Original subtitles

(intense music)

- [Narrator] Billions of years ago,

our nearest planetary neighbor, Venus,

may have had oceans and rivers,

life-giving habitats,

like the ones that graced the early Earth.

(dramatic music)

(thunder rumbling)

Today, it's burned-out surface

is a global fossil of volcanic destruction.

(lava sloshing)

Concealed beneath a dense toxic atmosphere.

(wind whooshing) (soft dramatic music)

Scientists are now unveiling daring new strategies

to explore its tortured landscapes,

to search for clues from a time when the planet was alive.

How did Venus descend into this hellish state?

And how did our planet, Earth, manage to survive?

(mysterious music)

(thunder rumbling) (lightning clapping)

(slow music)

At the surface, the air is 95% carbon dioxide.

So dense, it's more like an ocean than an atmosphere.

And it's hot enough to melt lead.

Overhead, thick clouds of sulfuric acid.

(eerie music) (thunder rumbling)

No spacecraft has lasted longer than two hours on Venus.

(lightning clapping)

All we have seen close up,

a few images transmitted by the Soviet Venera landers

back in the 1970s.

(mystical music)

Radar images from the Magellan Mission in the '90s

have stoked our desire for more.

- We think of it

as this sort of ugly, forbidding, scary place,

because we've seen these few Russian lander images

of these sort of eroded rocks and crushing atmosphere.

But when you look at the Magellan imagery

and you see this bewildering array

of volcanic forms and flows

and bizarre and interesting canyons and valleys and channels

that have been carved by liquid lava

and volcanoes of different sizes and shapes,

I think the surface of Venus is probably really beautiful.

(mystical music) (thunder rumbling)

(dramatic music)

- [Narrator] Thousands of years ago,

Asian skywatchers saw Venus as two separate stars.

(speaking in foreign language),

the Beginner of Brightness in the morning.

And (speaking in foreign language),

the Exalted Western One in the evening.

- It's always sort of flirting with the edges of night.

You'll never see Venus at midnight up high in the sky.

It's always just in the edges of dawn and dusk.

- [Narrator] Sumerian priests saw it as one, Inanna,

goddess of sex, fertility, physical beauty, and attraction.

(woman singing in foreign language)

Inanna became Aphrodite to the Greeks.

Born from the foam of the sea,

protector of those who sail,

the Hellenic goddess of love

beckoned us to venture into the unknown.

(dramatic music)

Roman leaders declared April the month of Venus

throughout the empire.

During the festival of Veneralia,

Romans honored purity and piety and affairs of the heart.

Across the Atlantic,

Venus was seen as the little brother of the sun.

- In a way, the most astute Venus observers

were the Mesoamericans, the Aztecs, the Maya, the Toltecs.

For those cultures, Venus was this male macho warrior

with a spear going down into the underworld

and doing battle against the enemies of humanity.

(light music)

We think of Venus as Botticelli's Venus,

the feminine figure in our culture.

But, around the world,

Venus has had a lot of very different manifestations.

- [Narrator] In time, the Venus of myth

gave way to the scrutiny of science.

(soft upbeat music)

In 1610, Galileo Galilei observed crescent Venus

changing through phases like the moon.

Yet another clue that planets orbit the sun.

But even through his telescope,

Galileo could not discern its features.

Finally, in May 1761, thanks to a rare alignment,

the Russian astronomer, Mikhail Lomonosov,

glimpsed its distinctive character.

(shimmering music)

- About once every 100 years or so,

Venus actually passes in between the sun and the Earth,

and you can see that little dot of Venus

moving across the face of the sun.

(dramatic music)

At the moment when the dot of Venus

starts to cross the disk of the sun,

the very first moment and the very last moment

when the dot of Venus moves off the disk of the sun,

something weird happens.

There's a distortion, what is called the black drop.

(mystical music)

It's almost like the edge of the sun

kinda leaps off towards it,

and then is distorted inwards.

And Lomonosov realized that that would make sense

if Venus had an atmosphere.

Once you know that,

then you can deduce the diameter of Venus.

And they said, "Oh, wow!

"It's basically the same size as Earth.

"It's the size of Earth, and yet it's so bright.

"Why is it so bright?

"Oh, because it must be covered in clouds."

And they were right about that, as well.

And the assumption at that time, which was reasonable,

was that the clouds are made out of water.

(intense music)

- [Narrator] The idea that Venus was like Earth

sparked the imaginations

of fantasy and science fiction writers.

(dramatic music)

(singers vocalizing)

- I remember seeing a book when I was a kid

that said that Venus might have prehistoric jungles

and dinosaur-type animals.

I thought that would be really exciting,

but turned out to be nothing like that.

(engines roaring)

- [Astronaut] (indistinct) be advised, three.

- [Dispatcher] 30 seconds, this is programming, over.

- [Narrator] At the dawn of the Space Age,

Mariner 2 was the first successful mission

to another planet.

- Way back in 1962, Mariner 2 became the first spacecraft

to explore Venus on a flyby.

And it made a fantastic discovery

that the Venus surface was very hot.

Unfortunately, that was not very good for Venus exploration

because all the classical ideas about Venus being habitable

with people and forests living on the surface

just went out the door.

(intense music)

- [Narrator] Stranger still,

NASA's Mariner 2 found that the planet rotates very slowly

and backwards compared to Earth.

Americans celebrated the triumph.

(soft music)

Not to be out done,

the Soviet Union attempted at least 16 missions to Venus

between 1961 and 1984.

(air whooshing)

December 1970, Venera 7 made the first successful landing

and the first transmission of data from its surface.

Nearly five years later,

Venera 9 and Venera 10 captured the first images

of Venus's tormented landscape.

In December 1978,

the American Pioneer Venus Orbiter

began a 13-year radar mapping mission.

It was followed by a separate Pioneer MultiProbe

dropped into the atmosphere.

- [Dispatcher] Atlantis, Houston.

Sunnyvale had a good direct check

and their command is complete.

- [Narrator] And a decade later,

NASA's flagship, Magellan,

was deployed from the Space Shuttle

on an unusually long 15-month journey inward to Venus.

- [Command Control] Okay, Peter, we copy checkout.

(intense music)

- [Narrator] Looping pole to pole more than 4,000 times,

Magellan's imaging radar

mapped more than 97% of Venus's landforms.

Among Magellan's findings, the volcanic surface of Venus

overlies a surprisingly thick crust.

(mystical music)

Remarkably, about three billion years ago,

Venus was probably much more like Earth.

(gas hissing)

Each one nurtured the promise of life.

(water bubbling)

- They're the same size.

They formed in roughly the same place.

There's a lot of circumstantial evidence

that Venus had a more Earth-like environment

when it was young.

So they may have both had warm oceans

and all the other conditions necessary for an origin of life

at the time when Earth apparently had an origin of life.

- Our biggest problem with Venus

is that we have so little information

about its ancient past.

The surface of a planet,

and to some extent the atmosphere of a planet,

are like a crime scene.

Venus is the worst crime scene imaginable

because it's been almost completely disrupted over time.

And the big question for those of us

who are trying to study Venus and understand its history

is, when did Venus's evolution really diverge

from Earth's evolution?

(mysterious music)

- Venus and Earth are the twin planets.

It's the only planet

that is likely to be still geologically active

in some of the same ways as the Earth.

It's really a huge laboratory for understanding the Earth

because of its many similarities,

as well as some differences.

(lava sloshing)

- So Venus is kind of the Earth's evil twin sister.

It shows how two worlds of similar size, similar density,

not that far apart in terms of distance to the sun,

evolved very differently.

(mysterious music)

- [Narrator] Early on,

Venus was on a path similar to its sister planet, Earth.

The radar data hold intriguing clues to a watery past,

including vast plateaus made up of granite rocks

that likely formed at the bottom of an ocean.

- There are areas on Venus that look like continents.

They're high-standing. They're kinda wrinkly.

They are also the oldest rocks on Venus

because Venus has had this weird history

where much of the surface is new.

So these rocks are the only rocks

from the first 80% of the history of Venus.

That's where we want to go.

That's where I wanted to go is to these areas,

which are called tessera terrain

because of their tile-like appearance.

Whatever it is that spurred the formation of life,

in my mind, why wouldn't it have happened everywhere?

And that's the record that we're looking for

in the rocks of the tessera.

Were there evidence of ancient environments

that would support life?

(gas hissing) (mystical music)

(light music)

- [Narrator] Venus is close to Earth

in size, mass, and rocky composition,

but that's where the similarities end.

- Now, one thing we've learned in general

about the formation of the planets

is that the final stages of planetary formation

were characterized by a small number

of really huge collisions.

(planets sizzling)

Those last few violent collisions happened on Earth

in just such a way as to leave it with a moon

and on Venus, through a different combination

of impact size and impact angles,

it may have had been left with no moon,

but with its very slow rotation.

- [Narrator] Compared to our 24 hour day,

it takes more than 243 Earth days

for Venus to rotate just once,

the longest day in the solar system.

- And it rotates in the opposite direction,

so that if you were on the surface of Venus

and could see the sun,

it would rise in the west and set in the east.

(light music)

On Earth, the sun moves along in the sunrise

at hundreds of miles per hour.

So you have to be up in a jet airplane

if you wanted the sunset to last forever.

On Venus though, it's four or five miles per hour.

So you could kind of walk along at a good clip

and keep the sunset going

or the sunrise going as long as you wanted.

The pace of the planetary rotation is a human walking pace.

- [Narrator] Even that slow pace may be winding down.

- The very thick atmosphere of Venus

is actually being pushed and pushing back

from the typography

and changing its rotation rate very slowly

and very slightly over the decades.

And that has been measured by ground-based units

and by Magellan, and even by Venus Express,

the most recent European mission.

(wind whooshing)

(mystical music)

- [Narrator] Though the surface creeps along without hurry,

the clouds move quickly and their speeds change.

The Japanese Akatasuki probe,

also known as the Venus Climate Orbiter,

recently found that the atmosphere spins much faster

than the planet itself,

a phenomenon called superrotation.

This movement is driven by heat

rising continually from the surface.

(gentle music)

Mysteriously, the planet lacks a key feature

that protects Earth and its atmosphere,

a strong magnetic field.

(intense music)

Without this protective shield,

intense ultraviolet rays from the sun

split water molecules apart in the high atmosphere.

Then the sun's intense stream of charged particles,

the solar wind carries much of the hydrogen away into space.

- In the very early solar system,

When the sun was not quite as bright as it was now,

Venus was much more habitable.

But just a little bit of increase in solar brightness

kicked off a runaway greenhouse effect

where the oceans evaporated,

put water vapor into the atmosphere

that increased the greenhouse,

and it ran away to become the very hot, very dry planet

we see today.

- We definitely believe that Venus had much more water

at the surface than it does today.

Potentially, a shallow ocean's worth of water.

We really have no information

about when that water was lost.

- That water could have persisted

for billions of years, billions.

And, on Earth, life evolved incredibly quickly, actually,

considering the meteorites that were hitting the Earth.

So why not Venus?

And if the answer is, "It didn't happen on Venus,"

that tells us something, too.

(eruptions rumbling)

- [Narrator] As Venus lost its water,

its atmosphere gave way

to the noxious outpourings of volcanoes,

the primary source of carbon dioxide.

(soft music)

But Venus lost its ability to recapture that carbon,

trapping itself in a terrifying loop.

- Ultimately, the oceans boiled off

partly because of the greenhouse property

of water vapor itself,

leading to this positive feedback

where the more water that evaporates,

the more the planet heats up

because of the greenhouse effect of the water vapor.

And then, once the oceans boil off

and Venus loses its oceans,

then that destroys the carbon cycle

because you still have CO2 coming out of volcanoes on Venus,

but there's no way to remove that CO2.

The sink disappeared when the water disappeared

and then the thermostat sorta gets pegged in the red zone

because you're just adding more and more CO2

to the atmosphere from volcanoes.

And it can't lose it anymore

without that mediating effect of water,

and it ends up in that runaway greenhouse state.

(eruptions rumbling)

(dramatic music)

- [Narrator] Why did volcanic CO2 runaway on Venus

while Earth kept it at trace levels?

One reason is a basic difference

in the geology of the two planets.

Earth, like Venus, harbors a core of molten iron.

Over time, the radioactive decay of uranium, thorium,

and potassium generates heat.

Hot liquid rock punches through

where Earth's crust is thinnest, in the middle of oceans,

and floods the seafloor with lava.

In a process called plate tectonics,

growing undersea mountains push aside

raft-like sections of the crust, the plates.

(plates rumbling)

When they collide with thicker continental crust,

they can dive beneath them.

As they do, grinding friction causes rocks to melt,

hasten by oceanic water, forming reservoirs of magma.

The pressure increases,

until finally, a volcano erupts.

(intense music) (eruption booming)

Plate tectonics generates

up to 80 volcanic eruptions each year.

Sometimes, this process can release energy

on such a large scale

that it changes the course of Earth's history.

(lava sizzling)

Today, Yellowstone National Park in North America

is an open laboratory of Earth's remarkable geology.

(geyser rumbling)

The rich mix of organic chemicals and geothermal energy

in places like this may have given rise

to living organisms billions of years ago.

The hot springs and geysers we come here to admire

are fueled by a dome of magma

that is building deep below ground.

(soft dramatic music)

At intervals of roughly every 600,000 years,

this dome pushes the land up into a broad plateau.

Inevitably, hot gas and lave explode

through the cracks in the land.

The plateau suddenly and violently collapses,

triggering eruptions so large,

they'd blanket the continent with ash.

(eruptions booming) (intense music)

- Plate tectonics is really what makes Earth tick.

Plate tectonics is the sort of overall organizing principle,

almost, of Earth's behavior.

Not just the obvious continental drift

and the movement of the continents,

but even the carbon cycle and the climate cycle

and all of these other aspects of Earth

that have to do with the way things cycle

between the interior and the surface

come down to plate tectonics.

And Venus doesn't have plate tectonics.

(slow music)

- [Narrator] You can see why

by cutting into the crust of these sister planets.

Earth has only about eight kilometers

of hard crust under the oceans

and no more than 30 kilometers under land.

But the skin of Venus is about twice as thick,

acting as a rigid lid, locking the surface in place.

- If Earth did not have volatile cycling

throughout most of its history,

and we had a stagnant plate mode,

like we have on Venus today,

then all of the nitrogen and all the carbon dioxide

that we see in Venus's atmosphere

would be in our atmosphere in the same way.

(soft dramatic music)

- [Narrator] The dense shell of Venus

may never have separated into continental plates,

but the heat must still find a way out.

(foreboding music)

- Venus is a really volcanic planet.

Most of the surface of Venus is actually covered

by volcanoes and several different types of volcanoes.

- From the smallest things that we can measure

in the radar data,

which is about a hundred meters per pixel,

to huge lava flows,

just lava flows that stretch for thousands of kilometers.

(slow music)

(mystical music)

- [Narrator] The Magellan Mission

documented a zoo of volcano types.

- Venus has some shield volcanoes.

Shield volcanoes have low slopes,

and they look like warrior shields laid on the ground.

That's where the name came from.

In fact, it came from Iceland.

Venus also has several other types of volcanoes

that we don't see in other places in the solar system.

For example, one very unusual type

are some domes that we call pancake domes,

and that's because they really look like pancakes.

They are flat on top.

They have steep sides.

Some of these domes have some fractured and jagged edges,

and they are known as ticks

because they look like squashed bugs.

Arachnoids because they have a fracture pattern

similar to a spider's web,

with some concentric fractures and some radial fractures.

- [Narrator] The enormous volcanic features called coronae

hint that Venus may be subducting rocks

back into its interior, after all.

- [Rosaly] Corona on Venus are in a way similar

to calderas on Earth,

where you have a pattern of concentric fractures.

There is a collapse or successive collapses.

- A plume of hot stuff comes up,

pushes up the surface, breaks the surface,

and a bunch of vulcanism forms at the surface,

much like what we see at Hawaii.

- [Narrator] The two volcanoes that built the Big Island

are, together, about 150 kilometers across.

But a few of the coronae on Venus have grown much larger.

- [Sue] There are a couple of very big features.

One is 2,600 kilometers across.

- On Venus, where we don't see

evidence of plate tectonics,

volcanoes are really the primary way

that we believe Venus is trying to lose its heat

or has lost its heat.

And so that record that we see of volcanism on the surface

is essentially telling us how Venus is trying to cool off,

how its interior is losing heat

and moving that hot magma and hot lava onto the surface.

(eruption rumbling) (sinister music)

- [Tony] About 500 million years

to a billion years or so ago,

something catastrophic happened.

When you look at Venus's surface from satellites

and you count the craters that you see on the surface,

we find out that the vast majority of Venus's surface

is geologically very young.

(lava sloshing)

- Half a billion years ago,

Venus had cause to erupt volcanoes over its entire surface,

its entire Earth-sized surface.

And we're still trying to figure out

how that happened and why.

(lava sloshing) (mysterious music)

- [Narrator] With its steadily declining stores of water,

lack of magnetic field,

no plate tectonics to drag carbon down into its crust,

but hordes of volcanoes blasting CO2 into its skies,

Venus sentenced its surface to climatic death.

A planet turned inside out.

(dramatic music)

(light music)

Meanwhile, its sister planet, Earth, managed to survive,

to nurture a complex and evolving biosphere.

The contrasting stories of Earth and Venus

are central to one of the greatest quests in science today.

The search for solar systems

and life-bearing worlds out in the galaxy.

That search, conducted by a growing array

of ground and space-based observatories,

has so far turned up over 4,000 planets

with thousands more detected, but not yet confirmed.

(mystical music)

Increasing numbers of these exoplanets

are close in size to Earth and Venus.

With orbits within the habitable zones

of their solar systems,

where water can exist in all three states,

liquid, ice, and steam,

statistical studies indicate there could be billions of them

tucked into the nooks and crannies of our galaxy.

(gentle music)

Is life, even intelligent life,

a natural product of the laws of nature?

Or is it a fluke?

Its chances mostly killed off

by all that can go wrong with planetary evolution.

So far, Venus has not told us

all it can about these questions.

- For years and years,

after we began to really understand Venus,

all of the scientists and the space advocates

were saying, "Oh, no, Venus is the one place

"we don't want to go to because it's so hot on the surface.

"It's hot enough to melt land."

But it turns out that, like Earth,

as you get higher in the atmosphere,

the temperature gets cooler.

And there's a region where Venus

is very, very much like the conditions

at the sea level on Earth.

(wind whooshing) (gentle music)

- [Narrator] The presence of hospitable conditions

in Venus's upper atmosphere

led to one of the most innovative mission ideas

on the books.

Imagine an entry vehicle penetrating

the outer most layers of Venus's atmosphere.

Instead of continuing to the surface,

it deploys an inflating envelope.

(balloon flapping)

An airship up to 130 meters in length.

(soft dramatic music)

This platform,

called the High-Altitude Venus Operational Concept, HAVOC,

would host a series of experiments and sensors.

(low intense music)

And its most far-ranging.

It's the start of a permanent human outpost.

A science city built in the clouds of Venus.

(light music)

The concept takes advantage

of the unique profile of Venus's atmosphere.

At ground level, the pressure is like being 900 meters deep

in Earth's oceans.

It's hotter than anywhere else in the solar system,

except the sun.

But 50 kilometers up, among the clouds,

the pressure has dropped to that of sea level on Earth.

It's no warmer than a day in June in Europe or the U.S.

(propellers clicking)

That's where balloon-born habitats could perpetually float.

(gentle music)

Consider a crew of scientist astronauts aboard an airship.

(thunder rumbling)

Making their way through the sulfuric acid clouds,

their ships' electric motors powered by the sun,

on their way home from a survey mission.

- [Geoff] The nice thing about the balloons

in the atmosphere of Venus, if they're very, very large,

is that they'd be riding with the inside pressure

about the same as the outside pressure.

It wouldn't burst like a balloon in a catastrophic failure.

- [Narrator] These sequences show a well-developed version

of a Venus cloud city.

The construction of stations this large and complex

depends on our ability to harvest materials,

like carbon fiber, directly from the atmosphere.

(propellers clicking)

Its core mission to serve as a field station

for deploying research probes around the planet

and down to its hostile terrain.

From their floating platforms,

these experimenters would investigate

the deep history of Venus's surface

and how land and atmosphere connect.

(lava sloshing)

They would check for signatures of live volcanoes

and the interaction of the atmosphere's physical, chemical,

and possibly biological components.

(lava sloshing) (light music)

(air whooshing)

All in a quest to discover

what went so terribly wrong on Venus.

(soft dramatic music)

Soviet researchers first demonstrated balloon science

with the 1985 Vega 2 probe,

riding turbulent hurricane-force thermal currents

at 240 kilometers per hour.

Vega 2's three and a half meter diameter balloon

found a stable altitude at about 54 kilometers.

(lightning clapping)

In the most active of the planet's three cloud layers,

it traveled more than 7,400 kilometers

around to the day side of Venus.

(thunder rumbling)

(propellers clicking) (gentle music)

Vega 2's descendants may deploy from airships

to sample specific altitudes.

Triangulating their positions,

they could form a global sensor network

to pick up signs of seismic activity.

(ground rumbling)

- So our balloon can listen for earthquakes

as it floats around Venus,

taking advantage of the fact

that the Venus atmosphere is so thick

that sound waves from earthquakes will propagate up

through the atmosphere

and can be measured from balloon or from orbit.

It's astounding.

(ground rumbling)

(dramatic music)

- [Narrator] Though it may appear

as a featureless planet-wide cloud bank,

Venus's atmosphere has its own complex stories to tell.

- One of the most important things

in the atmosphere of Venus that we need

is to understand some of the molecular fossils

that are left in Venus's atmosphere.

The noble gases of argon, neon, xenon, krypton,

those gases that don't react with anything else.

They've been in the atmosphere since Venus formed,

since the atmosphere formed.

And if we can measure those,

they provide us these fossil evidence

of what that original atmosphere on Venus looked like.

(gases hissing)

- [Narrator] A profile of the early atmosphere

would tell us whether Venus once held lakes and rivers

on its surface.

(propellers ticking) (intense music)

If humans do someday visit the acidic clouds of Venus,

they may find, strictly speaking, that they're not alone.

For more than 100 years,

astronomers on Earth have noticed enigmatic dark patches

that appear only at ultraviolet wavelengths.

- My Venus atmosphere friends talk about

the fact that there is a signature in the atmosphere

that they can't explain.

And there is something in the clouds

that absorbs UV radiation and they don't know what it is.

That signature may be due to absorption by biota,

by cloud creatures.

- There are nutrients. There's liquid.

There are a lot of the elements

that we think of being requirements for life.

The one impediment to life

may be the strong acid in the clouds,

but we've learned more recently

of all these extremophile organisms on Earth,

including what we call acidophilic organisms

or organisms that love living in strong acid.

(dramatic music)

- There are bacteria on Earth,

Thiobacillus ferroxidans, for example,

which have an absorption spectrum

which is very similar to what we see on Venus.

- [Narrator] Could life have developed and migrated upward

as the surface became lethal?

In Earth's atmosphere,

microbes have been found at altitudes up to 40 kilometers.

(air whooshing) (light music)

- [Mike] Earth has had 4 billion years

for life to fill every ecological niche possible.

And we find life everywhere we go.

We dig deep wells, we find life down there.

We find life in the clouds.

- If we have the opportunity to go to Venus

and float in the Venus clouds,

we should at least think about

if we can measure the conditions

that might be reasonable for life today

and also any signature of life in the clouds of Venus today.

(soft dramatic music)

(intense music)

- [Narrator] Though the cloud tops may be temperate,

the hot surface is merciless.

Venus engineers must take a mechanical approach

because even the most robust electronics

developed for the military

stop working at about 125 degrees Celsius.

- Let me show you something.

So this it's a clock

that is fully made out of stainless steel.

It's been baked out at 460 degrees Celsius

and has been operated in an oven.

And our goal with this was to start to understand,

what are the challenges when you build mechanisms

to operate in Venus conditions?

First of all, the spring material was really important.

We have a clock spring here,

as well as also a balance spring right here,

that is built out of Inconel.

Even when you're going to extreme temperatures,

like Venus, 462 degrees Celsius,

the entire assembly would just expand and contract together

and you wouldn't get jamming.

And we were able to demonstrate and show that

by operating this clock.

(clock clicking)

(dramatic music)

- [Narrator] As on Mars,

exploring the surface of Venus will require mobility.

But strategies that work for other worlds

don't work on Venus.

Not even solar power.

- Venus does have some solar power on the surface.

Although, it's a fraction of what is available here on Earth

is actually available on the surface of Venus

due to the thick cloud layers,

due to also the red shifting of the light,

and that solar panels don't work as efficiently.

The other big challenge with using solar on Venus

is that Venus has a very long night, about 60 days.

Therefore, one of the places we're looking to get energy

is from the wind.

The idea is you have low-speed, high-density wind

coming at you, collect that with a wind turbine,

and then directly transfer that to the wheels

to drive you at low speed, high torque.

You want to directly use that wind energy

to move yourself mechanically around the surface of Venus.

- We have the sensors on the surface.

We have the radio on the surface.

But most of the processing power, most of the computers,

most of the things that run the mission

would be high overhead.

We could either put them perhaps in an airplane

flying 50 kilometers above, or maybe in a satellite.

(propellers clicking)

The winds of Venus are pretty ferocious.

The winds of Venus are going 100 meters per second.

Well, one thing we could do is just fly around

and let the wind take us where the wind takes us,

as long as we can fly faster than that wind

so we can stay in the sunlight.

(dramatic music) (singers vocalizing)

- [Narrator] Venus's extreme surface heat

drives a blustery atmosphere of toxic acid rain.

(lightning clapping) (thunder rumbling)

But the planet lies within

what astronomers would think of as the habitable zone.

- Venus is a really interesting case

because it forces us to think of where is the inner edge

of the habitable zone.

Presumably now, in our solar system,

it's somewhere between Earth and Venus.

And yet, there was a time earlier

in the life of our solar system

when Venus was within the habitable zone,

when there probably was an ocean on Venus.

(dramatic music) (singer vocalizing)

- [Lori] Mars, we think at some point,

used to have more atmosphere

and was potentially warmer and wetter.

And Venus probably had less atmosphere,

at some point in the past,

and had significantly more water than we see today,

which today it seems very, very dry.

And so it's possible, when Venus did have less atmosphere,

it was habitable when it had more water available

and less atmosphere.

We very definitely think that it has changed over time.

(soft dramatic music) (lava crackling)

- [Narrator] Looking out in our Milky Way galaxy,

astronomers are seeking a broader understanding

of how planets form and what conditions can nurture

an Earth-like world.

All other exoplanets lie too far away

for us to know much about them.

But Venus is giving scientists a chance

to understand the alien planet next door.

(gentle music) (singer vocalizing)

(water sloshing)

- There's probably millions, likely billions, of planets

that are like Earth and like Venus.

And we'd like to understand

what made the one turn into a hot desert.

And Earth, on the other hand,

turn into a planet with liquid water and habitable for life.

(soft dramatic music)

- Different aspects of Venus's evolutionary history

would put it into the habitable zone.

Then that tells the exoplanet astronomers

that they should not just discard these worlds

as hot inhospitable planets,

and that they should go take a look at them.

And that would expand the idea of the habitable zone

to much closer reaches to the star

than we would otherwise think.

(gentle music) (water sloshing)

- When we look out in the galaxy

and we see all these Earth-like planets,

we're looking for another Earth, right?

We're looking for another place that is like our own.

We're looking at Earth-sized planets

and the closest Earth-sized planet

happens to be the closest planet to Earth.

(laughs) It's Venus. It's right there.

So it tells us about how Earth-sized planets form

and whether or not they're habitable.

(gentle music) (singer vocalizing)

(soft dramatic music)

- [Narrator] Two planets born together,

nearly the same size and mass.

(lava crackling)

Built of the same stuff.

At the start of their lives, each harbored oceans,

and atmospheres laden with water vapor.

Over time, one increasingly draped itself

in the greens and blues of life.

While the other became enshrouded in a toxic haze.

Its surface paved over with volcanic outpourings.

Which type of world is most common in the cosmos?

- We know a lot about Mars.

We've sent a dozen probes to Mars.

We've sent rovers across the surface.

But Venus, we have very little knowledge of,

so we need to start learning how to go to Venus.

Learn about the circulation of Venus,

learn about the climate, learn about the weather.

Venus needs to be not the forgotten planet

in the solar system.

If we understand Venus, we learn about the Earth.

(soft dramatic music)

- I imagine, imaged with the right kind of whatever drone

or spacecraft or even humans ultimately going there

in very well-protected suits,

when we get to see it in the right way,

we're gonna be like, "Wow, nobody told us.

"This place is beautiful and it's massive."

Remember, there's much more land area on Venus

than there is on Earth since Earth is two-thirds ocean,

and it's going to be just a thrilling place to explore fully

when we get the opportunity.

(soft dramatic music)

(light music)