Minimum amount of light for photosynthesis

[enioguedes]

Hi, i'm a new member of this community.

I'd like to know the maximum distance plants could grow in our solar system. For exemple, it is possible for plants to grow in a colony built on Ganymede or Rhea? I know that Saturn receives about 1% the light Earth receives from the sun. Is that amount enough to drive photosynthesis? And on the contrary, what is the maximum amount of light plants can withstand? Say, if a plant grew on Mars, due to it's larger distance to the sun, would the color still be green or another color would be best suited?

I imagine that if a plant grew on a habitable Venus with similar atmosphere of Earth it would be bluish because Venus is closer to the sun, so it receives more radiation and more blue light, as i think, might be bad for such plant life and such would prefer absorbing more energy light such as green or even red instead of blue, that would be less absorbed. On the other hand if a plant grew on Mars with a similar atmosphere that of Earth, it would be reddish because less light reach Mars compared to Earth and Venus, so it would need to absorb even more blue and green light. Is that correct? I mean, does the distance of a planet from it's star influence in the color of plants, not only the spectrum of the star?

Oh, by the way, just another matter... I'm creating a planetary system for my own book. The system orbits a M3V star with about 25% sun mass, that is in the limit of becoming a red giant or not in the future. One of the planets orbit at around 0,155 AU. It has an atmosphere denser than Earth's, with 7.5 air pressure. Such planet also has 4 moons, 3 small near the planet and another farther away but still somewhat distant from the hill sphere limit. The vegetation of this planet is between yellow and green, a very dark one, but not black since that vegetation is also capable of using near infrared to produce oxygen and it's not chlorophyll.

The thing is... that planet would be tidal-locked to it's star or not? Because i've read that planets closer to it's star but with enough atmosphere and at least a massive moon could avoid being tidal-locked. Eccentricity might do the trick too, in the case of Mercury.

[Jens]

I could be completely wrong about this, but apparently photosynthesis requires about 5,000 lux, which comes out to about 40 watts/square meter, which would fall somewhere in between Jupiter and Saturn. So it seems that photosynthesis would be (barely) possible on Ganymede but not Rhea.

But of course, I don't think you'd want to just use ambient light on a colony. It wouldn't be comfortable for the colonists. So you would probably want to harvest the light with solar panels, and use those to power artificial lighting anyway.

And I'm really not sure about the color of leaves. I don't think that the amount of light is the only reason for different color leaves, but I'm not an expert at all on plants.

ETA: Welcome to the board.

[profloater]

Ok, you need an astrobotanist, but the basic chemistry of photosynthesis should work with low light levels, with some heat and with CO2 and water. On Earth it evolved to use the energy radiation available but I imagine different molecules would be better with a different spectrum and of course with different gases. With water chemistry, photosynthesis here really slows down around freezing point and zooms away at 30 to 40 C with CO2 enhanced to say 1000 ppm, in greenhouses using visible light. Many plants need some blue light and have evolved cycles to match our environment but on other planets I think with current knowledge we would modify plants and so would natural evolution. Maybe in outer planets you need to devise alternatives since sunlight is much lower. Heat from geology and different chemistry would suggest alternative paths to plant or bacterial life. We tend assume liquid water is the first step. Then some chemistry and heat energy.

[enioguedes]

Fun fact is the reactions in the atmosphere of Titan that create those large complex substances, even at a large distance to the sun UV light is enough to break down molecules. Maybe on a cold and distant world like Titan a process similar to photosynthesis should be possible in the higher parts of the atmopshere by microrganisms that would use UV light, that is stronger than visible light and so could be enough to drive photosynthesis, I think. What i'm saying is the fact that visible light is too weak in greater distances, but UV light would still be energetic enough to drive photosynthesis, of course through different substances instead of chlorophyll. And a more volatile substance in place of water like ammonia or methane or even hydrogen sulfide may work best with low light levels.

[Swift]

Hi, i'm a new member of this community.

Hi enioguedes, welcome to CQ.

Say, if a plant grew on Mars, due to it's larger distance to the sun, would the color still be green or another color would be best suited?

Current belief is that the color of the plant would not be dependent upon the distance from the planet's star, but by the color of the light that reached the surface, which would depend on the spectrum (color) of that star, and to a lesser extent, the composition of the planet's atmosphere.

A NASA article about it.

NASA scientists believe they have found a way to predict the color of plants on planets in other solar systems.

Green, yellow or even red-dominant plants may live on extra-solar planets, according to scientists whose two scientific papers appear in the March issue of the journal, Astrobiology. The scientists studied light absorbed and reflected by organisms on Earth, and determined that if astronomers were to look at the light given off by planets circling distant stars, they might predict that some planets have mostly non-green plants.

Since all of the planet and other bodies in our solar system orbit the same sun, the light absorbed would have a similar color, and the plant color would be the same.

[Swift]

But of course, I don't think you'd want to just use ambient light on a colony. It wouldn't be comfortable for the colonists. So you would probably want to harvest the light with solar panels, and use those to power artificial lighting anyway.

I suspect at such a distance from the sun that a nuclear power plant might work better than solar panels as a power source (we do that with space craft that operate so far from the sun), but I completely agree that colonies would supplement with or completely use artificial lighting.

[profloater]

I suspect at such a distance from the sun that a nuclear power plant might work better than solar panels as a power source (we do that with space craft that operate so far from the sun), but I completely agree that colonies would supplement with or completely use artificial lighting.

One efficient light is sodium or high pressure sodium , very yellow in the low pressure type. You can grow lettuce in that almost monochromatic light if you add just a little blue, from blue leds. The blue stops the lettuce “bolting”.

[eburacum45]

Purple light is often used for LED grow lights, although adding a little green is even better.

[profloater]

Purple light is often used for LED grow lights, although adding a little green is even better.

Yes but low pressure sodium puts out about three times more lumens per watt and the blue needed for phototropisms is a tiny fraction. If stuck a long way from the sun, generate electricity somehow, and grow your plants with SOX.

[cjameshuff]

Yes but low pressure sodium puts out about three times more lumens per watt and the blue needed for phototropisms is a tiny fraction. If stuck a long way from the sun, generate electricity somehow, and grow your plants with SOX.

White LEDs now produce more lumens/W than sodium lights, and plants don't care about lumens. The portions of the spectrum the human eye is most sensitive to are not the ones most useful for photosynthesis. Those pink LED mixes produce relatively poor illumination for human vision, but are considerably more efficient than sodium lighting.

[profloater]

You are right ; current white led up to 350 lumen per watt, beating sodium. The old lettuce plant will have to be updated.

[publiusr]

Wasn’t there talk about really large collectors allowing plants to grow far from the sun?

[cjameshuff]

Wasn’t there talk about really large collectors allowing plants to grow far from the sun?

Sure, but even at Earth's distance, you may be better off with solar panels and LEDs instead of windows.

[Noclevername]

Sure, but even at Earth's distance, you may be better off with solar panels and LEDs instead of windows.

Which would make both radiation shielding and thermal control less complicated than windows.