Finding Life Beyond Earth Page #2

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hydrogen and oxygen,

among others.

Although organic molecules

aren't alive themselves,

they are the basic building

blocks of every living organism.

Life also needs a liquid,

like water.

In water, the basic organic

molecules can mix, interact

and become more complex.

The last ingredient is

an energy source like the sun

to power the chemical reactions

that drive all life,

from the smallest microbe...

to us.

When these three ingredients

came together

billions of years ago, life

found a way to take hold...

and today persists even in

the most extreme environments,

like here.

This is the Mojave Desert,

Nevada.

It is one of the hottest,

driest places on our planet.

McKAY:

This part of the desert is

particularly interesting to me,

because it's the driest part.

There's an axis of dryness here.

If we go either east or west,

it becomes wetter.

NARRATOR:

Surprisingly, even here, with

only a foot of rainfall a year,

all three ingredients

for life are present.

The rocks provide

just enough shade

to prevent water from

evaporating completely.

McKAY:

Underneath the white rocks,

we can find

the most amazing thing.

We see this layer of green.

This is bacteria.

The rock provides

a little shelter.

It's a little wetter

and a little nicer

living under the rock than it is

in the soil around it.

In addition, the white rocks

are translucent.

Hold them up to the sun and see

light coming through.

These organisms are

photosynthesizing

here in the desert where nothing

else will grow.

So they're living in a miniature

little greenhouse.

NARRATOR:

This place shows

that even in some of Earth's

most extreme environments,

under the right conditions,

life has a chance.

For scientists like Chris McKay,

the question is:

Is Earth the only planet

with the essential conditions

for life?

One way to know is

to investigate

how planets like ours formed

to have these ingredients

in the first place.

That story starts

4.6 billion years ago,

with the birth

of our solar system.

As a vast cloud of dust and gas

collapses in on itself,

pressures increase.

Temperatures at the center rise

to millions of degrees...

until energy from the early sun

blasts away some of the cloud.

This lights up

the young solar system,

revealing the beginnings

of planets.

The mystery has always been

how did this spinning

cloud of dust

become the massive planets

we see today?

SCOTT SANDFORD:

How does one go from microscopic

grains to golf-ball size things,

and how do golf-ball size things

go from there

to ten-meter size things?

How do those go to planetary

embryos?

And there's

a lot of steps in there

we don't quite understand.

NARRATOR:

Many scientists believe

the answers are hidden

in asteroids...

the oldest rocks

in the solar system,

leftover debris

from its earliest days.

In 2003 the Japanese probe

Hayabusa sets out

on an audacious mission.

The goal:

to land on an asteroid,

collect samples of dust and then

return them to Earth.

The target is asteroid Itokawa,

a third of a mile long

and speeding through space

at 56,000 miles per hour.

Landing on it would be like

trying to hit a speeding bullet

with another speeding bullet.

SANDFORD:

Hayabusa in Japanese

means falcon,

and the idea was to do

like a falcon grabs a rabbit--

swoop down, sort of just touch

the surface,

get your sample and go.

NARRATOR:

In 2005, 1 80 million miles from

Earth, Hayabusa makes contact.

It stays just long enough

to grab a sample.

It will take five years

before Hayabusa returns

asteroid dust to Earth.

But in the meantime,

using lasers on board,

Hayabusa takes measurements

of Itokawa's size and mass.

These allow scientists

to determine the asteroid's

internal structure.

What they discover could be

a blueprint

for how planets like Earth

first formed.

SANDFORD:

It's not one solid lump of rock,

but, in fact,

it consists of a pile

of smaller rocks,

of many sizes all the way from

houses down to dust grains.

NARRATOR:

If we could see

inside asteroid Itokawa,

this is what it would look like:

a loose mixture

of smaller asteroids

that are held together

by gravity.

SANDFORD:

Maybe 40% of the internal volume

of the asteroid is empty space.

You probably could just take

your hand and just go like this

and just push it down

into the asteroid.

NARRATOR:

Is this the first step

in building rocky planets

like Earth?

GREEN:

Asteroids are just

not lumps of rock.

These are the basic parts

or building blocks of planets.

NARRATOR:

Over hundreds

of thousands of years,

asteroids like Itokawa continue

to collide,

growing bigger and hotter.

As their gravity increases, they

attract even more asteroids

until eventually,

as temperatures rise,

they become spheres of rock

with hot molten cores--

protoplanets.

Computer simulations suggest

that within ten million years

of the solar system's birth,

up to a hundred protoplanets

ranging in size

from our moon to Mars

were orbiting close to the sun.

So why does the solar system

look so different today?

This is proto-Earth four-

and-a-half billion years ago.

Planetary geologist

Stephen Mojzsis believes

this world was very different

from the one we see today.

MOJZSIS:

Looking at the surface here,

this landscape is dominated

by lava, black and blasted

by impacts.

Underfoot we find

mostly basaltic rock.

It is the frozen product

of molten rock.

These planetary surfaces weren't

molten boiling cauldrons.

But instead, for most

of their early histories,

they were solid and cool.

NARRATOR:

The atmosphere is thick

with carbon dioxide

and laced with sulfuric acid,

the result of intense

volcanic activity.

MOJZSIS:

The embryonic Earth

would have an atmosphere

denser than the one we have

and a sky yellow and red and

thoroughly unbreathable to us.

NARRATOR:

How does this toxic

and inhospitable world

eventually become the Earth

we know today?

Ironically, it will take

a cataclysmic event

to create a planet capable

of harboring life.

A protoplanet the size of Mars

slams into early Earth.

The collision is so violent

it melts the surface,

creates an even larger planet,

and blasts molten rock back

into space that will coalesce

and eventually form our moon.

Earth isn't the only planet

that gets transformed

by giant impacts.

Over tens of millions of years,

all the protoplanets

of the early solar system

repeatedly collide,

becoming larger bodies

with each impact

in a destructive game

of planetary billiards.

This process eventually formed

the four rocky planets

seen today:

Mercury...

Venus...

Earth...

and Mars.

SARAH STEWART:

So the final planets

that we have today

are really the ones

that won the competition

in that some planets were

literally destroyed

or thrown out

of the solar system

and others survived

to be here today.

NARRATOR:

Sarah Stewart is

a planetary scientist.

She's trying to determine

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