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So... Mars has a thin atmosphere, and is far from the sun, so it's too cold for humans. Venus is close to the sun, has a very thick atmosphere, and therefore is too hot for humans. What would happen if it were the other way around? Mars's thin atmosphere allowing solar heat to escape, while Venus is wrapped in a nice cozy blanket to keep warm. (Not concerned about how the movement would be done.) Would either planet have at least a human-hospitable temperature?

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Mars would be about 50C warmer, thus still bitingly cold. But the polar CO2 icecap should evaporate, so the atmosphere will be about twice as dense, thus still a pretty good vacuum.
Expect to see much more interesting duststorms, due to about four times the solar energy input plus double the atmospheric density.

Venus would be about 15-20C colder, thus still hotter than the hinges of hell.
Some careful science experiments might be able to measure a difference.

Remember that in their current positions, less sunlight reaches Venus surface than Mars.
YES despite their differing distance to the sun.
and YES despite mars being much smaller than Venus.
Even with this, the sheer reflection from the clouds of Venus throws away all but about 30% of incoming solar energy, and 90% of that gets stuck in the cloud layers without reaching the surface, whereas Mars's rusty surface quite greedily gobbles up 84% of all incoming light.

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  • $\begingroup$ Shouldn't Mars get 16x the solar input instead of 2x? $\endgroup$ Jun 11 at 23:57
  • $\begingroup$ @TheSquare-CubeLaw it's actually about 4.4x (590 vs 2662) $\endgroup$
    – PcMan
    Jun 12 at 0:01
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Let's assume that the switch happened early in the solar systems history. Maybe Jupiter came in closer than it did in our time line and disturbed the inner solar system.

Young Mars would still lose the atmosphere, but the younger and fainter sun would keep its surface cosy for during its early history. Things depend on the time line, meaning when does the core get too cold for a magnetic field. At that point the planet will turn into present day Mars.

However the loss of the atmosphere could prevent a runaway greenhouse. Desert planets will have larger habitable zones, because they do not have to deal with any of the water climate feedbacks. Assuming that life develops as fast as on Earth, meaning as soon as the planet's lava oceans freeze out and the steam atmosphere rains down, this Mars would have life.

Venus could be interesting in an orbit further out. Its slow rotation might have been cased by solar tides in its thick atmosphere. Meaning that if the runaway greenhouse effect never happens, it might have a rotation rate of anything between a few hours and a few days. Though, as mars and earth do rotate at a similar rate, let's say Venus went for something close to 24 hours.

Venus would probably again be doomed by its water. It would freeze out and turn the planet into a hyper reflective snowball world. Any hope will be lost if the CO2 freezes out of the atmosphere entirely. Vulcanism, especially if the tectonic history goes towards plate tectonics, but even with its current regime, could keep habitable oceans beneath the ice warm. Think black smokers on earth. The world might even experience limit cycling, given the the CO2 in the atmosphere isn't frozen out and experience periods of liquid surface water, before the weathering cased by the higher temperatures will cool the planet again.

If Venus has significantly less water, it might be even more habitable, as the ice albedo feedback loop can be avoided. A desert tundra world with a temperate equator under a thick (think 10 atm) CO2 atmosphere is conceivable.

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