaTorre carbon-dated grass

from the nest, what he found

was amazing. The grass was

more than 11,000 years old.

>> What I have in my hands

here is an ancient ecosystem.

This is about as clear an

indicator you can get, better

than anything else you can

think of, that water increased

in the past in this area.

>> The nest reveals strong

evidence that plants and

mammals did exist here, and

they weren't alone.

Underneath the thick layer of

nest is another layer, rich

with tiny handmade tools.

>> If we look around, you

know, we can find actually

evidence of this past human

occupation, there's just--full

of little shards here on the

floor.

>> Some regions of Atacama

have been constantly dry for

23 million years. But this

evidence shows that other

regions, like Guanaqueros,

were very different 11,000

years ago.

It's a fossilized snapshot of

a diverse ecosystem briefly

bursting into life. Grasses

grow and wetlands flourish in

this wetter time. Tiny mammals

thrive and breed, while game

like vicuña and llamas meant

humans could live in this rich

and fertile environment.

>> So it's wonderful to know

that, by looking at something

as mundane as, uh, a rodent

nest, you can actually find

clues that enable you to

understand the past human

colonization of the Atacama

Desert, which is no mean feat

in itself, given the fact that

it's such a harsh climate today.

>> The date of the rat's nest

gives scientists a possible

theory of where the water came

from.

11,000 years ago, the last Ice

Age was at an end.

The global climate was changing.

More rain fell high in the

Andes, flowing down to the

desert in rivers.

In some places, groundwater

pooled, forming wetlands.

Others remained untouched by

water, as they had for

millions of years.

But, just a thousand years

later, the climate changed

again. Rivers dried up.

Grasses died. Rats and humans

disappeared. Now, every drop

of groundwater has been sucked

down into the parched earth.

LaTorre demonstrates how deep

that water is today.

>> So, just to give you an

idea of how much change has

gone on since the wetland was

formerly at the surface,

here's a little experiment

that we can do. This is a

well, and I'll drop this

little rock, and we're going

to count and we're going to

see how long it takes for that

rock to hit the water.

[Splash]

So that takes almost four

seconds to reach the water,

that's well over 200 feet

below the surface is where the

water table is today. It's

about as dry as it gets. It's,

uh, it's what we call absolute

desert. No plants, no wildlife,

nothing, no surface running

water whatsoever.

>> The investigation of this

driest place on Earth took a

surprising turn. Tools show

humans lived here. Diatomite

reveals the climate was once

wetter. Rats' dung and grass

dates a diverse ecosystem to

11,000 years ago.

Yet this extraordinary desert

has more secrets to tell, not

just about life in one of the

most extreme environments on

our planet, but also about

life on other planets.

Today, scientists suspect

Atacama is the driest it has

ever been, so they're

investigating whether there's

any source of water left here

at all. And NASA scientist

Alfonso Davila knows that if

there's water, there's a

chance there could be life

here too. But when he first

arrived, the signs didn't look

good.

>> When I came here for the

first time. I drove for a

couple of thousand miles, and

when I got, uh, back to my

base camp, I realized that I

didn't have a single insect

smashed against my windshield.

That has never happened to me

anywhere else in the world and

I--and I think that's a very

good example of, uh, how hard

this environment is for life.

>> Since the 1960s, NASA

scientists have been hunting

for bacteria life in the

desert's thin soils, yet they

found nothing...

...until 2005, when they came

across a strange white

landscape.

By chance, one of Davila's

colleagues picked up a rock,

smashed it open and discovered

something completely

unexpected.

>> Yeah, you can see very

nicely a--a green layer inside

the crust.

>> Under the microscope, the

significance of this pale

green blur zoomed sharply into

focus.

>> To our surprise, we saw a

green microorganism living

inside the rock. So that came

as a big surprise, uh, because

nobody was expecting

microorganisms in the middle

of the driest place on Earth.

>> Completely by accident,

hidden inside a rock they'd

discovered life.

>> This mineral is, uh, sodium

chloride, otherwise known as

halite. It's a very common

mineral in the Atacama Desert

and it's also a very common

mineral in kitchens around the

world, as this is exactly the

same salt we use to spice our

food.

>> Salt can preserve food by

killing off bacteria. But

here, strangely, it was

harboring a colony of green

microbes. To find out how they

survive, Davila laid out a

series of sensors that measure

humidity. His research shows

that, although, on average,

the air in the desert is

around ten per cent humidity,

on rare occasions, it rises as

high as 75 per cent.

This momentary increase in

water vapor is the only

source of water.

And it's this water that gives

rise to life.

>> The distinctive property of

salt is its capability to

extract water vapor from the

atmosphere and forms a liquid

solution inside the rock.

>> As moisture from the air is

sucked into the salt, the

microbes allow the rock to

bring the water to them.

>> Life is actually very

robust, it's, uh, very

flexible and it can really

adapt to some of the most

extreme conditions that we see

on Earth.

>> NASA believes this

discovery in the Atacama

desert can reveal something

about life on Mars.

In 1976, the Viking Lander

detected water in Mars's thin

atmosphere.

In 2008, NASA's Mars Odyssey

Orbiter found evidence of salt

on the planet's surface.

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[Radio chatter]

Now, when humans finally get

to Mars, they won't be looking

for life in the thin Martian

soils, but inside the rocks.

>> Unfortunately, it's gonna

be a long time until we see

humans walking on Mars.

Until then, we come to the

Atacama Desert, uh, and we

study this type of rocks,

which likely hold the clue to

understanding life on Earth

and also to understanding the

potential for life in other

planets in our solar system.

>> So it's possible that an

accidental discovery in the

driest place on Earth will one

day lead scientists to crack

open a Martian rock and

discover little green alien

life.

The investigation into how the

driest place on Earth was made

has revealed an awesome Earth

story spanning 150 million

years.

Gypsum, a rock which forms in

water, shows the desert was

once a sea bed. Hot geysers

show that immense volcanic

activity under the desert

raised it above the ocean.

Tiny pyroxene crystals reveal

the first areas of the desert

which became completely dry 23

million years ago. Rat nests

reveal a small pocket of life

that bloomed in the desert at

the end of the last Ice Age.

Tiny green organisms in salt

show that even here, life

clings on.

Today this place is unique on

earth--absolute perfect desert,

and the investigation into how

it formed has shed light

on another chapter in the story

of how the Earth was made.

== sync, corrected by elderman ==