Venus has long been called Earth’s twin, because our sizes are so similar. But as we learned in the early 60s, on our surface we couldn’t be more different. This wasn’t always the case though.

It’s thought that up until about 500 million years ago, our planets were very similar, in fact Venus may have supported life. So what happened? Intro produced by Cooper Carr, edited by Nick Turnbow, with special makeup effects by Sara Bachmeyer


Astronomical Disappointment

Imagine you’re planning a vacation. You pick a resort with a tropical setting: lush jungles, beautiful beaches. The last thing you do before buying tickets is check the weather.

And good thing you did! Because this place has changed since Travelocity put those photos up. Instead of a gorgeous vacation spot, it’s a barren, toxic hellscape, so hot it can melt lead.

This must have been how astronomers felt when the Mariner 2 probe first measured Venus’s temperature in 1962.

Because up to that point, the prevailing wisdom was that Venus was a lot like Earth. It was a rocky planet, it was the same size, had a reflective atmosphere. Why wouldn’t it have life like ours?


Venus in Fiction

In fact in the earliest days of science fiction, aliens were more likely to have come from Venus than Mars. And they were often portrayed in glowing terms, beautiful, intelligent civilizations. The very name evokes the Roman goddess of beauty.

We’ve known Venus has clouds since the 17th century, and ever since, scientists have theorized about what the clouds are hiding — this mentions that crediting Mikhail Lomonosov with the discovery is controversial, as it hinges on a translation

Some suspected there was a planetwide ocean on Venus. Others thought it was an endless desert.

In between were theories that featured ocean and land, with most of the land being jungle or swamp

For example, C. S. Lewis and Isaac Asimov covered their fictional Venuses in ocean.

Edgar Rice Burroughs added continents like here on Earth, complete with sky-scraping trees.


Plurality of Worlds

If there was going to be life outside of Earth, it was going to be on Venus, Earth’s sister planet. They were arguing this all the way back in ancient Greek days.

Aristotle insisted that there could be only one world, and because he was Aristotle, his view became accepted. The early Christian church adopted the same policy.

In the Late Middle Ages, several scholars insisted that God could create all the aliens he wanted. I mean… He was God.

One of those scholars was William of Ockham, by the way. The razor guy.

Point is, belief in aliens didn’t start in the 20th Century, people have been considering this for a long, long time, and Venus was always the best opportunity out there.

Which is why, by the way, one of the very first things both the US and the Soviet Union did once they figured out rockets was to send a probe to Venus. What paradise or advanced civilization might we find under those clouds?

Inhospitable Conditions

All that went flying out the window the second Mariner 2’s first readings came back.

No oceans, no paradise. Just a rocky surface as hot as a pizza oven, at 425 degrees Celsius (800 Fahrenheit).

Now to be clear, there were some astronomers who predicted that Venus had experienced a runaway greenhouse effect and would be way too hot for life. Carl Sagan was one of them.

But even Carl probably didn’t fully expect what an utter hellscape Venus is. It is, somehow one of the most inhospitable places in the solar system.

The longest operational lifetime ever managed by a Venus lander is the Soviet Union’s Venera 13, at 127 minutes.

It was finally done in by hurricane force winds, a 457 degree Celsius temperature, and 89 standard atmospheres of pressure.

One standard atmosphere is roughly what you’re feeling right now. That is one atmosphere. Venus’ air is 90 times more dense than our air.

In fact, it’s similar to the pressure at 1 kilometer under the ocean. For comparison, advanced SCUBA divers generally go no deeper than 60 meters, and anything below 100 meters is a record attempt

CO2 Atmosphere

Most of Venus’s crushing atmosphere is carbon dioxide, in fact, 96.5 percent of the atmosphere is CO2.

Earth’s atmosphere is 0.041 percent C02, and you know what kind of trouble that can cause when it goes up just a little bit. Carbon dioxide is a great insulator.


Weird Rotation

The crazy thing is that Venus wasn’t always like this. It’s thought that the majority of its history, it was a lot more like Earth. But before we talk about why, there’s one more weird thing about Venus worth talking about.

It rotates backwards… Very slowly.

Looking down at the north pole, the planet rotates clockwise. This is opposite Earth, and all other planets in our solar system except Uranus.
But we always knew Uranus was weird.

And when I say slowly, I mean very, very slowly. One solar day on Venus is about 117 Earth days long.

In other words, it takes about 117 Earth days for the Sun to reach the same point in the Venusian sky.

Its rotation is basically the same as walking speed so if you continuously walked eastward, you would never see the sun go down.

Which, if there were civilizations on Venus, they might want to be nomadic, just constantly moving to stay in the sun and out of the 117 days of nighttime.

For just a little more context, their year is 225 Earth days, so they basically have 4 seasons a year. First Day Season, First Night Season, Second Day Season and Second Night Season.


A Habitable Past?

Interestingly, computer models suggest that slow rotation could be a sign of a planet hospitable to life.
This was the conclusion of Mike Way and Anthony Del Genio of NASA’s Goddard Institute for Space Studies.

They hypothethized that slow rotation would favor forming dense clouds on a planet with the right chemistry.

Clouds raise a planet’s albedo, helping it reflect heat. Obviously it didn’t work out like that on Venus, but the study authors think things were different for the first 3 billion years of Venus’s history.

They describe a Venus with cooler temperatures and liquid surface water, and this is based on findings from NASA’s Pioneer Venus mission.

Basically that mission determined that there should be more water present on Venus. Only 0.002 percent of the Venusian atmosphere is water vapor, compared to 0.4 percent on Earth.

So very little water, but a very high concentration of Deuterium was found by the Pioneer Venus mission.

Deuterium is an isotope of hydrogen, meaning it had a lot of hydrogen at one point – probably bounded to oxygen in H2O.

And of course the only way it could have H2O was for it to be much, much cooler. The amount of deuterium suggested there could have been oceans hundreds of meters deep.

So what changed Venus from a vacation spot to a toxic pizza oven? There are several ideas.

But none of these ideas exist in a vacuum, most experts agree it wasn’t any one single thing that did it, the answer is kinda all of the above.

Volcanic activity, wonky magnetic fields, and ancient impacts all may have played a part in Venus’s past


Volcanic Supereruptions

Let’s start with volcanoes; Venus has plenty of those. In fact, no planet in the solar system has more volcanic features.

An estimated 75% of Venus’s surface is thought to have been formed by lava flows.

Here on Earth, liquid hot mag-ma usually breaches Earth’s crust at tectonic plate boundaries, though, as you may remember from my Supervolcanoes episode, there are times when magma finds its way out in the center of a tectonic plate, usually to impressive results.

The crust of Venus is thicker and more solid than Earth’s crust, they once thought it was one seamless lid, but this was disproven by the Magellan probe.

So there are plates on Venus’ crust. But they don’t act like they do here.

Because they’re so much thicker, they don’t subduct at the boundaries like they do here on Earth.

On Earth, we have this ongoing cycle, where plates subduct at the boundaries, pushing the crust down into the magma to melt down, only to ooze out somewhere else where two plates are spreading apart. This maintains a steady rotation of CO2, and it’s a significant part of the carbon cycle that has made life on Earth possible.

They also cause earthquakes, tsunamis, landslides, and volcanoes, but I mean… Life.

Without no subducting plate boundaries, volcanic eruptions on Venus build up longer, happen less often, and cover more ground when they do.

According to one theory, the magma that formed something like 75% of Venus’s surface started erupting about half-a-billion years ago.

For something like 100,000 years, it fed a surging, magma ocean that resurfaced the entire planet. That’s a blink of the eye in geologic time.

There are other theories that say the eruptions were less extreme, that they ejected magma at a slow, steady rate over millions of years.

This is one of the biggest mysteries in the solar system, I covered it a little bit in a previous video.


But whether it happened suddenly or gradually, the effects are the same.

First, that Venus’ surface looks super young, with most impact craters less than 500 million years old.

And second that all those volcanic eruptions released massive amounts of CO2 that then didn’t have anywhere to go and built up in the atmosphere, leading to a runaway greenhouse effect.

But here’s the weird part – on Earth, large volcanic eruptions usually cause temperatures to fall.


This is because the heating from carbon dioxide is counterbalanced by a different effect involving sulfur dioxide.


Volcanoes also pump out massive amounts of sulfur dioxide, which then combine with water vapor in the air and become sulfate aerosols. And sulfate aerosols absorb solar radiation.

This is why a big enough volcano could create a volcanic winter.


But the study authors suggested that SO2 didn’t cool Venus possibly because it was already too hot for sulfate aerosols to form? I think that’s what the paper says. It does get difficult, I’ll put the link down below.


So the planet warmed, the atmosphere thickened, a runaway greenhouse effect set in, whatever water was on the surface evaporated.

Of course, water evaporates all the time on Earth too and gets recycled as rain and snow. The amount of water stays constant.


It stays constant on Venus too… at practically none.
So, if there was water on Venus and it’s not there now, where did it go?


No Intrinsic Magnetic Field

The answer, most likely, is space. Over time, water in the atmosphere was broken down by photodissociation, that’s when ultraviolet rays split molecules apart.

Long story short, the Sun broke H2O into H, H, and O. Hydrogen being the lightest atomic element eventually just floated off into space.

Except for the heavier deuterium atoms, which is why there are higher amounts of them in the atmosphere.

This doesn’t happen on Earth for a couple of reasons, one is what’s called the Cold Trap – basically a layer of the atmosphere where it’s too cold for water vapor to stay a gas and condenses into clouds.

The other reason, you’ve probably already guessed is our handy little magnetic field, which Venus lacks.

Actually it has one, but it’s way different from ours.

Our magnetic field is generated by currents of molten iron in our planet’s core, making it intrinsic, it comes from the Earth.

And it’s super strong, extending out about 65,000 kilometers, directing particles around the planet like iron filings around a bar magnet.

But Venus has an induced magnetic field.

It’s pretty complicated but when solar wind hits the upper atmosphere, it strips electrons from atoms, making the atmosphere positively charged.

This forces the sun’s magnetic field to wrap around Venus, creating a teardrop shape. This protects the inner atmosphere but not the upper atmosphere.

Like I said, it’s complicated and research is ongoing.

But if you’re wondering why Venus doesn’t have that iron-core magnetic field like we do… it might have something to do with its weird backwards rotation.


Giant Impact Theory

So we all know that Earth got its moon because a Mars-sized planet named Theia smashed into us in the early days.

It’s thought that pre-Theia, our planet’s layers were stratified like the layers of a latte, with heavy elements sinking to the center and lighter stuff up top.

But the Theia smash might have stirred up the insides enough to get a convection current going that generates a magnetic field to this day.

So maybe Venus wasn’t smacked around enough as a kid. Because not only does it not have a magnetic field, it doesn’t have a moon either. No moon, no field, no smash.

But in typical Venus fashion, there’s a flipside to this because it’s thought that the reason it has this weird backwards rotation… Is because it got smashed.

Either smashed with enough velocity to slow and eventually reverse its rotation, or to flip it vertically, causing the rotation to go in an opposite direction.

Maybe with a little more oomph it really would have been Earth’s twin. Assuming the Venusian impact theory is true. It’s hotly debated.

Get it, hotly debated? Because Venus is… you get it…

Experts dispute pretty much every theory I’ve talked about here, Venus is a cornucopia of contradictions and mysteries.


Parallel History

Considering how different and weird Venus is, it’s kinda weird to think about the fact that before the major resurfacing event 500 million years ago, it still may have had the right conditions for life.

Earth was teeming with single-celled life long before that point. Maybe Venus did too.

And scientists think that maybe Mars did as well, so it’s possible that once upon a time, there were three planets in a row with life on them.

Hell there may be an advanced alien species 500 million light years away that is looking at our solar system right now and thinking they’ve hit the jackpot.

But both Venus and Mars suffered from a lack of tectonic activity. Maybe Earth’s superpower is its ability to change and remake itself slowly over geologic time, preventing sudden shifts that can spiral out of control.

Some worry that our pumping of carbon into our atmosphere at such a fast rate could be a similar threat, that we could hit a tipping point and spiral out of control and turn into Venus.

This is unlikely. But still Venus stands as cautionary tale. Of how fickle the conditions for life really are. Of how fleeting paradise can be. And how it needs to be protected.

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