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I've read about plants on worlds orbiting red dwarfs potentially having black leaves due to the higher infrared radiation, but what kind of changes would be expected of plants on a planet like Mars which orbits a star like ours but only receives about 40% of the luminance from the sun that Earth receives?

Are there certain kinds of plants that we would expect to succeed or not succeed if we introduced Earth plants into such an environment for terraforming purposes?

Assume for the purposes of this question that the rest of the environment (gravity, atmospheric composition, etc.) is broadly Earthlike.

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  • $\begingroup$ All other things being equal, how could less light not mean less life; more plants dying? $\endgroup$ Commented Apr 1, 2023 at 19:54

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Many Earth plants are at home in light below 40% of full sunlight. Those that grow in a forest on the ground, for instance, may see only 10% of what grain in an open field would receive. House plants not immediately by a south-facing window (for the northern hemisphere) will thrive in as little as 1% of full Earth sunlight. And redder light will have little effect, since chlorophyll uses mainly visible light (other than green, which is reflected/scattered away) in its process of converting CO2 and water to simple sugar and dumping the leftover oxygen overboard.

So, many low-growing forest plants will do well and might even still want some level of shade for best growth. Most of what we consider house plants will do well, assuming temperatures and water availability are suitable.

In contrast, plants that are adapted for sub-tropical deserts or tropical forest climates where they receive the maximum level of sunlight in their natural habitat might do less well -- this would likely include many species of tropical trees, some cacti and other succulents, and similar plants that, despite the UV, glare, and heat, grow in unsheltered areas near the equator. I don't know that this has been studied, but these would be the plant types one might expect to genuinely need high levels of light to thrive.

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  • $\begingroup$ Thanks! Are there any types of plants that we would expect not to do well? $\endgroup$
    – DMacc1917
    Commented Mar 29, 2023 at 18:08
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    $\begingroup$ @DMacc1917: Maybe some kinds of grasses which only grow open land between the tropics. Maybe, but I'm pretty sure that even most of said grasses will do just fine if it doesn't get too cold. In general, the geographical distribution of plants is limited by their temperature and humidity requirements, much more than by the amount of sunlight. For example, you can grow most tropical plants in St Petersburg in glasshouses with no artificial illumination provided you keep them warm enough. $\endgroup$
    – AlexP
    Commented Mar 29, 2023 at 18:34
  • $\begingroup$ A lot depends on what sort of planet you are thinking of. Having Mars level light levels is one thing, but what about gravity, atmospheric composition and pressure, temperature, water levels, nutrients, rotational period etc etc. Can we assume its Earth with Mars level illumination? Or is it Mars like? Because if its Mars like everything dies due to virtual lack of atmosphere and extreme temperatures etc $\endgroup$
    – Slarty
    Commented Mar 29, 2023 at 20:44
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I upvoted @ZeissIkon's answer and you should, too.

There are many plants that grow in the shade here on Earth. A quick visit to your local garden supply business will get you a wealth of information as will a quick Google search for "list of shade loving plants". But I'd like to build on Zeiss' answer.

No serious effort to colonize Mars (or any other planet) would be expected to just uproot existing plants (OK, take seeds...) and haul them along. That's bad planning. Every gardener and farmer on Earth wold be shaking their heads in contempt and disdain.

Major undertakings are planned for.

And when it comes to plant life, you can believe somebody would (OK, should...) be taking the time to seriously wonder, "what do I need to do to Winter Wheat to make it a viable food crop on Mars?"

Therefore, I'm going to suggest a bit of a spin on your question: "What should be done to plants intended for Mars to maximize viability?"

Disclaimer: I am not a botanist. I like electrons, they like chlorophyll. Things in their world die, things in my world release a bit of smoke before causing heartburn. These are observations from someone who's lived the last 38 years with a horticulturist. Fair warning. Who knows the value of what I've absorbed. If you know of a better solution, please let me know in comments to get your name in lights and corrections made.

Note: @JohnMcD pointed out that the atmosphere on Mars is only about 1% of Earth's, meaning wind isn't a problem. But dust is. That's going to change my answer a bit.

Desirable Terrestrial plants will be those that...

  • Convert CO2->O2 quickly.
  • Release atmospheric O2 from ground nutrients.
  • Release lower CO2 through respiration.
  • Act as quick-growing ground cover.
  • Are drought tolerant.
  • Provide food & materials for settlers.

And through either genetic manipulation or good hybridization, goals for Mars plantations can be plants that...

  • Grow deep roots to bind the soil.
  • Grow broad, tough leaves to capture more sunlight and deal with the abrasive nature of dust.
  • Grow taller with fewer low branches or leaves so collected dust falls off without the pile having a significant consequence.
  • Have a wide leaf spread.
  • Have superior nitrogen fixation.
  • Have superior water retention.

And if we don't have these plants in stock, we need to create them. And you should believe that we would.

Asking for candidates is a list too long.

There are thousands of plants that are good candidates for all of that. Some would be more work than others to develop into good Martian species and varieties. Some will meet a specific need: like food crops (human and stock animals). Others for textiles (clothing) and still others for pharmaceuticals (medicine) and construction (e.g. wood). But your question about the nature of the plants is a good one!

But don't forget that most basic of rules: location, location, location.

After all the modifications we might make to plants for Mars, in the end, we need to care greatly about where we plant them. This means you'll be planting in the lee of hills to avoid collected dust. It also means that one type of plant life will break most of the "rules" I just gave you...

  • Windbreak foliage

Off the top of my head, I can't imagine a more important plant than windbreak foliage on Mars. I call it "windbreak" because that's what we have on Earth. It would be better to call it "dustbreak." Ideally, this is a densely bushy tree standing as tall as you can get it with deep roots. The tree doesn't need a lot of leaves, but it does need to protect crops from the dust.

A Word about the Dust

To give you an idea of the problem, a Martian dust devil can reach 5 miles or 8 kilometers into the air. There isn't a tree that can stop that... but that isn't quite the point. No windbreak tree on Earth can stop all the wind, which can easily reach miles into the air as well. The goal of the break is to break up the effects of the storm, basically getting them to hop over the protected area.

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  • $\begingroup$ It's a good answer, but I think that the Martian atmosphere is so thin that wind isn't really a problem - it may have high wind speeds but there is little to it. The opening storm in The Martian is the single least scientific thing in that whole movie according to their scientific advisor $\endgroup$
    – John McD
    Commented Mar 29, 2023 at 18:30
  • $\begingroup$ @JohnMcD That's a good point. A quick search revealed the existing atmosphere is only about 1% the density of Earth's. That makes the problem the dust. Name in lights... $\endgroup$
    – JBH
    Commented Mar 29, 2023 at 18:35
  • $\begingroup$ Well, the BIGGEST problem is the 1% atmosphere coupled with conditions that make liquid water in surface conditions almost impossible (deep down in one valley, as I recall, the pressure is above triple point and sometimes the temperature gets that high). $\endgroup$
    – Zeiss Ikon
    Commented Mar 29, 2023 at 18:53
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    $\begingroup$ @ZeissIkon I have to admit, I didn't want to turn this into a full discussion of what it would take to botanically terraform Mars. I can easily imagine plantings being riddled with a pressure-controlled subsurface drip water grid. Along with the issues of atmosphere, I don't think we still know where to get sufficient water on the planet to grow an outdoor garden, much less terraform it. Since the OP asked about plants, I tried to focus only on that. $\endgroup$
    – JBH
    Commented Mar 29, 2023 at 18:56
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    $\begingroup$ Well, true, but you'd still need about 50x as much air to grow anything outside a pressure dome. Which I ignored in my answer too. And dust is the big issue for those "greenhouses". $\endgroup$
    – Zeiss Ikon
    Commented Mar 29, 2023 at 19:05

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