back of the envelope: daylighting on Mars

Sometimes I need to work out a rough calculation to check whether my idea of something science-fictiony has any basis in reality. It doesn’t need to be super-rigorous*, but close enough to tell if my conception is way off the mark.

In this case, I’ve been thinking about how it might feel to walk around a city on Mars. It’s likely to be mostly underground to help shield against radiation, but there should be as much daylight as possible to save energy. On Earth, that kind of daylighting comes from skylights, windows, and (my personal favorite) light tubes.

But what about on Mars? Mars is farther from the Sun than Earth is, so it gets less light on the surface, but how much less? Is walking down a Martian street destined to feel like a gloomy overcast day?

First I had to get a grip on how to measure daylight. An obvious comparison is solar radiation, measured in Watts per square meter. Depending on time of year, Mars gets between 1/3 and 1/2 as much solar radiation as Earth, because it’s about 50% farther from the Sun. That’s handy for figuring solar power output, but the human eye isn’t so linear.

Another way is illuminance, measured in lux. Though the exact conversion factor between solar radiation and lux is a bit tricky due to the eye’s reaction to different wavelengths, I gather that the relationship is linear. Thus, using some standard Earth values and scaling them:

on Earth on Mars (min) on Mars (max)
direct sunlight 110,000 lux 38,000 lux 55,000 lux
indirect daylight 20,000 lux 6,800? lux 10,000? lux
clear sunrise/sunset 400 lux 130 lux 200 lux

(I’m assuming that indirect daylight is scattered as well in Mars’s pink sky as it is in Earth’s blue. Something to check later.)

Filling in a few other Earthly values for comparison:

bright overcast 25,000 lux
dark overcast 10,000 lux
studio lighting 1000 lux
office lighting 500 lux
cloudy sunrise/sunset 40 lux

So it looks like daylight on Mars wouldn’t look too different from daylight on Earth. It’s orders of magnitude more light than during “golden hour” on Earth, which is plenty to get around by. It would probably feel like a partly-cloudy day, since there would be more light than even the brightest overcast day, with sharply-defined shadows.

For daylighting, this probably means that Martian interiors would need twice as many Solatubes to get the same level of illumination, but we’re still talking about a fraction of the available daylight. In other words, using Earth-style lighting techniques should keep a Martian city street from feeling gloomy.

*Note the use of Wikipedia sources. Kids, don’t use Wikipedia as a source if you want anyone to take you seriously.