Sunlight

The Sun is on average 228 million km away from Mars. Sunlight takes 13 minutes to travel from the Sun to Mars with a power of 589 W/m². It is only 43% as bright as on Earth (1367 W/m²).

Sunset photographed by Mars Rover Spirit

As shown by the images transmitted by the various probes and rovers, the sky is caramel colored most of the time. At sunrise and sunset, the sky can be blue near the horizon, as per the Spirit photograph. This is due to sun scattering of other wavelength by the dust in the Martian atmosphere.

The Martian atmosphere does not filter sunlight as efficiently as on Earth, letting more UV ray and high energy particles reach the surface. Life directly on the Martian surface is considered unlikely without additional shielding against UVs and cosmic radiation from the solar winds. An early warning system) may be useful for an eventual settlement.

The amount of usable energy on the surface is much lower than the solar constant due to the rotation of the planet and is also reduced the further away you are from the equator due to the incident angle of the light at the surface. Reflection and absorption (albedo) in the atmosphere must be considered. On Earth, the annual usable energy per m² is about 1000 kWh/y in Middle Europe and 2400 kWh/y in the Sahara.

Due to the dust, sunlight on Mars is usually fairly diffuse. It comes from many directions at once, rather than directly from the sun. Rather like an overcast day on Earth. This makes it harder to concentrate sunlight using reflectors ^[1].

Sunlight can be used for power generation by solar panels and for growing plants in greenhouses. It may also be used to warm a colony, especially if the habitat has Smart Windows.

Note that the sunlight is bright enough for growing plants (though they may grow slower). There is enough sunlight for crops out to the asteroid belt, but at Jupiter and further from the sun, artificial lights are needed for crops.

Facts and figures

"in the UK we receive an average power of about 125 Wm^-2 whilst the Sahara Desert gets 250-300 Wm^-2" ^[2]. With 24*365 hours per year, this results in an average usable energy per m² and year of 1095 kWh in the UK and 2190-2628 kWh in the Sahara.

The Mars solar constant is 590 W/m3, while the Earth solar constant is 1350 W/m2. Mars gets a bit less than half the sunlight Earth does. The Martian atmosphere is dusty; the usable energy at the Mars equator may be about 1100-1300 kWh per year per m2 (3960 to 4680 MJ), or somewhat like England or Northern Europe. For 22% efficient solar cells this would translate to about 240-286 kWh/year/m2.

Psychological and physiological effects of sunlight

The generally lower levels of sunshine may require the use of artificial lighting to treat seasonal depression disorder, as in commonly done in Northern countries on Earth.
The lower lighting levels will affect plants and reduce their agricultural yield and rate of growth.
Humans can make vitamin D via Ultraviolet light in our skins. Unless there are UV lights inside the base, humans will need to take vitamin supplements.