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Atmospheric pressure increases the deeper you go & the highest growing plants on Earth have been found at a height of 6 km so how deep a trench would we need on Mars to provide similar air pressure? World’s highest plants discovered growing 6km above sea level

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  • $\begingroup$ There's basically no O2 in the Martian atmosphere so no matter how deep you dig your trench, you still won't get an Earthlike atmosphere. $\endgroup$ – Organic Marble Dec 2 '18 at 1:33
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    $\begingroup$ That would be a different question, I'm only asking about air pressure in this instance. $\endgroup$ – Pelinore Dec 2 '18 at 1:35
  • $\begingroup$ Then you need to edit your question and make that clear. You just say "conditions". $\endgroup$ – Organic Marble Dec 2 '18 at 1:36
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    $\begingroup$ Edited to clarify. Both of you play nice. $\endgroup$ – Russell Borogove Dec 2 '18 at 1:43
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    $\begingroup$ Since the OP is interested in growing plants, he does not need the air pressure. He needs the CO2 partial pressure. One would have to calculate the CO2 partial pressure at 6000m altitude on Earth, and find the equivalent partial pressure depth on Mars. $\endgroup$ – dotancohen Dec 2 '18 at 12:04
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Mars' atmosphere scale height is, depending on who you ask, 10.8 to 11.1 km.

  • Pressure at the bottom of Hellas Planitia: 1.16 kPa
  • Earth sea level: 101.3 kPa
  • Earth 6km altitude: ~50 kPa.

So we need air pressure to increase by a factor of about 43; ln 43 = 3.76 scale heights -- so we need a trench about 41km deep. Start digging!

This gets equivalent air pressure, but there's almost no oxygen. Partial pressure of CO2 on the other hand is about 2400 times higher -- Mars's surface level atmosphere offers more CO2 than Earth as it is.

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  • $\begingroup$ "Start digging!" got my shovel, just waiting on my ticket from SpaceX :) $\endgroup$ – Pelinore Dec 2 '18 at 2:08
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    $\begingroup$ And note that at this depth your trench almost certainly collapses in on itself, the rock can't take the load. $\endgroup$ – Loren Pechtel Dec 2 '18 at 3:16
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    $\begingroup$ @LorenPechtel that depends on how steep the sides are, it has little to do with the depth. $\endgroup$ – Level River St Dec 2 '18 at 8:44
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    $\begingroup$ This answers the question asked! At 1.16kPa water boils at around 8C, at 50kPa, it boils at around 83C. Note that humans can tolerate pressures down to 6kPa (for periods short enough to avoid asphyxia) before their eyeballs and lungs are damaged by water boiling. That said, article cited by the OP states that the problem for plants at 6km altitude is not the pressure but rather "drought and frost." These are still going to be problems in a 40km deep trench on Mars. $\endgroup$ – Level River St Dec 2 '18 at 8:53
  • $\begingroup$ @LevelRiverSt No, that trench is getting down to the point where the rocks will flow under the pressure. How steep the sides are has nothing to do with that. $\endgroup$ – Loren Pechtel Dec 2 '18 at 21:33

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