Inside Planet Earth Page #9

in the Walla Walla Valley.

It revealed many layers of silt.

Waitt realized that this area

must have been in the path

of the great floodwaters

that formed

the channeled scablands.

This white layer here

is an ash that we know

is from Mount St. Helens.

We've analyzed it chemically,

mineralogically,

and it clearly came

from Mount St. Helens.

And we know its date.

By radiocarbon dating,

we know that this is

15,000 years old approximately.

Therefore, we have

a beautiful time line

running through this section

of 15,000 years.

The ash could only have fallen

after the flood had passed

and the sediment settled,

yet it's covered by many layers

of sediments-- 39 in all.

This was the final piece

of evidence

that explained the creation

of the scablands--

The catastrophic outburst flood

must have happened

time and again.

Once each flood

had left its mark,

the vast glacial dam

would advance

and the waters of Lake Missoula

would start to rise again,

continuing the cycle of flood

followed by calm.

Only when the time of the ice

was over did the floods stop.

The change from ice age

to warmer times

is governed by how close

the Earth is to the sun.

Every 100,000 years,

the shape of the Earth's orbit

around the sun changes.

This has led some scientists

to wonder if the recurring cycle

of catastrophe and extinctions

on our planet

is governed

by extraterrestrial forces.

Mass extinctions and

the catastrophes that cause them

seem to follow a periodicity

of about 30 million years.

It's that cycle that suggests

that the cause of a catastrophe

lies outside the Earth.

On a clear night here in Africa

or in other places where

you can see the night sky,

we can see

astronomical evidence

for the cause of these

geological catastrophes

every 30 million years.

Our solar system

is on a voyage

through the disc-shaped

Milky Way Galaxy.

Every 30 million years,

we pass through the densest part

of the galactic disc.

During that time,

the comets of our solar system

can become disturbed and fall

inward toward the inner planets.

During this period, the Earth

is more likely to be impacted.

This cycle may explain

the catastrophic history

of the Earth.

The last major mass extinction

was 35 million years ago.

We're in the densest part

of the galactic disc now,

and the next mass extinction

may include us.

Asteroids are a very real threat.

They even formed our own moon.

Soon after its violent beginning,

our planet suffered

a ferocious assault.

A rogue asteroid bigger than Mars

smashed into it with enough power

to blast much of the Earth's

surface into space.

The debris was drawn together

by gravitational forces

and formed a proto-lunar disc.

From that,

the new moon grew rapidly,

sweeping up the debris in orbit

around the Earth.

Computer models suggest

it only took a year

for the fragments in orbit to

coalesce and form a single moon.

This is the Clark telescope

at the Lowell Observatory

in Flagstaff, Arizona.

It's the telescope that was used

to take pictures of the moon

in preparation

for the Apollo moon landings.

To see the moon tonight,

we need the telescope

up in this position.

We can make

some fine adjustments.

The density of the craters on

the moon's surface was measured

using the photographs taken

by the Clark telescope.

What's more, the astronauts

visited some of these regions

in their 6 Apollo missions.

Man, does this thing

have steep walls.

They said 60 degrees.

Now, I tell you, I can't

see to the bottom of it,

and I'm as close to the edge

as I'm gonna get.

That's the truth.

I can't believe

we came over those mountains.

The largest impact feature

on the moon,

the Imbrium basin, is more

than 1,000 kilometers across

and was produced by the impact

of a comet or asteroid

more than 100 kilometers across.

Yeah, those are

pretty big mountains

to fly over, aren't they?

The basin is surrounded

by mountains

more than 5 miles high.

This is where

the Apollo 15 astronauts

landed and took samples.

They show that these mountains

are not like the mountains

on the Earth,

but they're piles of rubble,

fragmented rock,

thrown out by this giant impact.

Look at that.

Guess what we just found.

I think we found

what we came for.

Crystalline rock, huh?

Yes, sir. You better believe it.

The pristine moon rocks

that were brought back

by the Apollo astronauts

have been radiometrically dated

and studied,

and they show that impact

has been an important process

in the formation and evolution

of the Earthlike planets.

What's more, the radiometric

dating of the rocks

has shown that the moon

underwent a hellish bombardment

between 4.5

and 4 billion years ago.

After that time,

the bombardment died down.

By that time,

most of the asteroids moving

around in the inner solar system

had collided with the planets,

and the process of the formation

of the planets was over.

200 space rocks

large enough

to cause global devastation

are known to be on

Earth-crossing orbits,

and there may be many more.

Here in Namibia,

meteorites have always been

considered special objects

worthy of veneration.

These meteorites

have been put on display

as the centerpiece of the city.

The impact of

small iron meteorites like this

would have little effect

on the Earth as a whole.

But the impact

of a much larger object--

a 6-mile-diameter asteroid or comet--

would cause a mass extinction,

as we know

from the geological record.

The impact of

a quarter-mile diameter object,

like the one that just missed

the Earth in 1996,

would cause

widespread destruction.

A hit in the ocean

would cause tsunami

that would devastate

coastal cities.

And a hit on land

would produce a dust veil

that would produce

nuclear-winter like conditions

that would threaten

civilization as a whole.

The geological record

shows impacts brought

catastrophe and devastation

which wiped out many species.

There have been 5 key mass

extinctions in Earth's history.

Best known are the dinosaurs.

Why did they simply disappear

from the face of the Earth?

India may provide the answer.

65 million years ago,

it was still an island

drifting towards Asia.

This was when

the layered landscapes

of the Deccan Traps

were created.

A great volcanic rift

spewed out half a million

square miles of lava.

Layer after layer of lava

lies on the land,

in places

up to 8,000 feet thick.

Some scientists believe that the

scale of this volcanic activity

was so great

it killed off the dinosaurs.

Professor Michael Rampino

thinks the traps were triggered

by an extraterrestrial visitor.

This is the Hoba iron meteorite

in Namibia.

It's the largest meteorite

known on Earth.

And like most meteorites,

it contains the rare element

Iridium.

And iridium has proven to be

the clue, the connection,

between the impact event and

the extinction of the dinosaurs.

Iridium is found near impact craters

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