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We've found a habitable planet around Procyon and do have the technology to settle it. However, it is in the outer part of its systems habitable zone. So while it has a well-developed and mostly biocompatible ecosphere, its atmosphere is not very earth-like.

  • 11.6 atm CO2
  • 2.4 atm N2
  • 0.18 atm O2
  • traces of Ar, H2O, dust, and O3

furthermore

  • 0.8g surface gravity
  • -5C average temperature
  • 78° axial tilt
  • 37% surface water
  • stagnant lid tectonics, (drip and plume, as on Venus) but the carbon cycle is stable
  • star is a young F-Type (1.85 Gyr)
  • the land ecosystems are young and primitive (unicellular and colonial organisms dominate, multicellularity is only a few million years old)
  • life is centered around the polar seas and the large regions around them, that get flooded by the fall rains, frozen over during winter and dry up during summer

Terraforming is out of the question because a number of powerful artilects and organizations want to study the biosphere. So, bioforming it is. The winters are dealt with with hibernation capabilities, greater temperature tolerances are easy as well and the low oxygen issue has been resolved.

However, how does one change a human to make the CO2 tolerable? This sounds bad.

When carbon dioxide dissolves in water, it forms a bit of carbonic acid. And that acid is a key part of our respiratory system. When carbon dioxide builds up in our blood, carbonic acid forms and the acidity of the blood rises. And the human body tends to handle pH changes rather poorly. The blood should have a pH of 7.35 to 7.45, and any significant deviation from that interferes with the blood’s function. Once the pH falls below 6.8, which is surprisingly easy, irreversible cell damage occurs. This acidity can be reached quite easily by breathing 30% CO2 at atmospheric pressure.

So what kind of modifications cod be made to humans to deal with this? Genetic engineering is preferred for ideological reasons, but cybernetics could be used as well.

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    $\begingroup$ Can't be done, short of having an entirely different metabolism (and the result of that could hardly be called human). The problem is that ANY aerobic organism, not just humans, produces CO2 as a waste product. Getting rid of that waste depends on having a lower concentration outside the body. $\endgroup$
    – jamesqf
    May 23, 2021 at 18:46
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    $\begingroup$ @jamesqf that could easily be a good answer $\endgroup$
    – SJuan76
    May 23, 2021 at 20:17
  • $\begingroup$ @SJuan76: Someone with more biological expertise than I have would need to write it, though. Though I can think of possibilities, as for instance an active CO2 "pump" to get rid of the CO2 (like the various channels that maintain the balance of sodium, potassium, and other ions), but that would require metabolic energy to drive them... $\endgroup$
    – jamesqf
    May 24, 2021 at 4:12

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That level of carbon dioxide is fatal in several different ways

You mention blood PH, but there are multiple problems that would have to be fixed somehow.

Nitrogen narcosis - the stated partial pressure of nitrogen would be enough to cause mild nitrogen narcosis, but CO2 is about 20 times as powerful as N2 in terms of narcosis. The level of narcosis is well beyond the fatal level. Note that the mechanism of narcosis is not well understood.

Cerebrospinal Fluid (CSF) PH - this as the direct input to the bodies respiratory regulation system. If you breath into a paper bag, it raises you CO2 levels quickly, and you start breathing rapidly and experience significant distress despite the fact that their is still lots of oxygen in the air you are breathing. Your body is responding to the increased CO2 alter the CSF PH, and it is trying to increase respiration to compensate. The high levels of CO2 would result in a fatal response (coma followed by death)

Hypocalcemia The high levels of CO2 will severely deplete the calcium your need for basic body function. In addition to well-known use in bones and teeth, calcium is essential for the function of nerves as well as the ability for all muscles to contract. Not sure whether you would die faster from muscular problems or nervous system failure from this level of calcium depletion.

I suspect there are other ways in which this level of CO2 would be fatal

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  • $\begingroup$ Yes, the planet is very harsh on humans. But what could be done about this with genetic engineering? You just list any number o reasons why this environment would kill someone. $\endgroup$ May 23, 2021 at 20:47
  • $\begingroup$ Maybe that's the point, there is so many ways to kill a person, we have no idea how to engineering around the biological limitations. This is certainly true with out current knowledge of genetic engineering. I specifically mention narcosis, because we don't even understand the mechanism of this narcosis - much less how to redesign around it. This is a frame challenge - you are asking for something that it impossible to answer in a positive way. $\endgroup$ May 23, 2021 at 22:20
  • $\begingroup$ Hmm, someone would have to vat-grow meat puppets from local genetic material, which is per definition adapted to the CO2, and either transplant a human brain in a jar with the proper interface or mind upload the person onto a brain made of the local stuff. Interesting Frankenstein vibes. $\endgroup$ May 23, 2021 at 22:28

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