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So theoretically, if Mars were to suddenly gain an Earth-like atmosphere, then what would the impact be on the surface lighting conditions? How much light would be making it to the surface? Would noontime at the surface have the same low level of light as dusk on Earth?
It would be neat to have a little web-based Mars terraformation simulator. It would show one of these Curiosity Rover panorama scenes, and as you adjusted various physical parameters thru web-controls, the scene would change accordingly. I think that would be a cool thing to get people more interested in Mars.
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Mars gets 47% as intense light as Earth, just due to it's distance from the Sun. It does have an ozone layer, but it's pitiful. Mars surface has roughly 0.7% as much pressure as Earth, and it's over 95% CO2. Most of Earth's ozone comes from UV light interacting with oxygen. That is 3 O2 -> 2 O3. So almost all UV gets to the Martian surface.
A thick atmosphere won't affect visible light much. High humidity does cause diffusion, causing soft lighting and a blue sky. Once Mars has the same partial pressure O2 at the surface, I expect UV will create ozone until intensity of UV that reaches the surface is the same as Earth. That's the key: once O2 is the same, UV will also be the same. But that's UV, not visible light.
Clouds will cause cloudy days, but you know how that works. That's about it.
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So you feel that conditions will be just as bright on the Martian surface as they are now, as with an Earth-like atmosphere. So if terraforming could somehow be achieved, it wouldn't leave us all stumbling around in the dark.
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How much light reaches a terrestrial planet's surface depends on what the atmosphere is made of and how high the pressure is. Rayleigh scattering starts to reflect substantial amounts of light away from Mars once you give it a CO2 atmosphere thicker than ~0.1 bar. This is because CO2 scatters light more effectively than N2 or O2 (it is a larger molecule), and Mars' low gravity means that 0.1 bar translates into a pretty large column of atmosphere (almost 3 times larger than it would be on Earth). By the time you reach a 1 bar CO2 atmosphere, Mars (without clouds) would reflect a greater fraction of incoming light than modern Earth does (with clouds). This is actually a major problem for climate modeling scientists (like me) because the cooling effect from increased reflection overcomes the additional greenhouse effect of CO2 and makes it difficult to explain how Mars was ever warm enough for liquid water in the past.
An N2/O2 dominated atmosphere would have less of a problem with Rayleigh scattering, but you would still want to keep your total surface pressure lower than that of Earth. Of course, as RobertDyck mentioned, clouds are also a very important determinant of how much light gets to a planet's surface. Unfortunately, they are also the least well understood aspect of climate modeling, and predicting what the cloud cover of a terraformed Mars will be is a very uncertain business.
"Everything should be made as simple as possible, but no simpler." - Albert Einstein
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