It's hot enough to melt lead, the acid rain will scorch the flesh from your bones – and it's the perfect place to raise a family. Venus, not Mars, might be the off-world destination of choice for future space colonists.
Despite Elon Musk's enthusiasm, any colony choosing Mars as its new home would face big problems. The wispy-thin atmosphere on the surface of Mars offers colonists nothing to breathe, bitter cold, lethally low air pressure, and almost no shielding from dangerous solar radiation.
But Venus has its problems too: it's an insanely dangerous environment. Life as we know it is impossible.
Venus bears some superficial similarity to Earth – its gravity is about 90% that of Earth, and it's only about 30% closer to the Sun than we are – but there is a crucial and horrifying difference. Where Mars has too little atmosphere, Venus has far, far too much. Venus's atmosphere is 90 times thicker than Earth's, composed mostly of carbon dioxide (CO2), and shrouded in clouds of pure sulphuric acid.
The greenhouse effect of Venus's CO2 atmosphere traps an enormous amount of the Sun's heat, meaning that the surface of Venus is astonishingly unpleasant – the hottest place in the Solar System other than the surface of the Sun itself. With a surface temperature over 450C (842F), it's hot enough to melt zinc, lead, and most organic material, and with atmospheric pressures equivalent to a kilometre under the ocean, even a nuclear submarine would be crushed.
So how could we ever possibly hope to live there? The key is to avoid the surface
In fact, the pressure is so high that the CO2 itself is crushed at the surface into an exotic state known as a "supercritical fluid," a strange substance that's neither gas nor liquid, but has properties of both states. Here on Earth, supercritical CO2 is a dangerous and exotic substance, used as an industrial solvent and steriliser – and the surface of Venus is literally covered in an ocean of the stuff.
The USSR sent several probes to Venus in the 1960s, 70s, and 80s; unsurprisingly, most failed before reaching the surface, and others crash-landed. The most successful, Venera 13, managed to last just over two hours before succumbing to the intense pressure and heat. The pictures it sent back show a rocky, dry, and deeply alien world.
So how could we ever possibly hope to live there? The key is to avoid the surface. "The problem with Venus is that the surface is too far below the one-Earth-atmosphere [of air pressure] level," says Geoffrey Landis, the Nasa scientist and science fiction writer who was among the first to propose the idea. "The atmosphere of Venus is the most Earth-like environment in the Solar System (other than the Earth)." Some 50 kilometres (30 miles) above the surface, Venus is surprisingly hospitable.
First of all, the air pressure is right, coming in at one Earth atmosphere. And there's still enough atmosphere above to provide reasonable levels of radiation shielding, comparable to the shielding we receive from the atmosphere here on Earth. The temperatures are pretty close to comfortable too – around 60C, which is hot, but we have technology that can handle that reasonably well. And going just a few kilometres higher lowers the temperature to a very reasonable 30C without sacrificing much pressure or radiation shielding. And since Venus's gravity is nearly as strong as Earth's, colonists living there for years on end probably won't develop the brittle bones and weak muscles associated with low-gravity environments.
There's still the problem of staying afloat in a suffocating atmosphere dotted with clouds of drain cleaner. But the solution is perhaps the happiest coincidence behind the entire audacious scheme. CO2 is heavier than air on Earth – which means a balloon on Venus filled with an Earth-like atmosphere of nitrogen and oxygen is lighter than the Venusian air. Fill a Venusian balloon with Earth air, and it will fly into the sky like a helium balloon.
A big enough balloon will have enough lifting power to support you and your supplies – and a really big balloon could do even more
To live on Venus, then, just fill a balloon with nitrogen and oxygen, and live inside the balloon. A big enough balloon will have enough lifting power to support you and your supplies – and a really big balloon could do even more. "A one-kilometre diameter spherical [balloon] will lift 700,000 tons – two Empire State Buildings. A two-kilometre diameter [balloon] would lift six million tons," says Landis. "The result would be an environment as spacious as a typical city."
But what if the balloon rips? "It's not going to be like popping a balloon," assures Landis. Because the pressure inside the balloon would be the same as the pressure outside, a rip would slowly leak air, rather than leading to an immediate catastrophic explosion. "It would be like opening a window, and slowly the outside gas leaks in and the inside gas leaks out… The larger the habitat, the slower the process will be."
Protecting the balloon from the clouds of sulphuric acid turns out to be even easier. The solution has already been tested by the Russians – and you already have it in your kitchen. In 1985, the Soviet Vega mission flew by Venus on its way to intercept Halley's Comet. "[Vega] put two balloons into the atmosphere of Venus that floated in the atmosphere right at the levels we're talking about, for two days," says Landis. "The outermost layer of the balloons was simply Teflon. Teflon is completely robust against sulfuric acid."
There are still problems that any space colony would need to face, such as finding the raw materials needed to survive, and finding a way to jump-start the massively complex biosphere needed to support long-term human habitation. But the atmosphere of Venus has much of what we need to get started. The CO2 atmosphere could be split into oxygen and carbon; the sulphuric acid could be split into water, oxygen, and sulphur.
And while the surface of Venus would remain inaccessible to humans, robots could explore and mine the rocky terrain. The robots could be controlled easily by the cloud city's citizens itself in real time – an impossible feat to accomplish here on Earth, since it takes radio signals about 20 minutes to cover the millions of kilometres from Earth to Venus and back.
As our technology has improved, we've turned our sights to other planets, especially Mars
But don't pack your bags just yet – we don't know enough about Venus to be sure that we can actually build cloud cities there. "The first thing we would need to do is make Venus a subject of some space probes so that we simply learn more about Venus," says Landis. "Venus is one of the less-studied planets in the Solar System."
We don't know as much about Venus because so much of the exploration of Venus was done early in the space programme. As our technology has improved, we've turned our sights to other planets, especially Mars. "We've learned so much now about Mars. We know so much about its early history, we know so much about how its atmosphere works, we know so much about its climate," says Lori Glaze, a Nasa scientist who specialises in Venus. "Venus has fallen a bit behind."
Glaze is working to change that: she is the lead researcher on DaVinci, a proposed mission to send a robotic probe down into the depths of Venus's atmosphere, to better understand our nearest neighbour. "DaVinci is really focused on trying to answer fundamental questions about Venus… [It] will put better bounds on the composition and dynamics of [Venus's] atmosphere," says Glaze, "which would help us understand how you might send something that would live a long time in that upper part of the atmosphere."
"Venus is a fascinating planet that has so many mysteries yet for us to discover, and I think we need to better understand [it]," says Glaze. "We need to get back to Venus!"
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