All language subtitles for How.the.Universe.Works.S06E06.Secret.History.of.Mercury.720p.HDTV.x264-W4F[eztv]

For thousands of years,

planet Mercury
has baffled astronomers,

but now its secrets
have been revealed.

It's a bizarre world
unlike any other.

When you look at
the family of planets

that make up our
solar system, you know,

Mercury does seem to be
a little bit of a weirdo.

This is one tough world,

surviving brutal attacks
from comets, the Sun,

and even other planets.

You could see by
the surface of Mercury

that it has a lot
of battle scars.

The solar system
did not treat it well.

And yet, this world

has been brought to life
with water ice,

volcanism,
and tectonic activity.

If I had to describe
Mercury in a word,

it would be "surprising."

At first, it may seem
that you could write this off

as a dull, little dead rock
close to the Sun,

but Mercury has a story to tell.

And this story could end
with Mercury threatening

the very existence
of planet Earth.

Ignore it at your peril
because there may come a day

when Mercury makes its presence
very well known indeed.

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2015, the messenger spacecraft

completes its final orbit
around planet Mercury.

And the images
sent back to Earth

during its mission
left scientists stunned.

Our view of Mercury from before
and after the messenger probe

is like having terrible
vision your whole life

and then finally going
to the optometrist

and getting glasses
that clear everything up,

and it completely
opened our eyes

to what this planet looks like.

Mercury orbits
the innermost solar system,

the closest planet to the Sun...

around three times closer
than Earth,

in a scorching environment
of lethal heat and radiation.

Capturing images was
a monumental challenge

for the messenger mission
team led by Sean Soloman.

Mercury has been among
the most difficult planets

in the solar system to study.

The Hubble space telescope
is forbidden from viewing

planet Mercury because it's
too close to the Sun,

and their optics would be
severely damaged.

Messenger looks closer
than we ever have before.

It reveals a strange world,
just 5% the mass of the Earth,

closer in size to our moon.

At first glance,
Mercury and Earth,

they're nothing alike, right?

They're both made of rocks.
That's about it.

They're both orbiting
the same sun,

but when you start looking
a little bit closer at Mercury,

you start to see some
really surprising similarities

to Earth.

When scientists look at Mercury,

they spot something
that shouldn't be there.

Mercury has water on it,
which if you were going to make

a long list of all
the discoveries about Mercury,

I think water would be right
at the top of

"what? What? Seriously?"

If you thought in advance
of the last place in the world

to find water,
you'd think of Mercury

because it's so close
to the Sun.

It gets even stranger.

The water exists
in the form of ice.

There could even be
a trillion tons of it,

enough to encase
Washington, D.C.,

in a frozen block 2 miles thick.

So how do you get frozen water

onto the closest planet
to the Sun?

For me, I think the most
interesting thing about Mercury

is the fact that what should be
the hottest planet

in the solar system is,

in some parts,
among the coldest,

and I love that paradox
because I love the unexpected.

Mercury is a world
of brutal extremes.

The Sun-facing side is blasted
by solar radiation...

With temperatures rocketing
to 800 degrees Fahrenheit,

hot enough to melt lead,

but the side facing away
plummets to -300.

It's all because of Mercury's
almost nonexistent atmosphere.

Planetary scientist Dan Durda
demonstrates the effect

this has using a campfire.

So right now, with this jacket
on, holding the warmth in,

I'm a bit like a planet
with an atmosphere.

The atmosphere of a planet,
it's a thermal blanket,

a thermal insulator, that
helps make more uniform

the temperature
of the entire planet.

So at the moment,
I'm pretty warm uniformly,

but if I take my jacket
off altogether,

well, you know, I can already
feel my back cooling off.

Sitting here without my jacket,

without an atmosphere,
if you will,

I'm like the planet Mercury

which has no effective
atmosphere,

and therefore the only warmth
that it can hold

is the warmth radiating directly
on it from the Sun itself.

Any parts of Mercury facing
away from the Sun

very rapidly radiate
that heat off to space

and cool to very,
very chilly temperatures.

So could water ice
survive on the side of Mercury

facing away from the Sun?

The nighttime side is
very bitterly cold.

If that were the end
of the story,

you might be able to have,

you know, frozen water,
icy water, on the side

of Mercury facing away
from the Sun.

But Mercury doesn't keep one
side always locked to the Sun

and one side always facing away.

It actually does rotate.

It rotates three times
for every two trips

it takes around the Sun.

As Mercury turns,

the Sun would vaporize
any water ice facing it.

But are there secret
hiding places

where the Sun cannot reach?

To find the answer,
planetary scientist Nina Lanza

searches fissures in Iceland.

So here we can see,
at the surface, there's no ice.

It's too warm, but if we look
down in this fissure here,

it's only about 20 feet deep,

but at the bottom,
there's actually some ice.

So if we measure a rock
that's been in the Sun,

let's say this one, we can see
it's actually pretty warm.

It's about 61 degrees
Fahrenheit,

but if we aim, now, for the
bottom of this hollow,

we can see now,
it's about 33... nope.

It's dropping.
It's 32 degrees Fahrenheit,

and this is because
the Sun doesn't really

get to the bottom of this place,
and that's the key.

While this ice
is hidden in a fissure,

on Mercury the ice
survives in craters

on the planet's north pole,

forever safe from the glare
of direct sunlight,

because these craters
sit in a perfect spot.

Mercury's axis doesn't tilt
very much compared to the Sun.

The Earth's axis tilts
about 23 degrees,

but Mercury is pretty much
straight up and down,

and that means that as large
as the Sun is in the sky,

there are craters on the poles
that sunlight never gets to.

They're always cold.

So the surface of
Mercury is heated

and then cooled
as it moves around the Sun,

but there are craters that are
always dark and always cold.

Against all the odds,
despite everything that suggests

this shouldn't happen,

Mercury is a safe haven
for this water ice.

The bottoms of these craters
are called cold traps

because they're so cold
that any water

that gets there stays
there basically forever.

It can last literally
for billions of years.

But where would it
have come from?

It probably came from
comets and asteroids.

Comets are giant chunks of ice.

So if a comet hits Mercury,

that's going to deliver
a lot of ice,

and, in fact, we know
a lot of asteroids

have ice on them as well.

So both of these things
could deliver water to Mercury.

Comets and asteroids
brought water ice

to the innermost planets
of the solar system.

This same water delivery system

gave Earth the elements
needed for life.

And though Mercury has
pockets of frozen water,

the planet can never use it
to develop living organisms.

What Mercury is really
showing us is that you can start

with the same basic building
blocks for planets, right?

You can have essentially
the same materials,

but end up with very
different environments.

Compared to the other planets,

Mercury ended up as
the runt of the litter.

But there's evidence
this wasn't always the case.

Could this little world have
once been much, much bigger?

We have theories for
how the planets first formed

in our solar system,

but Mercury just doesn't fit in.

When you look at
the family of planets

that make up our solar system,
you know,

Mercury does seem to be
a little bit of a weirdo.

On Earth, we have
a relatively small core

and a thick mantle
and a thin crust.

So you compare the core of the
Earth to the size of the Earth,

it's relatively small.

If you look at Mercury,
it's not that way at all.

The core is absolutely huge
compared to the planet itself.

Mercury's huge
iron core is surrounded

by an unusually
thin mantle of rock.

It just looks odd.

It's funny.

You don't expect to get
a core that large

in a planet that small.

It's almost as if Mercury lost
some of its mantle somewhere.

I mean, maybe it left it
behind the couch.

Who knows?
But it's gone now.

But how can this
material just vanish?

Mercury could be
the way it is now

because it started as
a much larger planet

and then something happened to
strip away the top layer of it,

and the only way
we know how to do that

on a planetary scale is
with planetary impacts.

Picture the early solar system
when Mercury is still forming.

It's completely different from
the little world we know now.

Up to four times more massive,
twice the mass of Mars,

but it orbits
in a shooting gallery.

Impacts are inevitable,
and before long,

an object the size of our moon
smashes into Mercury

in a giant impact event.

This is apocalyptic.

This is the sweatiest
nightmare you can have.

You're resurfacing
an entire planet.

The energies are vast.

One of these giant impacts
probably would have remelted it

all the way through.

What's going to remain
after it's done

is completely different

than what started
in the first place.

It's hard to overstate just
how impactful these events are.

Huge chunks of Mercury's mantle
are flung into space.

The result?

Two-thirds of its mass
are now made up by its core,

but where did the rest of it go?

Billions of years ago,
there was a planet

that we will never know,

a planet that was destroyed when
the current Mercury was formed.

Is it possible that some of it
may still be out there?

The lost mantle debris
could still exist

in the innermost solar system.

Scientists call these
hypothetical objects vulcanoids.

Vulcanoids can be very valuable

for understanding
the formation of Mercury

because some of these vulcanoids

may be pieces of
Mercury's missing mantle.

But in order to survive,

these vulcanoids
would have to orbit

on a gravitational tightrope.

You know, like this marshmallow,
vulcanoids exist

in kind of a precarious
position in the solar system,

a little too close to the fire,
a little too close to the Sun,

those objects actually
would vaporize away,

kind of like that.

If you orbit too far
from the Sun,

you're going to approach
too closely to Mercury

and have gravitational
encounters,

maybe get flung out of the solar
system or maybe just impact

Mercury itself
and get eaten, if you will.

But in-between
those two extremes,

at just the right
distance from the Sun,

a little closer
to the Sun than Mercury,

but not so close
that you get fried,

is this vulcanoid region where
objects in orbit around the Sun

could remain
gravitationally stable

over the entire age
of the solar system.

That's the place to look
for this potential population

of little asteroid-like objects.

We know where they should be,

but there's a problem.

You may wonder why haven't

we seen these vulcanoids
if they actually exist?

If they're actually orbiting,
why don't we just look for them?

Well, it's because they're
really close to the Sun.

Unlike the asteroids
or the Kuiper belt objects

where we're looking out
away from the Sun

from our perspective
here on the Earth,

we're looking out into
the dark nighttime sky,

in the case of the vulcanoids,
we're looking for something

very, very close to the Sun,

in close to that really
brilliant light source.

Our sun is around
900,000 miles across.

Mercury is just 3,000.

When our telescopes see Mercury
passing in front of the Sun,

it's little more
than a pinprick.

With vulcanoids, we're searching
for something

thousands of times
smaller than Mercury.

If vulcanoids exist,
they orbit in a blind spot,

but scientists will keep
searching for them

because they could be
the last surviving remnants

of Mercury's lost mantle.

If we were to discover
vulcanoids, that would offer us

an entirely new population
of objects to study.

This material could offer
some rather unique insight

into the formation
of Mercury itself.

A giant head-on impact
has been our best explanation

for Mercury's weird structure.

But now there's a
startling new theory

for Mercury's missing mantle.

It was stolen,
but who is the thief?

For years,
we assumed that Mercury lost

most of its mantle
in a giant head-on collision.

But the messenger spacecraft
turned everything on its head.

One of the exciting things
that we've learned

about Mercury recently

by sending probes
to study its surface

is that its surface
is littered with material

that we didn't think
should be there,

things that we'd call volatiles.

The volatiles are chemical
elements like potassium.

They're called volatile...

Because they evaporate
easily in high temperatures,

just the sorts of temperatures
generated by a giant impact.

If I had to describe
Mercury in a word,

it would be "surprising."

The idea was that a smaller
object ran into Mercury

and knocked off material.

Well, that actually would work
to make it lose material,

but it would also generate
a lot of heat,

and because of that, the
volatile materials on Mercury

would also have been lost.

But today, we see that
they're still there.

So it meant that most of the
basic scenarios that have been

laid out for how Mercury was
assembled had been disproven.

We had to go back
to the drawing board

and rethink how
Mercury was assembled

and how Mercury evolved.

How did Mercury lose its mantle

but retain these volatiles?

Mercury is like
a detective case.

We have the body.
We have some clues,

but we really have to
piece it together.

Planetary scientist Erik Asphaug

tackles this giant riddle.

The original idea was that

Mercury was hit by something
smaller than itself.

But whatever process
made Mercury

somehow preserved
all these volatiles

that should have
vaporized and gone.

Erik has a bold alternative,

a hit-and-run collision.

In the hit-and-run collision
idea, you actually have

Mercury hitting something
bigger than itself

without losing
all of its volatiles.

Hitting something
bigger may sound even worse,

but it all depends
on how you hit it.

In the early solar system,
there were a lot of players,

and just like a hockey match,
things got brutal.

You would have had planetesimals
growing through collisions.

The bigger objects
would have dominated

because not only are they
running into smaller objects,

they're drawing them
to themselves gravitationally,

and two objects will sweep up
most of the matter.

Those two objects
became Venus and Earth.

Earth and Venus
are the enforcers

of the inner solar system,

wiping out most
of the competition.

Whoo!

Little Mercury is one
of the last players left,

and it could have
a faceoff with Earth.

Mercury is the little guy
in this thing,

and this hockey puck
is our volatiles.

The little guy comes in
at a high speed

and has a head-on collision
with the big guy.

It is catastrophic.

But suppose now,
instead of hitting him

in a head-on collision,

he hits him in a glancing blow.

So he might knock off
a little bit of equipment,

but Mercury just keeps on going
and keeps his hockey puck.

A head-on impact would send
a shockwave across Mercury,

melting the entire surface.

But a glancing blow
is less ferocious.

The more grazing
you hit something,

the less energy you bring
to bear, the less violent it is.

In the grazing collision
is where piece of the surface

has grazed off
and blasted into space,

and the other hemisphere of
the body is largely unaffected

and allowed some
primordial material

to remain on the surface.

Most of Mercury's
mantle is stripped away,

but this grazing impact

doesn't send a shockwave
through the planet,

and so the volatiles remain
along with enough of the mantle

to reshape this world
with a thin outer layer of rock.

Mercury was transformed
into the smallest planet

in the solar system,

and its lost mantle was stolen
by an unexpected thief.

That mantle accretes onto
the biggest object around,

but the biggest object
around isn't Mercury.

When Mercury's mantle got
knocked off in this collision,

it had to go somewhere.

So if you're looking for
Mercury's missing mantle,

look no further than right
under your feet.

To this day, part of
Mercury could be part of Earth,

stolen in a hit-and-run
collision.

Mercury was able to survive the
formation of the solar system,

but it paid a cost.

It was battered.

Since day one,
Mercury has had a tough ride.

It's been pounded by the Sun
and planets,

and things have not
improved since.

You can see by the surface
of Mercury

that it has a lot
of battle scars.

The solar system did not
treat it well.

The planet has been
bombarded and fried,

but these events
could help explain

one of Mercury's
biggest mysteries.

Why is the planet so dark?

The solar system is full of
beautiful, colorful planets...

but Mercury is different.

Its dark gray surface
baffles scientists.

One of the most
intriguing things

is how dark the surface
of Mercury is.

A big mystery in
the solar system is why?

Why is Mercury like that?

The rocky inner planets
all formed in the same region

of the solar system
from similar materials,

and yet Mercury is darker
than all of them.

Some of the darkest surfaces
here on Earth are lava fields.

Planetary scientist Nina Lanza
visits one in Iceland

to find some common ground.

The surface of a planet
records the history

of all the processes
that have acted upon it.

So when we look at
the surface of Mercury,

we can piece
together that story.

Right now, we're standing
on a basalt lava flow.

Basalt is a type
of volcanic rock

that's the building
block of all planets,

so we know just by seeing
this basalt here,

there was volcanic activity.

On Earth, most basaltic rock
is covered by oceans.

On Mercury,
the basalt is exposed.

Old liquid lava flows
are visible

as smooth channels
of solid rock.

For a billion years
after Mercury's formation,

lava explodes
from volcanic vents

and leaks out from fissures.

Could the volcanism explain
the dark world we see today?

Basalt is a pretty
dark rock already,

but what's so interesting
about Mercury

is that it's actually
darker than basalt.

So from the recent
messenger mission to Mercury,

we had some ideas about what may
be making the surface so dark,

and it's really strange.

It's actually carbon
in the form of graphite,

the mineral graphite, which
is what you find in a pencil.

So that material is all over
the surface of Mercury,

and it's incorporated
into the rocks.

Carbon doesn't come
from basaltic rock,

but there is something else

that could have brought
carbon to Mercury...

Comets.

These dirty ice balls
contain carbon,

and they've bombarded the planet
for billions of years.

The evidence of these attacks
is etched into the surface.

It's hard to look at Mercury
and not wince on occasion.

It is really covered
with craters.

It has been battered
and bruised.

It really has just
been terribly,

terribly mistreated
over its lifetime.

Mercury has been hit
often and hard.

Mercury is moving more rapidly

around the Sun
than the Earth is,

so a head-on impact
is going to be faster

than a head-on impact
of a comet in the Earth.

So pound for pound,
a comet impact on Mercury

is much more energetic,

will do much more damage
than an impact on Earth.

If you've ever been to, like,
meteor crater in Arizona

in the United States,
it's around a mile across.

It's this huge crater.

If you were to stick that crater
on Mercury, it would disappear.

Mercury has so many craters
that are so much bigger

than a mile across.

Mercury's biggest
impact site is Caloris basin.

Over 1,000 times larger
than meteor crater,

it's so big there are now other,
newer craters inside it...

And Caloris basin
is coated in carbon.

Comets have a lot
of carbon in them,

and we know that
comets hit planets.

So it's kind of obvious to say,

"where did the carbon
come from?"

Well, comets, but maybe not.

In 2016, the messenger team
reveals an exciting new theory.

Are the largest impact craters

actually exposing
something deeper?

Impact craters are windows
into the lower parts

of the crust of a planet,

and the larger
the impact crater,

the deeper the impact event

has excavated
material from depth

and brought it to where
we can see it at the surface.

The craters reveal a twist.

The carbon that makes
Mercury so dark

had been there all along

as part of the material
that first formed the planet.

When young Mercury is hit,
parts of the surface

are transformed
into an ocean of molten rock.

As this magma ocean cools,
it solidifies,

forming a crust,

and sitting on top
of this crust, graphite,

the crystallized form of carbon.

Carbon containing
minerals like graphite

would end up near
the surface of Mercury

at the top of that magma ocean

because those minerals
are a lot less dense

than the conventional
rocky minerals

that contain a lot of, you know,

iron and nickel.
Those are more dense.

They tend to sink to
the center of the planet.

Those lighter elements
like graphite

would tend to float to the top.

But volcanism covers the planet

in new basalt lava flows

coming from deep
below the surface

where the carbon didn't sink,

and this lava buries
Mercury's ancient crust.

Over time, that carbon
is covered up

by subsequent lava flows,

but there was
this layer of carbon

waiting underneath
the surface of Mercury,

and as objects hit Mercury and
gouged out holes in the surface,

it exposed this hidden
darker layer underneath.

Covered by a billion
years of lava flows

and revealed
by giant impact events,

one this is for sure...

Mercury's surface has been
a truly hellish landscape.

Imagine that we're on
the surface of Mercury

during that first billion years

when volcanism was very active.

Raining down from the sky,
all these comets and meteorites

just pelting
the surface mercilessly,

and then, beneath your feet,

there'll be all this
molten rock bubbling up.

Really an awful place to be

in that first billion years
on Mercury.

Over billions of years,

comets and volcanism reshaped
the surface of Mercury,

and this planet is not done yet.

Scientists find giant cliffs
stretching hundreds of miles.

It seems Mercury is alive.

Mercury, a small,
dark world covered in craters

and ancient lava flows,

but scientists find
something else on the surface.

Towering cliffs around 2 miles
high known as fault scarps.

If you were walking
along one of these scarps

on the surface of Mercury,

there would be
this giant cliff face

that would go on for miles
and miles well over the horizon,

so you would be walking next
to an almost endless cliff.

The largest fault
scarp is enterprise Rupes.

At over 600 miles long, it would
span the width of Texas.

We see fault scarps on Earth,

evidence of tectonic activity.

Planetary scientist
Jani Radebaugh visits a scarp

in death valley to demonstrate.

This long, straight
line of shadows behind me

formed because death valley
is still spreading apart

and the material on the right

has dropped down from
the material on the left

and left a really
sharp fault scarp.

The fault scarp
you see behind me,

and the ones we see on Mercury,
are formed by tectonic forces,

and this just means
that there are forces

inside of the planets,

and those forces cause
breaks in the crust.

On Earth, transfer
of heat from the mantle

drives the movement
of continental plates.

These plates move around
the surface of the planet,

interacting with each other...

Building mountains, rift
valleys, even continents.

Do the fault scarps on Mercury

mean it also has
plate tectonics?

When you think about
the Earth's plate tectonics,

there are multiple
plates of rock

that are moving around
on a layer of liquid rock below.

Mercury, however, has basically
just one big plate.

There's a solid surface
that covers the entire planet.

Mercury has a
different kind of tectonics.

Over billions of years,
its liquid core cools,

and as the interior cools, the
planet shrinks around 9 miles.

When something cools,
it contracts.

It actually becomes smaller.

Because of this contraction,
the rock and the crust wrinkles,

creating massive scarps.

You can imagine, if you take
a balloon and cover it in mud

and let the mud dry,

and then you let a little bit
of air out of the balloon,

what's going to happen?

Well, that mud is going to
try to contract as well,

but it can't, and so it'll crack
and snap like that,

and you'll get these
thrust faults, these scarps.

But for Mercury,
the story doesn't end there.

We understand that the large
fault scarps on Mercury formed

a long time ago
when the planet was cooling

and the crust was
shrinking and buckling,

but more recently, messenger
has come very close to Mercury

and has found really
small fault scarps.

Unlike the giant
mile-high scarps,

these small ones are less
than 200 feet in length.

What does this mean?

These are small scarps,
and the thing is,

if those were really old,

impacts would have erased them,

and so these should be
long gone off the surface.

So that means that
they're recent.

They're new.

Right now, Mercury
has the record

of being the smallest official
planet in the solar system,

but is it possible the smallest
planet is still shrinking?

Throughout its life,
Mercury has been bombarded.

Impacts have cratered
the large, old fault scarps.

The small scarps should be
covered in impact scars as well,

but they're pristine, which
means that they're young,

and that means Mercury
is still shrinking.

Mercury is actually
still tectonically active.

So the Earth is no longer

the only tectonically active
planet in the solar system.

We've seen that Mercury
has a surprising history.

The thing is, the planet
itself is still cooling,

still contracting, even after
all of these billions

of years after it formed.
That's amazing.

At first, it may seem
that you could write this off

as a dull, little, dead rock
close to the Sun,

but Mercury has a story to tell.

A story that continues
to this day

despite everything the
planet has gone through.

Mercury has had it pretty
rough. It's had a tough time

over the history
of the solar system.

It has been tossed
around by planets.

It has been impacted by
gigantic asteroids and comets.

It is shrinking.

It's bombarded
by solar radiation.

That would tick anybody off,

and it's entirely possible
that in the future,

Mercury will get its revenge.

Mercury could have
a final trick up its sleeve,

one that threatens
our very existence.

Messenger completes
its final orbit of Mercury...

and crashes into
the planet's surface,

a fitting conclusion for a world

with a history of impacts.

At the end of
the messenger mission,

our view of Mercury had
been substantially changed.

We suddenly had a picture
of a complete world,

a place that had hellishly
high temperatures

and yet ices of water,

an evolution that didn't match

that of any of its
sibling planets.

Could Mercury's future be just
as unpredictable as its past?

Right now, the future
of Mercury looks bright...

Very, very bright.

It's all thanks to
Mercury's massive neighbor.

The Sun is a giant ball
of hydrogen and helium,

and in the core, there's
a nuclear fusion reaction,

but over time, the fuel that
the Sun runs on, hydrogen,

will begin to die out.

It will actually burn
through all of its fuel.

When that happens,
in the final phases of its life,

the Sun will bloat up
to become a red giant star,

hundreds of times the size
that it currently is.

As the Sun expands,
it will engulf Mercury.

Being on the surface of Mercury
is bad enough right now.

Now put it inside of a star.

All of that heat
bombarding the planet

will literally boil it away.

But is there a chance
that Mercury could escape

this roasting?

There's not just
the evolution of the Sun

that we have to take
into account.

The planets themselves
and their orbits are evolving.

They look stable,
but over long time periods,

they can change drastically.

The Sun exerts the
strongest gravitational pull

of any object
in the solar system.

It's why the planets orbit it,

but planets can also
pull on each other.

Mercury is tiny,
making it most vulnerable

to these gravitational tugs.

After the Sun, the next
biggest object is Jupiter.

We can take computer
models and simulate

Mercury's orbit
as it's affected by Jupiter

to see what happens to Mercury,

and depending
on initial conditions,

a lot of different
things can happen.

But in some percentage
of the models, what can happen

is Mercury's orbit
changes so much

that it actually swings out

and can reach the orbit
of the Earth.

Jupiter is often seen as the
bully of the solar system.

Here, it's Mercury's bodyguard.

Over time, its
gravitational influence

stretches Mercury's orbit
out farther and farther,

and the little world escapes
the Sun's clutches

long before it expands
to a red giant.

But this tale
has one final twist.

It's easy to dismiss Mercury
in the pantheon of planets,

but ignore it at your peril
because there may come a day

when Mercury makes its presence
very well known indeed.

I'd like to think that Mercury
does not bear the solar system

any ill will for
all the hard time

it's been given over
the last 4.5 billion years,

but, you know, that's a long
time to build up a grudge.

Mercury has had
such a difficult past.

It's been beat on by the Sun,
collided with other planets.

Now Mercury could come in and
start wreaking havoc with us,

so maybe we can think of this
as sort of Mercury's revenge.

Billions of years ago,

Earth could well have collided

with the young Mercury,
changing Mercury forever.

And now, it comes face-to-face
with Earth once again...

Smashing into our planet
in one, final impact,

wiping out any trace
of the world we know and love.

It would melt our entire crust.

It would wipe out all life
on our planet.

So how worried should
we be about this threat?

The odds of this happening
aren't that high,

and if it happens,
it's going to happen

billions of years in the future.

So I'm not terribly worried
about it on a personal scale,

but as an astronomer and as
a scientist, that's fascinating.

Mercury may continue
to surprise us,

even to its dying day,

a world born in fire that
might also go down in flames.

But the fact that
it made it this far

is nothing short of remarkable.

Living in the toughest
neighborhood imaginable,

Mercury has made it through
the history of the solar system,

beaten, but not broken.

Mercury is one of the solar
system's great survivors.

Despite all of the things
that happened to it,

it's still hanging in there.

It's been subject to
the harshest environment

in the solar system,
and it's still around.

Mercury, you know, has been
attacked from all sides, right?

It's been roasted by the Sun.

It's been pummeled by impactors,
and yet it's still there,

still being a great planet,

this plucky, little survivor
who's still orbiting

despite everything
it's gone through.

This plucky, little survivor
who's still orbiting