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NARRATOR: Across the solar system,

spectacular storms are raging.

JAMES DOTTIN: What we've learned about weather on other planets

has rewritten the rulebook on what we thought was possible.

NARRATOR: From corrosive clouds...

GEORGE DRANSFIELD: If you were to go skydiving through the clouds of Venus,

there wouldn't be a lot of you left at the end.

NARRATOR: ...to global dust storms.

DAVID GRINSPOON: On Mars, the whole atmosphere basically freaks out,

and the storm grows to the point

where it engulfs the entire planet.

We don't have anything like that on Earth.

NARRATOR: Storms found in unexpected places...

DERRICK PITTS: It's a moon that looks like this!

How do you have these features on the moon of a planet?

NARRATOR: ...and made of bizarre materials.

SHANNON MACKENZIE: Buckets and buckets of rain

are falling on the surface, and yet,

it's not water raining out of the atmosphere,

but liquid methane.

NARRATOR: Extra-terrestrial weather

makes exploration challenging.

ABIGAIL FRAEMAN: Seeing the skies get darker and darker and darker,

we started to realize that,

"Oh, this is actually, this is a really bad one."

NARRATOR: But can teach us so much.

PETER GAO: It opens up the possibilities of, "What is weather?"

NARRATOR: What drives these exotic storms raging across the solar system?

And how do they shape a planet's destiny?

"Solar System: Storm Worlds."

Right now, on "NOVA."

♪

NARRATOR: It's midsummer.

♪

A storm is brewing.

♪

It unleashes torrents of rain...

...forming floods that drain into rushing rivers...

♪

...and giant lakes.

♪

This is the only world in our solar system

where rainstorms pummel the surface.

♪

The only world other than Earth.

Here, more than 800 million miles from the sun...

...orbits the largest of Saturn's moons.

In some ways, Titan is more similar to Earth

than any other place in the solar system.

But upon closer inspection,

you will find that it's actually exceptionally alien.

The surface temperature of Titan

is around - 300 degrees Fahrenheit.

NARRATOR: Far too cold for liquid water.

DOTTIN: What's fascinating is that

the rain that's falling out of the sky on Titan

and filling up the lakes

is usually found on Earth as a flammable gas.

NARRATOR: Methane.

Titan is a world shaped by alien storms,

and it is not alone.

MACKENZIE: It's pretty mind-blowing

that as we look elsewhere in the solar system,

we see these weather patterns

that look very familiar to us,

and yet there are also these wild extremes.

♪

DRANSFIELD: Some of the storms we're seeing

are like supersized versions

of the storms that we see here on Earth.

♪

NARRATOR: And the more we explore,

the more weird and wonderful weather we're finding.

GRINSPOON: Pretty much every planet with weather

has some oddity to it that's surprised us.

NARRATOR: So what creates the wild diversity in weather

across the solar system?

GAO: We're so used to dealing with our specific brand of weather.

But there's just so much more out there,

if you look at these stormy worlds.

♪

♪

ALEJANDRO SOTO: When I hear the word "weather,"

I get a memory

of when I was a kid, standing out in a field...

♪

...when the wind would pick up, the sky would darken a bit,

and you knew it was about to rain.

[wind gusting]

And yet it would also be the best.

NARRATOR: From towering thunderstorms

to howling blizzards

to choking dust storms,

our atmosphere is a restless place.

But what causes all this chaos?

JEN GUPTA: Earth, like all the planets,

is essentially a big ball floating in space

being heated by the sun.

It's pretty simple, really.

But because of the shape of a planet,

its spherical geometry,

it's actually really hard to heat the planet evenly.

For a start,

there's always a side of the Earth

in darkness, the night side of the planet.

DOTTIN: And some parts,

like the Equator,

receive more sunlight

than others, like the poles,

and that creates an energy imbalance.

But because the Earth is surrounded by this atmosphere,

this envelope of gas that's free to move around,

it can carry the heat from the hot places to the cold

in an attempt to equalize this imbalance.

LEIGH FLETCHER: And without that conveyor belt of energy motion,

we wouldn't have

the incredible diversity ofweather

that we can see around us today.

♪

NARRATOR: Wind, rain, and lightning are all side effects

as our atmosphere balances hot and cold.

♪

And what applies here applies elsewhere, too.

CLARA SOUSA-SILVA: In our solar system, wherever there's an atmosphere,

there's weather, no matter how different

an atmosphere from Earth's.

FRAEMAN: Some are thicker, some are thinner,

sometimes they're made of different things.

And all of these combine

to give these crazy cool weathersystems

that are all so different

across the solar system.

♪

NARRATOR: To see this alien weather in action

across our vast solar system,

we head away from our shared star...

♪

...passing a planet so small and close to the sun

that it has no atmosphere,

to encounter a planet

smothered by one.

PITTS: Venus looks so beautiful in the evening sky.

Brilliant, clear, white,

just stunningly lovely to see,

yet it's such a horribly nasty environment.

NARRATOR: Looking down from orbit,

we can't actually see Venus's surface at all.

Instead, all we see are endless, thick, churning clouds

that completely conceal the rest of the planet.

♪

GRINSPOON: Historically, we thought, "Oh, Venus has clouds.

"They're probably made out of water.

It might be sort of nice there."

[laughing]: But then,

when we started to actually get data from spacecraft,

we discovered, no, they're not water.

In fact, they're battery acid.

♪

SOUSA-SILVA: The clouds on Venus are

made up of extremely concentrated sulfuric acid,

one of the most corrosive substances we know of.

If you were to go skydiving

through the clouds of Venus,

there wouldn't be a lot of you left at the end.

NARRATOR: But if we could withstand the acid

and took the plunge down through Venus's clouds...

...we'd reach a landscape that is eerily calm...

...but with air pressure more than 90 times greater

than at Earth's surface.

GAO: Even though we don't realize it,

air has weight.

It's being pulled down bygravity,

just like the rest of us.

And so, on Venus, where the atmosphere

is so thick and so massive,

all that air is being pulled down,

and if we're on the bottom of it,

then we'll feel all that pressure.

SOUSA-SILVA: If you find yourself on the surface of Venus,

you would almost immediately be crushed.

But if somehow you survived

and tried to go for a stroll, it would be exhausting.

The atmosphere is so thick,

it would be like walking through molasses.

NARRATOR: The surface of Venus

is comparable to the bottom of Earth's oceans,

but with a key difference.

At about 900 degrees Fahrenheit,

this is the hottest planetary surface in the solar system.

And again, Venus's extreme atmosphere is responsible.

SOUSA-SILVA: Carbon dioxide and other molecules

are able to absorb and radiate heat,

and so, in large enough quantities,

they can act like a planetary blanket

that traps that heat in a planet.

That's what we call the greenhouse effect.

NARRATOR: And Venus's incredibly dense atmosphere

is 96.5% carbon dioxide.

♪

Even 0.04% CO2 in our atmosphere

is enough to initiate a climate crisis

in our world right now.

So you can imagine how bad it is on Venus.

NARRATOR: Venus's extreme greenhouse effect

means it's even hotter than Mercury.

Which is why what we would find there

is so extraordinary.

♪

What look like snow-capped mountains

towering over the scalding surface.

For generations, we could only guess

at what lay beneath Venus's clouds.

But, thanks to radar images

beamed back by NASA's Magellan orbiter,

we now have detailed maps of the planet's surface.

So, here we have an image taken by Magellan,

and what we're looking at

is actually Maxwell Montes.

Maxwell Montes here

is the highest region on Venus.

And what you'll notice is that around this image,

the surface looks dark.

But in the center, where Maxwell Montes is,

it's much brighter-- it almost looks white.

We weren't expecting

to have such radar-bright regions

on the highlands, in the mountaintops of Venus.

♪

GUPTA: When you see those radar images of Maxwell Montes,

it's hard not to imagine snow-capped mountains.

♪

NARRATOR: But given the conditions on Venus,

snow as we know it is impossible.

♪

In places, it's so hot,

the ground actually glows,

like metal coming out of a forge.

GRINSPOON: On Earth, you can imagine

waiting till nighttime to cool off.

That wouldn't work on Venus.

There's no relief at night on Venus from the temperature.

And in fact, if you go from the equator to the pole,

it doesn't get any cooler, for the same reason--

that thick atmosphere is very efficient

at redistributing the heat.

NARRATOR: It is a world where there is no place for snow to hide.

But there's one way to cool off on Venus,

and that's to go up a mountain.

♪

NARRATOR: A phenomenon that we here on Earth can also relate to.

♪

PORT: Everyone knows when you go up a mountain,

you should always pack a jacket.

And the reason for that is because it's much colder.

But why is it colder?

Well, when you go up a mountain, there's less air,

and that means there's less pressure.

♪

NARRATOR: And as the pressure decreases,

so does the temperature.

PORT: So, as we go up in elevation,

we cross a threshold

where the temperature drops low enough

that water can be a solid.

And this is what we call the snow line.

NARRATOR: Venus's snow line suggests

something may be freezing up there.

But even its highest peaks

are far too hot for that something to be water.

And that points us to substances

that solidify at much higher temperatures.

PORT: So, instead of water,

it actually could be something like this.

So this is a piece of galena,

also known as a lead sulfide.

On Earth, it's present as a solid,

as you can see here.

But on Venus,

it may only be a solid in the highlands.

NARRATOR: Deep in Venus's lowlands,

like valleys,

it's hot enough for compounds like lead sulfide

to vaporize,

drift on the wind,

and, as the winds flow up

into the mountains of Maxwell Montes...

...the temperature drops just enough for the compounds

to freeze out of the air.

At 820 degrees Fahrenheit,

Venus's atmosphere may coat its mountaintops

not in snow,

but in a metallic frost.

PORT: We still don't know for sure

if metal frost can explain the bright regions

that we're seeing on the mountaintops.

There's still a lot that we don't know about Venus,

and that's because we haven't even sent a lander

to the surface of Venus since the 1980s.

But it's amazing to think

that something like this could be condensing

on the mountaintops of Venus.

When we think of the possibility of metallic frost on Venus,

it sounds crazy, but that's only crazy for us Earthlings.

If you have a different atmosphere,

you get different weather, different phenomena playing out.

♪

NARRATOR: Leaving this crushing world behind...

...we head out in search of an atmosphere

that could almost be Venus's opposite.

To reach it, we bypass our own planet.

Passing two potato-shaped moons

orbiting our other next-door neighbor,

we encounter an atmosphere

more than 10,000 times thinner than Venus's.

With little in the way of cloud cover,

we don't need radar to get a look at the Martian surface.

Yet, images sent back by our orbiters

suggest rain may have once fallen here.

These pictures show that Mars is covered in features

which appear to have been created by flowing water.

DOTTIN: With such a thin atmosphere,

it's really difficult

to retain heat, and certainly

not enough heat to have

liquid water on the surface.

NARRATOR: So if Mars was once home to lakes and rivers,

it must have had a much thicker atmosphere in the past.

FRAEMAN: In the early 2000s,

we'd seen that there were hints

that liquid water had once been

on the Martian surface

from our orbiters,

but we didn't have definitive proof

from the surface.

So the goal of the Mars Exploration Rover mission

was to follow the water.

MAN: T minus five, four, three,

two, main engine start,

zero, and liftoff

of the Delta rocket with Opportunity,

a chance to explore

and unlock the secrets of our neighboring planet.

FRAEMAN: I was 16 years old when Spirit and Opportunity

launched and landed on Mars.

But I had the amazing opportunity

through an outreach program to actually go to be at NASA,

at the Jet Propulsion Laboratory,

when the rovers landed.

MAN: Getting a bounce to signal.

[cheering]

FRAEMAN: And I was in the room with the science team

when Opportunity landed on Mars,

and it was the most amazing experience.

[cheering and applauding]

NARRATOR: Two rovers named Spirit and Opportunity

were sent to opposite sides of the Red Planet

to search for definitive proof

that Mars's dry landscape

was once a water world.

At first glance, modern Mars doesn't appear

to be a very dynamic place.

With an average surface temperature

of about - 80 degrees Fahrenheit

and an atmosphere just one percent as thick as Earth's,

it is a freezing desert.

Typically, we think of Mars as this dead world.

But we do see changes on the surface over time.

NARRATOR: Strange, overlapping lines,

often dozens of feet wide,

can stretch for miles.

Unlike many of Mars's features,

these are not relics.

We see them appear and disappear.

What are these bizarre, shape-shifting patterns?

♪

In January 2004, Spirit and Opportunity

began to investigate Mars.

♪

But their mission wasn't expected to last very long.

GUPTA: Mars is covered in dust.

It gets everywhere,

including on the solar panels of Mars rovers.

As the solar panels got dustier and dustier,

the power levels that the rover would have

would get lower and lower and lower,

and eventually, they would just stop working.

NARRATOR: The team hoped the rovers

would last at least three months,

a feat they surpassed.

But, as the months wore on,

the power levels continued to drop.

What was really exciting

was, at one point in the mission,

as we were seeing the power levels

getting lower and lower and lower,

all of a sudden, the power levels were

back up high again.

♪

Almost as high as they were when the rovers had landed.

And you can take a look at the before and after pictures,

and you can see that it looks like the solar panels

have just been wiped clean.

You could call up AAA and send them to Mars

with the rag to clean everything off.

They couldn't have done a better job.

NARRATOR: Finally, a clue to what might be happening

was spotted by Spirit's Navcam.

♪

In the mid-afternoon sun...

♪

...heating of the Martian surface

creates towering vortices,

which can grow miles high.

Dust devils.

GUPTA: I imagine it would be absolutely terrifying

to be standing on the surface of Mars

and see one of these dust devils coming straight at you.

FRAEMAN: You might think that there was this

tornado-force wind coming towards the rover,

and it might pick the rover up and spin them around

and do all sorts of horrible things.

But in reality,

the Martian atmosphere is actually very, very thin,

and so you can have a dust devil pass over you

and, and you would barely even notice it.

But even though a Martian dust devil

might not be able to pick you up,

it can certainly pick dust up.

NARRATOR: It's thought that

as these whirlwinds traveled over the rovers,

they sucked dust off the solar panels,

like a vacuum.

So a mission expected to last just a few months

was extended again,

and again,

and again.

But the cleaning power of the dust devils

doesn't just work on rovers.

[wind roaring]

♪

As they travel,

these huge vortices

suck up dust from Mars's surface,

exposing the darker bedrock beneath.

Leaving trails so large,

we can see them clearly from space.

♪

There are no Martians creating the lines.

The culprit is the Martian atmosphere.

Dust devils might create the tracks,

but something else wipes them away.

♪

Just like Earth,

Mars has a tilt that gives it seasons.

Summer in one hemisphere means winter in the other.

And with no oceans or thick atmosphere

to help spread heat around the globe,

an extreme temperature difference.

So here, dust helps drive the action.

♪

As summer arrives in the southern hemisphere,

warmer temperatures drive stronger winds,

which kick dust up into the air.

The dust absorbs sunlight and heats the air around it...

...further strengthening the winds,

which lift even more dust...

...until a storm forms...

...wiping away dust devil trails.

And these storms can combine and grow so large...

...that they engulf the entire planet,

sometimes blocking more than 99% of the incoming sunlight

from reaching the surface...

...and the solar panels of Mars rovers.

In May 2018, with Opportunity still active on the surface,

a storm began to build.

FRAEMAN: It only took a couple of days

of seeing the skies get darker and darker and darker

before we started to realize that,

"Oh, this is actually, this is a really bad one."

Um, and by the end of the week,

we knew that this was the worst dust storm

that we were ever gonna have to survive.

We knew we were probably gonna lose communication with Oppy.

So we let her go dark

and we just waited and waited.

NARRATOR: Mars's low gravity means the dust in the air

can remain there for months.

♪

Eventually, with so little sunlight

reaching the Martian surface,

the temperature differences which fueled the storm

are dampened,

cutting off its own energy supply,

and the dust finally settles.

But by the end of 2018,

even after the Martian skies had cleared,

there was no signal from Opportunity.

And with winter looming,

it was unlikely the rover's electronics

would survive the cold.

♪

On February 13, 2019,

NASA declared its mission complete.

FRAEMAN: I know it sounds weird,

'cause it's a robot... [laughs]

...but it's sort of the same feeling you have

when you might lose a loved one

who's lived a very full life.

I was a high school student when they landed.

By the time Opportunity's mission ended,

I had gone to college, I had gotten a Ph.D.,

I had done a post-doc,

and then I had been hired by NASA

to work on this very same Opportunity rover

that had inspired me when I was a high school student.

NARRATOR: Sent to Mars to hunt for ancient evidence of water,

Opportunity not only succeeded, but exceeded expectations,

lasting over 14 years thanks to the Martian weather.

♪

Despite their differences,

Venus and Mars have one thing in common.

Their weather is powered by solar energy.

But beyond the asteroid belt...

...ruled by the dwarf planet Ceres,

the sun's influence wanes.

And yet, the next planet we encounter

is a storm world...

...on the grandest scale.

PITTS: If you were just arriving at Jupiter in a spacecraft,

you would be stunned by what you see.

First of all, there's the size.

You can fit 1,300 planet Earths inside Jupiter.

In fact, you could fit all of the planets

in the solar system insideJupiter

and still have room to spare.

NARRATOR: Jupiter is a gas giant

on which storms can grow bigger than Earth itself.

Since 2016, NASA's Juno spacecraft

has been exploring this gargantuan planet

and found its weather is anything but subtle.

[thunder rumbling]

Jupiter's thunderstorms can produce lightning bolts

ten times more energetic than anything recorded on Earth.

[thunder rumbling, lightning crackling]

Most of the flashes lie beneath thick ammonia ice clouds.

But the most powerful storms break free,

soaring to the edge of space.

[thunder booming, lightning crackling]

The scale of Jupiter's storms may be hard to comprehend.

But Juno found that the frequency

of its lightning strikes is remarkably similar

to Earth's thunderclouds.

DRANSFIELD: It's odd that lightning is such a common occurrence

here on Earth,

but it's taken us centuries

to really understand how itworks.

[thunder claps, lightning crackles]

NARRATOR: The substance responsible

for our planet's thunderstorms is water.

The way that lightning getsformed

is that it relies on collisions

between icy particles and watery droplets.

And as the watery droplets are moving upwards and colliding

with the icy particles that are falling downwards,

they're exchanging charge with each other.

So you get a separation of charge

from the top of a cloud to the bottom of the cloud.

And as that separation of charge

becomes bigger and bigger and bigger,

eventually, the atmosphere can't cope anymore.

PITTS: And when that occurs,

now a flow of electricity can happen.

[lightning crackling]

NARRATOR: A single bolt of lightning is so powerful,

it can heat the surrounding air

to temperatures greater than the surface of the sun.

♪

So how can clouds produce so much energy?

HEIDI BECKER: For all of our history, lightning has been

something that humans have feared.

But what's driving it really is the water cycle.

The sun heats the water,

it evaporates, becomes water vapor,

rises up into the atmosphere,

cools, and eventually becomes

water droplets in clouds.

And intuitively, as humans,

we actually understand what's happening very well.

We wet our hand,

and it feels cold because

the water is stealing energy from our hand,

breaking up the water molecules,

turning them into water vapor

so that they can

rise up intothe sky.

NARRATOR: Water evaporation requires energy,

whether it's from our body heat or the sun.

As water turns from a liquid to a gas, it stores this energy.

And when it turns back into a liquid, in the form of clouds,

it releases this energy back into the atmosphere.

What's kind of incredible to believe

is that a, a small little cumulus cloud that you might see

floating in the sky, looking so graceful and peaceful,

is actually the result

of an enormous amount of energy being released.

It's the same as hundreds of tons of TNT.

NARRATOR: And for an average thunderstorm,

the energy released is equivalent to a nuclear blast.

It's this huge release of heat

that makes thunderclouds so turbulent,

causing the countless icy collisions...

[thunder rumbling, lightning crackling]

...which power Earth's lightning.

Several missions to Jupiter have detected water

in its atmosphere.

But the planet receives just four percent of the sunlight

that reaches Earth,

too little energy to account for its ferocious lightning storms.

♪

But looking at the planet in infrared

reveals a different source of energy.

Jupiter is radiating heat,

almost twice the energy it receives from the sun.

FLETCHER: So, Jupiter itself is slowly cooling,

and has been over the entire age of the solar system.

And as it's cooling, the entire gas giant is slowly shrinking.

And as it shrinks, it's relinquishing that energy

that was left over from when Jupiter was born.

That's enough to power

the gargantuan storms that we see.

[crackling]

NARRATOR: This internal heat means water.

The chemical powering Earth's storms

also powers storms on Jupiter.

In vapor form, water rises,

carrying heat energy from deep inside the planet...

...until it reaches a place under the ammonia ice clouds...

[thunder crashes]

...where it's cool enough

to condense into liquid water droplets

and ice crystals.

The energy released by this condensing water

powers the growth of violent thunderstorms.

But because Jupiter has so much internal heat

that's trying to escape,

in some places, enough energy is released by condensing water...

...that ice crystals are swept up into the ice clouds above.

Here, ammonia acts as an antifreeze...

...allowing thunderclouds to grow

to more than 35 miles tall

and lightning to strike at minus 150 degrees Fahrenheit.

♪

Jupiter and Earth may be worlds apart,

but both play host to lightning storms

which are driven by water.

I just find it wicked cool that on a planet

as different as Jupiter,

the weather is being formed in much the same way

as it is here on Earth.

NARRATOR: However, there is one more place in the solar system

with even more Earth-like weather.

PITTS: But what's really interesting and exciting is,

the liquid that's being shiftedaround

isn't water.

NARRATOR: To find it, we travel 400 million miles farther

from the sun

and encounter another gas giant.

But this isn't our destination.

♪

Saturn is orbited by an amazing variety of moons,

so many that it can be hard to keep track of them all.

Iapetus, Rhea,

Enceladus...

GAO: Tethys, Mimas, Epimethys.

Helene, Prometheus, uh, Calypso.

Shoot, there's that really funny one.

Phoebe.

There's Pan, which is really cool

because it looks like a ravioli.

GRINSPOON [laughing]: I didn't cram for this.

Did I mention Mimas?

GRINSPOON: What's the little, really weird...

Hyperion.

Uh... [chuckles]

Um...

Um, uh, let's see.

And a bunch more. [laughs]

NARRATOR: But one is special.

More than 20 times the mass of all the others combined.

It's the only moon in the solar system with a thick atmosphere.

In November 1980, NASA's Voyager 1 spacecraft

revealed Saturn and its many moons

in greater detail than ever before.

PITTS: When the Voyager 1 spacecraft flew by Titan,

we could see that it had an atmosphere,

and a thick atmosphere,

but this was only a tantalizing view.

SOTO: The images from Voyager of Titan

were exciting but also, in a way,

infuriating, right?

It confirmed everything that we had thought about Titan,

but it didn't allow us to see down to the surface.

With Voyager, we could do some remote sensing

and start to tell that the surface was, was very cold,

but in the right temperature range

so that you might actually have liquid methane on the surface.

And so we, we just knew that we had to go back

and, and learn more.

MAN: Five, four, three, two, one,

and liftoff of the Cassini spacecraft

on a billion-mile trek to Saturn.

NARRATOR: The legendary Cassini-Huygens mission

took almost seven years to reach Saturn

and spent over a decade

exploring the planet and its moons.

And on December 25, 2004,

nearly a quarter of a century after Voyager 1's flyby...

...the Huygens probe detached from Cassini

in a bid to become the first probe ever to land on a body

in the outer solar system.

♪

You know, there was so much anticipation

of what that surface might be like.

But I think in none of our wildest dreams

was the surface of Titan as interesting and beautiful

and exotic and exciting as it turns out to actually be.

NARRATOR: During its two-and-a-half-hour descent to Titan,

Huygens captured images of intricate ridges,

deep gullies,

and, perhaps most strikingly, river channels.

GRINSPOON: It was truly like a dream, but there they are.

These are real pictures coming

from our first-ever descent to Titan.

And, uh, it's unmistakable what we're seeing.

And I'll, I'll just never forget that moment.

PITTS: It's a moon!

It's a moon that looks like this!

Not a planet.

How do you, how do you have these fluvial erosional features

on the moon of a planet?

♪

NARRATOR: The team had designed Huygens to float,

but it landed in what appeared to be an empty riverbed.

And the river channels it imaged during its descent were dry.

The probe continued to transmit data

from Titan's surface for more than an hour

before its breathtaking mission came to an end.

But Cassini continued to explore the surface from afar.

♪

It saw a landscape of rolling dune fields,

vast plains,

and more dried-up rivers.

It seemed Titan was a desert world.

But in 2006, after many months of searching...

...Cassini's radar instrument found

what everyone had been waiting for:

lakes and seas of liquid methane at the north pole.

Titan's equatorial region may be a desert,

but Cassini had proved that its poles were anything but.

FLETCHER: So that difference we could see

between the deserts

down around the equatorialregion

and the seas that were present

up at high latitudes

had us scratching our heads.

NARRATOR: What could be carving channels

over a thousand miles away from the poles?

MACKENZIE: We think about deserts as dry places,

and most of the time they are, but they're carved,

they're shaped, they're defined by these seasonal rainstorms

that may happen only once or twice a year.

The question is, could something similar be happening on Titan?

Saturn happens to be tilted

at about the same angle that the Earth is.

So as it's going around the sun,

it experiences seasons much like we do here on the Earth.

But it takes Saturn and Titan a lot longer

to go around the sun than we do.

It takes 29-and-a-half years.

Titan shares the same orbital plane as Saturn.

And so it also experiences seasons

as it's going around the sun.

If we take the southern hemisphere, for example,

there are seven years of summer

where the southern hemisphere is pointed

towards the sun and receiving more energy.

That extra bit of energy,

even though it's only a 100th

of what we experience here on the Earth,

is enough to pump the atmosphere full of methane,

thanks to evaporation

from the lakes in the south pole.

♪

NARRATOR: Cassini saw this happening.

It flew by Titan during the southern summer...

...and saw methane clouds swirling around the south pole.

But the clouds didn't stay there.

MACKENZIE: In 2010, there was a giant cloud

that formed over the equatorial region in the shape of an arrow.

And as it moved across the surface of Titan,

we saw that land that was bright before

darkened, uh, after the cloud passed over.

And the area that darkened

is equivalent to about Utah and Arizona put together.

So how did that happen?

What is responsible for that?

Well, we think the best explanation

is a giant rainstorm.

♪

NARRATOR: In Titan's deserts, autumn brings change to the air.

Monsoons arrive from the south...

...unleashing torrents of methane rain.

But because of Titan's low gravity and dense atmosphere,

the raindrops fall about six times slower than on Earth.

These storms dump enough liquid methane onto the surface...

...to create flash floods...

...which drain into dry riverbeds...

...carving into the landscape...

...before spilling out onto vast floodplains.

MACKENZIE: So, thanks to Cassini, we know

that massive floods are happening on Titan.

And just a few months later,

which is only, like, five Titan days, that same land

reverted to what it looked like before, it brightened.

And the clouds themselves

that were responsible for that rain had already moved on.

♪

NARRATOR: In 2022, five years after the Cassini mission ended,

the James Webb Space Telescope

turned its infrared gaze towards Saturn's largest moon.

By now it was late summer in Titan's northern hemisphere,

and the telescope spotted something remarkable:

huge clouds over the north pole.

The traveling storms had reached their destination.

It is now believed Titan's storm belt

goes on an epic 29-and-a-half-year journey

from one pole to the other and back again.

Slow-motion seasons

unleashing massive, slow-motion storms,

creating seasonal rivers of methane,

in a desert on one of Saturn's moons.

♪

MACKENZIE: It's pretty incredible to be standing here

in a desert landscape on the Earth

and think about how similar that is

to a landscape a billion miles away

elsewhere in the solar system,

the same processes, the same desert monsoon systems

we think are at work on Titan,

creating vistas much like this one that we see here.

And yet the compounds, the materials

that are involved in that process are totally different,

and the temperature's totally different,

and the timescales are totally different.

And they're leading to a phenomenal landscape

like the one that we see here.

♪

SOUSA-SILVA: On all these worlds, all this complexity,

all these storms

ultimately come from one fundamental law of nature.

FLETCHER: That need to move energy

from where it's plentiful to where it's scarce.

DOTTIN: What makes these worlds so different

are the unique conditions

that are found in their atmospheres.

[thunder rumbling]

GUPTA: Because that determines the chemistry

that drives their storms and shapes their surfaces.

[thunder rumbling]

GRINSPOON: You know, it's a pretty amazing time to be alive

in, in that we have the ability

to be monitoring weather on other planets.

And yet I feel like we're in our early stages

of understanding weather beyond Earth.

DRANSFIELD: There are hundreds of billions of stars

out there in the galaxy.

And we now know that planets around stars is the norm.

GAO: So that's potentially billions and billions

of planets orbiting out there,

all each with their own weather systems and storms.

FRAEMAN: And I think there are things that are happening

on these planets that we haven't even begun to imagine,

because they're gonna be so different

from anything else that we've been able

to experience in our own solar system.

GAO: And so that just makes me think of,

what possibilities exist out there?

What have we not thought of?

♪