[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.