Would we need to use natural lighting to grow crops on Mars instead of just going with artificial LED lights? What are the pros and cons of using natural lighting, and do you think that a future Mars habitat would utilize natural lighting to grow crops?
Yes. While Mars only receives about half of the light that Earth does, it doesn't have clouds or other similar items to deal with. Assuming you can have a dome that is very clear, you should have about 600 W/m^2, while the surface of Earth has 1000 w/m^2 (source). The site linked to has some great pictures, I'm not sure if I can use them here.
Assuming equatorial Mars vs about 36 degrees latitude, the lighting levels should be fairly equal. So you can grow crops that would grow perfectly fine at that latitude on the equator, assuming you can get the temperature correct.
Note that if plants primarily grow in the early spring or late fall, they would also be fine on Mars.
Roughly speaking Mars gets between 1/2 and 1/3 the light that Earth gets (depending on time of year) meaning you could probably grow plans that handle full shade on earth pretty well but most crop plants are more on the full sun side of things. Growing crop plants would probably require concentrated light if you were going to use natural light. LED grow lights are probably a lot more practical than huge lenses to concentrate light on rows and rows of plants.
millions km perihelion semimajor aphelion Earth 147.1 149.6 152.1 Mars 206.7 227.9 248.2 (Earth's SMA / x)^2 perihelion semimajor aphelion Earth 1.03 1.00 0.97 Mars 0.52 0.43 0.36
There's some current scientific interest in figuring out photosynthesis in resource-short environments, such as low light.
A recent example is the paper "Photochemistry beyond the red limit in chlorophyll f–containing photosystems" DJ Nürnberg et al Science 15 Jun 2018: Vol. 360, Issue 6394, pp. 1210-1213 (link might be paywalled, sorry) for which the summary is:
Plants and cyanobacteria use chlorophyll-rich photosystem complexes to convert light energy into chemical energy. Deep-water organisms do not receive the full spectrum of light and have adaptations to take advantage of longer-wavelength photons. Nürnberg et al. studied photosystem complexes from cyanobacteria grown in the presence of far-red light. The authors identified the primary donor chlorophyll as one of a few chlorophyll molecules in the red light–adapted enzymes that were chemically altered to shift their absorption spectrum. Kinetic measurements demonstrated that far-red light is capable of directly driving water oxidation, despite having less energy than the red light used by most photosynthetic organisms.
That's more about light spectrum than intensity. But note that Mars surface sunlight has more energy (a larger fraction) in the longer wavelengths due to air-borne dust which has blue-absorbing iron oxides.
Any comparison with photosynthesis on Earth is made more complicated by the differences in other factors. For example, Earth-life photosynthesis can be limited by other things than sunlight: Water, trace nutrients, CO2, etc. An early paper even showed the the morphology (shape and organization, both at the big and small scales) of plants can optimize for changes in sunlight and CO2 levels.
More recently, the work tends to look at light in concert with other shortfalls, for example "Effects of light intensity on photosynthesis and photoprotective mechanisms in apple under progressive drought" MA Ping et al, Journal of Integrative Agriculture Volume 14, Issue 9, September 2015, Pages 1755-1766 which takes about low light not so different from Mars:
The effects of light intensity on photosynthesis and photoprotective mechanisms under progressive drought were studied on apple trees (Malus domestica Borkh.) Fuji. The potted trees were exposed to drought stress for 12 days and different light conditions (100, 60 and 25% sunlight). During the progressive drought, the relative water content (RWC) in leaf declined and was faster in full light than in 60 and 25% sunlight. However, the decrease in the net photosynthetic rate (Pn), stomatal conductance (Gs) and Rubisco activity were slower under 100% sunlight condition than other light conditions. After the 6 days of drought, the maximum PSII quantum yield (Fv/Fm), the capacity of electrons move beyond QA− (1–VJ) and electron move from intersystem to PSI acceptor side (1–VI)/(1–VJ) decreased, with greater decline extent in brighter light. While RWCs were >75%, the variations in different light intensities of Gs and Rubisco activity at identical RWC, suggested the direct effects of light.
Bottom line: Multiple stressors, i.e. less light, less water, less CO2 are bad; individual stressors not so bad, but not great.
Plants will grow in 25% light, it's just a question of how well, and what else you have to do to make it worth it.
In hot sunny climates like Australia vegetables are often grown under shade cloth in the 30-50% range (that is 30-50% of sunlight blocked) - this is actually comparable with the sunlight available on Mars during the northern hemisphere summer.
Now, the reason to use shade cloth in hot climates is to reduce temperature, not to reduce illumination, it is not so much that too much sunlight is bad, but that getting too hot is bad, using shade cloth prevents sun-scorch and reduces water requirements allowing higher productivity at lower cost.
Commercial vegetable growers employ shade cloth to help them increase production, as can home gardeners. Tomatoes and peppers will stop growing at temperatures of 95 degrees Fahrenheit and fail to set fruit. For pepper and tomato production in hot summer areas, 30 percent shade cloth is recommended. Likewise, high temperatures can keep cool-season crops like lettuce (Lactuca sativa) and spinach (Spinacia oleracea) from growing properly. For cool-season crops, 30 percent shade cloth is used in cool-summer climates and 47 percent in hot-summer climates.
On Mars the available illumination should be comparable with this kind of shaded greenhouse on Earth, the reduced illumination may or may not be ideal, but it ought to be sufficient provided that other growth factors such as temperature, humidity and atmosphere are appropriately managed.
It is also worth considering that crops such as corn and wheat are grown at such a density that allows each individual plant to get sufficient light and at this density each plant is significantly shaded by its neighbors. I have a suspicion that growing these crops under Martian illumination levels might be as simple as using a wider spacing allowing each individual plant to sprawl out more and gather more light.
The other answers do a great job of talking about what plants could grow in Mars natural light, but it's also worth mentioning that by using something as simple as a lens or a mirror, you could easily give plants their full light needs using Martian light. If you imagine a greenhouse that's half growing area and half walking/storage space, you could easily lens/mirror the light in a way that plants would get double the native Martian light while the walkways/storage areas get just that which is reflected by the plants or bounced around the room.