Ice Worlds
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00:00If I asked you what is a planet made of, then you'd probably say, well, rocks and iron.
00:30And for the planets of the inner solar system, like Earth, close to the heat of the sun, you'd be right.
00:42But if you head out into the frozen outer reaches of the solar system, then even the gases that make up our atmosphere, nitrogen, carbon dioxide, and of course water, are all frozen solid.
00:57And the planets and moons out there, the mountains and glaciers and even the crust of the worlds themselves, are made of that, solid, frozen, pristine ice.
01:13Ice that in the extreme conditions we find beyond Earth, behaves in ways we never imagined possible.
01:27As we've explored the solar system, our spacecraft have encountered moons torn apart by great canyons of shifting ice.
01:43Dwarf planets, where mountains of solid ice float across the surface.
01:49Worlds where ice appears to cover one face, but leaves the other entirely alone.
02:02And elsewhere, alien aurora hang above the clouds, all thanks to a strange, newly discovered form of ice.
02:19Even here, on Earth, the behaviour of something as simple as ice has had, we think, profound consequences.
02:29Because without ice's counterintuitive behaviour, life on our planet may not have survived.
02:48We begin our journey to the ice worlds, at the freezing edge of the solar system.
02:55We begin our journey to the ice worlds, at the freezing edge of the solar system.
03:26Out here, the Sun is so far away, it resembles just another star.
03:39Pluto is so remote, that it was only in July 2015 that we had our first, and to date only, close encounter.
03:55As it flew by, the New Horizons spacecraft sent back the first close-up images of this mysterious frozen world.
04:11It discovered a great heart-shaped plane, a thousand kilometres across, that dominates one face.
04:26Around the edge of this plane, mountains made of solid ice tower over Pluto's surface.
04:43And amongst its rugged uplands, ice was detected in a form no one ever expected to see on Pluto.
04:56Glaciers.
05:00Flowing rivers of ice on a world so far away, we expected nothing would be moving.
05:09That's because the temperature here is only 40 degrees Celsius or so, away from absolute zero.
05:17A temperature at which nothing should move.
05:21And yet New Horizons discovered regions of Pluto that, for all the world, look like the frozen reaches of our planet.
05:52So when you fly here over the years, how much does it change?
05:55It changes every day.
05:56Yeah?
05:57Yeah.
06:00The shape of the glaciers change.
06:03It's never the same.
06:08I'm just imagining flying over Pluto, actually.
06:12Because in light years, it's going to be the same.
06:15I'm just imagining flying over Pluto, actually.
06:19Because it looks remarkably similar.
06:22Really?
06:25What is the topography like?
06:27It's like this. Same topography, same mountain heights.
06:30That's incredible.
06:33The discovery of Pluto's dynamic, icy landscapes came as a huge shock.
06:40It forced us to rethink our understanding of this so-called dwarf planet.
06:47We're coming in for landing, so I'll just be talking to the aeroplane and not you.
06:51I respect that choice.
06:53Have a nice day.
06:55We're coming in for landing, so I'll just be talking to the aeroplane and not you.
06:59I respect that choice.
07:01I've had a lot of landings, but none quite like this.
07:06In fact, none like this at all.
07:09This is the world's most beautiful runway.
07:18Welcome to the glacier, you guys.
07:26Wow.
07:29You know, when we arrived, they said, oh, it's zero degrees.
07:33And I thought, that's great.
07:35Zero degrees, it's warm.
07:37Fahrenheit.
07:38Zero degrees Fahrenheit.
07:40It's about minus 20 up here.
07:42It's just...
07:43And you've got a real sense, actually, coming in.
07:46It looks frigid and frozen.
07:48Yeah.
07:49It's just...
07:50And you've got a real sense, actually, coming in.
07:53It looks frigid and frozen.
07:55Yeah, unmoving, unchanging.
07:57But it's so dynamic.
07:59You can feel it in the wind as you land.
08:02And then, you know, you can see.
08:04You can see the way that everything flows.
08:14Just look at that glacier.
08:16You can almost feel or see it moving.
08:20It looks like a slow-motion river.
08:23And indeed, it is moving.
08:25It does flow very, very slowly.
08:30And the reason this great mass can grind its way down the valley
08:34is because of the unique properties of the ice from which it's made.
08:43That bright blue ice certainly looks solid, immovable.
08:48It's formed by pressure.
08:50So these snowflakes are falling down onto the top of the glacier.
08:54And over time, they build up.
08:56Their weight presses down, increasing the pressure.
08:59And you get that particular crystalline structure of ice,
09:03which looks transparent and blue.
09:07But actually, at those pressures and temperatures,
09:09it's not completely solid.
09:11The crystals are sort of arranged in planes,
09:14a little bit like a deck of cards.
09:16And that means that as gravity is acting,
09:19trying to slide this whole thing down the valley,
09:22those planes can slip and slide over each other.
09:26And that allows the whole glacier to move.
09:29You can also get liquid water between the rock and the ice,
09:32and that sort of lubricates the glacier,
09:34and that allows it to slip as well.
09:36So although this looks fixed and immovable,
09:40at the conditions we find on Earth,
09:42this can almost behave like a fluid,
09:46sort of sliding very slowly and deforming down the valley.
10:01Glaciers were the last things we expected to see on Pluto.
10:05It's so cold here that we'd expect the ice crystals
10:09to be too brittle to flow.
10:13Nothing should slip.
10:15Nothing should slide.
10:22Yet that's precisely what these glaciers are doing.
10:30So if they can't be made of water ice,
10:33what are they made of?
10:43As New Horizons flew past Pluto,
10:46its detectors picked up an important clue.
10:51These are images of Pluto's surface,
10:53and the colours correspond to different molecules,
10:56different substances that New Horizons detected on the surface.
11:00The purple is methane.
11:03The yellow is nitrogen,
11:05and the green is carbon monoxide.
11:09All these gases are frozen solids.
11:13Now, the glaciers on Pluto are primarily made of nitrogen,
11:18solid nitrogen.
11:20The nitrogen is something that we're all familiar with.
11:22It's this stuff.
11:24Our air is pretty much made of nitrogen,
11:26so our familiar experience of it is just we can't see it.
11:30If we cool it down...
11:32..then we can get it pretty easily to turn into a liquid.
11:36Now, if we carried on cooling that down,
11:39it would turn into a solid.
11:43Nitrogen freezes at minus 210 degrees C,
11:47and Pluto's surface temperature at around minus 230 degrees
11:51ensures that glaciers remain solid.
11:55But, crucially,
11:57the nitrogen ice is just 20 degrees or so
12:00away from its melting point.
12:05That's very similar to the situation here on this glacier.
12:09The glacier is about, well, the air temperature today
12:12is about minus 10, minus 20 degrees Celsius,
12:15heated up by about 20 degrees, and it'll melt.
12:19So the temperature difference between the two glaciers
12:23and it'll melt.
12:25So the temperature difference
12:27between the solid nitrogen ice and the nitrogen gas
12:31and the solid water ice and the water is about the same.
12:37That means that with just a small rise in temperature,
12:41Pluto's nitrogen ice should be able to move.
12:48So the discovery of its glaciers
12:50tells us something remarkable about Pluto.
12:54This tiny world must have a little heat at its core,
12:58a faint warming from radioactive decay,
13:02just enough to gently melt the bottom of these rivers of ice,
13:07sending them on their way down the valley.
13:15I think, for me, there are two lessons from the exploration of Pluto.
13:19One is that geology finds a way,
13:22even so far away from the sun,
13:24where temperatures are only 40 degrees or so above absolute zero,
13:28pretty much the coldest it can be,
13:31there can still be active geology,
13:33particularly where something is close to its freezing point.
13:37In Pluto's case, nitrogen ice.
13:40The second lesson, I think, is perhaps even more profound,
13:44is that nature's imagination far exceeds our own.
13:48Nobody expected that they would see such a beautiful, active world
13:54so far away from the sun, on the far icy edge of the solar system.
14:08The similarities between Earth and Pluto are striking,
14:12but New Horizons discovered a wonderful difference.
14:19Pluto's glaciers flow through mountains
14:22reminiscent of Alaska's Great Ranges.
14:27But unlike Mount Denali's granite spires,
14:31Pluto's mountains are made from frozen water.
14:39You can't imagine something that big, that high, being made of water.
14:44That's the thing that amazes me. That's crazy.
14:47All water, no rock.
14:49Yeah, pretty much, yeah.
14:52And this leads to a surreal twist.
14:56Water ice in the mountains is less dense than the nitrogen ice in the glaciers.
15:02So in places, we've seen mountains floating on the glaciers,
15:07carried away like icebergs onto the vast ice plain below.
15:18Pluto, a world sculpted by ice.
15:32Leaving Pluto and heading back towards the sun's glow,
15:36we enter the realm of the ice giants.
15:43Vast gaseous worlds.
15:47Where ice storms rage.
15:54And on the innermost of these planets,
15:56we've discovered a phenomenon eerily reminiscent of home.
16:13Just above Uranus's ice clouds,
16:16hang beautiful, ethereal aurora.
16:24Found not at the poles of the planet, as on Earth,
16:28but scattered across its face.
16:36Even around the equator.
16:39So what's creating this beautiful, rare display?
16:54The basic physics of the aurora on Uranus
16:57is the same as the physics of the aurora on Earth.
17:00The sun's constantly emitting a rain of high-energy charges
17:05constantly emitting a rain of high-energy charged particles,
17:09which is called the solar wind.
17:11When those charged particles reach the Earth,
17:13most of them are deflected around the Earth,
17:16harmlessly off into space by our magnetic field.
17:20Now, the Earth's magnetic field looks very much
17:23like the field around a bar magnet.
17:26I'll show you that by sprinkling some iron filings
17:31around the bar magnet.
17:36And the iron filings line up with the magnetic field lines.
17:40When the solar wind hits this magnetic field,
17:43most regions of the Earth is deflected harmlessly off into space.
17:47But at the poles, those charged particles can become trapped,
17:51and then they can be accelerated down into the upper atmosphere
17:55and hit molecules in the atmosphere, oxygen and nitrogen,
17:59and that can cause those molecules to emit light, to glow.
18:03And that's what, if you're lucky,
18:05you see as the northern and southern lights.
18:11So somewhere deep inside our planet
18:14lies the equivalent of that bar magnet.
18:19And, of course, it does, in the form of a hot molten iron core
18:23spinning away as the Earth rotates...
18:28..creating electrical currents
18:30and the magnetic field that projects out into space.
18:36But Uranus is different.
18:38The aurora are not found at the poles,
18:41and we don't think it has a molten iron or metallic core
18:46to support those electrical currents.
18:49And so the fact that Uranus does have aurora,
18:52and therefore some kind of magnetic field,
18:56is a tremendous mystery.
19:01But we do have theories that allow us to piece together
19:05what might be going on.
19:11Imagine diving beyond the clouds of Uranus,
19:14beyond the slushy ice layer that flows around the planet...
19:22..and keep going towards the core.
19:26We enter a region where the pressure
19:28approaches several million times that of Earth's atmosphere...
19:33..and where it's almost as hot as the sun's surface.
19:41Rather than molten rock or metal, like we find inside our own planet,
19:46we instead find yet more frozen water...
19:51..in a bizarre form of matter.
19:55It's water known as superionic ice.
20:03Normal water ice has a crystal structure like this.
20:07So the reds are oxygen atoms, the whites are hydrogens,
20:12and you can see that they're bonded together
20:14into this regular crystal lattice.
20:18Within the lattice,
20:20nothing that could carry an electrical current can flow,
20:23so a magnetic field can't be created.
20:28This is the crystal structure of superionic ice.
20:34The oxygens are still there, bonded together into a crystal,
20:38but now there are hydrogen nuclei,
20:41electrically charged protons,
20:43that can move freely through the crystal lattice.
20:47That means that this is an electrical conductor.
20:53It's this movement of the protons
20:56that could be contributing to Uranus's magnetic field.
21:03If so, then the superionic ice is, at least in part,
21:08driving the planet's mysterious aurora.
21:14Now, this story is still far from fully understood.
21:19For a long time, this strange form of ice
21:22was only a theory.
21:23But then a team pointed one of the world's most powerful lasers
21:27at a droplet of water
21:29and recreated the conditions
21:31that are present deep down inside Uranus
21:34and, just for a moment,
21:36caught a glimpse of superionic ice.
21:43Uranus.
21:45A world illuminated by ice.
21:53On the journey between ice worlds,
21:56we edge ever closer towards the sun
21:59for an encounter with one of the most thoroughly explored
22:03planetary systems of them all.
22:22Saturn's rings are constructed of countless crystals,
22:26ranging in size from just a few microns to vast boulders.
22:33And all of them made almost entirely from frozen water.
22:45The rings are joined in their orbits by at least 146 moons.
22:53And out towards the edge of the system,
22:56NASA's Cassini probe made one of its most surprising discoveries.
23:13The moon Iapetus resembles a walnut
23:16with a mountain ridge around its middle.
23:20But that's not its strangest feature.
23:27Back in the 17th century, only about 60 years or so,
23:30actually, after the invention of the telescope,
23:33Giovanni Cassini discovered Iapetus.
23:36But he immediately noticed something strange about the moon
23:39as he watched it orbit the planet
23:41because he could see the moon on one side of the planet,
23:44but then on the other side, he couldn't.
23:47Now, being sensible, as a scientist, after all,
23:50he said, well, it's not somehow disappearing.
23:53There must be another explanation.
23:55And he guessed that one side of the moon must be very bright
23:59and the other side must be very dark.
24:02And 300 years later,
24:04we sent a spacecraft to Saturn bearing his name.
24:10And we discovered that he was right.
24:15Cassini sent back proof that one side of Iapetus is icy white
24:20whilst the other looks as if it's been painted black.
24:29So what could be creating such a sharply defined monochromatic world?
24:36Tremendous mystery.
24:38But a clue could be found
24:40in looking at the line between the two hemispheres
24:43because there are jet black regions on the surface there
24:47that are also some of the hottest places in the Saturnian system.
24:54Hot is relative, of course.
24:56It's still minus 140 degrees Celsius on the dark side of the moon.
25:02But that's about 20 degrees warmer than the moon's icy face.
25:13And we think that this difference is just enough
25:16to move ice around the moon in a very particular way.
25:26The sunlight falls on that dark surface
25:29as Iapetus rather languidly rotates, actually,
25:33about once every 79 Earth days, and it heats it up.
25:37The water molecules rise up,
25:40surface the light side, and then condense and fall onto the surface,
25:45making it brighter and brighter and brighter.
25:48It's not like what's happening here.
25:50So out there in the Pacific Ocean,
25:52the water is turning into water vapour,
25:55drifting over the cold land and falling as snow,
25:59making the whole surface bright.
26:10On the dark side of Iapetus,
26:13ice is warmed and creates a thin atmosphere of water vapour.
26:25And where this vapour meets the colder, white side of the moon,
26:29it freezes to the surface again, resembling fresh snow.
26:34Maintaining the bright icy white of this hemisphere.
26:41But a mystery remains, because Iapetus is an ice moon.
26:48So what is the dark material covering its other face?
26:54In 2009, the Spitzer Infrared Space Telescope discovered this.
26:59This is another ring around Saturn, but it's enormous.
27:03It's one of the largest structures in the solar system.
27:06This is about 12 million kilometres across.
27:16Later observations showed that this ring,
27:19and its more familiar icy rings, are barely visible.
27:28It might seem strange that no-one had seen
27:31such a large, icy ring before.
27:34But it's not the first time that we've seen it.
27:37It's the first time that we've seen it.
27:40It's the first time that we've seen it.
27:43It's the first time that we've seen it.
27:47It might seem strange that no-one had seen
27:49one of the largest structures in the solar system until 2009.
27:53The reason is that that ring is very dark and very diffuse.
27:57If you're transported into the ring,
28:00you can look around and you wouldn't know you were in it.
28:03Spitzer saw it, because Spitzer's an infrared telescope,
28:07and so it detected not visible light, but infrared light,
28:11the glow, the heat emanating from the ring.
28:17The giant outer ring is therefore very different
28:20to Saturn's ice rings.
28:23So what is it made of, and where did it come from?
28:36Phoebe is another of Saturn's outer moons,
28:39and each time a passing asteroid gets too close...
28:46..the resulting impact throws dark material out into space.
28:57Over billions of years, numerous impacts have resulted
29:01in the dust from Phoebe spreading around Saturn,
29:05forming its vast, dark ring.
29:09Iapetus passes through the ring as it orbits,
29:14and so that dark material from the ring
29:17gets deposited on the surface of Iapetus.
29:21This is a really slow process.
29:24Material falls onto Iapetus and increases the size of that dark layer
29:29by about four hundredths of a millimetre every million years.
29:34It's not a bad analogy, this, actually.
29:37Some of those dust particles in the ring are about this size,
29:41about the size of pepper grains.
29:43Some are bigger, a few centimetres across or something,
29:46but it is pretty much stuff like this.
29:51And yet a puzzle remains.
29:54Why half black and half white?
30:00Iapetus spins on its axis once every 79 days,
30:05and orbits around Saturn once every 79 days.
30:09It's what's called spin-orbit-locked.
30:11It's like our moon.
30:13So it always leads with one hemisphere
30:16as it orbits around Saturn and passes through the ring.
30:24Iapetus, then, is a fluke of nature
30:27that exists thanks to the interaction of two moons
30:31within a dark ring right at the edge of Saturn's domain.
30:38A world painted by ice.
30:56As we return ever closer to the sun,
30:59ice becomes increasingly rare.
31:08Jupiter has 95 known moons...
31:15..including three large ice worlds.
31:20And one of these is a promising target
31:23in our search for life beyond Earth.
31:41In 2022, NASA's Juno space probe,
31:46In 2022, NASA's Juno spacecraft flew by Europa
31:52and photographed a world crisscrossed with mysterious red lines.
32:01Grand canyons.
32:05Some 100 metres deep and tens of kilometres wide.
32:11In places coated in a red substance
32:14that may be a newly discovered compound of salt and water.
32:29Juno is the latest NASA mission to fly by Europa
32:33and take detailed photographs of its peculiar fractured surface.
32:44The canyons on Europa are quite unlike anything seen on Earth.
32:50Or, indeed, anywhere in the entire solar system.
32:55The markings are geometric.
32:57They form lines that crisscross over the surface.
33:01So, what could be causing that?
33:06Europa's surface features are an active area of research,
33:09taking NASA scientists to the frozen reaches of our own planet
33:13in search of answers.
33:16This is an image of a region on Europa's surface called Phaedra Linea.
33:20And see this feature?
33:23It almost looks like the Grand Canyon on Earth.
33:26It's actually about 50 kilometres across.
33:29A clue to what this is can be seen...
33:32If you look at the top line and the bottom line,
33:35and just in your mind's eye just draw these together,
33:39you'll see that they knit together perfectly.
33:43So this looks like the crust has just spread.
33:46Now, there's only one other place in the solar system
33:49where we see features like this, and it's here, on Earth.
33:53It's caused by plate tectonics.
33:58On Earth, it's the internal heat of the planet
34:01as it forces its way through the crust,
34:04which is the driving force of plate tectonics.
34:10No-one expected to see behaviour like this on a moon.
34:24On Europa, it's not molten rock that's driving its plates apart.
34:32Density measurements of the moon
34:35suggest that beneath the thick, icy crust
34:39lies a different liquid.
34:41A global subsurface ocean of water.
34:45Up to 150 kilometres deep,
34:48it may contain two or three times all the water
34:51in Earth's oceans combined.
34:56So how can all that liquid water exist
34:58just below the surface of this frigid ice moon?
35:05The answer lies with two other moons of Jupiter,
35:11with Io and Ganymede.
35:13So here's Jupiter,
35:16and then Io goes around four times,
35:23as Europa goes around two times,
35:27and Ganymede, farthest out,
35:30goes around once.
35:32It's called an orbital resonance.
35:35Four orbits to two orbits to one orbit.
35:38That means that these three moons line up periodically
35:43and give each other gravitational kick,
35:46which means that the orbits don't stay as nice circles.
35:50They're all ellipses.
35:52And that means that tidal effects,
35:55just like the tides here on Earth,
35:57stretch and squash the moons and heat them up.
36:00Now, the effect is strongest for Io
36:03because that's closest to the giant planet,
36:05and so that turns Io into essentially one giant volcano.
36:09For Europa, further out, that heat melts the ice.
36:15But the energy that goes into Europa
36:18from this eccentric elliptical orbit around Jupiter
36:22sort of trickles into Io,
36:25sort of trickles into the moon.
36:28So it really isn't enough on its own
36:31to produce the very active geology that we see on the surface.
36:40We estimate the surface ice on Europa
36:43is somewhere between 10 and 25 kilometres thick.
36:48So whilst the tidal forces are enough
36:51for the subsurface ocean to remain liquid,
36:54they're not enough to split apart all this ice.
36:59So to drive the high-energy geological processes
37:03we see on the surface of Europa,
37:05then there must be some kind of energy storage in the moon itself.
37:10So I've got two camping stoves here.
37:13These two pans are filled with water
37:15and it's at the same temperature, zero degrees.
37:18The only difference is that this water has ice in it
37:22and this has no ice in it.
37:24The only difference when we start the stoves.
37:31This is a thermal camera here
37:33because, you know, I wouldn't travel without one.
37:36So it will tell us that the temperature of the water is right
37:40and it will tell us that the temperature of the water is rising,
37:44eight, nine degrees already,
37:47whereas this one is still zero degrees,
37:51even though we're putting all the energy into it.
37:54Now look.
37:56You see that?
37:58So why?
38:00Well, this is a model of ice.
38:03You can see the water molecules here and here and here
38:07and they're bonded together by these longer bonds,
38:10which are called hydrogen bonds.
38:12They're the thing that hold the crystal lattice in place
38:15and they're pretty strong.
38:17So to melt the ice, you've got to break all these bonds.
38:21You've got to put a lot of energy into it.
38:24So all the energy from this camping stove at the moment
38:26is going into breaking bonds in the ice.
38:29It's not going into making all the molecules move around faster,
38:32which is what temperature is.
38:35So this one's getting hotter and hotter and hotter.
38:37Nothing is happening to this one.
38:39Now these have been cooking away now and I'll show you.
38:42I have confidence.
38:44I have confidence in physics.
38:46I believe in it.
38:48I would not put my hand in there.
38:50I can see it would be a stupid idea.
38:52But there, there you go.
38:54Physics works.
38:56It's actually freezing.
38:59Now reverse that idea.
39:02Reverse that argument.
39:04What happens then when I freeze water,
39:07when I turn it from a liquid to a solid?
39:10I get all that energy back out again.
39:13Huge amounts of energy as the bonds form.
39:17And this is what we think may be happening on Europa.
39:22The subsurface ocean is warmed by tidal forces
39:25from Jupiter and its moons,
39:28storing energy.
39:37Then, thanks to its elliptical orbit,
39:40the tidal forces of Jupiter and its moons
39:43start to pull the water out of the ocean.
39:48Then, thanks to its elliptical orbit,
39:51as Europa periodically cools,
39:54the ice begins to freeze,
40:00releasing the stored energy.
40:07The volume of the icy crust grows as it freezes,
40:12increasing the pressure
40:23until the entire canyon is cleaved apart.
40:35And briny water from the ocean below
40:39surges up through the cracks,
40:44where, bathed in Jupiter's intense radiation,
40:48it turns red.
40:59You're actually very familiar with this process.
41:02If your pipes burst in your house because they freeze,
41:05where does the energy come from to burst the pipes?
41:08It comes from water freezing into ice.
41:18Europa is far more dynamic than we'd imagined.
41:28And it's this dynamism that makes it a tantalising target.
41:35In our search for life under the ice.
41:42At its simplest, life needs three things.
41:46Water, energy and the right chemical ingredients.
41:53Europa has the first two in abundance,
41:56but the chemistry for life is missing.
42:01But fortunately, Europa is not alone.
42:12Orbiting close by, Io has the missing ingredients
42:16we believe necessary for life in abundance.
42:22Erupting into space in enormous quantities around Jupiter.
42:31Here's where the story gets even more wonderful.
42:35Because the volcanoes of Io
42:38are constantly producing chemicals, materials
42:41that rain down onto the frozen surface of Europa.
42:44But if it wasn't for the geology,
42:46then they'd be separated forever from the ocean below
42:49by 10 or 20 kilometres of ice.
42:53But that active geology,
42:55creating the plate tectonic-like behaviour,
42:58can bring those materials, those chemicals, into the ocean.
43:02And then we have all the conditions we think are necessary
43:07for the origin of life.
43:13So Europa's dynamic surface
43:15may form part of an extraordinary ecosystem.
43:21One that stretches from one moon to another.
43:25And work is already under way
43:27to send robotic probes into that distant icy ocean.
43:37It would be a profound discovery to find life on Europa.
43:41But it would also be profound if we didn't.
43:44Because everything we think we know about the origin of life,
43:48all the ingredients that are necessary,
43:51seem to be present on Europa.
43:53So if we go there and send a cryobot into the oceans of Europa
43:58and find nothing at all,
44:00then it may be far more likely
44:03that we are alone for millions or even billions of light-years
44:09in every direction.
44:23Europa, a world completely encased in ice,
44:27couldn't exist much closer to the sun.
44:31Because just a little closer in lies the solar system's ice line.
44:39Cross it and temperatures become too warm
44:42for ice to stay frozen for long.
44:46When comets fall inwards towards the sun,
44:49some of the ice they carry is transformed into water vapor.
45:03Forming tails that streak through space for hundreds of kilometres.
45:09Inside the ice line, then, ice is rare.
45:14But there are places where it can hold on at the margins.
45:28Most of the ice in the world is frozen.
45:31But there are places where it can hold on.
45:35Most of the ice on the surface of Mars is held at poles.
45:47Here, NASA's Mars Reconnaissance Orbiter
45:50has captured these extraordinary images
45:53of a strange phenomenon that takes place on the southern ice cap.
45:58Dark spider-like formations
46:01that we think are being formed as the seasons turn.
46:07During the winter, it gets so cold on Mars
46:11that the carbon dioxide in its thin atmosphere freezes...
46:20..creating crystals of dry ice that fall as snow.
46:28On the pole.
46:33Snowfall on Mars is nothing like snowfall on Earth.
46:37Every winter, between three and four trillion tonnes
46:42of carbon dioxide freezes out onto the surface.
46:45That's about 15% of the entire Martian atmosphere.
46:50And then, in the springtime, everything changes.
46:59As the sun returns in the spring, the ground is warmed...
47:05..and the frozen carbon dioxide vaporises in an instant,
47:09from solid to gas.
47:11Geysers of gas that lift dark Martian dust high into the air.
47:23And it's this dust, as it settles,
47:26that's causing the fan-like spidery marks that we've seen from orbit.
47:43From Mars, it's just a short hop to our own world.
47:48And Earth, too, has permanent ice caps at its poles.
48:01But there, the similarity ends.
48:18If an alien astronomer got a powerful telescope
48:21and pointed it at our solar system,
48:23they would immediately see there's something interesting
48:26and very rare about the third planet from the sun, about Earth.
48:31Because they'd see a place like this,
48:33a place with mountains covered in snow
48:36and flowing rivers and clouds and rain.
48:40It's a place where water exists in all three of its phases,
48:45both solid, liquid and gas, at the same time.
48:49And that's extremely unusual.
48:52Let me show you what I mean.
48:54So I'm going to draw what's called a phase diagram for water.
48:59It has pressure there and temperature along here.
49:05The Earth sits at one atmosphere pressure,
49:08so atmospheric pressure, there.
49:12And it sits at around zero degrees Celsius, give or take.
49:18So the Earth exists somewhere in this region here.
49:22I'm going to draw a line.
49:24I'll tell you what it is after I've drawn it.
49:28So these two lines mark out the region of pressure and temperature
49:32where water can be either a solid, a liquid or a gas or a vapour.
49:39And the Earth is here, a little tiny range
49:43where you can have solid, liquid and vapour.
49:47Mars sits somewhere around here.
49:53So that means that on Mars, water can either be frozen as a solid
49:57or it can be a vapour.
49:59But it can never be a liquid because the atmospheric pressure is too low.
50:03Pluto sits around here, minus 230 degrees,
50:07where water can only be a solid, frozen, hard as steel,
50:11building the mountains of Pluto.
50:14On the other hand, Uranus sits somewhere over here
50:17at, what, millions of times atmospheric pressure
50:20and extremely high temperatures.
50:22And there we get these strange structures of ice, the supraionic ice.
50:27So Earth sits in a very narrow range of temperature and pressure
50:33where water can exist in all three phases.
50:36And that's what makes the Earth unique, certainly in our solar system.
50:40And perhaps for hundreds or even thousands of light years beyond,
50:45it's this that allows a complex ecosystem to exist
50:49on the surface of our planet.
50:58Earth's snow-covered mountains,
51:02great oceans,
51:05rivers
51:08and storm clouds
51:11can only exist together thanks to the rare and very narrow
51:15temperature and pressure range that our planet enjoys.
51:28And that's surely necessary for complex life to have emerged
51:33on just one of the solar system's ice worlds.
51:41But there is one more twist to our story of ice,
51:45a strange property of the everyday ice with which we are so familiar.
51:51Ice on Earth has the unusual property that it floats on its own liquid.
51:56It's due to that complicated crystal structure with all those hydrogen bonds.
52:01Now, there are times in Earth's history when the planet almost froze solid.
52:06But because ice floats, there was always a bit of liquid water
52:10at the base of the ocean.
52:12And that liquid water was the ice.
52:15And because ice floats, there was always a bit of liquid water
52:19at the base of the ocean.
52:21And life could cling on in that liquid.
52:28That means that there has been an unbroken chain of life
52:31for 3.8 billion years, culminating in us.
52:36So, next time you stick a few ice cubes in your drink,
52:42pause for a second and give a thought to the wonder of ice.
53:13The three key ingredients that you need for life are,
53:19number one, liquid water,
53:22two, a source of energy,
53:24and three, various chemical elements that we associate with life.
53:29And we think that Europa has all of these ingredients.
53:35Juno has deepened our knowledge of Europa.
53:39But the mission is due to end in 2025.
53:45While we are yet to find any evidence of life on Europa,
53:49it's clear that this icy moon is worth a closer look.
54:00Sam Howell is part of a NASA team scoping a hugely ambitious attempt
54:05to explore the moon and its subsurface oceans
54:09in search of that elusive proof.
54:13It's all a guess until you go swimming in it,
54:15but we're building this picture up
54:19where we understand how saltwater and rock interact on Earth
54:24and the chemistry that produces,
54:26which is likely important to the emergence of life.
54:30How are we going to prove that?
54:32We're launching the Europa Clipper mission,
54:34and that will, in the early 2030s,
54:37arrive at Jupiter and orbit Jupiter
54:39surveying the entirety of the world.
54:45Europa Clipper is the first dedicated mission to Europa.
54:48And in fact, it's the first dedicated mission to any icy moon.
54:54Europa Clipper is set to launch from the Kennedy Space Center
54:58in October 2024.
55:00And we have a payload of 10 instruments
55:03that are going to work together
55:05to characterize Europa's icy surface, its ocean,
55:09what their compositions are,
55:11and to figure out if Europa has environments
55:14that could support life.
55:20Anytime you're anywhere near Jupiter, it's really dangerous.
55:23These high-energy particles that are zipping around
55:26can hit your spacecraft and damage it.
55:28And that's one of the big challenges facing Europa Clipper.
55:35And it's not just the spacecraft that must survive Jupiter's onslaught.
55:39We know that this intense radiation,
55:41that's bad for life as we know it, life as we are.
55:44But maybe life on Europa doesn't mind it too much.
55:47But what's more likely is that the thick layers of ice
55:50that are at Europa's surface shield life in space.
55:55We think that the icy crust is 18 miles thick,
55:58but Europa Clipper will tell us more.
56:06I'm really excited about learning about Europa's plumes.
56:09I think they could be the key to sampling the subsurface ocean
56:13and figure out if there are traces or ideas of life in those plumes.
56:17And that's something we're going to be able to do
56:19with this amazing spacecraft.
56:25Clipper will only ever survey Europa from afar.
56:29But future missions are being developed
56:32that one day may land on the surface and explore beneath the ice.
56:38There is no ice on this planet that behaves like the surface of Europa.
56:44There, the ice is so thick and so hard
56:47that the upper few miles are like concrete or rock.
56:52What we really look at are ways to pack enough heat
56:56into a cylindrical probe so that it can melt all the way to that ocean,
57:01but also carry along the scientific payload with us
57:04that we want to use to explore.
57:06So is this recording video now?
57:09Yes, absolutely.
57:11I don't know if you've ever seen yourself on camera before,
57:14but there you go.
57:16We're just going to deploy it there right into the hole
57:20and pluck.
57:23And then there we go.
57:25We're down in the lake.
57:28We're looking around the interface of the ice and water just beneath us.
57:40Finding life on Europa would be extraordinarily profound
57:44because it's almost guaranteed that that would be a separate instance
57:48of an origin of life.
57:51So Europa could teach us a lot about how life begins across the universe.
58:00Next time...
58:05..a powerful force creating oddball worlds...
58:11..of bizarre shapes...
58:13..and hidden secrets.
58:16These are the solar system's strange worlds.
58:46NASA Jet Propulsion Laboratory, California Institute of Technology