CMV: Mars is a poor choice for terraforming.

It's not that I didn't consider it... It's that there are other problems with Venus to overcome which made me consider it and dismiss it as feasible.

As I already mentioned, daytime on Venus is over a hundred days long. Then over a hundred days of night. Its day is actually longer than its year, but that doesn't really matter as much. Venus' axial tilt is effectively only 2.7 degrees... not enough to give it seasons. The only "seasonal effect" would be constant daylight or constant night. (But because Venus' actual axial tilt is 177.3 degrees, it means that the sun on Venus rises in the West and sets in the East... basically the planet is upside-down.)

Outgassing Venus would be a lot harder than rebuilding an atmosphere on Mars. We'd have to remove the sulfur dioxide and carbon dioxide and replace it with oxygen. That's complicated enough. As I already mentioned, we'd have to accomplish this with current temperatures higher than Mercury, pressure that's like a mile of Earth's ocean, and the problems presented with the day/night cycle. Another complicating factor is that Venusian winds ... at cloud levels they are more than 200 miles per hour. In the middle cloud bands it's more than 400 mph. And while wind speeds on the surface aren't well established, one thing to consider is that the considerable pressure means that even a light breeze would hit like a tidal wave.

Finally let's say we magically overcome all of this. We reduce the atmosphere to Earth-normal pressures and temperatures. We transform the current composition to Earth-normal levels of oxygen, nitrogen, and other gases. Problem solved, right?

Wrong.

Venus gets about twice as much sunlight as Earth does. That's problematic. Everything would have to be protected. Humans wouldn't be able to venture out onto the surface without protection. Plants wouldn't be able to exist outside of greenhouses that mimic an Earth-normal day/night cycle and reduce the amount of solar radiation they receive. Or we'd have to set up massive space-based reflectors to reduce the levels. Atmospheric reflectors might be possible, but they'd be less effective, requiring constant power to keep them in proper position and at least periodic maintenance to keep them flying.

On Mars, researchers have already proven that photosynthesis is possible with the reduced level of sunlight it receives. The only factors the experiment didn't consider was the reduced gravity (which wouldn't be a problem, plants have been grown on the ISS with no gravity at all) and the cosmic rays (which a thicker atmosphere would help protect them from).

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