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My Universe Sandbox Almost Terraformed Mars (My Universe Sandbox Almost Terraformed Mars)

How high can plants grow on a low gravity planet like Mars? Since lower gravity planets tend to have a greater height differences in terms of geography but at the same time, a lower climate difference with regards to the vertical axis, at what altitude can plants grow before reaching their limit?

If Mars theoretically is habitable, can plants grow on Tharsis Rise or even on the base of Olympus Mons?

In this case my Mars has an:

  • Average surface pressure of 0.975 atm and still rising
  • An artificial magnetic field orbiting the Lagrange 1 point
  • Has sufficient amounts of greenhouse gases to keep it at an average temperature of 12 to -3 Celsius
  • Sea Level 11.7 Km
  • Genetically modified vegetation to withstand colder temperatures
  • All other data should follow what Mars already is, such as Elevation Rate
  • I don't know what would the atmosphere height, precipitation rate and several other things, this is why I ask on StackExchange otherwise I will solve my own problem because I were asked to find these solution by my own

Satellite Map of my Mars Satellite Map of my Mars

Mars Topographic Data (Mars Topographic Data)

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    $\begingroup$ Please tell us about the atmosphere, precipitation etc.. $\endgroup$
    – Escaped dental patient.
    Apr 17, 2022 at 18:41
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    $\begingroup$ As a minimum, we'd need to know the air pressure at the datum level (eg. the equivalent of sea-level on Earth), and what the major chemical constituents of the atmosphere are. As a starter-for-ten, you could go with "75% Earth surface pressure at same chemical mix", for example. $\endgroup$
    – Starfish Prime
    Apr 18, 2022 at 10:37
  • $\begingroup$ One of the problems of a low gravity planet would be its ability to maintain an atmosphere leading to issues concerning the pressure of the atmosphere & the gases it would be composed of. This then leads to issues about the presence of liquid water in soil, if soil exists. Regarding plants, don't expect trees or shrubs on such a planet. If anything, maybe algae, lichen, moss or liverworts. $\endgroup$
    – user81881
    Apr 18, 2022 at 12:08
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    $\begingroup$ Welcome Baselin ! that Mars looks really nice :d here are some tips to improve this question: describe in detail, how you terraformed the planet. Take a good paragraph, explain us what you did. Atmospheric pressure ? how high does your atmosphere reach ? Do you have oceans ? do they reach Olympus Mons, how high is the top in respect to sea level ? How high do clouds reach ? What's the chemical composition of the atmosphere now ? And did you genetically adjust your plants, or do you expect earth plants to survive.. We need more info. $\endgroup$
    – Goodies
    Apr 18, 2022 at 14:02
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    $\begingroup$ The depth to the lowest point is interesting for ocean trench life forms, but what's really of interest is the height above sea level (which the answerer should supply) and the pressure at sea level. I think you're saying the 0.975 atm is the average atmospheric pressure at sea level (since that's how it's usually given), so the question can readily be answered at this point. Note the scale height is larger on a warm N2/O2-rich Mars - given the highest point is 29429 m above the lowest (17.2 km altitude) it'll be more like Earth at roughly half that. $\endgroup$
    – Mike Serfas
    Apr 20, 2022 at 1:17

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(Mike Sertas answered most of this, but let's pull it together.)

On Earth, plants are found about as high as they could be -- "the highest-altitude plant species is a moss that grows at 6,480 m (21,260 ft)" according to Wikipedia. This is likely more limited by temperature and just how few opportunities there are to grow higher -- but let's use it as a starting point.

The scale height of a Martian atmosphere with the same temperature, composition, and pressure as Earth's goes with the relative surface gravities, so it's about 2.64x higher than Earth. However, our reference pressure is a bit lower, so we'll scale by that 0.975 factor to have equivalent pressure at 2.57x the height of reference. Therefore, our plant that is comfortable at 21,260 ft (6,480 m) is comfortable on Mars at 54,630 ft (16,650 m). This is from whatever altitude the 0.975 pressure holds -- if. we want to call that sea-level pressure altitude 11.7 km as the question suggests, this puts our moss at around 28.5 km above the lowest point on mars.

Let's sanity check this, and for this we have to get into "sea level" references a bit. Martian reference altitude is defined today somewhat arbitrary as the 610.5 Pascal isobar. The lowest point on Mars, Hellas Planitia, is about 6 km below this; so your sea level is 5.7 km above the Martian reference altitude. So our plants are growing at 22.5 km Martian reference altitude.

This is an interesting number, because it just exceeds the altitude of Olympus Mons at 22.1 km.

So, recap: We used a somewhat conservative starting point for where plants can live, because it's likely that plants on Everest are as much limited by temperature as by pressure. Looking at a Mars as your described, we modeled the atmospheric density variation with altitude and found that our Everestian plants would be happy (barely!) at the top of Olympus Mons. In summary, due to the scale height of the Martian atmosphere, if you manage to build an atmosphere that is approximately Earth-density at some reasonable sea level, there is no altitude limit to plants at all on the planet -- not just the base of Olympus Mons is possible, but even the crown.

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