Strange Worlds
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00:00We all know the familiar faces of our solar system.
00:17The worlds we grew up with.
00:21But there's another side to our solar system we're now discovering.
00:29The misfits and oddballs.
00:36Worlds of freakish shape and size.
00:43Of extreme landscapes.
00:49Mysterious phenomena.
00:56And hidden secrets.
01:04Our neighbourhood is far stranger than we ever imagined.
01:11So how did all these weird worlds come about?
01:19Well to answer that question, we'll have to explore the force that sculpted and created
01:24them, gravity, and the forces that resist its relentless inward pull.
01:32And also, at a deeper level, because there's always a deeper level, we'll be forced to
01:37contemplate why there is anything of complexity and beauty in our universe at all.
01:45Welcome to the solar system of the weird.
02:16From a cloud of gas and dust, gravity, the great sculptor of our universe, fashioned
02:24our star and all the worlds and moons around it.
02:35Shaping the solar system.
02:49And gravity has continued to shape these myriad worlds ever since.
03:03So I'm going to give you a little 30 second lecture on gravity.
03:09And I'm going to use Newton's picture, not Einstein's, because we don't need the additional
03:13accuracy delivered by relativity.
03:17Gravity is a force of attraction between objects, and it only attracts.
03:22So that means that it tends to clump things together.
03:26And it's a force that only depends on the distance between objects, not the angle.
03:32And so it tends to make spheres.
03:39It's this property of gravity that shaped the moons and planets.
03:55But beyond the near perfect spheres that dominate our solar system.
04:05Out past the giant orbs of gas and ice.
04:16In a distant realm of the solar system.
04:21We found something strange.
04:40Only one craft has been sent to explore the worlds of this distant region.
04:50And on its epic ongoing journey, the probe caught a glimpse of something truly bizarre.
05:05Moving in the dark.
05:14Not a sphere like Earth or even Pluto, but a giant 2,000 kilometre long egg shaped world.
05:33Orbiting around it, two glittering moons.
05:38Mountains, an icy rock, and a faint ring.
05:45If you were standing on Haumea's surface, the stars would wheel above you.
05:51Six times faster than here on Earth.
05:57Haumea is a truly unexpected and bizarre shaped object.
06:05The first like it ever discovered.
06:17Leaving the question.
06:21What created such a seemingly gravity defying world?
06:32Now for rocky worlds, the force resisting the inward pull of gravity is created by this rigidity of the rock.
06:43And the thing about pressure is that it acts equally outwards in all directions.
06:50So if you have a force that's squashing everything inwards equally in all directions,
06:54and a force that's resisting that squashing equally in all directions,
06:59then the shape that's naturally produced is a sphere.
07:04You might say, well, why is something like that not a sphere then?
07:08I mean, it's made of rock, it's got a gravitational pull,
07:11but it's a very weak gravitational pull because it's not very massive.
07:15And that's the point.
07:17So the gravitational forces on the surface here, trying to squash it down,
07:22are nowhere near big enough to overcome the strength of the rock.
07:27So how big does a thing have to be such that the gravitational force is strong enough
07:35to overcome the strength of the rock and allow it to deform into a sphere?
07:40And you find, if you wave your hands around a bit, that that size, the radius,
07:46is something like 200, 300 kilometres.
07:51It's called the potato radius.
07:53And indeed, you find that if you look out into the solar system,
07:56anything that's smaller than about a couple of hundred kilometres in radius looks like that.
08:03And anything that's bigger than a couple of hundred kilometres in radius looks like the Earth.
08:13From what we observe, it seems that the potato radius is a pretty strictly followed rule.
08:20The larger worlds are, the more spherical they become.
08:27Yet it's a rule how Maya breaks.
08:39Way over the potato radius, there's a new world.
08:45Way over the potato radius, Haumea should be a round world.
08:54So if its egg shape is not down to its size, then what is it?
09:01There is a clue, found by looking at our world in a slightly unusual way.
09:09This is a photograph of us working on the beach today.
09:14I use the term loosely.
09:16And what we did is we took a time lapse, but it's an interesting time lapse.
09:21We used an astronomical mount.
09:23And so we fixed the camera at a single point in the sky, the sun.
09:29And then, you see what happens.
09:32So it's holding its position.
09:34It doesn't look right because the whole ground is rotating around.
09:41So usually our experience on the surface of the Earth is watching the sky and the sun and the moon and the stars rotate around us.
09:49But if you take that motion out, then what you're seeing here is the Earth rotating beneath the sky.
10:05This unusual view really brings home the fact that we live on a spinning ball of rock.
10:20And there are consequences for sitting on the surface of something that's spinning.
10:26New forces are introduced, forces that are so-called fictitious forces.
10:32But there's nothing fictitious about them, actually.
10:35You'll know that if you try to hang on to a spinning roundabout.
10:38If you let go, you go flying off.
10:40That's not a fiction.
10:42And that force is called the centrifugal force.
10:48Like the Earth, all worlds in the solar system spin.
11:01But Haumea is spinning incredibly quickly.
11:10The entire 2,000 kilometre long world whips around once every four hours.
11:32And that makes the centrifugal force very powerful indeed.
11:37And I can show you by taking a small thing, let's say that's Haumea, and spinning it really fast.
11:51So you see what's happening is it was a sphere and now it's bulging out.
11:59And it's bulging out along its equator.
12:01Look at that.
12:02That's because the centrifugal force tends to flatten things.
12:16You see that?
12:17I mean, there it is, right?
12:18Those are fictitious forces at work.
12:21And that's essentially actually what happened to some bits of Haumea, we think.
12:26We think it was spinning so fast that some bits got thrown off.
12:32Right, there it is.
12:33So what you just saw there was a demonstration of how we think this system was created.
12:37This is the best photo we have of that system.
12:40And these bits are essentially that bit that's now over there somewhere.
12:50There we are.
12:51See, look.
12:52Haumea.
12:56Haumea.
13:02The battle between spin and gravity has created a truly strange world.
13:12Gravity shapes everything in the solar system.
13:15And our next destination has the scars to prove it.
13:20Let the pole from our star draw us inwards.
13:26Past Neptune.
13:29Until we reach the innermost ice giant.
13:44Uranus is pretty odd to begin with.
13:48The entire planet is knocked over on its side,
13:52likely by a giant impact in the past.
13:58But it's not only the planet that's strange.
14:10Voyager 2 is the only spacecraft to have visited the moon Miranda.
14:18As it flew past the South Pole,
14:23its cameras saw a truly weird patchwork landscape.
14:35A jumble of towering mountains the height of Everest
14:40and plunging chasms deeper than the Grand Canyon.
14:48One of the most astonishing surfaces in all the solar system.
14:57Where strange cliffs rise to unimaginable heights.
15:06Unlike anything seen on Earth.
15:18So what created the truly bizarre face of Miranda?
15:30The geology of our world is awe-inspiring,
15:33even though we're really familiar with it.
15:35I mean, this island rises two and a half kilometres
15:39from the surface of the Atlantic Ocean.
15:41But just imagine what it would be like
15:43standing on the surface of Miranda.
15:45I mean, there's a slope, not unlike this,
15:48that stretches for something like 10,000 metres.
15:53I remember when Voyager 2 arrived at Miranda in 1986
15:58and sent back images like this.
16:01Well, that slope is up here.
16:03But one of the scientists at the time said that this world is exotic
16:07and you can see why.
16:08One of the explanations for why it's like this
16:11was that it must have been hit by something
16:13and then reassembled.
16:15It's like a Frankenstein world.
16:17But we now know that the explanation for this strange geology
16:23is, if anything, even more exotic.
16:37We're pretty sure that Miranda must receive the occasional impact.
16:43The result would look like it was playing out in slow motion.
16:51Debris taking the best part of ten minutes
16:54to slowly tumble to the bottom of those great slopes.
17:03On Earth, it would take only 50 seconds to fall the same distance.
17:13Because on this moon, smaller than the width of the UK,
17:18the pull of gravity is much weaker.
17:23One hundredth of the strength on our world.
17:28Now, the basic explanation for Miranda's strange surface
17:32really is just basic physics.
17:34Miranda's very small.
17:36It's only about 470km in diameter,
17:38not too far away from the potato radius.
17:41And so its gravity is just not quite strong enough
17:44to squash it down into a sphere.
17:47But there's more to the geology, to the surface of a world,
17:51than just basic physical principles.
17:54There's also the history of the world.
17:59Miranda's weak gravity is what makes this landscape possible.
18:04But it's not alone responsible for sculpting it.
18:12Something must have happened to Miranda
18:15to create its battered and scarred surface.
18:21All we have to go on are the glimpses of this world
18:24captured as Voyager 2 flew by...
18:31..which suggest this moon had a troubled past.
18:42The key to unlocking the mystery of Miranda
18:45is to notice that this surface is not as chaotic as it looks.
18:50It's not entirely random.
18:52There are these three distinct regions, which are known as corona.
18:57And at least on these two external regions,
19:00there are ridges, fault lines, that surround them.
19:04And to a geologist, that's a smoking gun.
19:07What it suggests is that this surface was not created by external forces,
19:12by impacts from the outside.
19:14It was created from within.
19:17And it's similar to this landscape here.
19:20This is new land. These are volcanoes.
19:23They were created by a hot spot deep underneath the surface of the Earth
19:27and by buoyant hot material
19:29rising up through the surface of the Atlantic Ocean.
19:33And we think that's what's happened here.
19:36Buoyant, less dense material rising to the surface,
19:40creating these features.
19:50It's thought that it was this internal turmoil
19:53that left ruler-straight canyons running for hundreds of kilometres
19:58across the face of the moon.
20:02Formed when warm material, pushing up from the interior,
20:05caused the surface to crack along fault lines.
20:12Part of the active geology that, over millions of years,
20:16created this Frankenstein world.
20:22But that raises another mystery,
20:24because Earth's geology is driven by the heat stored away
20:28from its formation 4.5 billion years ago,
20:31along with the energy released by radioactive decay.
20:35But Miranda is far too small
20:38to have retained any of the heat from its formation.
20:42So where did all that energy come from?
20:54For the answer, you have to look at Miranda's relationship
20:58with its parent planet
21:00and another quirk of gravity.
21:05Probably several times in its history,
21:07Miranda was in a more elliptical orbit around Uranus.
21:11That meant that it went close to the planet, far away,
21:14close and far away,
21:16and the changing gravitational forces
21:19injected the heat into the moon,
21:21and that's what drove its geology.
21:23And that's what we're going to look at today.
21:35Gravity, sculpting one of the most tortured landscapes
21:40in the solar system.
21:47I think the story of Miranda
21:49reveals something quite deep, actually,
21:51the way that they sculpted the strange worlds
21:53in our solar system,
21:55and actually the way that they sculpt everything in the universe.
21:58Because the basic shape, in this case a sphere,
22:02reflects the simplicity and beauty and symmetry
22:06of the laws of nature that created it,
22:08in this case gravity.
22:10But the detail of the surface,
22:13the complexity reflects a turbulent and often chaotic past.
22:17So you're seeing history frozen in time.
22:20And it is this interaction
22:22between simplicity and symmetry and complexity
22:26that truly makes our universe beautiful.
22:29Beautiful and strange.
22:43Travel further into the solar system
22:46and we enter the realm of the outer gas giant.
23:02Home to a site unrivaled in the solar system.
23:07A structure of outrageous size and shape.
23:12Rings of rock and ice.
23:20Split into hundreds of ordered, repeating tracks and gaps.
23:26Almost engineered in their precision.
23:36And looping for thousands of kilometres through the void.
23:42So how did nature create the intricate, ordered beauty?
23:47The spiralling gaps and tracks of Saturn's rings.
24:03One of the most obvious things you can say about our universe
24:08is that at first sight it is very complicated indeed.
24:12But one of the deepest things you can say about it
24:15is that complexity emerges from the action of very simple laws.
24:20Think about this desert landscape.
24:22There's all these beautiful sand dunes and ripples.
24:27But if you look more closely, there's regularity in the ripples.
24:31And if you look at the sand dunes,
24:33this angle that they fall away at is always the same.
24:37There's regularity and beauty and structure
24:40emerging from the action of simple laws.
24:43In this case, it's just the wind blowing sand grains
24:47and gravity pulling them down to the ground.
24:51And I think the best and certainly the most evocative example of that
24:56in the solar system has to be the rings of Saturn.
25:08Yet at first sight, there's nothing simple about Saturn's rings.
25:21We think they formed when an icy moon strayed too close to Saturn...
25:31..and was pulled apart by its gravity.
25:38..creating a jumble of trillions of individual fragments of ice.
25:50So what turned such chaos into the ordered beauty of Saturn's rings?
25:57NASA's Cassini probe captured the rings in stunning detail.
26:08And orbiting within them,
26:10it saw one of the most startling objects in the entire Saturnian system.
26:17Pan is the most wonderful, bizarre object.
26:20I mean, look at these photographs taken by Cassini.
26:23It looks like a cross between a UFO and a piece of pasta.
26:27And it's really small.
26:29It's a very small object.
26:31It's a very small object.
26:33It's a very small object.
26:35It's a very small object.
26:37It's a very small object.
26:39It's a very small object.
26:41It's a very small object.
26:43It looks like a piece of pasta.
26:45And it's really small.
26:47It's less than 30km in diameter.
26:50But its impact on the rings is profound.
26:53The shape, actually, is the key
26:56to understanding how it is that Saturn's rings are so wonderfully complex.
27:02And you can see the basic idea here.
27:07So there's Pan.
27:09And the moon is orbiting inside the ring.
27:12And so that means that ring particles can, it can essentially hit them, they fall onto the surface.
27:19And because Pan has got a very weak gravitational field, it's too small,
27:23way below the potato radius, they don't get squashed into a sphere, they stay there, sort of a ridge.
27:29So part of the explanation for the gaps is that the rings are slowly being eaten.
27:48For millions of years, Pan has been nibbling away, clearing icy particles out of its orbit.
28:00And yet, Pan is only 28 kilometres across,
28:05but it sits within a track that is over 300 kilometres wide.
28:14Clearly far broader than Pan could clear through snacking alone.
28:19This moon doesn't just create a tiny gap in the rings, it creates a very big gap. Indeed,
28:25it's so big, in fact, it's called the anchor gap, that that gap was discovered using 19th
28:30century telescopes. It's about 10 times the diameter of the moon. And the way it does that
28:35is really key to understanding the complexity of Saturn's rings. So I have to tell you one thing,
28:45very important thing, about orbits. Here's Saturn, and here is Pan orbiting around. Now it's a
28:54property of orbits that the further away from the planet you are, the slower you move. That's
29:01actually traced back all the way to the beautiful simplicity of Newton's law of universal gravitation.
29:07So that means that ring-particle distances, the distance between a ring and a planet,
29:14that ring particles on the inside of Pan are orbiting faster. They're overtaking the moon.
29:24These particles get a gravitational tug that tends to slow them down. They are pulled back
29:32by Pan's gravity. And ring particles further out are moving slower. Now Pan is overtaking them,
29:40and that tends to give them a gravitational kick which speeds them up.
29:45And the effect of that is that Pan's gravitational pull on the particles that are overtaking it
29:53tends to cause them to fall down towards the planet. And its gravitational pull on the particles
30:00outside that it's overtaking tend to get raised to a higher orbit around the planet. And so Pan
30:08clears a much bigger gap in the rings than you might otherwise expect.
30:23And Pan is not alone.
30:31Daphnis, a moon a mere eight kilometres across, clears its own track.
30:38Tiny worlds creating structures on a staggering scale.
30:50What's more puzzling is that so far these are the only moons we've seen directly clearing a track
30:57like this. But there are thousands of looping spirals and gaps seemingly created by nothing at
31:06all. Including one of the biggest, the Cassini division, over 3,000 kilometres wide.
31:24So what's creating these other structures?
31:31Surprisingly, the answer lies not within the rings, but out beyond the discs of ice.
31:40There really is tremendous complexity and structure in Saturn's rings. Not only gaps,
31:47but also sort of structures, density waves that wrap around the planet often several times like
31:54the grooves on a record. And all those structures ultimately are caused by hundreds of moons,
32:02actually over 140 large-ish moons at the last count and countless smaller ones. And all those
32:10have a gravitational influence on the particles in the rings. One of the key culprits or drivers
32:17of complexity is this moon, which looks like a space station, but it's not a space station.
32:25It's a moon, it's called Mimas.
32:30Another truly odd, almost science fiction world with its dominant impact crater.
32:38Yet it's not obvious why this moon should influence the rings, as it's about 40,000 kilometres away.
32:47So Mimas, it's orbiting outside the rings such that it goes around Saturn once for every two orbits
32:57of particles that would be inside the Cassini division. So that means that those particles
33:05would regularly meet Mimas on its orbit. There's a gravitational interaction that disrupts the orbits
33:12of these particles and moves them out of the division.
33:26Each time the moon and the ice particles align, Mimas's gravity tugs at the fragments of the
33:32Mimas's gravity tugs at the fragments of ice and rock like an invisible hand,
33:41over millions of years opening up the giant gap.
33:51And Mimas is just one of over 140 known moons,
33:55each capable of creating their own resonances with the rings.
34:02Look at this picture. This is an image from the Cassini spacecraft,
34:05and you see the complexity here. It's mind-boggling. This is a resonance with a moon called Prometheus
34:13that orbits 14 times around Saturn every 15 orbits of the particles in there. And that
34:21causes this disruption, this structure in the rings. Here's a moon called Janus that creates
34:27a recognisable structure in the rings, and so on. And these are just the structures that we've observed.
34:39The orbital dance of Saturn's moons recorded in the rings,
34:45creating a pattern we're lucky to see.
34:50Imagine how complicated the gravitational field is around Saturn, and that's what you're seeing.
34:56It's very beautiful. It's as if someone had sprinkled ice crystals over the gravitational
35:02field so that we can see it. I suppose that a vinyl record really is a bit like Saturn's rings.
35:08There's a structure in the rings that's a bit like a vinyl record.
35:15Physical structure, which can give rise to something that we can perceive now.
35:21Sound made solid, in a sense, when you put a needle on there and stylus needle. All right, grandad.
35:30But also there is, of course, a sense of history
35:34about a recording on a record. It tells you something about the past.
35:39And so it is, with a pattern that we see in the rings.
35:43With a pattern that we see in the rings.
35:45In Saturn's rings, we can see gravity at work, shaping our solar system.
36:16Over half a billion kilometres closer to the sun is a planet on a mind-boggling scale,
36:34so huge you could fit all the other planets inside it.
36:46Jupiter's immense gravity has helped shape an astonishing world.
37:00Since 2016, NASA's Juno spacecraft has been exploring Jupiter and its moons,
37:12including the largest moon in the solar system.
37:15Ganymede is a very strange world indeed.
37:28A moon playing at being a planet.
37:34It's the only moon we know of with an internally generated magnetic field, producing strange aurora.
37:45And elsewhere on its surface, Juno witness bizarre scars gouged into its icy crust.
38:04These phenomena suggest Ganymede may be hiding an extraordinary secret.
38:16Ganymede is becoming, I think it's fair to say, one of the most fascinating places in the solar system.
38:22This is one of our best images of Ganymede taken by Juno.
38:26It is a big moon. This is the eighth largest object orbiting the sun.
38:32It's bigger than Mercury and not much smaller than Mars, but it doesn't look particularly different from our moon.
38:38But a series of observations are beginning to suggest to us that there may be something extremely interesting indeed going on below the surface.
38:54One clue comes from Ganymede's aurora.
39:02Detailed observations have shown that it behaves in an unexpected way.
39:08To have an aurora, then a planet or moon needs two things, basically.
39:14It needs a tenuous atmosphere and it needs a magnetic field.
39:21So what's happening on Ganymede is that charged particles, primarily from Jupiter,
39:26have been funneled down the magnetic field lines to the poles.
39:31And there they hit particles in the atmosphere, they excite them and cause them to emit light, to glow.
39:37And that's the same process that we see here on Earth in the northern and southern lights.
39:42However, Jupiter also has a magnetic field and that will affect the aurora on Ganymede.
39:49And so what was done is some computer modelling. You get Ganymede with its field and its aurora
39:54and you get Jupiter with its magnetic field and you put it all into the computer and you see what happens.
39:59And you find there is a prediction that the aurora on Ganymede should kind of wobble around,
40:04wander in the vicinity of the pole.
40:08And we observed that, but we observed that the aurora wanders far less than it should.
40:14So that implies there's something else going on.
40:17If Ganymede had an additional second magnetic field,
40:23it would interfere with the aurora, causing it to wander less.
40:33But the only way to generate that extra field would be if another layer of magnetic field
40:40within the moon conducts electricity.
40:54I really was never very good in the lab. That doesn't work. Have we got another battery?
41:01Yeah.
41:02I really was never very good in the lab. That doesn't work. Have we got another battery?
41:09Yeah.
41:09Let's plug another battery in.
41:15Here's an electrical circuit. There's a battery and a bulb.
41:18And if I connect it, the electrons flow and the bulb lights up.
41:23But now look what happens if I take these two wires,
41:28but connect it by dipping the wires into salt water.
41:38Pretty cool, isn't it?
41:40So in here, the circuit is being completed. Salt water is a conductor of electricity.
41:47An electrical current flows and that can produce a magnetic field.
41:53So we think that is the origin of that third magnetic field
41:57that's making the aurora wander far less than it should.
42:03The implication is that beneath the surface of Ganymede,
42:08there's a saltwater ocean.
42:15Welcome to the largest ocean of water in the solar system.
42:23It's estimated that there's a layer of water over 100 kilometres deep
42:29wrapped around the moon. One that never sees the light of day,
42:34hidden beneath 150 kilometres of rock-hard ice.
42:43But how can liquid water exist in such enormous quantities beneath the frozen surface?
42:50One fascinating theory involves those strange gouges in the surface.
43:05These are impact craters.
43:10Not single craters like those found in other worlds.
43:13But long chains.
43:27Yeah, quite a lot of the answer, actually, of how it came to be that Ganymede has an ocean
43:31is that it's an ocean of water.
43:33And that's what we're going to look at today.
43:35We're going to look at how the Ganymede has an ocean of water.
43:38We're going to look at how the Ganymede has an ocean of water.
43:40Of how it came to be that Ganymede has an ocean, it's the presence of Jupiter.
43:46I can see clouds on the surface of Jupiter through this pretty small telescope,
43:52even though tonight it's about 600 million kilometres away.
43:56You can fit over 1,000 Earths inside it.
43:59It's massive.
44:00And being massive, it means it's got a strong gravitational pull.
44:05And Jupiter tends to attract things, suck things in that come within its vicinity
44:11and rip them apart.
44:13And we've seen that.
44:15This is a great image.
44:17It's one of the most famous images in astronomy in recent times, actually.
44:22Can you see that?
44:23So that is Comet Schumacher-Levy 9.
44:26This is a comet that came too close to Jupiter,
44:29was drawn in by its gravitational field,
44:31whipped to bits by its gravitational field.
44:34And then ultimately hit Jupiter.
44:37And it hit Jupiter with such ferocity that we saw the impacts in the clouds
44:42and some of them were bigger than the Earth.
44:46Now you look at that.
44:52And then look at that, the surface of Ganymede.
44:57Being so close to Jupiter puts Ganymede in the firing line.
45:04ROCKET SOUNDS
45:14Ferocious impacts.
45:18That create the chain craters.
45:26These scars are just a fraction of what Ganymede has suffered
45:30living so close to Jupiter.
45:34And that's the key to understanding how it may have got its hidden ocean.
45:47The early solar system was a much more chaotic place than it is today.
45:52Impacts were common.
45:58Everything got hit.
46:00Jupiter's immense gravity drew in countless asteroids and comets,
46:09and Ganymede was caught in the crossfire.
46:18Impacts delivered enough energy to heat the moon
46:21and kick-start a process that caused it to melt and separate into layers.
46:29Dense, heavy metals at the core,
46:33and an outer shell made of water and ice.
46:40And we think Ganymede has retained enough of that heat
46:43to produce a saltwater ocean with more water, actually,
46:47than all the oceans of the Earth combined, below the frozen surface of Ganymede.
46:55A strange, giant moon with an ocean and aurora
47:02nearly a billion kilometres away from the sun.
47:10We're talking about, potentially, a habitat for life.
47:13This is a very, very small planet.
47:15Potentially, a habitat for life.
47:17This is a big world, a planet-sized moon,
47:20which has a magnetic field, and a saltwater ocean,
47:23and a ready source of energy, it seems.
47:26All the things that we think are necessary for the origin of life.
47:30And it's important because we used to think of what's called a habitable zone around a star,
47:35which is where the Earth orbits, and indeed, Mars and Venus, just about,
47:40which is the zone where you could potentially have liquid water on the world,
47:45on the surface of the world, in that case.
47:48But now, looking at places like this,
47:51we understand that there might be habitable zones far away from stars.
47:55In this case, a habitable zone around a gas giant.
48:00And that habitability, here, is delivered by gravity.
48:10Leaving this distant ocean moon behind,
48:14we head inwards, on the final leg of our journey.
48:19Passing through the asteroid belt,
48:21rubble left over from when gravity failed to pull a planet together.
48:32Until we reach the inner rocky planets.
48:40The world's here, a home to phenomena and landscapes that are mesmerising.
48:56So strange and alien.
49:11But amongst all these wonders lurks, perhaps, the strangest world of all.
49:32Welcome to Earth.
49:34It is the biggest rocky world,
49:36it is the biggest rocky world, radius about 6,370 kilometres or so.
49:44It's a bit unusual in that it's got a single moon,
49:47but the thing that makes it very unusual, indeed,
49:51is the presence of that liquid water on the surface.
50:01You might not think of Earth as strange,
50:04because we live on it,
50:06but it is, in fact, a very rare world.
50:17This is a really wonderful and unusual thing to be able to do in our solar system,
50:24because there is no other world where the conditions of temperature and pressure on the surface
50:29allow liquid water to exist.
50:32It's a very narrow range, and that range is set by the details of our atmosphere.
50:39There are tons, tons of atmosphere pressing down on this rock wall to stop it from boiling away.
50:47The nature of our atmosphere is defined by the history of our world,
50:52our place in the solar system, and gravity.
50:56If you imagine that you'd reduce the mass of the planet just a bit,
51:01then the pressure would fall and this would boil away.
51:05If I carried on doing that and reduced the gravitational pulse some more,
51:09the whole atmosphere would disappear off into space.
51:19All the myriad properties of our planet have combined
51:23to allow liquid water to persist here for over four billion years.
51:31Leading to planet Earth's most unique feature.
51:41Life.
51:43Life.
51:58As we explore the solar system, we are discovering ever stranger places.
52:06All born of the interplay between beautifully simple laws of nature
52:12and the deep history of each and every world.
52:20Creating endless wonders of the solar system.
52:31Including...
52:34us.
52:35Just look at these telescopes, our eyes on the universe.
52:43I find it so remarkable that on one strange world in our solar system,
52:48collections of atoms have come together that can do astronomy.
52:52Because there's nothing particularly special about the Earth.
52:55It is just another lump of stuff that's found a way to avoid gravitational collapse.
53:02But somewhere in between the relentless inward pull of gravity
53:06and the sheer bloody mindedness of matter,
53:11some of that stuff has found a way to contemplate its place in the universe.
53:31No other planet has rings quite like Saturn does.
53:35They're beautiful, but it's odd to think that they might not be there forever.
53:39Far from a permanent structure,
53:41we now know that these strange loops of rock and ice are constantly changing
53:47and may one day disappear completely.
53:51We have big questions about Saturn's rings.
53:54How old are the rings?
53:55How old are the rings?
53:57We have big questions about Saturn's rings.
54:00How old are the rings?
54:01How did they form?
54:03And what is their evolution like?
54:04How long are they going to last?
54:09NASA's Cassini spacecraft studied Saturn and its rings for 13 years in search of answers.
54:17Cassini allowed us to see Saturn from closer up than ever before,
54:21but also from new vantage points that we had never been able to access from the Earth.
54:27Cassini witnessed a series of bizarre moons clearing paths in the rings.
54:36But one of the biggest insights came from its encounter with a strange kind of rain
54:42falling onto Saturn.
54:45It was Voyager that gave us the first hints that particles could be falling into Saturn.
54:50But towards the end of the Cassini mission,
54:52when we flew the spacecraft between the rings and the planet,
54:56we were able to detect small ring particles that were falling into the planet,
55:00so-called ring rain.
55:03The immense gravity of Saturn is pulling on these particles, eroding the rings.
55:10Ring rain causes the rings to slowly die.
55:15But what we don't know is the rate at which the rings are perishing.
55:18We just know that they are.
55:20Flying through the icy rain falling from ring to planet was one of Cassini's last endeavours.
55:28In 2017, the mission came to an end before Cassini could find out how long the rings had left.
55:43To get a definitive answer on the lifespan of the rings,
55:47To get a definitive answer on the lifespan of Saturn's rings,
55:51we needed a brand new mission.
55:56So JWST isn't like a normal telescope that you would find on Earth.
56:00It's not at the top of the mountain like the big telescopes that we have here.
56:04Instead, it is 1.5 million kilometres away in space.
56:09The Space Telescope is designed to peer into the depths of the universe.
56:14But its infrared cameras are also showing us our solar system in a strange new light.
56:24Illuminating the faint rings around the outer planets normally invisible to us.
56:34It's extremely difficult to get to the outer solar system,
56:38and so an instrument like JWST that can look at these distant objects is invaluable.
56:45Amongst its targets is Saturn and its rings.
56:51Where the hope is that the telescope will be able to help answer how fast the ring rain is falling.
56:58So the rings are made of mostly water ice,
57:01and some of the smallest pieces flow up the magnetic field and fall into the planet.
57:06That happens all the way around.
57:08So in our observations, we see this kind of infrared glow all the way around the planet,
57:12that location, which indicates that there is ring material flowing in.
57:19In the next few years, JWST will measure the intensity of the infrared glow in that band,
57:25revealing how fast the rings are losing particles.
57:30I'm very excited to find out how quickly Saturn's rings are eroding today.
57:36Because finding out what's going on today is really important
57:39for mapping their past and predicting their future.
57:43Bringing us ever closer to understanding exactly how long
57:47Saturn's stunning rings of ice are likely to last.
57:54There's something about seeing Saturn's rings.
57:57You have this almost childlike fascination
58:00and a professional curiosity that come together in a very unique way.
58:06Knowing that Saturn's rings won't be around forever
58:09and that we're here at the exact moment when they are here is really amazing.
58:13I feel really lucky that we get to experience them.