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(WIND WHISTLES)

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If | asked you,
"What is a planet made of?"

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then you'd probably say,
"Well, rocks and iron."

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And for the planets
of the inner solar system,

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like Earth, close to the heat
of the sun, you'd be right.

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But if you head out
into the frozen outer reaches

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of the solar system,

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then even the gases
that make up our atmosphere,

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so nitrogen and carbon dioxide
and, of course, water

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are all frozen solid.

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And the planets and moons
out there,

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the mountains and glaciers,

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and even the crust
of the worlds themselves,

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are made of that -

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solid, frozen, pristine ice.

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Ice that,
in the extreme conditions

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we find beyond Earth,

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behaves in ways
we never imagined possible.

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As we've explored
the solar system,

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our spacecraft have encountered
moons torn apart...

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...by great canyons
of shifting ice...

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... dwarf planets
where mountains of solid ice

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float across the surface...

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...worlds where ice appears
to cover one face...

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...but leaves the other
entirely alone.

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And elsewhere, alien aurora
hang above the clouds,

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all thanks to a strange,
newly discovered form of ice.

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Even here, on Earth,

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the behaviour of something
as simple as ice has had,

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we think,
profound consequences.

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Because without ice's
counterintuitive behaviour,

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life on our planet
may not have survived.

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We begin our journey
to the ice worlds

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at the freezing edge
of the solar system.

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Out here,
the sun Is so far away,

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it resembles just another star.

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Pluto is so remote

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that it was only in July 2015

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that we had our first and,
to date, only close encounter.

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As it flew by,
the New Horizons spacecraft

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sent back
the first close-up images

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of this mysterious
frozen world.

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It discovered
a great heart-shaped plain

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1,000km across

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that dominates one face.

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Around the edge of this plain,

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mountains made of solid ice
tower over Pluto's surface.

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And amongst its rugged uplands,

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ice was detected in a form

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no-one ever expected to see
on Pluto.

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Glaciers.

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Flowing rivers of ice
on a world so far away,

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we expected
nothing would be moving.

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That's because
the temperature here

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is only 40 degrees Celsius or
so away from absolute zero...

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...a temperature
at which nothing should move.

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And yet New Horizons
discovered regions of Pluto

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that for all the world

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look like the frozen reaches
of our planet.

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So when you fly here
over the years,

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how much does it change?

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It changes every day.

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- Yeah?
- Yeah.

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The shape of the glaciers
change.

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It's never the same.

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I'm just imagining flying
over Pluto, actually...

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(CHUCKLES)

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...beCause
it looks remarkably similar.

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Really?

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- What is the topography like?
- It's like this.

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Same topography,
same mountain heights.

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LEIGHAN: That's incredible!

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PROF COX: The discovery of
Pluto's dynamic icy landscapes

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came as a huge shock.

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It forced us
to rethink our understanding

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of this so-called dwarf planet.

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LEIGHAN:
We're coming in for landing,

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so I'll just be talking
to the airplane and not you.

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PROF COxX: | respect
that choice.

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(LEIGHAN LAUGHS)

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I've had a lot of landings,
but none quite like this.

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In fact, none like this at all.

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It's the world's
most beautiful runway!

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LEIGHAN: Welcome
to the glacier, you guys!

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(PROPELLER SLOWS)

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(WIND WHISTLES)

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Wow!

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You know, when we arrived,

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they said,
"Oh, it's zero degrees,"

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and | thought, "That's great.
Zero degrees, it's warm."

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Fahrenheit.
Zero degrees Fahrenheit.

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It's about minus 20 up here.

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It's just...

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And you got a real sense
actually, coming in,

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of...it looks frigid
and frozen,

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you know, unmoving, unchanging,

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but it's so dynamic.

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You can feel it in the wind
as you land.

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And then,
you know, you can see...

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...you can see the way
that everything flows.

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Just look at that glacier.

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You can almost feel
or see it moving.

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It looks like
a slow-motion river,

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and indeed it is moving.

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It does flow very, very slowly.

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And the reason

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this great mass can grind
its way down the valley

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is because of the unique
properties of the ice

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from which it's made.

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That bright-blue ice
certainly looks solid.

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Immovable.

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It's formed by pressure.

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So these snowflakes
are falling down

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onto the top of the glacier,
and over time they build up,

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and their weight presses down,

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increasing the pressure,

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and you get that particular
crystalline structure of ice,

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which looks transparent
and blue.

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But actually, at those
pressures and temperatures,

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it's not completely solid.

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The crystals are
sort of arranged in planes,

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a little bit
like a deck of cards.

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And that means
that as gravity is acting,

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trying to slide
this whole thing

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down the valley.

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those planes can slip
and slide over each other,

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and that allows
the whole glacier to move.

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You can also get liquid water
between the rock and the ice,

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and that sort of lubricates
the glacier,

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and that allows it
to slip as well.

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So although this looks fixed
and immovable,

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at the conditions
we find on Earth,

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this can almost behave
like a...like a fluid,

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sort of sliding very slowly
and deforming down the valley.

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Glaciers were the last things
we expected to see on Pluto.

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It's so cold here
that we'd expect

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the ice crystals
to be too brittle to flow.

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Nothing should slip,
nothing should slide.

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Yet that's precisely
what these glaciers are doing.

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So if they can't be made
of water ice,

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what are they made of?

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As New Horizons flew
past Pluto,

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its detectors picked up
an important clue.

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These are images
of Pluto's surface,

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and the colours correspond

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to different molecules,
different substances

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that New Horizons detected
on the surface.

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The purple is methane,

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the yellow is nitrogen,

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and the green is
carbon monoxide.

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All these gases are
frozen solid.

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Now, the glaciers on Pluto are
primarily made of nitrogen.

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Solid nitrogen.

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Now, nitrogen is something
that we're all familiar with.

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It's this stuff. Our air is
pretty much made of nitrogen.

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So our familiar experience
of it is just we can't see it.

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But if you cool it down...

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(SIZZLING)

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...then we can get it pretty
easily to turn into a liquid.

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Now, if we carried on
cooling that down,

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it would turn into a solid.

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Nitrogen freezes
at minus 210 degrees C,

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and Pluto's
surface temperature,

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at around minus 230 degrees,

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ensures that glaciers
remain solid.

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But crucially, the nitrogen ice

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is just 20 degrees or so
away from its melting point.

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That's very similar to the
situation here on this glacier.

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The glacier is about...
Well, the air temperature today

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is about minus 10,
minus 20 degrees Celsius.

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Heat it up by about 20 degrees,
and it'll melt.

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So the temperature difference
between the solid nitrogen ice

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and the nitrogen gas,

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and the solid water ice
and the water,

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is about the same.

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That means that,

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with just a small rise
in temperature,

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Pluto's nitrogen ice
should be able to move.

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So the discovery
of its glaciers tells us

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something remarkable
about Pluto.

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This tiny world must have
a little heat at its core,

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a faint warming
from radioactive decay,

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just enough to gently melt

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the bottom
of these rivers of ice,

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sending them on their way
down the valley.

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| think, for me,
there are two lessons

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from the exploration of Pluto.

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One is that geology
finds a way,

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even so far away
from the sun,

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where temperatures are

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only 40 degrees or so
above absolute zero,

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pretty much the coldest
it can be,

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there can still be
active geology,

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particularly where something is
close to its freezing point.

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In Pluto's case, nitrogen ice.

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The second lesson, | think is
perhaps even more profound,

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is that nature's imagination
far exceeds our own.

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Nobody expected
that they would see

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such a beautiful, active world

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so far away from the sun,

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on the far icy edge
of the solar system.

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The similarities between Earth
and Pluto are striking...

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...but New Horizons discovered
a wondertul difference.

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Pluto's glaciers
flow through mountains

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reminiscent of Alaska's
great ranges.

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But unlike Mount Denali's
granite spires...

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...Pluto's mountains are made
from frozen water.

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But you can't imagine something
that big, that high,

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being made of water.
That's the thing

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- that amazes me.
- That's crazy!

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- All water, no...no rock?
- Yeah, pretty much, yeah.

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And this leads
to a surreal twist.

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Water ice in the mountains
is less dense

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than the nitrogen ice
in the glaciers.

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So in places,

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we've seen mountains floating
on the glaciers,

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carried away like icebergs
onto the vast ice plain below.

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Pluto, a world sculpted by ice.

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Leaving Pluto and heading back
towards the sun's glow,

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we enter the realm
of the ice giants...

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... vast gaseous worlds...

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...where ice storms rage.

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And on the innermost
of these planets,

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we've discovered a phenomenon
eerily reminiscent of home.

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Just above Uranus's ice clouds

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hang beautiful,
ethereal aurora...

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... found not at the poles
of the planet, as on Earth,

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but scattered
across its face...

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...even around the equator.

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So what's creating
this beautiful, rare display?

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The basic physics
of the aurora on Uranus

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is the same as the physics
of the aurora on Earth.

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So the sun's
constantly emitting

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a rain of high-energy
charged particles,

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which is called the solar wind.

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And when those charged
particles reach the Earth,

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most of them are deflected
around the Earth,

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harmlessly off into space,
by our magnetic field.

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Now, the Earth's magnetic field
looks very much like the field

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around a bar magnet.

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| can show you that
by sprinkling

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some iron filings...

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...around a bar magnet.

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And the iron filings line up
with the magnetic field lines.

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When the solar wind hits
this magnetic field,

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00:17:50,40 --> 00:17:51,560
most regions of the Earth,

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is deflected harmlessly
off into space.

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00:17:54,400 --> 00:17:55,680
But at the poles,

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those charged particles can
become trapped.

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And then
they can be accelerated

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down into the upper atmosphere

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and hit molecules
in the atmosphere,

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oxygen and nitrogen.

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00:18:05,720 --> 00:18:08,00
And that can cause those
molecules to emit light,

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00:18:08,120 --> 00:18:09,400
to glow.

256
00:18:09,520 --> 00:18:11,680
And that's what,
if you're lucky,

257
00:18:11,800 --> 00:18:14,920
you see as the northern
and southern lights.

258
00:18:17,600 --> 00:18:20,320
So somewhere
deep inside our planet

259
00:18:20,440 --> 00:18:23,400
lies the equivalent
of that bar magnet.

260
00:18:24,640 --> 00:18:26,520
And, of course, it does,

261
00:18:26,640 --> 00:18:29,720
in the form of a hot,
molten iron core,

262
00:18:29,840 --> 00:18:32,640
spinning away,
as the Earth rotates...

263
00:18:34,680 --> 00:18:36,600
...creating electrical currents

264
00:18:36,720 --> 00:18:40,400
and the magnetic field
that projects out into space.

265
00:18:42,560 --> 00:18:44,920
But Uranus is different.

266
00:18:45,40 --> 00:18:47,400
The aurora are not found
at the poles,

267
00:18:47,520 --> 00:18:49,560
and we don't think

268
00:18:49,680 --> 00:18:52,680
it has a molten iron
or metallic core

269
00:18:52,800 --> 00:18:55,520
to support
those electrical currents.

270
00:18:55,640 --> 00:18:58,920
And so the fact that Uranus
does have aurora,

271
00:18:59,40 --> 00:19:02,160
and therefore some kind of
magnetic field,

272
00:19:02,280 --> 00:19:03,920
is a tremendous mystery.

273
00:19:08,880 --> 00:19:11,240
But we do have theories
that allow us

274
00:19:11,360 --> 00:19:13,440
to piece together
what might be going on.

275
00:19:17,920 --> 00:19:21,280
Imagine diving
beyond the clouds of Uranus,

276
00:19:21,400 --> 00:19:24,720
beyond the slushy ice layer
that flows around the planet...

277
00:19:24,840 --> 00:19:26,600
(RUMBLING)

278
00:19:29,120 --> 00:19:31,280
..and keep going
towards the core.

279
00:19:32,880 --> 00:19:35,880
We enter a region
where the pressure approaches

280
00:19:36,00 --> 00:19:39,200
several million times
that of Earth's atmosphere...

281
00:19:40,880 --> 00:19:43,880
...and where it's almost as hot
as the sun's surface.

282
00:19:48,640 --> 00:19:50,960
Rather than molten rock
or metal,

283
00:19:51,80 --> 00:19:53,680
like we find
inside our own planet,

284
00:19:53,800 --> 00:19:57,560
we instead find
yet more frozen water...

285
00:19:59,160 --> 00:20:04,00
..In a bizarre form of matter
known as superionic ice.

286
00:20:08,480 --> 00:20:12,680
Normal water ice has
a crystal structure like this.

287
00:20:12,800 --> 00:20:17,80
So the reds are oxygen atoms,
the whites are hydrogens,

288
00:20:17,200 --> 00:20:19,280
and you can see
that they're bonded together

289
00:20:19,400 --> 00:20:22,520
into this regular
crystal lattice.

290
00:20:23,760 --> 00:20:25,400
Within the lattice,

291
00:20:25,520 --> 00:20:29,00
nothing that could carry
an electrical current can flow,

292
00:20:29,120 --> 00:20:31,560
so a magnetic field
can't be created.

293
00:20:33,00 --> 00:20:35,80
And this...

294
00:20:35,200 --> 00:20:38,880
..1S the crystal structure
of superionic ice.

295
00:20:39,00 --> 00:20:41,280
The oxygens are still there,

296
00:20:41,400 --> 00:20:43,280
bonded together into a crystal,

297
00:20:43,400 --> 00:20:45,920
but now
there are hydrogen nuclei -

298
00:20:46,40 --> 00:20:48,480
electrically charged protons

299
00:20:48,600 --> 00:20:52,440
that can move freely
through the crystal lattice.

300
00:20:52,560 --> 00:20:57,880
That means that this is
an electrical conductor.

301
00:20:59,440 --> 00:21:02,400
It's this movement
of the protons

302
00:21:02,520 --> 00:21:05,800
that could be contributing
to Uranus's magnetic field.

303
00:21:09,480 --> 00:21:14,520
If so, then the superionic ice
is, at least in part,

304
00:21:14,640 --> 00:21:17,720
driving the planet's
mysterious aurora.

305
00:21:20,280 --> 00:21:24,800
Now, this story is still
far from fully understood.

306
00:21:24,920 --> 00:21:29,120
For a long time, this strange
form of ice was only a theory.

307
00:21:29,240 --> 00:21:30,280
But then a team

308
00:21:30,400 --> 00:21:32,680
pointed one of the world's
most powerful lasers

309
00:21:32,800 --> 00:21:34,680
at a droplet of water

310
00:21:34,800 --> 00:21:37,960
and recreated the conditions
that are present

311
00:21:38,80 --> 00:21:40,40
deep down inside Uranus,

312
00:21:40,160 --> 00:21:42,400
and, just for a moment,

313
00:21:42,520 --> 00:21:45,960
caught a glimpse
of superionic ice.

314
00:21:49,200 --> 00:21:54,680
Uranus,
a world illuminated by ice.

315
00:22:00,00 --> 00:22:02,160
On the journey
between ice worlds,

316
00:22:02,280 --> 00:22:04,880
we edge ever closer
towards the sun,

317
00:22:05,00 --> 00:22:07,560
for an encounter
with one of the most

318
00:22:07,680 --> 00:22:11,840
thoroughly explored
planetary systems of them all.

319
00:22:28,360 --> 00:22:31,600
Saturn's rings are constructed
of countless crystals,

320
00:22:31,720 --> 00:22:34,560
ranging in size
from just a few microns

321
00:22:34,680 --> 00:22:36,240
to vast boulders...

322
00:22:39,400 --> 00:22:43,680
..and all of them made almost
entirely from frozen water.

323
00:22:51,640 --> 00:22:53,560
The rings are joined
in their orbit

324
00:22:53,680 --> 00:22:56,920
by at least 146 moons.

325
00:22:59,520 --> 00:23:02,400
And out towards
the edge of the system,

326
00:23:02,520 --> 00:23:04,840
NASA's Cassini probe made

327
00:23:04,960 --> 00:23:07,960
one of its most surprising
discoveries.

328
00:23:19,40 --> 00:23:22,400
The moon lapetus resembles
a walnut,

329
00:23:22,520 --> 00:23:24,720
with a mountain ridge
around its middle.

330
00:23:26,160 --> 00:23:28,640
But that's not
its strangest feature.

331
00:23:32,800 --> 00:23:34,400
Back in the 17th century,

332
00:23:34,520 --> 00:23:37,00
only about 60 years or so,
actually,

333
00:23:37,120 --> 00:23:38,720
after the invention
of the telescope,

334
00:23:38,840 --> 00:23:41,680
Giovanni Cassini
discovered lapetus.

335
00:23:41,800 --> 00:23:43,720
But he immediately noticed
something strange

336
00:23:43,840 --> 00:23:46,160
about the moon as he watched it
orbit the planet,

337
00:23:46,280 --> 00:23:48,320
because
he could see the moon

338
00:23:48,440 --> 00:23:50,80
on one side of the planet,

339
00:23:50,200 --> 00:23:52,880
but then on the other side,
he couldn't.

340
00:23:53,00 --> 00:23:54,680
Now, being sensible -

341
00:23:54,800 --> 00:23:56,480
because he was a scientist,
after all! -

342
00:23:56,600 --> 00:23:59,00
he said, "Well, it's not
somehow disappearing.

343
00:23:59,120 --> 00:24:00,920
"There must be
another explanation."

344
00:24:01,40 --> 00:24:05,40
And he guessed that one side
of the moon must be very bright

345
00:24:05,160 --> 00:24:07,680
and the other side
must be very dark.

346
00:24:07,800 --> 00:24:09,720
Now, 300 years later,

347
00:24:09,840 --> 00:24:13,560
we sent a spacecraft to Saturn,
bearing his name...

348
00:24:15,960 --> 00:24:17,640
...and we discovered...

349
00:24:17,760 --> 00:24:19,360
...that he was right.

350
00:24:22,600 --> 00:24:24,520
Cassini sent back proof

351
00:24:24,640 --> 00:24:27,760
that one side of lapetus
is icy white,

352
00:24:27,880 --> 00:24:30,680
whilst the other looks
as if it's been painted black.

353
00:24:35,880 --> 00:24:37,280
So what could be creating

354
00:24:37,400 --> 00:24:40,760
such a sharply defined
monochromatic world?

355
00:24:43,360 --> 00:24:46,920
A tremendous mystery,
but a clue can be found

356
00:24:47,40 --> 00:24:49,360
in looking at the line
between the two hemispheres,

357
00:24:49,480 --> 00:24:51,840
because there are
jet-black regions

358
00:24:51,960 --> 00:24:53,200
on the surface there

359
00:24:53,320 --> 00:24:55,80
that are also
some of the hottest places

360
00:24:55,200 --> 00:24:57,360
in the Saturnian system.

361
00:25:00,00 --> 00:25:02,40
Hot is relative, of course.

362
00:25:02,160 --> 00:25:05,960
It's still
minus 140 degrees Celsius

363
00:25:06,80 --> 00:25:08,80
on the dark side of the moon.

364
00:25:08,200 --> 00:25:10,760
Now, that's about
20 degrees warmer

365
00:25:10,880 --> 00:25:12,760
than the moon's icy face.

366
00:25:19,00 --> 00:25:21,560
And we think that this
difference is just enough

367
00:25:21,680 --> 00:25:23,720
to move ice around the moon

368
00:25:23,840 --> 00:25:25,680
in a very particular way.

369
00:25:32,80 --> 00:25:34,680
The sunlight falls
on that dark surface,

370
00:25:34,800 --> 00:25:38,240
as lapetus rather languidly
rotates, actually,

371
00:25:38,360 --> 00:25:42,600
about once every 79 Earth days,
and it heats it up.

372
00:25:42,720 --> 00:25:47,360
The water molecules rise up,
drift over to the light side,

373
00:25:47,480 --> 00:25:49,120
and then condense,

374
00:25:49,240 --> 00:25:51,80
and fall onto the surface,

375
00:25:51,200 --> 00:25:53,640
making it brighter
and brighter and brighter,

376
00:25:53,760 --> 00:25:55,680
sort of like
what's happening here.

377
00:25:55,800 --> 00:25:58,560
So, out there
in the Pacific Ocean,

378
00:25:58,680 --> 00:26:01,40
the water is turning
into water vapour,

379
00:26:01,160 --> 00:26:04,720
drifting over the cold land,
and falling as snow,

380
00:26:04,840 --> 00:26:08,240
making the whole surface
bright.

381
00:26:16,240 --> 00:26:20,160
On the dark side of lapetus,
ice is warmed

382
00:26:20,280 --> 00:26:23,760
and creates a thin atmosphere
of water vapour.

383
00:26:30,920 --> 00:26:32,240
And where this vapour meets

384
00:26:32,360 --> 00:26:34,160
the colder,
white side of the moon,

385
00:26:34,280 --> 00:26:36,360
it freezes to the surface
again,

386
00:26:36,480 --> 00:26:38,800
resembling fresh snow...

387
00:26:42,600 --> 00:26:46,320
...Maintaining the bright
icy white of this hemisphere.

388
00:26:48,240 --> 00:26:50,440
But a mystery remains,

389
00:26:50,560 --> 00:26:53,720
because lapetus is
an ice moon...

390
00:26:55,920 --> 00:27:00,00
...SO What is the dark material
covering its other face?

391
00:27:06,400 --> 00:27:09,600
In 2009, the Spitzer
infrared space telescope

392
00:27:09,720 --> 00:27:11,520
discovered this.

393
00:27:11,640 --> 00:27:14,480
This is another ring
around Saturn,

394
00:27:14,600 --> 00:27:15,800
but it's enormous.

395
00:27:15,920 --> 00:27:18,200
It's one of the largest
structures in the solar system.

396
00:27:18,320 --> 00:27:22,200
This is about
12 million kilometres across.

397
00:27:27,880 --> 00:27:31,680
Later observations
from NASA's WISE telescope

398
00:27:31,800 --> 00:27:34,240
suggest
that the disc may extend

399
00:27:34,360 --> 00:27:38,400
a further 20 million kilometres
out into space.

400
00:27:40,720 --> 00:27:42,400
At this vast scale,

401
00:27:42,520 --> 00:27:46,840
Saturn and its more familiar
icy rings are barely visible.

402
00:27:53,00 --> 00:27:55,560
It might seem strange
that no-one had seen

403
00:27:55,680 --> 00:27:58,800
one of the largest structures
in the solar system until 2009.

404
00:27:58,920 --> 00:28:03,320
The reason is that that ring
is very dark and very diffuse.

405
00:28:03,440 --> 00:28:05,800
If you were transported
into the ring,

406
00:28:05,920 --> 00:28:07,120
you could look around,

407
00:28:07,240 --> 00:28:09,280
and you wouldn't know
you were in it.

408
00:28:09,400 --> 00:28:10,920
Spitzer saw it

409
00:28:11,40 --> 00:28:13,80
because Spitzer is
an infrared telescope,

410
00:28:13,200 --> 00:28:15,680
and so it detected
not visible light,

411
00:28:15,800 --> 00:28:17,600
but infrared light.

412
00:28:17,720 --> 00:28:21,240
The glow, the heat
emanating from the ring.

413
00:28:22,920 --> 00:28:25,280
The giant outer ring is
therefore very different

414
00:28:25,400 --> 00:28:27,400
to Saturn's ice rings.

415
00:28:28,960 --> 00:28:32,200
So what is it made of,
and where did it come from?

416
00:28:41,920 --> 00:28:45,280
Phoebe is another
of Saturn's outer moons,

417
00:28:45,400 --> 00:28:49,40
and each time a passing
asteroid gets too close...

418
00:28:51,680 --> 00:28:55,720
..the resulting impact throws
dark material out into space.

419
00:29:03,520 --> 00:29:05,160
Over billions of years,

420
00:29:05,280 --> 00:29:08,720
numerous impacts have resulted
in the dust from Phoebe

421
00:29:08,840 --> 00:29:13,520
spreading around Saturn,
forming its vast dark ring.

422
00:29:17,920 --> 00:29:22,400
lapetus passes through the ring
as it orbits,

423
00:29:22,520 --> 00:29:26,160
and so that dark material
from the ring gets deposited

424
00:29:26,280 --> 00:29:28,560
on the surface of lapetus.

425
00:29:28,680 --> 00:29:31,120
This is a really slow process.

426
00:29:31,240 --> 00:29:33,600
Material falls onto lapetus

427
00:29:33,720 --> 00:29:36,400
and increases
the size of that dark layer

428
00:29:36,520 --> 00:29:40,40
by about 4/100ths
of a millimetre

429
00:29:40,160 --> 00:29:42,160
every million years.

430
00:29:42,280 --> 00:29:44,320
It's not a bad analogy, this,
actually.

431
00:29:44,440 --> 00:29:47,120
Some of those sort of
dust particles in the ring

432
00:29:47,240 --> 00:29:48,240
are about this size,

433
00:29:48,360 --> 00:29:50,200
about the size
of...of pepper grains.

434
00:29:50,320 --> 00:29:51,280
Some are bigger,

435
00:29:51,400 --> 00:29:53,00
a few centimetres across
or something,

436
00:29:53,120 --> 00:29:55,720
but it is pretty much
stuff like this.

437
00:29:58,320 --> 00:30:01,400
And yet a puzzle remains.

438
00:30:01,520 --> 00:30:04,360
Why half black and half white?

439
00:30:07,360 --> 00:30:11,960
lapetus spins on its axis
once every 79 days

440
00:30:12,80 --> 00:30:16,360
and orbits around Saturn
once every 79 days.

441
00:30:16,480 --> 00:30:18,80
It's what's called
spin-orbit locked.

442
00:30:18,200 --> 00:30:19,440
It's like our moon.

443
00:30:19,560 --> 00:30:22,440
So it always leads
with one hemisphere,

444
00:30:22,560 --> 00:30:26,320
as it orbits around Saturn
and passes through the ring.

445
00:30:30,800 --> 00:30:34,520
lapetus, then, is a fluke
of nature that exists

446
00:30:34,640 --> 00:30:36,920
thanks to the interaction
of two moons

447
00:30:37,40 --> 00:30:39,00
within a dark ring,

448
00:30:39,120 --> 00:30:41,680
right at the edge
of Saturn's domain.

449
00:30:44,320 --> 00:30:47,840
A world painted by ice.

450
00:31:02,880 --> 00:31:05,640
As we return
ever closer to the sun,

451
00:31:05,760 --> 00:31:08,120
ice becomes
increasingly rare.

452
00:31:14,960 --> 00:31:17,720
Jupiter has 95 known moons...

453
00:31:21,840 --> 00:31:25,00
.. Including
three large ice worlds.

454
00:31:32,560 --> 00:31:35,480
And one of these
is a promising target

455
00:31:35,600 --> 00:31:38,600
in our search
for life beyond Earth.

456
00:31:52,560 --> 00:31:57,200
In 2022, NASA's Juno spacecraft
flew by Europa...

457
00:31:58,560 --> 00:31:59,880
..and photographed a world

458
00:32:00,00 --> 00:32:02,840
crisscrossed
with mysterious red lines.

459
00:32:07,640 --> 00:32:09,440
Grand canyons...

460
00:32:11,120 --> 00:32:15,640
...some 100 metres deep
and tens of kilometres wide...

461
00:32:17,360 --> 00:32:19,760
..In places
coated in a red substance

462
00:32:19,880 --> 00:32:24,160
that may be a newly discovered
compound of salt and water.

463
00:32:34,800 --> 00:32:38,680
Juno is the latest NASA mission
to fly by Europa

464
00:32:38,800 --> 00:32:40,840
and take detailed photographs

465
00:32:40,960 --> 00:32:43,800
of its peculiar,
fractured surface.

466
00:32:50,00 --> 00:32:51,280
The canyons on Europa are

467
00:32:51,400 --> 00:32:53,880
quite unlike anything
seen on Earth...

468
00:32:55,400 --> 00:32:59,80
...Or, Indeed, anywhere
in the entire solar system.

469
00:33:00,920 --> 00:33:02,920
The markings are geometric.

470
00:33:03,40 --> 00:33:06,800
They form lines that crisscross
over the surface,

471
00:33:06,920 --> 00:33:10,440
though what can be causing
that pattern?

472
00:33:12,00 --> 00:33:14,960
Europa's surface features are
an active area of research,

473
00:33:15,80 --> 00:33:17,800
taking NASA scientists
to the frozen reaches

474
00:33:17,920 --> 00:33:20,560
of our own planet
in search of answers.

475
00:33:22,280 --> 00:33:24,920
This is an image of a region
on Europa's surface

476
00:33:25,40 --> 00:33:26,320
called Phaidra Linea.

477
00:33:26,440 --> 00:33:28,960
And you see this feature,

478
00:33:29,80 --> 00:33:31,600
it almost looks like
the Grand Canyon on Earth.

479
00:33:31,720 --> 00:33:34,560
It's actually
about 50km across.

480
00:33:34,680 --> 00:33:37,640
A clue to what this is
can be seen...

481
00:33:37,760 --> 00:33:40,960
If you look at the top line
and the bottom line,

482
00:33:41,80 --> 00:33:45,160
and just in your mind's eye,
just draw these together,

483
00:33:45,280 --> 00:33:48,600
you'll see that
they knit together perfectly.

484
00:33:48,720 --> 00:33:52,00
So this looks like
the crust has just spread.

485
00:33:52,120 --> 00:33:54,640
Now, there's only one
other place in the solar system

486
00:33:54,760 --> 00:33:56,360
where we see features
like this,

487
00:33:56,480 --> 00:33:58,720
and it's here, on Earth.

488
00:33:58,840 --> 00:34:00,920
It's caused by plate tectonics.

489
00:34:03,360 --> 00:34:06,440
On Earth, it's the internal
heat of the planet

490
00:34:06,560 --> 00:34:09,280
as it forces its way
through the crust

491
00:34:09,400 --> 00:34:12,640
which Is the driving force
of plate tectonics.

492
00:34:17,960 --> 00:34:22,120
| mean, no-one expected to see
behaviour like this on a moon.

493
00:34:31,560 --> 00:34:33,600
On Europa, it's not molten rock

494
00:34:33,720 --> 00:34:36,40
that's driving its plates
apart.

495
00:34:39,240 --> 00:34:42,40
Density measurements
of the moon suggest

496
00:34:42,160 --> 00:34:45,40
that beneath
the thick icy crust

497
00:34:45,160 --> 00:34:46,760
lies a different liquid,

498
00:34:46,880 --> 00:34:51,360
a global subsurface ocean
of water.

499
00:34:51,480 --> 00:34:54,00
Up to 150km deep,

500
00:34:54,120 --> 00:34:55,960
it may contain
two or three times

501
00:34:56,80 --> 00:35:00,00
all the water
in Earth's oceans combined.

502
00:35:02,80 --> 00:35:04,320
So how can
all that liquid water exist

503
00:35:04,440 --> 00:35:07,400
just below the surface
of this frigid ice moon?

504
00:35:15,480 --> 00:35:20,600
The answer lies with
two other moons of Jupiter,

505
00:35:20,720 --> 00:35:22,880
with lo and Ganymede.

506
00:35:23,00 --> 00:35:25,640
So here's Jupiter,

507
00:35:25,760 --> 00:35:31,160
and then lo goes around
four times...

508
00:35:33,200 --> 00:35:36,280
...as Europa goes around
two times,

509
00:35:36,400 --> 00:35:41,720
and Ganymede, farthest out,
goes around once.

510
00:35:41,840 --> 00:35:44,400
It's called
an orbital resonance.

511
00:35:44,520 --> 00:35:47,720
Four orbits, to two orbits,
to one orbit.

512
00:35:47,840 --> 00:35:52,160
That means that these three
moons line up periodically

513
00:35:52,280 --> 00:35:55,280
and give each other
a gravitational kick,

514
00:35:55,400 --> 00:35:59,40
which means that the orbits
don't stay as nice circles.

515
00:35:59,160 --> 00:36:01,40
They're all ellipses.

516
00:36:01,160 --> 00:36:04,240
And that means
that tidal effects,

517
00:36:04,360 --> 00:36:06,480
just like the tides
here on Earth,

518
00:36:06,600 --> 00:36:09,880
stretch and squash the moons,
and heat them up.

519
00:36:10,00 --> 00:36:12,600
Now, the effect is strongest
for lo,

520
00:36:12,720 --> 00:36:14,760
because that's closest
to the giant planet,

521
00:36:14,880 --> 00:36:18,320
and so that turns lo into,
essentially, one giant volcano.

522
00:36:18,440 --> 00:36:22,80
For Europa, further out,
that heat melts the ice.

523
00:36:24,360 --> 00:36:27,400
But the energy
that goes into Europa

524
00:36:27,520 --> 00:36:31,80
from this eccentric, elliptical
orbit around Jupiter,

525
00:36:31,200 --> 00:36:33,760
um, sort of trickles
into the moon.

526
00:36:33,880 --> 00:36:36,800
So it really isn't enough
on its own

527
00:36:36,920 --> 00:36:39,280
to produce
the very active geology

528
00:36:39,400 --> 00:36:40,880
that we see on the surface.

529
00:36:46,640 --> 00:36:49,280
We estimate the surface ice
on Europa is

530
00:36:49,400 --> 00:36:52,840
somewhere
between 10 and 25km thick.

531
00:36:59,40 --> 00:37:01,00
So whilst the tidal forces
are enough

532
00:37:01,120 --> 00:37:03,600
for the subsurface ocean
to remain liquid,

533
00:37:03,720 --> 00:37:07,560
they're not enough
to split apart all this ice.

534
00:37:09,200 --> 00:37:12,280
So to drive the high-energy
geological processes

535
00:37:12,400 --> 00:37:14,320
we see on the surface
of Europa,

536
00:37:14,440 --> 00:37:17,800
then there must be some kind of
energy storage

537
00:37:17,920 --> 00:37:19,160
in the moon itself.

538
00:37:19,280 --> 00:37:22,520
So I've got two camping stoves
here.

539
00:37:22,640 --> 00:37:24,360
These two pans are filled
with water,

540
00:37:24,480 --> 00:37:27,80
and it's at the same
temperature - zero degrees.

541
00:37:27,200 --> 00:37:31,200
The only difference is
that this water has ice in it,

542
00:37:31,320 --> 00:37:33,240
and this has no ice in it -

543
00:37:33,360 --> 00:37:36,800
the only difference
when we start the stoves.

544
00:37:39,880 --> 00:37:43,120
This is a thermal camera here
cos, you know,

545
00:37:43,240 --> 00:37:45,360
| wouldn't travel without one.

546
00:37:45,480 --> 00:37:47,960
So it will tell us
the temperature

547
00:37:48,80 --> 00:37:49,120
of the water is rising.

548
00:37:49,240 --> 00:37:52,800
Eight, nine degrees already.

549
00:37:52,920 --> 00:37:56,600
Whereas this one
is still zero degrees,

550
00:37:56,720 --> 00:37:58,680
even though we're putting
all the energy into it.

551
00:37:58,800 --> 00:38:00,320
Here, look!

552
00:38:00,440 --> 00:38:01,960
(CHUCKLES EXCITEDLY)

553
00:38:02,80 --> 00:38:03,760
You see that?

554
00:38:03,880 --> 00:38:05,200
So why?

555
00:38:06,160 --> 00:38:08,800
Well, this is a model of ice.

556
00:38:08,920 --> 00:38:12,360
You can see the water molecules
here and here and here.

557
00:38:12,480 --> 00:38:15,720
And they're bonded together
by these longer bonds,

558
00:38:15,840 --> 00:38:17,520
which are called
hydrogen bonds.

559
00:38:17,640 --> 00:38:20,800
They're the thing that hold
the crystal lattice in place.

560
00:38:20,920 --> 00:38:22,800
And they're pretty strong.

561
00:38:22,920 --> 00:38:24,720
So to melt the ice,

562
00:38:24,840 --> 00:38:27,160
you've got to break
all these bonds.

563
00:38:27,280 --> 00:38:29,360
You've got to put
a lot of energy into it.

564
00:38:29,480 --> 00:38:31,280
So all the energy
from this camping stove

565
00:38:31,400 --> 00:38:34,280
at the moment is going
into breaking bonds in the ice.

566
00:38:34,400 --> 00:38:36,760
It's not going into
making all the molecules

567
00:38:36,880 --> 00:38:40,80
move around faster,
which is what temperature is.

568
00:38:40,200 --> 00:38:43,160
So this one is getting hotter
and hotter and hotter.

569
00:38:43,280 --> 00:38:45,120
Nothing is happening
to this one.

570
00:38:45,240 --> 00:38:48,80
Now, these have been cooking
away now, and I'll show you,

571
00:38:48,200 --> 00:38:49,440
| have confidence.

572
00:38:49,560 --> 00:38:51,600
| have confidence in physics.

573
00:38:51,720 --> 00:38:53,160
| believe in it.

574
00:38:53,280 --> 00:38:55,00
| would not put my hand
in there.

575
00:38:55,120 --> 00:38:57,480
| can see it'd be
a stupid idea. But there...

576
00:38:59,00 --> 00:39:01,80
There you go. Physics works.

577
00:39:01,200 --> 00:39:02,600
It's actually freezing.

578
00:39:09,80 --> 00:39:12,920
Now reverse that idea,
reverse that argument.

579
00:39:13,40 --> 00:39:15,160
What happens, then,
when | freeze water,

580
00:39:15,280 --> 00:39:17,480
when | turn it
from a liquid to a solid?

581
00:39:19,480 --> 00:39:21,880
| get all that energy
back out again,

582
00:39:22,00 --> 00:39:25,40
huge amounts of energy,
as the bonds form.

583
00:39:27,160 --> 00:39:30,520
And this is what we think
may be happening on Europa.

584
00:39:38,40 --> 00:39:42,80
The subsurface ocean is warmed
by tidal forces from Jupiter

585
00:39:42,200 --> 00:39:44,40
and its moons...

586
00:39:45,280 --> 00:39:46,880
... Storing energy.

587
00:39:54,480 --> 00:39:57,120
Then,
thanks to its elliptical orbit,

588
00:39:57,240 --> 00:40:02,800
as Europa periodically cools,
the ice begins to freeze...

589
00:40:06,240 --> 00:40:08,320
... releasing the stored energy.

590
00:40:13,80 --> 00:40:16,760
The volume of the icy crust
grows as it freezes...

591
00:40:20,440 --> 00:40:22,400
.. Increasing the pressure...

592
00:40:31,400 --> 00:40:35,40
.. Until the entire canyon
is cleaved apart...

593
00:40:43,200 --> 00:40:45,880
..and briny water
from the ocean below surges up

594
00:40:46,00 --> 00:40:47,360
through the cracks...

595
00:40:50,200 --> 00:40:53,840
...where, bathed in Jupiter's
intense radiation...

596
00:40:55,480 --> 00:40:57,200
.../t turns red.

597
00:41:05,200 --> 00:41:07,880
You're actually very familiar
with this process.

598
00:41:08,00 --> 00:41:09,960
If your pipes burst
in your house

599
00:41:10,80 --> 00:41:11,280
because they freeze,

600
00:41:11,400 --> 00:41:14,400
where does the energy come from
to burst the pipes?

601
00:41:14,520 --> 00:41:17,120
It comes from water
freezing into ice.

602
00:41:24,80 --> 00:41:27,760
Europa is far more dynamic
than we'd imagined.

603
00:41:34,560 --> 00:41:36,80
And it's this dynamism

604
00:41:36,200 --> 00:41:38,280
that makes it
a tantalising target...

605
00:41:40,640 --> 00:41:43,600
...1N our search for life
under the ice.

606
00:41:47,720 --> 00:41:51,240
At its simplest,
life needs three things.

607
00:41:51,360 --> 00:41:53,720
Water, energy,

608
00:41:53,840 --> 00:41:55,840
and the right
chemical ingredients.

609
00:41:58,280 --> 00:42:01,40
Europa has the first two
in abundance...

610
00:42:02,920 --> 00:42:05,480
...but the chemistry for life
is missing.

611
00:42:07,920 --> 00:42:11,400
But fortunately,
Europa is not alone.

612
00:42:18,960 --> 00:42:20,760
Orbiting close by,

613
00:42:20,880 --> 00:42:24,720
lo has the missing ingredients
we believe necessary for life

614
00:42:24,840 --> 00:42:26,360
in abundance...

615
00:42:29,160 --> 00:42:30,640
...erupting into space

616
00:42:30,760 --> 00:42:33,440
in enormous quantities
around Jupiter.

617
00:42:39,200 --> 00:42:42,760
Here's where the story gets
even more wonderful,

618
00:42:42,880 --> 00:42:44,960
because the volcanoes of lo

619
00:42:45,80 --> 00:42:47,360
are constantly producing
chemicals,

620
00:42:47,480 --> 00:42:49,760
materials, that rain down

621
00:42:49,880 --> 00:42:51,440
onto the frozen surface
of Europa,

622
00:42:51,560 --> 00:42:53,480
but if it wasn't
for the geology,

623
00:42:53,600 --> 00:42:56,600
then they'd be separated
for ever from the ocean below

624
00:42:56,720 --> 00:43:00,240
by 10 or 20km of ice.

625
00:43:00,360 --> 00:43:02,480
But that active geology

626
00:43:02,600 --> 00:43:05,200
creating the plate
tectonic-like behaviour

627
00:43:05,320 --> 00:43:07,520
can bring those materials,

628
00:43:07,640 --> 00:43:10,40
those chemicals,
into the ocean,

629
00:43:10,160 --> 00:43:12,600
and then we have
all the conditions

630
00:43:12,720 --> 00:43:16,280
we think are necessary
for the origin of life.

631
00:43:20,00 --> 00:43:22,680
So Europa's dynamic surface

632
00:43:22,800 --> 00:43:25,920
may form part of
an extraordinary ecosystem...

633
00:43:28,160 --> 00:43:33,240
..one that stretches
from one moon to another,

634
00:43:33,360 --> 00:43:35,960
and work is already under way
to send robotic probes

635
00:43:36,80 --> 00:43:39,880
into that distant icy ocean.

636
00:43:44,880 --> 00:43:46,800
It would be
a profound discovery

637
00:43:46,920 --> 00:43:48,680
to find life on Europa,

638
00:43:48,800 --> 00:43:52,200
but it would also be profound
if we didn't,

639
00:43:52,320 --> 00:43:54,560
because everything we think
we know

640
00:43:54,680 --> 00:43:56,80
about the origin of life,

641
00:43:56,200 --> 00:43:58,600
all the ingredients
that are necessary,

642
00:43:58,720 --> 00:44:00,760
seem to be present on Europa,

643
00:44:00,880 --> 00:44:04,80
so if we go there
and send a cryobot

644
00:44:04,200 --> 00:44:07,160
into the oceans of Europa
and find nothing at all,

645
00:44:07,280 --> 00:44:10,520
then it may be far more likely

646
00:44:10,640 --> 00:44:13,480
that we are alone for...

647
00:44:13,600 --> 00:44:16,280
...millions or even billions
of lightyears

648
00:44:16,400 --> 00:44:18,280
in every direction.

649
00:44:29,280 --> 00:44:32,880
Europa, a world
completely encased in ice,

650
00:44:33,00 --> 00:44:35,920
couldn't exist
much closer to the sun...

651
00:44:37,240 --> 00:44:39,640
... because
just a little closer in

652
00:44:39,760 --> 00:44:42,200
lies the solar system's
ice line.

653
00:44:45,520 --> 00:44:48,320
Cross it, and temperatures
become too warm

654
00:44:48,440 --> 00:44:51,360
for ice to stay frozen
for long.

655
00:44:57,280 --> 00:45:01,120
When comets fall inwards
towards the sun,

656
00:45:01,240 --> 00:45:03,760
some of the ice they carry
is transformed

657
00:45:03,880 --> 00:45:06,320
into water vapour...

658
00:45:13,560 --> 00:45:16,720
.. forming tails
that streak through space

659
00:45:16,840 --> 00:45:18,600
for hundreds of kilometres.

660
00:45:24,120 --> 00:45:27,440
Inside the ice line, then,
ice Is rare.

661
00:45:28,720 --> 00:45:32,440
But there are places where
it can hold on at the margins.

662
00:45:42,920 --> 00:45:47,840
Most of the ice on the surface
of Mars Is held at the poles.

663
00:45:54,480 --> 00:45:57,440
Here, NASA's
Mars Reconnaissance Orbiter

664
00:45:57,560 --> 00:46:00,320
has captured
these extraordinary images

665
00:46:00,440 --> 00:46:02,480
of a strange phenomenon

666
00:46:02,600 --> 00:46:06,00
that takes place
on the southern ice cap.

667
00:46:07,920 --> 00:46:12,720
Dark spiderlike formations
that we think are being formed

668
00:46:12,840 --> 00:46:14,760
as the seasons turn.

669
00:46:16,880 --> 00:46:20,400
During the winter,
it gets so cold on Mars

670
00:46:20,520 --> 00:46:22,800
that the carbon dioxide

671
00:46:22,920 --> 00:46:24,920
in its thin atmosphere
freezes...

672
00:46:29,200 --> 00:46:34,840
...creating crystals of dry ice
that fall as snow on the pole.

673
00:46:38,840 --> 00:46:42,840
Snowfall on Mars is
nothing like snowfall on Earth.

674
00:46:42,960 --> 00:46:47,360
Every winter, between
three and four trillion tonnes

675
00:46:47,480 --> 00:46:50,960
of carbon dioxide freezes out
onto the surface.

676
00:46:51,80 --> 00:46:55,640
That's about 15% of the entire
Martian atmosphere.

677
00:46:55,760 --> 00:46:59,640
And then, in the springtime,
everything changes.

678
00:47:04,560 --> 00:47:06,920
As the sun returns
in the spring,

679
00:47:07,40 --> 00:47:08,960
the ground is warmed...

680
00:47:10,720 --> 00:47:13,00
..and the frozen
carbon dioxide vaporises

681
00:47:13,120 --> 00:47:14,600
in an instant,

682
00:47:14,720 --> 00:47:16,840
from solid to gas.

683
00:47:16,960 --> 00:47:22,00
Geysers of gas that lift dark
Martian dust high into the air.

684
00:47:28,360 --> 00:47:31,440
And it's this dust,
as it settles,

685
00:47:31,560 --> 00:47:34,520
that's causing
the fanlike spidery marks

686
00:47:34,640 --> 00:47:36,280
that we've seen from orbit.

687
00:47:48,640 --> 00:47:52,400
From Mars, it's just
a short hop to our own world...

688
00:48:01,360 --> 00:48:05,40
..and Earth too has permanent
ice caps at its poles.

689
00:48:12,640 --> 00:48:15,480
But there the similarity ends.

690
00:48:26,440 --> 00:48:29,480
If an alien astronomer got
a powerful telescope

691
00:48:29,600 --> 00:48:31,320
and pointed it
at our solar system,

692
00:48:31,440 --> 00:48:34,160
they would immediately see
there's something interesting

693
00:48:34,280 --> 00:48:37,480
and very rare about
the third planet from the sun,

694
00:48:37,600 --> 00:48:38,800
about Earth,

695
00:48:38,920 --> 00:48:40,720
because they'd see
a place like this,

696
00:48:40,840 --> 00:48:43,760
a place with...with mountains
covered in snow,

697
00:48:43,880 --> 00:48:47,200
and flowing rivers
and clouds and rain.

698
00:48:47,320 --> 00:48:51,360
It's a place where water exists
in all three of its phases -

699
00:48:51,480 --> 00:48:56,40
solid, liquid and gas -
at the same time.

700
00:48:56,160 --> 00:48:58,840
And that's extremely unusual.

701
00:48:58,960 --> 00:49:00,840
Let me show you what | mean.

702
00:49:00,960 --> 00:49:02,560
So I'm going to draw

703
00:49:02,680 --> 00:49:05,760
what's called a phase diagram
for water.

704
00:49:05,880 --> 00:49:08,520
It has pressure there

705
00:49:08,640 --> 00:49:11,120
and temperature along here.

706
00:49:12,320 --> 00:49:15,00
The Earth sits
at one atmosphere pressure,

707
00:49:15,120 --> 00:49:17,280
so atmospheric pressure...
there.

708
00:49:18,640 --> 00:49:22,800
And it sits at around zero
degrees Celsius, give or take.

709
00:49:24,320 --> 00:49:27,880
So the Earth exists somewhere
in this region here.

710
00:49:28,00 --> 00:49:29,560
I'm going to draw a line.

711
00:49:29,680 --> 00:49:31,440
I'll tell you what it is
after I've drawn it.

712
00:49:34,80 --> 00:49:36,120
So these two lines mark out

713
00:49:36,240 --> 00:49:38,480
the region of pressure
and temperature

714
00:49:38,600 --> 00:49:41,920
where water can be
either a solid,

715
00:49:42,40 --> 00:49:45,760
a liquid or a gas, or vapour.

716
00:49:45,880 --> 00:49:47,600
And the Earth is here.

717
00:49:47,720 --> 00:49:50,600
Little tiny range,
where you can have solid,

718
00:49:50,720 --> 00:49:53,400
liquid and vapour.

719
00:49:53,520 --> 00:49:58,800
Mars sits
somewhere around here.

720
00:49:58,920 --> 00:50:01,00
So that means that on Mars,

721
00:50:01,120 --> 00:50:03,400
water can either be frozen
as a solid,

722
00:50:03,520 --> 00:50:05,360
or it can be a vapour,

723
00:50:05,480 --> 00:50:07,320
but it can never be a liquid

724
00:50:07,440 --> 00:50:09,840
cos the atmospheric pressure
is too low.

725
00:50:09,960 --> 00:50:13,520
Pluto sits around here,
minus 230 degrees,

726
00:50:13,640 --> 00:50:16,240
where water can only be
a solid,

727
00:50:16,360 --> 00:50:17,760
frozen hard as steel,

728
00:50:17,880 --> 00:50:19,920
building the mountains
of Pluto.

729
00:50:20,40 --> 00:50:21,400
On the other hand,

730
00:50:21,520 --> 00:50:23,600
Uranus sits somewhere
over here,

731
00:50:23,720 --> 00:50:26,400
at, what, millions of times
atmospheric pressure

732
00:50:26,520 --> 00:50:28,440
and extremely high
temperatures.

733
00:50:28,560 --> 00:50:29,920
And there, we get

734
00:50:30,40 --> 00:50:33,440
these strange structures
of ice, the superionic ice.

735
00:50:33,560 --> 00:50:38,160
So Earth sits in a very narrow
range of temperature

736
00:50:38,280 --> 00:50:42,80
and pressure, where water can
exist in all three phases.

737
00:50:42,200 --> 00:50:44,440
And that's what makes the Earth
unique,

738
00:50:44,560 --> 00:50:46,160
certainly in our solar system,

739
00:50:46,280 --> 00:50:49,320
and perhaps for hundreds
or even thousands

740
00:50:49,440 --> 00:50:50,760
of light years beyond.

741
00:50:50,880 --> 00:50:54,640
It's this that allows
a complex ecosystem to exist

742
00:50:54,760 --> 00:50:56,840
on the surface of our planet.

743
00:51:04,240 --> 00:51:07,80
Earth's snow-covered mountains,

744
00:51:07,200 --> 00:51:09,360
great oceans...

745
00:51:11,120 --> 00:51:12,760
...f1Vers...

746
00:51:14,200 --> 00:51:18,160
..and storm clouds
can only exist together,

747
00:51:18,280 --> 00:51:20,840
thanks to the rare
and very narrow

748
00:51:20,960 --> 00:51:24,640
temperature and pressure range
that our planet enjoys.

749
00:51:34,440 --> 00:51:36,360
And that's surely necessary

750
00:51:36,480 --> 00:51:40,160
for complex life
to have emerged on just one

751
00:51:40,280 --> 00:51:42,840
of the solar system's
ice worlds.

752
00:51:46,760 --> 00:51:50,560
But there is one more twist
to our Story of ice -

753
00:51:50,680 --> 00:51:53,680
a strange property
of the everyday ice

754
00:51:53,800 --> 00:51:56,160
with which we are so familiar.

755
00:52:05,320 --> 00:52:08,80
Ice on Earth has
the unusual property

756
00:52:08,200 --> 00:52:10,240
that it floats
on its own liquid.

757
00:52:10,360 --> 00:52:12,600
It's due to that complicated
crystal structure

758
00:52:12,720 --> 00:52:14,680
with all those hydrogen bonds.

759
00:52:14,800 --> 00:52:17,400
Now, there are times
in Earth's history

760
00:52:17,520 --> 00:52:20,40
when the planet
almost froze solid.

761
00:52:20,160 --> 00:52:21,800
But because ice floats,

762
00:52:21,920 --> 00:52:24,320
there was always
a bit of liquid water

763
00:52:24,440 --> 00:52:25,840
at the base of the ocean,

764
00:52:25,960 --> 00:52:29,40
and life could cling on
in that liquid.

765
00:52:33,160 --> 00:52:36,520
That means that there has been
an unbroken chain of life

766
00:52:36,640 --> 00:52:41,160
for 3.8 billion years,
culminating in us.

767
00:52:41,280 --> 00:52:47,40
So next time you stick
a few ice cubes in your drink,

768
00:52:47,160 --> 00:52:48,960
just pause for a second

769
00:52:49,80 --> 00:52:52,800
and give a thought
to the wonder of ice.

770
00:53:25,440 --> 00:53:29,760
The three key ingredients
that you need for life are,

771
00:53:29,880 --> 00:53:32,280
number one, liquid water,

772
00:53:32,400 --> 00:53:34,480
two, a source of energy,

773
00:53:34,600 --> 00:53:37,960
and three, various chemical
elements

774
00:53:38,80 --> 00:53:39,640
that we associate with life.

775
00:53:39,760 --> 00:53:43,560
And we think that Europa has
all of these ingredients.

776
00:53:45,560 --> 00:53:48,560
PROF COX: Juno has deepened
our knowledge of Europa,

777
00:53:48,680 --> 00:53:52,360
but the mission is due to end
in 2025.

778
00:53:55,280 --> 00:53:58,640
While we are yet to find
any evidence of life on Europa,

779
00:53:58,760 --> 00:54:02,400
it's clear that this icy moon
is worth a closer look.

780
00:54:08,800 --> 00:54:11,520
Sam Howell is part
of a NASA team

781
00:54:11,640 --> 00:54:13,800
scoping a hugely ambitious
attempt

782
00:54:13,920 --> 00:54:17,360
to explore the moon
and its subsurface oceans

783
00:54:17,480 --> 00:54:20,960
in search
of that elusive proof.

784
00:54:22,80 --> 00:54:24,360
It's alla guess
until you go swimming in it,

785
00:54:24,480 --> 00:54:27,360
but, um,
we're building this picture up

786
00:54:27,480 --> 00:54:29,440
where we understand

787
00:54:29,560 --> 00:54:32,520
how saltwater and rock
interact on Earth,

788
00:54:32,640 --> 00:54:34,640
and the chemistry
that produces,

789
00:54:34,760 --> 00:54:38,200
which is likely important
to the emergence of life.

790
00:54:38,320 --> 00:54:40,80
How are we gonna prove that?

791
00:54:40,200 --> 00:54:42,680
We're launching
the Europa Clipper mission,

792
00:54:42,800 --> 00:54:45,760
and that will,
in the early 2030s,

793
00:54:45,880 --> 00:54:47,880
arrive at Jupiter
and orbit Jupiter,

794
00:54:48,00 --> 00:54:50,840
surveying the entirety
of the world.

795
00:54:53,40 --> 00:54:54,440
NEW SPEAKER: Europa Clipper is

796
00:54:54,560 --> 00:54:57,120
the first dedicated mission
to Europa.

797
00:54:57,240 --> 00:54:58,680
And, in fact, it's the first
dedicated mission

798
00:54:58,800 --> 00:55:00,80
to any icy moon.

799
00:55:02,440 --> 00:55:04,560
NEW SPEAKER:
Europa Clipper is set to launch

800
00:55:04,680 --> 00:55:07,440
from the Kennedy Space Center
in October 2024.

801
00:55:08,800 --> 00:55:11,440
DR QUICK: And we have a payload

802
00:55:11,560 --> 00:55:13,560
of ten instruments
that are going to work together

803
00:55:13,680 --> 00:55:16,560
to characterise
Europa's icy surface,

804
00:55:16,680 --> 00:55:19,400
its ocean,
what their compositions are,

805
00:55:19,520 --> 00:55:23,240
and to figure out
if Europa has environments

806
00:55:23,360 --> 00:55:24,760
that could support life.

807
00:55:28,800 --> 00:55:31,40
DR HOWETT: Any time
you're anywhere near Jupiter,

808
00:55:31,160 --> 00:55:32,240
it's really dangerous.

809
00:55:32,360 --> 00:55:35,80
These high-energy particles
that are zipping around

810
00:55:35,200 --> 00:55:36,960
can hit your spacecraft
and damage it.

811
00:55:37,80 --> 00:55:38,760
And that's
one of the big challenges

812
00:55:38,880 --> 00:55:40,640
facing Europa Clipper.

813
00:55:43,280 --> 00:55:44,920
PROF COx: And it's not
just the spacecraft

814
00:55:45,40 --> 00:55:47,320
that must survive
Jupiter's onslaught.

815
00:55:47,440 --> 00:55:50,200
DR HOWETT: We know
that this intense radiation,

816
00:55:50,320 --> 00:55:53,200
that's bad for life
as we know it, life as we are,

817
00:55:53,320 --> 00:55:55,720
but maybe life on Europa
doesn't mind it too much.

818
00:55:55,840 --> 00:55:59,240
But what's more likely is
that the thick layers of ice

819
00:55:59,360 --> 00:56:00,720
that are at Europa's surface

820
00:56:00,840 --> 00:56:03,320
shield life
in the subsurface ocean.

821
00:56:04,920 --> 00:56:07,840
DR QUICK: We think that the icy
crust is 18 miles thick,

822
00:56:07,960 --> 00:56:10,200
but Europa Clipper
will tell us more.

823
00:56:16,80 --> 00:56:17,360
DR HOWETT: I'm really excited

824
00:56:17,480 --> 00:56:19,200
about learning
about Europa's plumes.

825
00:56:19,320 --> 00:56:20,840
| think they could be the key

826
00:56:20,960 --> 00:56:22,680
to sampling
the subsurface ocean.

827
00:56:22,800 --> 00:56:25,840
We'll figure out if there are
traces or ideas of life

828
00:56:25,960 --> 00:56:27,640
in those plumes,
and that's something

829
00:56:27,760 --> 00:56:30,00
we're going to be able to do
with this amazing spacecraft.

830
00:56:32,80 --> 00:56:35,480
PROF COX: Clipper will only
ever survey Europa from afar,

831
00:56:35,600 --> 00:56:39,520
but future missions are being
developed that, one day,

832
00:56:39,640 --> 00:56:44,640
may land on the surface
and explore beneath the ice.

833
00:56:44,760 --> 00:56:47,680
There is no ice on this planet

834
00:56:47,800 --> 00:56:50,120
that behaves
like the surface of Europa.

835
00:56:50,240 --> 00:56:52,880
There, the ice is so thick
and so hard

836
00:56:53,00 --> 00:56:56,840
that the upper few miles are
like concrete or rock.

837
00:56:59,400 --> 00:57:03,640
What we really look at are ways
to pack enough heat

838
00:57:03,760 --> 00:57:05,600
into a cylindrical probe

839
00:57:05,720 --> 00:57:08,00
so that it can melt all the way
to that ocean,

840
00:57:08,120 --> 00:57:10,720
but can also carry along
the scientific payload with us

841
00:57:10,840 --> 00:57:12,640
that we want to use to explore.

842
00:57:13,760 --> 00:57:15,680
So is this recording video now?

843
00:57:15,800 --> 00:57:16,960
Yes, absolutely.

844
00:57:17,80 --> 00:57:18,640
Ah... Oh, yeah.

845
00:57:18,760 --> 00:57:20,520
| don't know if you've ever
seen yourself on camera before,

846
00:57:20,640 --> 00:57:23,40
- but there you go!
- (LAUGHS) Hello!

847
00:57:23,160 --> 00:57:25,480
We're just going to deploy it
there,

848
00:57:25,600 --> 00:57:26,920
right into the hole, and...

849
00:57:28,880 --> 00:57:30,280
...good luck.

850
00:57:30,400 --> 00:57:33,440
And then there we go,
we're down in the lake.

851
00:57:35,600 --> 00:57:38,520
We're looking around
the interface

852
00:57:38,640 --> 00:57:40,920
of the ice and water
just beneath us.

853
00:57:47,160 --> 00:57:49,280
DR WONG:
Finding life on Europa would be

854
00:57:49,400 --> 00:57:50,840
extraordinarily profound,

855
00:57:50,960 --> 00:57:52,920
because it's almost guaranteed

856
00:57:53,40 --> 00:57:54,960
that that would be
a separate instance

857
00:57:55,80 --> 00:57:57,200
of an origin of life.

858
00:57:57,320 --> 00:57:59,520
So Europa could teach us
a lot

859
00:57:59,640 --> 00:58:02,640
about how life begins
across the universe.