Solar Storm Worlds
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00:00It's mid-summer, a storm is breaking,
00:25unleashing torrents of rain,
00:32forming floods that drain into raging rivers
00:44and giant lakes.
00:51This is the only world in our solar system where raindrops fall to the ground,
01:04apart from our own planet.
01:121.4 billion kilometres from the sun lies Earth's strange stormy twin.
01:26Saturn's moon Titan is in some ways more like a planet than a moon.
01:30It is larger than Mercury and it is also in some ways more like the Earth
01:37than any other place in the solar system.
01:39I could stand on the surface of Titan in a spacesuit and look out over lakes.
01:48And I could look into the sky and I could experience droplets of rain landing on my visor.
01:56But appearances can be deceptive.
02:06Temperatures on Titan hover around minus 180 degrees Celsius,
02:11far too cold for liquid water.
02:18Instead, these clouds are made of a chemical that on Earth is a flammable gas, methane.
02:31Titan is a world sculpted by methane storms.
02:37Titan is the only moon in the solar system with a thick atmosphere.
02:41And that atmosphere is the stage for the methane cycle,
02:45an arena within which liquid can evaporate from lakes,
02:49condense as clouds and fall as rain.
02:53But Titan is certainly not the only storm world.
03:01Among the most dynamic and violent worlds in our solar system
03:09are the handful that, like Titan, have a thick atmosphere.
03:18Planets where sulfuric acid storms rage,
03:25giant dust vortices dance
03:30and lightning,
03:32ten times more powerful than anything found on Earth,
03:36lights up the sky.
03:39And worlds whose storms grow so large they are entirely engulfed.
04:01On a perfectly still morning like this,
04:05as the sun rises above the high plains of Utah,
04:09it's easy to forget that we live in an atmosphere.
04:15But our atmosphere is central to our existence
04:19and central to the character of our planet.
04:25Atmospheres behave like fluids.
04:29And that's extremely important for a planet like Earth.
04:34Let me show you what I mean.
04:39You can see that I'm a very experienced camper.
04:44I'm going to show you what I mean.
04:50You can see that I'm a very experienced camper.
04:55Now, if I build a model of the Earth's atmosphere,
05:00which is coffee,
05:02and then if I put some milk in,
05:06which is a denser fluid than the coffee,
05:09and so it's sinking down to the bottom of the pan.
05:13And the reason I want to do that is to show you something that happens
05:18when I heat a fluid.
05:22When you think about a planet like the Earth,
05:25then it's heated from the sun.
05:27That means that there are temperature differences
05:30naturally occurring in our atmosphere.
05:32And it's what happens in nature
05:35when there are temperature differences that matters.
05:38So what you see here is that that milk at the bottom of the pan
05:43is getting hotter than the coffee at the top.
05:48And that temperature difference is trying to equalise.
05:52So it's not that the temperature difference
05:55just equalises in a nice uniform way.
05:58Well, you can see it.
06:00There.
06:01See, there are patterns developing.
06:04Tremendous amount of complexity.
06:07And it's exactly the same in our atmosphere.
06:10But in our atmosphere,
06:12we call all those patterns and all that turbulence weather.
06:24Wind, rain and storms can almost be thought of as side effects
06:29of an atmosphere trying to move energy between hot and cold.
06:35If a world has an atmosphere, it will have some form of weather.
06:46But as the fleet of spacecraft exploring our cosmic backyard have revealed,
06:53atmospheres vary hugely by temperature, pressure and chemical make-up.
07:05And that means that while storms might be common in our solar system,
07:10on no two worlds are they ever the same.
07:27Journeying outwards from our shared star,
07:31searching for storm worlds.
07:37We dodge past a planet with no atmosphere
07:46and arrive at a world consumed by one.
07:49Venus
07:59From a distance, Venus appears to be a serene, pearlescent orb floating in space.
08:08But get a little closer
08:12and the planet begins to reveal its true nature.
08:20All that can be seen are clouds.
08:23Endless, thick, churning storm clouds
08:27that conceal the entire surface from view.
08:34This is just the top of a cloud deck that's 20km deep
08:41and whipped up by hurricane-force winds.
08:45Venus is a true storm world.
08:55Now, those clouds on Venus just look like storm clouds here on Earth.
09:00A bit foreboding, perhaps,
09:02but maybe nothing too much to worry about if you were sat beneath them.
09:07Well, they are actually something to worry about.
09:11They're not made of water.
09:14They're made of this.
09:16Concentrated sulfuric acid.
09:19Now, even saying that sounds nasty,
09:22but just wait till you see what this stuff does
09:26to the stuff that I'm made of, the stuff that you're made of,
09:29just organic material, in this case, sugar.
09:38Now, you see that's already starting to react.
09:41It's turning brown, it's starting to bubble.
09:44The concentrated sulfuric acid, and this is 90-odd% concentrated,
09:49which is precisely what we find in the clouds of Venus,
09:53is ripping the water out of the sugar.
09:56In fact, that dark material there is carbon.
10:00It's what's left over when you rip all the water out of the sugar.
10:04I can show you.
10:06So C12H22O11.
10:09So that's the sugar we've got.
10:11And then sulfuric acid, which is H2SO4.
10:15And that goes to carbon and water.
10:19And I can smell it, actually.
10:22Sulfur dioxide.
10:24And a lot of heat.
10:26But just think, that started off as sugar.
10:30Essentially organic material like this stuff.
10:34I mean, can you look at that?
10:37So if you were to skydive through the clouds of Venus for some reason,
10:42then that's what you'd turn into.
10:45So I suppose the moral of the story is, don't.
10:56But ignoring that advice,
10:58and taking the plunge down through Venus' storm clouds...
11:07..we reach a surface that's eerily calm.
11:11With air pressure so intense,
11:13it's like being one kilometre beneath the ocean.
11:17But with one key difference.
11:23At 460 degrees Celsius,
11:26Venus' surface is hotter than that of any other planet.
11:31Which is why one feature leaps out.
11:37Something that shouldn't be possible on this roasting world.
11:43A mountain that seems to be covered in snow.
11:52Could it really be snowing on Venus?
12:01As well as being really nasty and corrosive,
12:04those sulfuric acid clouds are extremely dense.
12:07That's why, if you look at Venus through a telescope,
12:10all you can see is clouds.
12:13Until the 1950s, actually,
12:15we imagined that Venus might be a tropical paradise.
12:19Now, NASA's Magellan probe arrived in the 1990s,
12:23equipped with radar that could peer through the clouds
12:26and image the surface.
12:28And it took pictures like this.
12:30This is a vast mountain range called Maxwell Montes,
12:34and it's huge.
12:35It's much bigger than the size of Wales,
12:37to use the standard measure of area.
12:39And there are points on here that are 11 kilometres in altitude.
12:43Now, I think it's very difficult using, as we do,
12:46our brains tuned to the landscapes of Earth,
12:49not to look at the clouds,
12:52not to use our brains tuned to the landscapes of Earth,
12:56not to look at that and see that.
13:03The material that coats Venus's mountains reflects radar,
13:07mimicking the appearance of snow.
13:14But Venus's extreme atmosphere means snow is unlikely.
13:20The atmospheric pressure on the surface of Venus
13:23is 90 times the pressure here on Earth.
13:26That's because its atmosphere is extremely dense,
13:29so 96% of it is carbon dioxide.
13:32Now, carbon dioxide is a powerful greenhouse gas.
13:36That means that although it lets the visible light in from the sun,
13:40which heats up the ground,
13:42the heat radiation coming back out again,
13:44the infrared light, is trapped.
13:47That is a runaway greenhouse effect.
13:53Venus is so hot, it's thought that the ground might actually glow,
13:58like metal coming out of a forge.
14:03It's a world far too hot for snowstorms.
14:10But the fact that this strange material imitates snow,
14:14being found only on mountaintops, is a clue.
14:26The key is altitude.
14:32As you go higher and higher in the Earth's atmosphere,
14:35then the atmospheric pressure falls.
14:38And the reason for that is pretty easy to understand
14:41because that's what the atmosphere is.
14:43It's just the weight of air pressing down.
14:46And so imagine going up to 100km, for example,
14:49then you'd be in space, there'd be no atmosphere at all,
14:53and the pressure would be zero.
14:59As you increase altitude,
15:01it's not only the pressure that falls, the temperature as well.
15:06The explanation for that, actually, is quite complicated.
15:09It's what a physicist would call slightly non-trivial.
15:12There are a lot of things happening.
15:14One is that if you imagine a piece of air,
15:17a volume of air down at sea level,
15:20and you lift it up higher and higher,
15:23and the pressure falls, and so that air expands,
15:26and therefore it cools.
15:28There's another thing happening as well,
15:30which is related to the greenhouse effect.
15:32So sunlight is coming down and heating up the ground,
15:35and then the ground is reradiating the heat
15:38up into the atmosphere, which is trapping it.
15:41And so the closer you are to the ground, the hotter it is.
15:44The point is that if you climb a mountain on Earth,
15:47then you can get to a point where the temperature is so low
15:50that water freezes out to form snow.
15:53And that dividing line between the two regions
15:57is called the snow line for obvious reasons.
16:00Now, on Venus, we also see something
16:02that looks for all the world like a snow line,
16:05but water isn't involved.
16:08So what is it?
16:14Venus's snow line suggests that something is freezing up there.
16:19Something that freezes at much higher temperatures than water.
16:23And that points us to chemicals that, on much cooler days,
16:27and that points us to chemicals that, on much cooler Earth,
16:31are only ever found as solids.
16:36Now, one of the candidates for that bright, snowy stuff
16:39that coats the mountaintops of Venus...
16:43..is this.
16:45This is lead sulphide.
16:47So the idea is that the lead and sulphur that are becoming vapour
16:51because it's so hot and heading up into the atmosphere
16:54cool and condense out onto the mountaintops and react
16:59to coat them in this bright silver.
17:03I mean, we don't really know for sure,
17:05and part of the reason for that is it's so difficult to explore Venus.
17:09It'd be wonderful to drop a spacecraft onto those mountains,
17:12but we haven't landed a spacecraft successfully on the surface of Venus
17:16since the Russian probes in the 1980s, and they didn't last very long.
17:20But just imagine if that's right.
17:23I mean, what a sight that would be.
17:27Instead of water, it's thought that on Venus,
17:31it's lead and sulphur that vapourise.
17:38In vapour form, they're carried on air currents from lower altitudes...
17:47..up into mountain ranges...
17:51..where, because of the altitude, the temperature drops just enough...
17:59..to allow them to crystallise out of the air...
18:06..coating Venus' mountaintops in glittering metallic frost...
18:16..creating snowy peaks...
18:20..on a hellish world.
18:34Leaving Venus' crushing atmosphere behind...
18:41..we head out in search of a world that could almost be Venus' opposite.
18:51Bypassing our own planet...
19:04..and dodging two potato-shaped moons...
19:10..we arrive at the farthest rocky planet from the sun.
19:21We've sent more spacecraft to explore Mars
19:24than any other world in the solar system.
19:29And thanks to this robotic army beaming back photographs...
19:37..we know that in the deep past...
19:41..Earth-like rainstorms carved at the Martian surface...
19:46..but around four billion years ago, Mars began to lose its atmosphere...
19:51..transforming it into a planet
19:53where you wouldn't expect to see any storms at all.
20:06Modern Mars' wisp of an atmosphere is just 1% the density of the Earth.
20:13And it's so dusty, the sunrise is tinted blue.
20:24Temperatures on the surface average minus 60 degrees Celsius.
20:31Mars might appear to be a frozen world.
20:36But all is not what it seems.
20:40Strange lines, often tens of metres wide, are etched on the surface.
20:52Unlike Mars' dry rivers, these are not relics.
20:58We see them appear and disappear...
21:03..almost as if they're being deliberately drawn and then wiped away.
21:10What, on dry, freezing Mars,
21:13could be behind these bizarre shape-shifting patterns?
21:18Oh, look at that.
21:20This is...Moab is a...
21:22It's just a fascinating place, the Uranium Building.
21:25This was known as one of the wisest buildings in the world.
21:29And it's still the place where people have the most fun.
21:33It's a place where people have the most fun,
21:35and it's a place where people are very passionate about the world.
21:39It's a place where people are very passionate about the world.
21:42It's a place where people are very passionate about the world.
21:45This was known as one of the wildest places in the Wild West,
21:49and then, in the 1950s, discovered uranium.
21:52So there was a boom. It was like the gold rush.
21:55But it was a uranium rush.
21:57And there's all these echoes of the atomic hair salon over there,
22:02and there's nuclear coffee.
22:05Nuclear coffee.
22:07I'm having some of that.
22:10Clues to solving the mystery of the Martian lines.
22:15Come from a pair of trailblazing Mars rovers.
22:25Spirit and Opportunity were small rovers,
22:28and unlike the big nuclear-powered rovers of today,
22:31they were purely solar-powered, and the solar panel was very small,
22:35only about this big.
22:37And those rovers were only designed to last around three months
22:40because Mars is a dry, dusty desert world,
22:44and all the engineers thought that, over time,
22:46those solar panels would be covered with dust and the power would drop.
22:51That's indeed what happened, for a while.
22:57Thanks to Mars' dusty atmosphere,
23:00at first, the solar panel's energy output dropped.
23:07But then, suddenly, the power started leaping up.
23:15Some think we're sweeping dust from the solar panels,
23:19keeping the rovers alive much longer than expected.
23:26Before long, images started to arrive at Earth...
23:34..that hinted at what was going on.
23:38On Mars, as on Earth, the sunlight passes through the atmosphere
23:42pretty much unhindered and hits the ground and heats it up.
23:46But on Mars, because the atmosphere is much lower pressure,
23:49much more tenuous, then the temperature gradient
23:52you get close to the ground can be far greater.
23:55So I could stand on the equator of Mars
23:58and the ground can be at 20 degrees Celsius,
24:03but my head can be in air that's at minus 10 degrees Celsius.
24:08And that temperature gradient has powerful effects,
24:11it has consequences, because the gradient wants to equalise.
24:15So the air in contact with the ground will heat up,
24:19and that means that it will rise, hot air rises.
24:23Under the right conditions, that rising air creates a lower pressure
24:29into which colder air can fall.
24:32And so you can get a system where air rises, air falls,
24:36the whole thing spins, and that can form a stable structure, a dust devil.
24:41So this is again a beautiful example of a gradient, an imbalance,
24:47creating temporary structure.
24:50In this case, a spinning storm of dust.
24:59Now spotted frequently by spacecraft on the surface,
25:05it's thought that dust devils passing over the rovers
25:09sucked dust off the solar panels like a vacuum.
25:17Keeping spirit roving for six years
25:20and making opportunity seem unstoppable.
25:25But the cleaning power of dust devils doesn't just work on rovers.
25:39Thanks to Mars' thin atmosphere,
25:42Martian dust devils can grow to up to 20km tall and 1km wide.
25:54And as they travel, these spinning vortices suck up dust from Mars' surface,
26:00exposing the darker bedrock beneath.
26:09Leaving trails so large,
26:12we can see them clearly from our orbiting spacecraft.
26:20There were no Martians behind the lines.
26:23The culprits are spinning Martian windstorms.
26:33But dust devils are just one half of the puzzle.
26:37They might create the tracks.
26:42But it's something else that wipes them away.
26:46Just like the Earth, Mars has a tilt that gives it seasons.
26:51Summer in one hemisphere means winter in the other,
26:55and a planetary temperature gradient that wants to equalise.
27:04But Mars has no oceans or thick atmosphere to help move heat around the globe.
27:11The one thing it does have, however,
27:15is dust.
27:18As summer progresses, a huge amount of dust is lifted into the air by the sun's heat.
27:29The dust absorbs sunlight, heating up the atmosphere.
27:35The dust absorbs sunlight, heating up the air around it,
27:40causing updrafts and more dust to be lifted.
27:47Until...
27:49a storm is formed...
27:54that wipes away any dust devil trails in its path.
28:05And every few years, these storms grow so large...
28:11they encircle the entire planet.
28:21In 2018, a monster dust storm darkened Mars' skies for months on end.
28:28And for solar-powered Opportunity, it was catastrophic.
28:34This is the last of over 200,000 photographs
28:37that Opportunity sent back from the surface of Mars to Earth.
28:41And it's certainly not the most beautiful photograph by any means,
28:45but it is, I think, remarkably poignant,
28:48because these speckles, these speckles of dust,
28:52but it is, I think, remarkably poignant,
28:55because these speckles, they're not stars in the sky.
28:59They're camera noise,
29:01because it was so dark when this photograph was taken.
29:05And this dark area here, it's not the Martian surface.
29:08It's actually nothing at all,
29:11because Opportunity ran out of power
29:14just before it finished transmitting this photograph back to Earth.
29:19So, after 14.5 years,
29:23this is the final thing that Opportunity saw,
29:27defeated by the Martian atmosphere that kept it alive for so long.
29:38But the darkness plays an important role for Mars.
29:42With less sunlight hitting the surface,
29:45the temperature difference between the hemispheres is reduced.
29:52And when the storms recede,
29:55they leave a slate white clean...
30:02..ready for dust devils to start etching the surface again.
30:16Leaving Mars and its dust cycle behind...
30:21..we head out in search of a completely different kind of atmosphere.
30:29But first, we must traverse the asteroid belt...
30:36..ruled by the dwarf planet Ceres.
30:40Until, three times further from the sun than Mars,
30:44we enter the realm of giants.
30:56Twice as massive as all the other planets of the solar system combined,
31:01this is a star that has never been seen before.
31:09A storm world on the grandest scale.
31:16Made mostly of hydrogen and helium,
31:19Jupiter is a gas giant...
31:26..on which storms can grow bigger than planet Earth.
31:39Since 2016, NASA's Juno spacecraft
31:42has been exploring this gargantuan planet...
31:47..and found that the violence of its weather matches its scale.
31:55Lightning strikes here in abundance...
31:59..with bolts ten times more powerful than those found on Earth.
32:04Most flashes are trapped under Jupiter's thick outer layer
32:08of ammonia ice clouds.
32:15But the most powerful storms break free...
32:22..allowing us to get a proper look...
32:26..at the fireworks.
32:34It's not really fully understood, in precise detail,
32:38how lightning forms on Earth.
32:40You need ice crystals rising and hailstones falling,
32:44and they collide, and in that process, electrons are exchanged,
32:48and so the electric charges separate.
32:50The top of the cloud and the bottom of the cloud
32:53become electrically charged.
32:57I mean, it's just like walking around on the beach
33:01and it's just like walking around on the wrong kind of carpet
33:05and then grabbing a doorknob and getting electrocuted,
33:08but the spark is much bigger.
33:10But what we do know is that in the same region of the atmosphere
33:14for lightning to form,
33:16you need all three phases of water to be present,
33:19the vapour, ice and liquid.
33:23Lightning is common on our planet because of Earth's water cycle.
33:32But Jupiter is a very different kind of world,
33:35five times further from the sun.
33:41Thanks to Juno, we know that its atmosphere
33:44does contain a trace of water...
33:47..around a quarter of 1%.
33:53But could this water really be the cause of Jupiter's lightning?
34:01We all learn about the water cycle at school.
34:03The sun shines down on the oceans and lakes,
34:06water evaporates, the water vapour rises and cools,
34:10condenses back to form clouds
34:13and then falls down to the ground again as rain.
34:16But there's something else to the water cycle
34:19that's extremely important
34:21because it is a very efficient energy transport mechanism.
34:26Let's take some water from the river
34:28and pour it into the hot frying pan on the camping stove.
34:36Then the water boils, turns into vapour
34:39and disappears off into the atmosphere.
34:41Let's think about what's happening here at a deeper level.
34:46So water molecules, H2O, are bonded together in the liquid.
34:51I have to put energy in from the flame
34:54to break those bonds and turn the liquid into vapour.
34:58The reverse must also be true.
35:01So the vapour, the steam, turns back into liquid again,
35:04the bonds reform and all that energy is released.
35:08And that's why steam...
35:11..burns.
35:12What's happening is the vapour is touching my cooler hand,
35:15turning back into liquid,
35:17and as the bonds reform, a tremendous amount of energy is released.
35:27The water cycle acts like a battery.
35:33On Earth, when water evaporates,
35:35it absorbs the sun's energy
35:37and stores it...
35:43..until re-releasing it into the atmosphere
35:46when it condenses into clouds.
35:51In a typical cloud, the energy release
35:54when the vapour turns to liquid is hundreds of tonnes of TNT.
35:59Just in a cloud like that,
36:01if you think of a big storm system like a hurricane,
36:04then over its lifetime,
36:06the energy release is more like thousands of atomic bombs.
36:17By funnelling the sun's energy into the atmosphere,
36:20the water cycle powers Earth's electrical storms.
36:27But the same can't be true on Jupiter.
36:30The planet receives just 4% of the sunlight that we do here on Earth.
36:36And the surface we see, the ammonia ice clouds,
36:40is at minus 100 degrees Celsius.
36:46But looking at the planet in the infrared
36:50provides a clue as to what's going on.
36:53Jupiter is radiating heat,
36:56double the amount of energy it receives from the sun.
37:02Jupiter is basically a giant ball of gas
37:04and there's nowhere, really, as you descend into the planet
37:07where the atmosphere ends, just that the pressure increases
37:10and, ultimately, those gases become liquids
37:13and, actually, at the core, strange sorts of metallic solids.
37:17Now, Jupiter is collapsing under its own gravity.
37:20It's been doing that since it formed,
37:22about 4.5 billion years ago,
37:24and even now, it's collapsing by about one millimetre per year.
37:28But that releases a tremendous amount of this.
37:36It's gravitational potential energy.
37:38It's a huge amount of energy.
37:40Look at that.
37:45It's gravitational potential energy.
37:48That release, this heat in Jupiter up at the core,
37:51is 24,000 degrees Celsius,
37:54a huge temperature gradient,
37:56and it's that that powers the storms on Jupiter.
38:05This internal heat allows water to drive storms on Jupiter,
38:09just as it does here on Earth.
38:16And that's why Jupiter has so much lightning.
38:23In vapour form, water ascends,
38:26carrying energy from deep inside the planet...
38:34..until it reaches a place under the ammonia ice clouds.
38:40Where it's cool enough for it to condense
38:43into droplets and ice crystals.
38:51The energy released as the water condenses
38:54powers the growth of violent thunderstorms.
39:09In places, so much energy is released...
39:15..that ice crystals are swept upwards into the ammonia ice clouds.
39:25Here, ammonia acts as antifreeze...
39:33..allowing liquid water to grow thunderclouds 60 kilometres tall.
39:40Even though it's minus 100 degrees Celsius.
39:51Jupiter's about as different from the Earth as you can possibly get.
39:54It's a gas giant.
39:56Extremes of temperature and pressure,
39:58a really different chemical composition.
40:00But there is a water cycle.
40:03There's a region in the atmosphere
40:05where the temperature and pressure is just right
40:08for it to exist in all of its three phases.
40:11And it's that region that plays the dominant role
40:14in allowing the energy from deep inside the planet
40:17to escape into the upper atmosphere,
40:20drive the storm systems that we see
40:22and ultimately allow energy to flow
40:25from inside the planet and out into space.
40:39Heading out from Jupiter...
40:43..we cross 700 million kilometres of empty space...
40:52..before we encounter the solar system's other gas giant.
40:57Taking the crown for the planet with most moons,
41:01Saturn is orbited by at least 146...
41:06..that we know of.
41:11But one stands out amongst the crowd.
41:17It's a gas giant.
41:20But one stands out amongst the crowd.
41:26More than 20 times the mass of all Saturn's other moons combined.
41:35The only moon in the solar system to have a thick atmosphere.
41:40But what makes Titan really special...
41:44..is it's the only place we know of other than Earth...
41:49..where you could see a sight like this.
41:59Thanks to Titan's thick nitrogen atmosphere,
42:02it's possible to see the sun rise and set
42:06Thanks to Titan's thick nitrogen atmosphere
42:09and temperatures of minus 180 degrees Celsius...
42:15..methane, naturally found as a gas here on Earth,
42:19can exist as a liquid.
42:26It forms clouds in the sky,
42:29falls as rain and pools in giant lakes.
42:36But lakes like these are not found everywhere on Titan.
42:40They're only located at the poles.
42:50Travel beyond and we find a very different world.
42:58Great plains.
43:01Rolling dune fields.
43:05These are Titan's deserts.
43:12But get down closer...
43:16..and a familiar shape comes into view.
43:29The reason we know so much about Titan
43:31is because of the iconic spacecraft Cassini.
43:35It arrived in the Saturnian system in 2004
43:38and spent over a decade exploring the planet and its moons.
43:42And it discovered not only that Titan is a desert world
43:45with methane lakes around the poles,
43:48but also...
43:53..it saw features like this meandering across the deserts.
43:58And this...
44:01..is one of those.
44:06This...
44:12Titan's desert regions are crisscrossed with dry riverbeds.
44:18Some 3,000km away from the methane lakes at Titan's poles.
44:26So we're faced with a mystery.
44:30What is carving these rivers?
44:36The southwestern United States is just covered in canyons like this.
44:41And they're very similar to the canyons
44:43that Cassini saw on the surface of Titan.
44:46Now here, they're caused by flash flooding.
44:49So in the summer months,
44:50the North American monsoon sweeps across this landscape.
44:53It's a huge amount of moist air
44:55that's risen up from the Gulf of California
44:58and dumps rain onto this otherwise parched desert.
45:02And it flows down and cuts these canyons.
45:10If Earth's dry rivers have a seasonal origin,
45:14could the same also be true for Titan?
45:23Just like Earth, Saturn is tilted on its axis,
45:27and that means that just like Earth, Saturn has seasons.
45:31But Saturn's year is 29 Earth years.
45:34And so that means that each season is something like seven years long.
45:38Now, Titan shares Saturn's tilt in Titan's southern hemisphere summer.
45:47The southern hemisphere points towards the sun.
45:50And even though it's a billion miles away,
45:52so there's not much energy falling on the southern hemisphere,
45:55there is enough for those seven years
45:58for methane to evaporate from the lakes and up into the atmosphere.
46:07Cassini saw this happening.
46:10It flew by Titan during southern summer
46:14and saw methane clouds swirling around the South Pole.
46:21All that methane condensing out in Titan's atmosphere
46:24releases a tremendous amount of energy,
46:27just like water condensing out in our atmosphere releases energy.
46:31And that energy release seeds the formation of storms.
46:37But the clouds didn't stay at the South Pole.
46:41In 2010, Cassini took this image of Titan,
46:45and I think it's just remarkable
46:47because this is a storm around Titan's equator.
46:51It's worthwhile sometimes just sitting back and realising what this is.
46:55It's a photograph of a storm in the atmosphere of a moon orbiting Saturn.
47:05In Titan's deserts, autumn brings change to the air.
47:11Storms like the one seen by Cassini arrive from the pole...
47:20..unleashing torrents of methane rain.
47:25But because the gravity on Titan is even less than that of our moon,
47:30the raindrops fall at one-sixth of their speed on Earth.
47:39Storms in slow motion,
47:42the most powerful thought to drop 30 centimetres of methane rain a day...
47:49..forming flash floods...
47:54..that over millennia carve canyons into the desert landscape...
48:03..before they spill out into vast floodplains.
48:19And Cassini also took these images.
48:24This one is an image of the surface,
48:27and these dark areas here have been interpreted as liquid methane,
48:33a flood of liquid methane.
48:35It's a few tens of centimetres deep,
48:38but the area of this flood
48:40is something like the area of Utah and Arizona combined.
48:45And then, just a few months later,
48:48this image was taken of the same region on Titan,
48:51and now you see that the flooding has disappeared.
48:54All that methane has evaporated back up into the atmosphere again
48:58over the period of just a few months,
49:01and the storm has moved on.
49:05In 2022, five years after the Cassini mission had ended,
49:10the James Webb Space Telescope turned its infrared gaze
49:14towards Saturn's distant moon.
49:19By now, it was late summer in Titan's northern hemisphere,
49:24and the telescope's close-up view of Saturn's distant moon
49:28By now, it was late summer in Titan's northern hemisphere,
49:32and the telescope spotted something magical.
49:38Giant clouds over the North Pole.
49:41The travelling storms had reached their destination.
49:49It's now believed that Titan's storms go on an epic 29-year migration.
49:56From one pole to the other, and back again.
50:03As they travel, they unleash methane floods
50:07that over millennia carve canyons into Titan's deserts.
50:14Seasonal rivers on a moon 1.2 billion kilometres from Earth.
50:25Titan is a fascinating world,
50:28and although it lives in permanent twilight,
50:31and so we might expect it to be frigid and frozen solid,
50:35it has a tremendously dynamic atmosphere.
50:38There are storms and seasonal monsoons that sweep across the surface,
50:42not unlike the monsoons that sweep across Utah and Arizona.
50:46It's just that because of those temperatures,
50:49it's not water that carries energy around the atmosphere.
50:53The chemistry is shifted, and it's methane that takes centre stage.
51:04This, then, is the story of the storm worlds of our solar system.
51:12The beautiful and complex structures we call weather
51:16emerge from each atmosphere trying to do the same thing.
51:24Move energy to balance out hot and cold.
51:33But what makes these worlds so dazzlingly different...
51:41..is which chemicals play the leading role in carrying that energy.
51:54Chemistry is what happens between the heat of the stars and the cold of space,
51:59and it plays out on the surface of planets and moons.
52:02The arena is the atmosphere.
52:05Storms sculpt the surface of worlds.
52:08On Venus, the mountains might be coated in metal,
52:12and methane falls as rain on Saturn's moon Titan.
52:16Here on Earth, the atmosphere has allowed life to emerge.
52:21But our solar system is only one of hundreds of billions of solar systems
52:25out there in the Milky Way galaxy alone.
52:28So just imagine what nature, that great tinkering chemist,
52:33might have created out there.
52:52The navigation has confirmed that the parachute has deployed.
52:57Vacuum manoeuvre has started. About 20 metres off the surface.
53:07For decades, NASA has used rovers to explore the Martian surface.
53:15But because Mars is a storm world,
53:19a world with an atmosphere,
53:23there is another way.
53:28Ingenuity is a helicopter.
53:30It's our first spacecraft that we built to fly on another world.
53:37In 2021, after hitching a ride to Mars with NASA's latest rover,
53:42Perseverance,
53:44Ingenuity made history.
53:50Ingenuity is reporting spin-up, take-off, glide.
53:54Altimeter data confirms that Ingenuity has performed its first flight
53:59of a powered aircraft on another planet.
54:03Ingenuity's first flight was so cool.
54:07It was the first time I'd ever flown in a helicopter.
54:11Ingenuity's first flight was so cool.
54:15It was one of these, oh my God, it worked moments, you know.
54:19You test and you test and you do your best to design something,
54:22but to actually see it work on the surface of Mars,
54:25we called it the Wright Brothers moment, but for another planet.
54:30APPLAUSE
54:37The helicopter was designed at NASA's Jet Propulsion Laboratory,
54:41a test vehicle to prove that extraterrestrial flight is possible.
54:48But the major challenge for engineers was Mars' atmosphere.
54:54Mars does not have a lot of atmosphere to speak of.
54:58It's not like here on the Earth, it's much thinner.
55:01A couple of molecules bouncing into each other every once in a while
55:05is not a lot of stuff to push against to generate lift.
55:11So you have to have a helicopter that's very, very, very, very light
55:15and you have to have rotors that spin very, very, very, very, very fast.
55:22Only designed to fly five short test flights,
55:26Ingenuity surpassed all expectations.
55:30That looks great. We're right in the vicinity of where we want it to be.
55:35Its mission was finally brought to an end
55:37when it sustained rotor damage on its 72nd flight.
55:43But during its active three years on Mars,
55:46the helicopter pioneered a new approach
55:48to exploring the storm worlds of our solar system.
55:53Helicopters like Ingenuity open up a new dimension to exploration
55:57on the surface of the planet.
55:59And I mean dimension literally.
56:02You can cover so much more ground.
56:04Instead of driving for meters every day, you can drive kilometers.
56:10You're also going to get this bird's eye view of the planet
56:13that's going to be very different.
56:16A rover on the surface has got to climb over boulders, climb up hills.
56:20With a helicopter like Ingenuity, you just fly right over it. No big deal.
56:26Which is why NASA's future mission to Titan is going to be a flying one.
56:34Titan is a fabulous place to explore by rotorcraft.
56:38It's smaller, so it has much lower gravity than Mars.
56:42But it also has a much thicker atmosphere.
56:46If you and I were sitting on the surface of Titan
56:49and strapped some wings and an oxygen mask to our face,
56:53we would be able to generate enough lift to fly.
56:58So you can build something that's a lot heavier,
57:00that has a lot more complicated, intense science instruments.
57:04In 20 to 30 minutes, Dragonfly will cover several kilometers.
57:09Compare this to the rovers on Mars,
57:11which go about 100 meters over the course of a day.
57:14This will allow the Dragonfly team to visit many sites with one spacecraft.
57:22Dragonfly's fundamental mission
57:24is to give us an understanding of the chemistry on Titan.
57:30What is the surface of Titan actually made of?
57:33That question has huge implications for our understanding
57:38of how complex chemistry can become
57:41which means it's important for our understanding
57:44of how life may emerge elsewhere in the universe.
57:49And it's, I think, going to be so cool.
57:52I can't even imagine what Dragonfly is going to see
57:54and what we're going to learn.
57:56I can't wait for that mission.
58:03Next time, the ice worlds.
58:06Where mountains of ice float across great frozen plains.
58:11Where strange aurora hang above an icy giant.
58:16Where a moon is torn apart by a monster planet.
58:36Next time, the ice worlds.