[ben_w]

A lot of mirrors in a 24h orbit might be easier.

Still, even then it will take, what, about a century to get rid of the CO2? (Does it even count as an "atmosphere" at ground level, given that it's past the critical point and the distinction between liquid and gas phases no longer exists?)

[indoordin0saur]

Oh, I'm sure it'll take at least several decades and more likely centuries but getting a new planet would be worth it. Seems like a harder project than terraforming Mars but in the end it would be a nicer place in my estimation. With only 7% less gravity, nearly as much surface area and without the dim sun that Mars has I think I'd rather live there.

[dylan604]

But that new planet has the exact same fate as Earth in that the Sun will eventually devour it. The only way for the species to survive longer than the sun is to find other planets around other stars at different stages of their life cycles. Yeah, I agree. Expecting the species to survive that long is a bit optimistic.

[aaronmdjones]

We have billions of years before that happens. We're talking about things we can do in the next hundreds.

[mcmoor]

Yeah seems like with our current physics, almost everything can be cheated out, except mass and gravity. There's no other solid Earth-sized mass now other than Venus so it may be our only real potential second home.

[ben_w]

You can cheat gravity with a centrifuge; when the alternative is on the scale of "let's freeze or boil away Venus's atmospheric", building a city-sized rotating cylinder to live in is trivial.

[projektfu]

I've always loved the idea of floating habitat in Venus' atmosphere but the temperature remains the largest part of the problem.

[indoordin0saur]

Kurzgesagt has a good youtube video of the idea. It's wacky and unrealistic in some ways but doesn't violate any laws of physics and seems feasible to the sort of advanced civilization we humans hope to have in many decades.

[mjevans]

If going that far, use the sunlight for power / in space solar ovens. Just make sure to limit how much power goes to Venus because that will increase the net energy in that envelope.

[ben_w]

One of the options is to use the mirrors to boil off the atmosphere, the other is to keep the sunlight away for so long the atmosphere almost entirely condenses and can then be paved over; either way, it was a long wait.

[cyberax]

You can't "boil off" the atmosphere. You need to accelerate the gas molecules past the planetary escape velocity, otherwise they'll just cool down and drop back onto the surface.

There's no realistic way to evacuate that much gas (the surface pressure on Venus is almost 100 atmospheres!).

One option is first to cover the surface of Venus with water, by first creating giant orbital mirrors to let the atmosphere to cool. Then you can sequester the carbon dioxide as elemental carbon under the water surface. Oxygen released in the process will be naturally consumed by all the underoxidized minerals present on Venus.

[indoordin0saur]

The idea I've heard is to dump many gigatons of hydrogen into the atmosphere to make H2O and elemental Carbon via the Sabatier reaction which will occur immediately with Venus' temperature. This gets rid of some of the CO2. The water will be part of the atmosphere, the carbon will rain down on the surface of the planet.

There will still be a lot of CO2 around though. Now you shade the planet with a giant sunshade and the atmosphere will cool down to the point where CO2 will snow out as dry ice after a couple decades of cooling. What you'll have left is a layer of carbon, followed by a layer of water ice, followed by a layer of dry ice. The atmosphere will be 2 - 3 times as dense as Earth's with almost entirely nitrogen gas. You use autonomous robots and mass drivers to collect and launch the excess dry ice into orbit.

Once you've got most of the CO2 ice out you remove the sunshade and the planet will very quickly thaw out. The planet will be covered mostly in oceans at this point with a thick nitrogen and CO2 atmosphere. At this point use genetically modified algae and microbes which, using the plentiful sunlight, can quickly convert the high CO2 atmosphere into oxygen and organics for the soil. The planet may still need some minor solar shading to keep the temps down until the CO2 approaches earth levels. Introduce Earth life to build a natural biosphere and voila! You've got a whole new Earth whose biosphere should be able to sustain life for millions of years!

[cyberax]

Sabatier reaction is an equilibrium reaction, so it can only remove a part of carbon. You need to somehow continuously remove the generated carbon to keep the reaction from transforming it back into CO2.

The second issue is sourcing H2, water ice is common, you "just" need to redirect enough comets. Comets can be also used to build a solar shade, by placing them into a polar orbit around Venus inside the Roche limit. They'll naturally fall apart and form a cloud around the planet.

[ben_w]

> You can't "boil off" the atmosphere. You need to accelerate the gas molecules past the planetary escape velocity, otherwise they'll just cool down and drop back onto the surface.

Velocity in a gas is a distribution; raise temperature and increasing fraction exceeds escape velocity.

This is why Earth has ~ no hydrogen or helium in the air.

> There's no realistic way to evacuate that much gas (the surface pressure on Venus is almost 100 atmospheres!).

"Realistic" for values including "let's build a mirror the size of a planet, in space, and keep it together for centuries".

It's currently scifi to send more than a mere few tons total mass that way, and mirrors wouldn't even survive decades, so "realistic" is a bad criticism.

[cyberax]

> Velocity in a gas is a distribution; raise temperature and increasing fraction exceeds escape velocity.

It's not going to be a significant fraction to matter, except of truly geological timeframes.

> "Realistic" for values including "let's build a mirror the size of a planet, in space, and keep it together for centuries".

Probably thousands of years. But not hundreds of thousands.

[mkl]

Venus hardly has any water, as it has already lost most of its hydrogen into space (due to its very weak magnetic field): https://en.wikipedia.org/wiki/Venus#:~:text=The%20solar%20wi....

Doing it to heavier gases on human time scales is a whole other scale of problem.