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(Intro reel)

Venus has been called Earth's twin and I guess you could say there's a certain resemblance. Placed side-by-side it would be hard to tell which planet is larger: the diameter of Venus is just 5% smaller than Earth's, and inside both planets have metal cores surrounded by a thin, rocky crust.

But once you get to know Venus, it seems a lot more like Earth's evil twin: you know something's fishy about a planet when it spins backward! Venus' rotation is retrograde, meaning it moves from east to west, although it takes it's own sweet time doing it. A day on Venus lasts 243 Earth days - eight days longer than it takes to orbit the Sun, which means that a Venusian day is longer than its year. 

But that's not exactly evil, it's just unusual.

The real thing to watch out for is Venus' totally hostile environment: probably the most inhospitable conditions in the entire solar system, and that's saying something! We're talking about the hottest temperatures of any planet, even Mercury, plus an atmosphere composed almost entirely of carbon dioxide, which creates a hellish greenhouse effect, and dense layers of clouds composed of sulfuric acid.

It's hard to even imagine a hypothetical scenario in which humans could set foot on Venus, because any explorer would be crushed by its atmospheric pressure and incinerated by its blazing temperatures long before getting anywhere near the surface.

I'm not exaggerating here, the average temperature is 465 degrees Celsius - that's hot enough to melt lead - and the atmospheric pressure on the Venusian surface is 90 times greater than Earth, similar to what you'd experience one kilometer under water here.

These conditions have even made it hard for probes to get a good look at the place. Both the US and Soviet Union sent spacecraft there in the 1960's, 70's and 80's with poor results. The last probe to land on the Venusian surface, Russia's Venera 14 in March of 1982, lasted for 57 minutes before being fried.

But it didn't use to be this way.

Billions of years ago, Venus was a much cooler place. Some scientists believe much of the surface was covered by oceans of liquid water for hundreds of millions of years, but Venus is 28% closer to the sun than Earth, and it receives twice the amount of solar energy as us.

Plus, probably because of the incredibly slow rotation, Venus wasn't able to internally generate a magnetic field, like the one that Earth has, to deflect the torrent of charged particles that make up the solar wind.

So at some point, the planet reached a tipping point and all that ocean water began evaporating into the atmosphere. Water vapor is a more potent greenhouse gas than carbon dioxide, capable of trapping more heat, which in turn could cause more evaporation - this is a classic positive feedback loop - over time, the oceans essentially boiled away.

Eventually, the surface became so hot that carbon dioxide trapped in the planet's rocks began sublimating into the atmosphere, essentially being BAKED OUT of the rock as temperatures rose.

Then, a few hundred million years ago, Venus became intensely volcanically active, spewing out sulfur and even more gasses, creating canyons and mountains, that today cover up to 85% of the planet.

Put it all together and you have a lifeless, volcanic surface wrapped in an atmosphere that's 97% carbon dioxide, with air so dense that it would instantly crush all your organs.

Venus might qualify as Earth's nasty little sibling, or, maybe, just a hot, smelly neighbor, but our twin - NO!

Thanks for joining us for this episode of SciShow Space, if you wanna learn how you can help us explore the universe, just go to and don't forget to go to and subscribe.