It's a basic fact that most plants need sunlight to generate growth. Sure, they also need water and nutrients, but their reliance on sunlight is the one sole thing that separates them from animals.

Now, compare our sun to our moon. Even at its fullest, moonlight can only get to an apparent magnitude of -12.90, which is 1/400,000 as bright as our sun. To make matters worse, it's just a ball of unbreathable rock, which has a fairly low albedo, despite what the view of the moon would have you thinking. In short, moonlight is nowhere near strong enough for photosynthesis to occur.

But let's say that some alternate Earth is orbited by a Pluto-sized ball of ice, which is more reflective than rock. From a view of the night sky on that planet, this new icy moon, in its crescent, is 12,000 times brighter than our full moon. Which means that when this icy moon gets full, it's going to get even brighter. But will it be bright enough to encourage plants to photosynthesize in the dark of night?

  • $\begingroup$ You say: "moonlight is nowhere near strong enough for photosynthesis to occur" - this is incorrect, it would just occur slowly. $\endgroup$ – Tantalus' touch. Aug 19 '19 at 21:13
  • $\begingroup$ Plants can photosynthesize in weak light, but they need more energy than that, so they will still consume oxygen and produce CO2, just a bit less $\endgroup$ – Juraj Aug 19 '19 at 21:25
  • $\begingroup$ if it is not bright enough for photosynthesis it is unlikely to be warm enough for liquid water. $\endgroup$ – John Aug 19 '19 at 23:41

To learn how low you can go, let us consider the illustrative example of the gentle Lampenflora.


Lampenflora (translated page from German Wikipedia)

Totality of all autotrophic plants, which are located in caves in the field of fixed lighting fixtures

These are plants which are adapted to very low light such as occurs in show caves. They occur on the wet surfaces around lights. They are studied because they are a nuisance and damaging to the cave. How low can light be to sustain a population of Lampenflora?

CONTROL OF LAMPENFLORA AT WAITOMO CAVES. NEW ZEALAND http://www.ackma.org/papers/Wait3.html

Light - The restricted occurrence of lampenflora to areas directly lit by the fixed lamp housings in the caves clearly shows that light is the primary controlling factor for lampenflora development. The extent of lampenflora growth around any one lamp housing depends on the number of bulbs in that housing and on the distance of that housing from a suitable substrate. Light intensity measurements made in the Glow-worm and Ruakuri caves suggest that the minimum light levels required for continued growth of the different autotrophic lampenflora organisms are: green and blue- green algae 0.1 to 1.0u E /m2 /sec ( 10 to 50 lux) , mosses 1.0 to 3.5u E / m2/sec ( 50 to 180 lux), and ferns 5.0u E/m 2/sec ( 250 lux).

So that is how much the lampenflora need. Now how much does the moon provide?


The intensity of moonlight varies greatly depending on its phase, but even the full Moon typically provides only about 0.05–0.1 lux illumination.[2] When the full Moon is at perigee and viewed around upper culmination from the tropics, the illuminance can reach up to 0.32 lux.[2]

A range of 0.05 to 0.32 lux. Our moon provides an order of magnitude less light than the lampenflora require. But the OP asserts that the glowy moon of this world provides 12,000 times the light of the moon, which means 600 to 3840 lux! Far more than the lampenflora require, even at the low end of the range, even the light hungry ferns!

So the answer is yes. Glowy moon as described is enough light for photosynthesis.

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As far as I know light is only necessary to release electrons making further reactions possible. So as long as there is some light it’s not impossible for a kind of plant like organism to develop the necessary organs.

That said I would expect that life will go down a path that is most efficient for its environment. That why it seems unlikely that we’d find many plant life specialized to produce energy under low light situation on earth, if it can do better at daylight. That doesn’t mean however that photosynthesis would be impossible but simply a lot slower. If a plant life has developed to cope with it ..why wouldn’t it be impossible. But as said, if it isn’t very efficient than maybe plants could use other energy sources that are easier to tap in like something that is result of volcanic activity etc...

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  • $\begingroup$ How do volcanoes create light, much less for long enough for plants to take advantage of? $\endgroup$ – JohnWDailey Aug 19 '19 at 21:39
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    $\begingroup$ @JohnWDailey There are microbes which engage in photosynthesis powered by infrared radiation from thermal vents in the deep ocean. But that is marginal compared to chemosynthesis, using the chemical energy gradient that results from volcanic and geothermal activity. $\endgroup$ – Logan R. Kearsley Aug 19 '19 at 21:45

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