NOVA

NARRATOR: When we look beyond Earth, we discover dynamic worlds.

KATHERINE DE KLEER: As we're exploring the solar system, we are finding remarkably diverse and ferociously active worlds.

CLARA SOUSA-SILVA: Where lava flows, where volcanoes erupt, where rumblings beneath crack the surface.

NARRATOR: From giant, dead volcanoes on the planet next door... JAMES DOTTIN: It's absolutely mind-boggling that there are volcanoes that can get that big that it can tip a planet.

That's absolutely crazy.

NARRATOR: ...to active ice-cold eruptions on frozen moons.

PAUL BYRNE: Where everything's frozen over, we see volcanic eruptions blasting out into space, and that's a real surprise.

NARRATOR: These explosive worlds could even provide a clue to one of the biggest questions of them all.

JEN GUPTA: Studying volcanoes in the solar system is incredibly important.

It can help answer that question of where you and I came from and even how life first began.

NARRATOR: But ultimately, volcanoes, fiery or frozen, reveal the incredible activity that lies within.

When I think of a volcano, I think of awesomeness and... how can I get a ticket to go there?

NARRATOR: How can there be so many different kinds of eruptions across our solar system?

And what might Earth's volcanoes tell us about the possibilities of life elsewhere?

"Solar System: Volcano Worlds."

Right now, on "NOVA."

NARRATOR: Humans have only set foot on one world beyond our own.

On the moon, we found a barren landscape, appearing unchanged for more than a billion years.

Its surface, scarred with craters and dark patches.

This was activity that occurred early on in the moon's history.

The moon now, there isn't much going on there.

NARRATOR: Our moon is an inactive world, frozen in time.

In stark contrast with Earth.

Which seethes with activity beneath the surface.

Who doesn't love a volcano?

I mean, hot stuff jumping out of the ground at you.

It would be really exciting.

OLUSEYI: I've seen erupting volcanoes, and I just love active geology.

It just tells me that the planet is alive, not literally, but figuratively.

NARRATOR: And when we look beyond Earth and its moon, out into our solar system, other dynamic worlds like ours do exist.

As we're exploring the solar system, both with telescopes and with spacecraft, we are finding remarkably diverse and ferociously active worlds throughout the solar system.

NARRATOR: Many hold clues to our own planet's story.

JASON HOFGARTNER: Looking at volcanoes elsewhere in the solar system, we see that Earth has a special type of volcanic activity, and so, by understanding other volcanoes, we might understand how Earth had eruptions in the past or in the future.

NARRATOR: And since many scientists think volcanic activity might have played a major role in the origins of life on Earth... ...other active worlds in our solar system are especially intriguing.

SOUSA-SILVA: We know that there's a relationship between volcanism and life, but we don't know where it ends and where it begins, and that's why we need to keep exploring.

NARRATOR: The largest volcanoes discovered in the solar system so far are right next door.

Mars is one of the most explored and photographed of all the planets beyond Earth.

We've captured its stark beauty.

From ice frosted dunes to canyons just over six miles deep.

And many of the most detailed images have come from the Mars Reconnaissance Orbiter.

Launched in 2005, it is still orbiting the planet today.

JORGE NEZ: It carries instruments, high-resolution instruments, that allows us to view and explore the surface of Mars in such detail that we could map out things the size of a desk on the surface.

OLUSEYI: It's the length of a school bus with these two big solar panels to power it, and as it's going about its business, it's sent back more data to Earth than any mission ever.

NARRATOR: These images show in greater detail than ever before giant volcanoes that dwarf anything seen on Earth.

One so wide... ...it would cover the length of Nevada.

And one so tall... ...it reaches over twice the height of Mount Everest.

It's the largest volcano in the solar system.

Lava once flowed from these giants, spilling across the land in all directions.

So much molten rock that the combined mass tipped the planet over on its axis by about 20 degrees.

How did these monsters get so big?

A hint comes from studying active volcanoes here on Earth.

OLUSEYI: If you want to understand volcanism on Earth, or if you just want to understand Earth at all, you need to start with plate tectonics.

NARRATOR: Earth's crust is made up of seven large plates and several smaller ones, which interact with each other at their boundaries.

AISHA MORRIS: And they interact in multiple different ways.

They can be smashing into each other, pulling apart from each other, or grinding against each other as they move around.

DOTTIN: So at plate boundaries, what happens is that you end up with rock instabilities and, ultimately, the rock melts.

NARRATOR: Creating the conditions that have formed the majority of volcanoes on Earth.

But it's the volcanoes that don't form at plate boundaries that tell us the most about Martian volcanoes.

GUPTA: On Earth, we also have volcanoes in the middle of a plate, not just the boundary.

These are called hotspot volcanoes, and Hawaii is a great example of this.

DOTTIN: Hotspot volcanoes are typically created by a plume, like a bubble of hot magma welling up directly from the interior, hitting the bottom of the plate and bursting through.

NARRATOR: Hawaii is a chain of hotspot volcanoes, created by a tectonic plate moving across a single plume of magma, erupting onto the surface forming a line of islands.

Like on Earth, Mars has hotspot volcanoes, but with one major difference, no plate tectonics.

On Mars, instead of the plate moving over the plume and making a chain of volcanoes, you get the plume stationary, with respect to the ground, and that means, over millions or even billions of years, Mars has been able to build up volcanoes that dwarf anything that we see on Earth.

NARRATOR: But all of those monster volcanoes have quieted.

Still and cold for millions of years.

Why?

To find the answer, scientists look deep within, tracing the source of heat that drives volcanism on rocky planets like ours.

Four and a half billion years ago, with the rest of the solar system, the Earth formed from the collapse of a cloud of gas and dust being smashed together under the force of its own gravity.

NARRATOR: Much of the energy that went into these collisions turned into heat, trapped inside Earth.

Added to that is heat from radioactive elements inside the planet, like uranium.

So over time, these elements actually decay into lighter elements and as they do so, they are constantly warming and heating our planet from the inside.

When you stop and think about it, it really just is incredible that all of this heat that is powering volcanoes here on Earth can be traced back four and a half billion years ago to the formation of the Earth.

NARRATOR: Mars shares Earth's origin story.

It formed in the same way... ...at the same time... ...capturing enough heat to drive volcanism on a staggering scale.

But now the volcanoes are cold and silent.

Where did all that heat go?

A clue exists within this volcanic landscape.

In Mars's northern hemisphere lies the Marte Vallis area... ...where cliffs are built from an intricate array of pillars.

Arranged in uniform patterns, these structures are made of solidified lava.

Similar structures also exist here on Earth... ...and they can help unravel the mystery behind why Mars's volcanoes are now silent.

PAUL BYRNE: We see columnar jointing in lots of places on Earth.

There's a particularly beautiful example in the Studlagil Canyon in Iceland.

NARRATOR: Hot lava once flowed here.

As it cooled, it did something extraordinary.

PAUL BYRNE: As that lava's cooling down, it's shrinking, it's contracting.

And as the lava pulls apart from itself, as it contracts, it makes these fractures and these fractures form this very regular, even pattern.

NARRATOR: Exactly what conditions cause these unusual structures to form is complex.

But they do reveal one of the most fundamental laws of nature.

DOTTIN: The simplest explanation for the second law of thermodynamics is that heat flows from hot to cold.

So if you were to go outside on a cold day with a hot cup of coffee, eventually, that hot cup of coffee will be the same temperature as your outdoor surroundings.

NARRATOR: More than 65 feet tall, these columns show this very principle in action.

BYRNE: So we can think of how, say, columnar joints form through that lens of energy flowing from hot to cold.

When the lava flow is flowing over the surface, in the case of Earth, at least, it's much hotter than both the ground and the air.

NARRATOR: As the lava cools, its heat rises into the atmosphere and then out into space, slowly cooling the entire planet.

TRIPATHI: The beautiful truth about physics is that the laws are the same everywhere.

So the laws of thermodynamics work on Earth, they work on Mars, they work beyond.

NARRATOR: The physics may be the same: volcanoes driven by ancient internal heat moving towards equilibrium with the coldness of outer space.

But one key difference caused a big change in how quickly Mars lost its heat.

OLUSEYI: If we look at these two balls as an example for Earth-- the baseball-- and Mars-- the golf ball-- then they're different sizes.

So as the object gets bigger, the volume grows faster than the surface area grows.

In spite of having a volume that's only 15% of the Earth, Mars's surface area is, actually, bigger proportionally.

It's got about 28% of the surface area of the Earth.

You can think of it this way.

If I make a big pot of coffee, it's gonna stay warm for hours.

If I pour some of that coffee into a mug, it'll cool down much faster because there's less of it, but there's more surface area.

And these differences have resulted in so much of what we see today, including the geologic and volcanic history on the two worlds.

NARRATOR: Earth and Mars started out with similar materials: heat and volcanic activity.

But while Mars's heat and its volcanism ebbed away, Earth's volcanoes continue erupting to this day.

And many scientists believe that on Earth, volcanism on the ocean floor played a role in life's beginnings.

So with such similar origin stories... ...could there once have been life on Mars?

NEZ: Mars is like Earth's cousin.

And so very early in their history as they formed, they had this volcanic activity, and we found evidence that Mars had liquid water on its surface.

It had a thicker atmosphere.

And around the same time that we know that life emerged here on Earth, there was liquid water on the surface of Mars and it had conditions very similar to early Earth.

NARRATOR: Today the Perseverance rover, on Mars since 2021, is creating packages of rock samples with the intention that a later mission will one day return them to Earth.

Could they contain evidence of Martian life?

DOTTIN: So generally, in these rock samples from Mars, we're looking for signatures that there was life on the planet.

But it would be absolutely amazing if we actually found cells or something similar in these rocks that indicated that there is life on Mars today.

NARRATOR: For now, any secrets of life on Mars remain a mystery.

But what Mars does demonstrate is the smaller the planet, the more quickly it loses heat.

Still, there is a rule breaker: a world even smaller than Mars that is somehow the most ferociously active body in the solar system.

Leaving the quiet volcanoes of Mars behind... ...passing through the asteroid belt, the cold rubble left over from the formation of the planets... ...we reach the first of the gas giants.

Twice the mass of all the other planets combined, Jupiter is circled by more than 80 moons.

Some are icy.

Like the frozen expanses of Ganymede.

And Europa.

But then, there is Io.

About the size of our moon, Io orbits closer to Jupiter than any other moon of its size.

Since 2016, the space probe Juno has been our eyes on Jupiter and its moons.

With an elongated orbit, it has made regular flybys past Io... ...witnessing violent eruptions, which shoot out columns of gas and dust that reach far out into space... ...and send rivers of lava pouring across the landscape... ...creating a surface dotted with lakes of lava.

Io is a volcanic powerhouse, defying all expectations.

DE KLEER: Considering how small Io is, the heat sources like we have on Earth, the heat of formation and the radioactive decay, those should be, essentially, completely gone on Io by now.

OLUSEYI: This little, tiny, small world of Io, you'd expect it to be geologically inactive.

Instead, it's the most volcanically active body in our solar system.

That's a surprise.

NARRATOR: Eruptions continue day in, day out, across the entire surface.

A world like no other in our solar system.

But similarities can be found by taking a closer look beneath the surface inside volcanoes here on Earth.

Astrophysicist Jen Gupta is entering a chamber created by a volcanic eruption over 4,000 years ago.

GUPTA: The incredible thing about this place is that as the magma drained away, it left behind these colors that we can see here on the walls, from elements and minerals that were dragged up from the interior of the Earth during that volcanic eruption.

The one that immediately grabbed my attention is the yellow.

This is from sulphates and sulphur, and these are the exact same colors that we see covering the entire surface of Io.

NARRATOR: Images of Io are dominated by its yellow surface, created by the same sulphates and sulphur that line the chamber walls here in Iceland.

GUPTA: The scale of this place is just astonishing, and to think that there are chambers like this under the surface of Io, filled with molten magma ready to erupt out.

It's easy to think that Earth, our home planet, is completely unique, but the more we discover about the solar system, the more we see these remarkable similarities, like having volcanoes on Io.

NARRATOR: And unlike Mars's ancient dead volcanoes, Io's are continually erupting, creating a thin atmosphere of volcanic gases.

As Jupiter blocks the sun, Io passes into its shadow.

Even with such constant activity, temperatures plummet, so low that the thin atmosphere begins to freeze, creating an incredible frost.

Made not of water, but of delicate crystals of sulphur dioxide.

A few hours later, they are gone, evaporating away as the sun returns.

DOTTIN: Sulphur is a major component of volcanic eruptions.

And so it's not surprising that the surface of Io is covered with sulphur crystals because there's so much volcanic activity going on there.

NARRATOR: Io isn't big enough to have retained heat from its formation.

So the heat that drives this much sulphur to the surface must be coming from somewhere else.

DE KLEER: So, the question is, why is an object this small still volcanically active?

Why hasn't it run out of heat yet?

GUPTA: The thing about Io is it doesn't orbit Jupiter in a circular orbit.

What it does instead is travel round in an elliptical orbit like this.

And it's that elliptical orbit that's one of the driving forces behind the level of volcanic activity on the moon.

NARRATOR: Jupiter is the largest planet in our solar system, and its gravitational pull on Io is enormous.

GUPTA: As Io comes in towards the planet, the gravitational pull of Jupiter is stronger, causing the very rock on Io to bulge out towards its host planet.

Then as Io moves further away, the bulge shrinks and moves, causing a tide of rock on Io that's over 300 feet tall.

That's rock the height of this chamber bulging and moving on that rocky moon.

This is rock rubbing against rock in a similar way to when we rub our hands together, and they warm up.

It's generating huge amounts of heat underneath the surface of Io.

NARRATOR: A process known as "tidal heating."

But this bulging movement generates only some of the heat that drives Io's violent volcanoes.

So where is the rest of the heat coming from?

DOTTIN: When we think about our moon, we understand that we only see one side of the moon, and this is because the Earth and the moon are tidally locked.

NARRATOR: But as the moon travels its elliptical path around Earth, we actually see it at slightly different angles.

OLUSEYI: It's sort of like if I was to shake my head and say "no."

Right?

You see my face the entire time, but because it rocks back and forth, you see slightly more of my head than if I just kept it pointing right at it.

So, it's like the moon is looking down on the Earth and going, "Mm, mm, mm.

Look at those humans."

NARRATOR: Just like our moon, Io is tidally locked to its planet and has an elliptical orbit.

Which means that as Io orbits Jupiter, it appears to rock back and forth by around one degree.

But there's one big difference between our moon and Io.

OLUSEYI: Jupiter is way more massive than the Earth, and Io is similar to the moon, so the tidal effects are more extreme.

NARRATOR: Io's tidal bulge is continuously pulled towards Jupiter.

So as Io faces the planet at slightly different angles throughout its orbit, Jupiter not only raises the rock tide 300 feet up and down, but also drags that tidal bulge back and forth 40 miles across Io's surface.

Earth's heat was captured billions of years ago, locked inside the planet at the time of formation.

But on Io, its heat is constantly replenished by its elliptical orbit.

NARRATOR: As Io deforms, the intense friction generated by these tidal forces produces enough heat to drive Io's spectacularly violent volcanism.

But could all that heat have driven out some common planetary ingredients, like water?

Io doesn't seem to have any water today, and we actually don't entirely know why that is.

Maybe it was never able to form with any water, but maybe it just lost all of its water over time because of this tremendous amount of heat that's produced in it.

FRANCK MARCHIS: So, there is no water on Io.

Is there life?

We don't know.

If there is life, it would be a life which is very different to the life we have on our own planet, and the only way for us to find it will be to go there and to explore directly.

DE KLEER: Every time I point a telescope at Io, I still have that new excitement because you don't know what's going to be happening.

You take an image of Io, and you're saying, "Which volcanoes are active?"

And you look at it, and you can just immediately see from the picture and identify which volcanoes are going off at that time.

NARRATOR: As scientists learn more, they are finding the effects of tidal heating on an even stranger world.

That same source of heat, caused by an elliptical orbit, drives volcanic eruptions that aren't even hot.

Even farther from Earth than Jupiter, across a great gulf of space, lies the next planet.

Saturn's rings loop for hundreds of thousands of miles.

And just beyond them, one of its largest moons changed everything we thought we knew about volcanic activity this far out in the solar system.

With a hard, frozen exterior, Enceladus' surface averages a chilly -330 degrees Fahrenheit.

It's one of the coldest places in the Saturn system.

An ice world, where we'd expect everything to be completely still, frozen and unchanging.

But in 2005, NASA's Cassini spacecraft travelled to the South Pole and discovered that stillness shattered.

It captured explosive jets constantly erupting from the surface.

NEZ: So, when we first saw those plumes coming out of Enceladus, it was just... mind-boggling that to see this tiny world spewing material out, indicating that it was geologically active.

Discovering the jets at Enceladus's south pole completely changed the way that we see icy worlds.

It changed the way that we see small, icy worlds in particular.

NARRATOR: Giant plumes far bigger than the moon they erupt from are an incredible sight.

But how is it possible to have such powerful eruptions on a frozen moon?

Our usual experience of volcanic eruptions is of molten rock bursting onto the surface.

But eruptions on Enceladus are different, more like what's happening here.

At a geothermal power plant in Iceland.

A mile below the surface lies a reservoir of superheated water.

Drilling down into it allows steam and water to erupt out and generate electricity.

HOWETT: To be here is amazing!

I spent most of my entire adult life studying Enceladus, but I've never heard it, I've never seen it, I've never felt it, and this is one-tenth of what one of the jets on Enceladus would be like.

It's astounding.

NARRATOR: Over 660 pounds of water erupt from the surface of Enceladus every second, creating visible eruptions that can thrust up to 6,000 miles into space.

It's called cryovolcanism.

"Cryo" from the Greek for cold.

Cryovolcanism is absolutely volcanism.

When we study the planets, we learned that, even though they have different chemistry or slight differences, that these processes are common throughout the planets, and volcanism is one of them.

MARCHIS: What is surprising is not only the presence of the jets, but their size, the fact that this is an extreme cryovolcanism coming from such a small body.

NARRATOR: But where does such an enormous amount of water come from on a frigid ice moon?

There is a hint.

As Enceladus orbits Saturn, it wobbles by about 0.12 degrees on its axis.

A tiny but significant movement... ...that tells us something about the moon's interior.

HOWETT: A simple way to think about what's happening inside Enceladus is to consider what happens when we rotate two eggs, one of which is hard-boiled and the other one isn't.

So, we'll start with this one.

If we spin the egg... (laughs) Just with eggs, doesn't seem like the most obvious solution.

You can see that it... As it rotates, it's got a bit of a wobble to it, and if we rotate it a bit faster and then stop it, it'll continue to rotate.

Whereas this one, we do the same thing.

It rotates more smoothly, and if we rotate it and stop it, you can see it stops.

NARRATOR: The reason the first one wobbles is because it is raw.

Whereas this one... ...is hard-boiled.

The raw one, of course, has a liquid in the middle, and so, even after you stop the shell, the liquid continues to rotate, and that causes the shell to rotate.

It also caused that wobbly rotation that we see all the way along.

Whereas the hard-boiled one is solid all the way through.

When you stop the shell, you stop all of it.

It also causes it to rotate very nicely.

But it's the liquid one, the raw one, that's like Enceladus.

NARRATOR: Enceladus is unlikely to crack like a raw egg.

But it does wobble like one.

And that tells us that there is an outer shell of ice, sitting on top of a global ocean of water.

But how does that water manage to erupt through a shell of around three miles of solid ice?

QUICK: When the liquid ocean is heated from below by energy from tidal heating, it expands, so the liquid wants to take up more space.

As it seeks to take up more space, it pushes up against the bottom of the ice shell.

NARRATOR: Once there, another force comes into action.

Once that water goes through a crack and then is exposed to the vacuum of space, it's like a suction.

That's what vacuum does.

BYRNE: The reason water behaves this way when it's exposed to zero pressure is because there's nothing acting to keep the water together.

So, the water will try its best to expand as much as it possibly can.

One force pushes the water up, and the other force pulls it out onto the surface, and that's why we have these magnificent jets.

NARRATOR: And they carry with them the secrets of what lies beneath the ice.

These plumes contain traces of silica and molecular hydrogen that could've come from the ocean water interacting with hot volcanic rocks.

Suggesting that, in the depths of Enceladus's ocean, there may be hydrothermal vents, towering structures created by water flowing into the volcanic rocks on the seabed.

I think the chances are pretty high that there are hydrothermal vents at the bottom of Enceladus' ocean.

Just like Earth's ocean, Enceladus's ocean sits right on top of its rocky mantle.

NARRATOR: And with probable hydrothermal vents, an exciting possibility for life.

NEZ: We think that life could've originated on Earth in hydrothermal vents.

And these hydrothermal vents have those ingredients that are essential for life.

NARRATOR: And it isn't just the possibility of heat that makes Enceladus a compelling incubator for life.

HOFGARTNER: There are three key ingredients to a habitable environment: liquid water, chemical nutrients, and an energy source to use, and now we know that Enceladus has all three within its ocean.

QUICK: A 2023 study showed that Enceladus's jets have phosphates in it, anywhere from 100 to 1,000 times the amount of phosphates that we find in Earth's oceans.

Phosphates, we know, are a key component of DNA, which is the building block of life.

NEZ: Does that mean that we found life?

Well, we don't know.

Just because we found phosphates, it's-it's a potential ingredient, but it's not a slam dunk, and so, we need to go back and really explore in more detail to see what is the origin of this phosphate.

NARRATOR: Any mission to Enceladus is a ways off.

But there is another world much closer to home that has more volcanoes strewn across its surface than any other planet.

To find it, we have to return all the way back into the inner solar system.

Past Mars's ancient peaks... ...and beyond Earth.

...to a mysterious, cloud-covered planet.

Venus's dense atmosphere of carbon dioxide and clouds of sulfuric acid obscure the surface.

For decades, orbiting probes have used radar to peer through the clouds.

Revealing a landscape dominated by more than 85,000 volcanoes.

But it was hard to see in the images if any were recently active, or if, like Mars, they were relics of the past.

So why does Venus have so many volcanoes?

And are they still erupting today?

To find out, scientists are studying Iceland, one of the most volcanically active places on Earth.

Here they are investigating recent lava flow activity to help prepare for future missions to Venus.

SUE SMREKAR: I have remained fascinated by Venus because it tells us so much about the Earth.

They really are twin planets evolving down different paths.

NARRATOR: Sue Smrekar is leading the Veritas mission preparing to launch to Venus in the next decade.

She uses radar data, which creates black-and-white images that reveal the texture of the surface.

SMREKAR: I try to see things with my radar eyes, just imagining what they look like.

When it's a nice smooth surface, the radar wave comes down, and it looks dark.

But these areas that are super rough, you'll get a lot more reflection, so, in radar, these areas will be much brighter.

NARRATOR: Whether new lava flows like we see on Earth exist on Venus was a much debated subject, as scientists had no direct evidence of recent volcanic activity on Venus.

Until now.

ROBERT HERRICK: It's really only been in the last decade or so that technology has made it possible to be able to zoom in and out and flip back and forth between data.

NARRATOR: Using today's faster computers, Robert Herrick and his team reanalyzed images taken by the Magellan orbiter... over 30 years ago, and spotted a volcanic crater that had doubled in size over eight months.

Proving that a volcano, about the size of Mount Everest, was erupting from its northern flank.

HERRICK: When an investigation actually works out, and you learn something important, and you're like, "Oh, this is... this is cool."

So, yes, it's a thrill to, you know, "Wow, I've-I've made my mark on the field."

I was so excited when we saw those images because what that tells us is that everything we've been thinking about Venus is right, that this world probably is volcanically active, and that just makes us hungry for more.

NARRATOR: But evidence of one probable eruption can't reveal if the rest of Venus's volcanoes are still active.

And another mystery remains.

Venus could have been Earth's twin.

It is roughly the same size and built of the same stuff.

So why are the two planets so different today?

Understanding where volcanoes are found may solve this puzzle and explain why Venus is now a planet with such extreme conditions.

SMREKAR: So, this map shows the distribution of volcanoes on the Earth.

And you can see there's a big line of them that goes down the coast of North America on down to South America, and their location is really controlled to a large degree by the location of plate boundaries.

NARRATOR: Most volcanoes on Earth are found where two plates meet, or are pulling apart, creating lines of volcanoes along their edges.

But on Venus, the pattern is very different.

SMREKAR: So, this is a map of the volcanism on Venus, the volcanoes, and you can see that they are all over the place.

They don't follow any nice, uh, tight line of volcanism the way we saw for Earth.

And, you know, the difference is that, uh, on Venus, we don't have plate tectonics.

Venus has some entirely different system.

NARRATOR: Not only are there numerous volcanoes scattered across Venus' landscape, they take on odd forms.

From lines of flattened volcanic domes that look like chains of pancakes... ...to strange volcanoes whose rutted sides make them look almost like insects, clamped to the surface.

Venus is a volcanic zoo, with a variety of volcanoes, including some that are unique in the solar system.

Venus doesn't have plate tectonics, but its crust has uneven thickness, and so, magma from within can climb and poke into the crust and then cool.

NARRATOR: The inner heat drives vast lava flows that can run for thousands of miles, and creates far more blemishes and bubbles on the surface than here on Earth.

But the lack of plate tectonics points to a far more profound difference between Earth and Venus.

DOTTIN: So on Earth, there have been times where there were volcanic eruptions that were so large and so constant that it was filling the atmosphere with tons of toxic gases and greenhouse gases.

NARRATOR: Venus's greenhouse gases trapped heat, increasing the temperature, but thanks to plate tectonics, Earth had a way to balance this effect.

MORRIS: On Earth, carbon dioxide is actually pulled out of the atmosphere when it rains, and those molecules interact with the rocks and the carbon dioxide gets deposited in the rocks, and as the plate tectonics cycle occurs, that material gets then pulled into the subsurface and removed from the atmosphere.

BYRNE: When we have these huge volcanic eruptions injecting all this CO2 into the atmosphere, we see substantial and severe climate change, at least for a while, until plate tectonics is able to regulate things and get them back under control.

DOTTIN: So, plate tectonics is probably one of the main reasons why we've been able to maintain the climate that we have on our planet.

NARRATOR: As Venus's volcanoes erupted, it seems there was no mechanism to remove those gases from the atmosphere back to the interior.

So the planet got hotter and hotter, turning it into the hellish landscape we see today.

Venus shows the importance of plate tectonics in helping to control not just volcanoes, but the climate, too.

MORRIS: So, the lesson that we can learn from Venus is that on Earth, the planet is actually able to stabilize itself through the process of plate tectonics, and we see on Venus what happens when the planet's not able to maintain that stability in the atmosphere.

NARRATOR: But human activity emits at least 60 times more carbon dioxide than all the volcanoes on Earth each year, and plate tectonics is a slow process.

OLUSEYI: What we see in Venus's atmosphere is a cautionary tale for the situation that Earth is in at this very moment.

We have a greenhouse effect taking place that's warming our planet.

Venus is a runaway greenhouse effect just gone rampant, and the whole planet is now a hotbox of it.

NARRATOR: Scientists don't think human emissions alone can send us to those extremes.

But with Venus, we have another planet that's identical in many ways.

And by understanding why it's so different today, we can appreciate the unique habitability of our own planet.

(explosive pop) SMREKAR: Venus is kind of a... like a teenager.

We go back in time to the early part of Earth's geology when we study Venus.

So, it's a... it's a really a fascinating, you know, look at our... the evolution of our own planet.

NARRATOR: Future missions like the Veritas orbiter will help explain the different processes behind volcanism on Venus.

SMREKAR: With Veritas, we have just all kinds of ways to look for new volcanism.

We can actually see the surface deforming at the scale of, you know, an inch or so.

And I think we're just going to see vastly more flows that are young and even, I predict, happening during our mission.

NARRATOR: With new insights into Venus, we can better understand the role plate tectonics plays in balancing our own environment, creating the conditions for life to thrive.

One of the revelations in our exploration of the solar system is what we are finding isn't a collection of inactive worlds.

HOFGARTNER: There is a great diversity of geological activity in the solar system, and sometimes nature makes it occur in surprising places.

NARRATOR: From the tiny, explosive moon of Io... to the stunning cryovolcanoes on Enceladus, they are telling us so much about Earth.

SOUSA-SILVA: When we look at other planets and moons, we see snippets of Earth's past and future.

We were once covered in volcanoes like Io is now, and in Venus, we see what happens when greenhouse gases get out of control.

NARRATOR: But they also tell us just how remarkable Earth is, for one very special reason.

Amongst all these geologically active worlds, it's still pretty amazing that the only place where we saw these active geology become biology is here on Earth.