Venus could have been a paradise but turned into a hellscape. Earthlings, pay attention.

“Hellscape” is the most appropriate word to describe the surface of Venus, the second planet from the sun. At 900 degrees Fahrenheit, it’s the hottest planet in the solar system, thanks to an atmosphere that’s almost entirely made up of carbon dioxide. Clouds made of highly corrosive sulfuric acid are draped over a volcanic landscape of razor-sharp lava flows. Most crushingly, the pressure on the surface of Venus is about 92 times the pressure you’d feel at sea level on Earth.

“It’s really almost entertainingly, comically horrible, like some sort of cosmic deity had a really, really grumpy day and just went ‘Nope, I’m gonna ruin this planet,’” Robin George Andrews, a science journalist and volcanologist, says. Andrews compares it to being in a pressure cooker a mile underwater. “If you stood on the surface, you would be pancaked and you would melt,” he says. Your eyes would explode due to the pressure — “which would be gross,” he adds.

Yet as Andrews relays in his new book, Super Volcanoes: What They Reveal about Earth and the Worlds Beyond, some scientists suspect Venus was once much like Earth, with a liquid water ocean like the ones that support life on our planet. For Andrews, the question of what happened to ruin Venus is captivating and even existential.

“Venus and Earth are planetary siblings,” he says. “They were made at the same time and made of the same stuff, yet Venus is apocalyptic and awful in every possible way. Earth is a paradise. So why do we have a paradise next to a paradise lost?”

Scientists know something on Venus triggered truly catastrophic levels of climate change, causing surface temperatures to shoot up hundreds of degrees. But they don’t know exactly what. I spoke to Andrews for an episode of Unexplainable, Vox’s science podcast about unanswered questions, about what could have triggered Venus’s apocalypse and why we should care about it. Our conversation has been edited for length and clarity.

The podcast episode also features a discussion with Sara Seager, an MIT planetary scientist and an expert on exoplanets, or planets outside our solar system, who raises the tantalizing question: What if, despite the cataclysm, something is still alive on Venus?

The origins of Venus could tell us a lot about our place in the universe

NASA/Jet Propulsion Laboratory-Caltech (left) and NASA (right)

Brian Resnick

You say the question of “what killed Venus” is existential. What makes it so?

Robin George Andrews

It really is a question about why are we here. Answering it will help us answer the question: How lonely are we? Are there other Venuses or Earths out there?

If Earth is the odd one out, how lucky are we to exist? If Venus is the odd one out, then maybe we’re not so special after all.

Brian Resnick

So Earth and Venus started off as similar planets, and then went down different paths. You want to know what path is more common out there in the cosmos?

Robin George Andrews

When you hear in the news that scientists have discovered an Earth-like exoplanet, they might as well be saying, we found a Venus-like exoplanet. We don’t know if this is like a habitable world by our human, surface-dwelling standards, or if it’s gone through this sort of apocalyptic climate change like Venus. A good way to work out what may be more common in the cosmos is to study Earth and Venus, because they are siblings.

Brian Resnick

How do scientists know, or suspect, Venus used to be more pleasant, habitable even?

Robin George Andrews

So even though Venus today looks and is apocalyptic in every meaning of the word, probes have looked at its atmosphere and found there’s a lot of “heavy water” in the atmosphere. Heavy water is exactly what it sounds like. The water we’re used to, this classic H2O, which is found commonly everywhere on Earth, is a more common type of water throughout the cosmos. Heavy water just kind of switches out that hydrogen for something called deuterium, which is like a heavier version of hydrogen.

Brian Resnick

What does finding heavy water mean?

Robin George Andrews

If you measure how much heavy water exists somewhere, you can make a reasonable guess as to how much classic water there is or was on that planet. It suggests that there once was a lot of classic water on Venus, at least an ocean’s worth of water on Venus. If that water existed in liquid form, what are the odds that Venus was habitable at some point? It’s not unlikely, even though today it looks impossible.

Brian Resnick

How long ago must this have been, this habitable Venus?

Robin George Andrews

I think there is a possibility that water was always steam, and it might have never been liquid water. But if it was, then it could have been habitable for billions of years. Maybe right up until the last billion years.

Brian Resnick

So that’s what we’re talking about when we say Venus used to be “alive.” What do we mean by “killed”?

Robin George Andrews

Death, in this case, is runaway climate change. Absolutely irreversible, world-ending climate change. The average temperature of the planet shot up by something like several hundred degrees Fahrenheit. It really sort of cooked itself to death.

Suspect No. 1 for the death of Venus: The sun

NASA/Solar Dynamics Observatory

Brian Resnick

So the question, I imagine, is how Venus cooked itself.

Robin George Andrews

There’s kind of two leading theories as to what killed Venus. Option No. 1 is the sun. From what we know of our sun and other stars, when they’re kind of in their teenage years, they get hyper-excitable, and they get hotter and brighter quite quickly. If the sun actually gets brighter and hotter too quickly, even though you might have that water sitting on the surface, the sun can boil it off. And that is a fate that lots of exoplanets, or planets outside our galaxy, are presumed to have gone through.

Brian Resnick

And once Venus’s water gets vaporized...

Robin George Andrews

That steam is a greenhouse gas that would have kicked up the greenhouse gas effect. And then the carbon dioxide coming out of Venus’s embryonic volcanoes would have just sealed the deal. That could explain why we see Venus as it is today.

Brian Resnick

If this young, very excitable sun is what killed Venus, the Earth would have been fine, right?

Robin George Andrews

Yeah, the Earth would have been fine. It seems that Earth was spared the worst of it.

Suspect No. 2 for the death of Venus: Tectonic plate-breaking volcanoes

NASA/JPL-Caltech/Peter Rubin

Brian Resnick

So that’s one suspect in our whodunit. What is the other suspect?

Robin George Andrews

The other suspect just happens to be my favorite thing: volcanoes.

Brian Resnick

How could your fave, volcanoes, kill a planet?

Robin George Andrews

Two hundred and fifty-two million years ago, Earth experienced its worst mass extinction, something called the Great Dying. At least 90 percent of all life was wiped out, and the primary suspects were these volcanic fissures that opened up in Siberia. It produced a continent-sized flood of lava that took about 2 million years to erupt.

So it causes giant explosions and also unleashed all these greenhouse gases into the atmosphere, and it created a global warming effect that raised the temperature by a dozen degrees [Celsius]. That caused 90 percent of all the life on Earth to die. Earth had to kind of reset itself. And the idea is, well, what if that happened on Venus, but worse?

Brian Resnick

That could kill the planet?

Robin George Andrews

If you just have one [Great Dying-scale] eruption, it might be okay, because Venus had plate tectonics. Plate tectonics is essentially a planet’s thermostat.

Brian Resnick

Plate tectonics — that’s how continents kind of float around and smoosh into each other. How do those act like a thermostat?

Robin George Andrews

Carbon dioxide can get soaked up in the ocean, and that filters down to these tectonic plates. If tectonic plates dive down beneath each other, then you’re burying carbon [and slowing the greenhouse effect].

Brian Resnick

Okay, but how do you break this carbon-burying system?

Robin George Andrews

You have two of these epic, Great Dying-like eruptions at the same time.

That will immediately trigger quite an intense period of global warming. And the oceans will just start to boil off.

Now, plate tectonics could bury carbon for a while. But if you boil off your oceans, that carbon dioxide has nothing to dissolve into. And if you get rid of that water, plate tectonics itself shuts down. If you dehydrate [tectonic plates], you make them brittle — they can’t bury the carbon anymore. That’s essentially game over. If you break plate tectonics you’ve broken the world.

The scientific jury is still out

JAXA/ISAS/DARTS/Damia Bouic

Brian Resnick

So we have our two suspects here. We have the sun and we have massive eruptions on Venus that broke Venus. Which one should we sentence here?

Robin George Andrews

The volcanoes seem more likely because we can see how that’s happened on Earth — just to a slightly lesser extent. But if this was put into a court of law, they would both be presumed not guilty, just because there isn’t a telltale bit of evidence yet.

Brian Resnick

So how do we solve this mystery?

Robin George Andrews

There’s basically a fleet of missions that is going to unravel the geologic makeup of Venus today. If it looks bone-dry and it was always bone-dry, then maybe the sun did it, because it’s been dehydrated for a long time.

But if it looks like there’s still some dehydration going on, that means you still have water somewhere. If Venus was still kind of soggy on the inside, and it’s still kind of belching water out, the volcanoes probably killed Venus.

Brian Resnick

Could the Earth pull a Venus one day?

Robin George Andrews

[Laughs.] Yeah, it could. Let’s not panic — I mean, these sorts of eruptions are like millions of years in timescales. So it’s not like no one would see this coming and we’d instantly be doomed. But it could happen. And the question is: Is it normal for a planet to have just one of these really epic, game-changer eruptions at one time? Or is it just a fluke?

The fact that no one knows the answer to this is weirdly, perversely exciting to me. We don’t know how often it is that volcanoes decide to trash the planets they’re on.

Brian Resnick

People sometimes bring up Venus in the context of climate change. It’s an example of how bad a planet can become when greenhouse gases start to accumulate in an atmosphere. Could we humans potentially be the volcano?

Robin George Andrews

The pace at which we’re putting carbon dioxide into the sky is worse than what was happening during the Great Dying, in terms of the amount of carbon per year.

Brian Resnick

But we would need to keep this up for millions of years to match, right?

Robin George Andrews

Right.

Brian Resnick

This is weirdly reassuring that humans are unlikely to break the Earth completely.

Robin George Andrews

Yeah. I think it would be a terrible idea to pay homage to what happened to Venus.

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