system
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human
ELI5: Why can't we terraform Mars and Venus?

Why can't we terraform Mars and Venus by taking the greenhouse gasses from Venus and moving them toward Mars? That would raise the temperature or Mars and lower the one on Venus, right? Or do greenhouse gasses not work in that way?
gpt
There are different reasons for each of the planets, but the main reason for both is that, while theoretically possible, it isn't economical in either case.

For Venus, you have a surface temperature and atmospheric pressure so high that even the most resilient materials only last a few moments before disintegrating. Also, Venus has a runaway greenhouse effect, where its thick atmosphere and surface temperature actually caused a further increase in levels of CO2, leading to even higher pressure and temperature. Terraforming Venus is likely impossible, but an interesting idea is to have platforms in Venus' atmosphere. The planet's atmosphere is so thick that it could possibly support massive dirigible-like platforms large enough for people to live and conduct research. Think of a much smaller scale verson of Cloud City from Star Wars.

Mars has pretty much the exact opposite thing going on. Mars has a runaway icehouse effect, where its thin atmosphere and cold temperatures have led to greater decreases in atmospheric gases and even colder surface temperatures. The exact reason for this is unknown, but it's likely that Mars at one time had a magnetic field which it has since lost. 

A planet's magnetic field protects its atmosphere from cosmic rays and the solar wind, both of which can excite atmospheric particles and cause them to escape the planet's gravity. So, even if you did introduce an atmosphere to Mars, you would have to have a magnetic field, otherwise the solar wind and cosmic rays would just cause all of the new gases to quickly escape.

Finally, neither planet has plate tectonics the way Earth does. Plate tectonics are responsible for introducing and sequestering many of the greenhouse gases people need to survive, so you would have to devise some kind of work around to keep the same mixture of gases on either Mars or Venus.