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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

Pelinore
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  • 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. – Organic Marble Dec 02 '18 at 01:33
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    That would be a different question, I'm only asking about air pressure in this instance. – Pelinore Dec 02 '18 at 01:35
  • Then you need to [edit] your question and make that clear. You just say "conditions". – Organic Marble Dec 02 '18 at 01:36
  • No I don't, the question is very clear, it asks for the depth needed to give the air density, the comments you've misinterpreted as the question are there to give context to the question (what prompted me to ask). – Pelinore Dec 02 '18 at 01:38
  • You're the asker, you get to do what you want. However, density and pressure are not the same thing. – Organic Marble Dec 02 '18 at 01:39
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    Edited to clarify. Both of you play nice. – Russell Borogove Dec 02 '18 at 01:43
  • Well, edited to make the question match the answer I know how to provide. – Russell Borogove Dec 02 '18 at 01:50
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    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. – dotancohen Dec 02 '18 at 12:04
  • @dotancohen : Plants are interesting & that article is what got my wheels spinning but I'm more interested in a comfortable air pressure for people to work in (& maybe breath at a later point if you can get the plants to convert enough of that CO2.. though possibly only if your from Tibet). : If someone doesn't beat me to it you may have given me my next question though :) – Pelinore Dec 02 '18 at 12:39
  • There is a difference between creatures that can exchange gasses already in the atmosphere and those that cannot. If you want something that inhales CO2, then you might not have to dig at all as the Martian atmosphere is already ~96% CO2 as opposed to <0.05% on Earth. Earth recently had only 0.02% CO2, but I don't know if plants grew at altitude then. However, if you want respirators for O2, then the best you could do is to assume a 100% O2 respirator and 20 kPa atmosphere pressure. Plugging the numbers into Russel's answer gives 16 KM. – dotancohen Dec 02 '18 at 12:49
  • @dotancohen : You'd still have to dig down some for 16 km on Mars (the Hellas Planitia & Valles Marineris are the deepest places at something like up to 7.1 & 7km respectively), 16 km might do as an interim stage to get the plants started but ultimately you'd want a depth you could potentially breath at unaided. – Pelinore Dec 02 '18 at 13:01
  • Breathe at unaided: 20 kPa of O2. I've addressed the 20 KPa, the respirator would provide the O2. If you want the O2 to come from the atmosphere, you had better start planting a lot of plants! – dotancohen Dec 02 '18 at 13:50
  • "a lot of plants!" Precisely, no more unlikely (probably a lot less actually.. if you can get them to the point where they start seeding themselves) than digging a 41 km (or even only 16 km) deep trench really :) – Pelinore Dec 02 '18 at 14:04

1 Answers1

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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; natural log of 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.

Russell Borogove
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    "Start digging!" got my shovel, just waiting on my ticket from SpaceX :) – Pelinore Dec 02 '18 at 02:08
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    And note that at this depth your trench almost certainly collapses in on itself, the rock can't take the load. – Loren Pechtel Dec 02 '18 at 03:16
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    @LorenPechtel that depends on how steep the sides are, it has little to do with the depth. – Level River St Dec 02 '18 at 08:44
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    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. – Level River St Dec 02 '18 at 08:53
  • @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. – Loren Pechtel Dec 02 '18 at 21:33
  • @LorenPechtel : you might be right but on earth or on mars, less gravity so the calculation changes? – Pelinore Dec 03 '18 at 19:49
  • The average thickness of the planet's crust is about 50 km (31 mi), with a maximum thickness of 125 km (78 mi). https://en.wikipedia.org/wiki/Mars – Pelinore Dec 03 '18 at 20:20
  • @LorenPechtel : You might like to comment on this question https://space.stackexchange.com/questions/32541/whats-the-deepest-a-trench-could-theoretically-be-dug-on-mars?noredirect=1#comment99467_32541 it is there as a result of your comments after all :) – Pelinore Dec 04 '18 at 01:47
  • @Pelinore Yes, the gravity is lighter, that means you can go deeper before the rocks flow. Earth rocks flow at about 10km, so Mars rocks should flow at about 30km. – Loren Pechtel Dec 04 '18 at 01:55
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    So what you're saying is that at a depth of over 30 km (ballpark figure of course) the rock at the bottom of our "trench" will begin to flow & well up into the bottom of the trench until it's only 30 km deep again (much like a bucket with a whole in it placed in a 6" deep puddle, you can bale the water in the bucket out into the puddle with a cup all you like but the water level in the bucket will keep returning to 6"). @LorenPechtel – Pelinore Dec 04 '18 at 02:37
  • @Pelinore Yup, but I don't know how fast your trench will close. We've seen deep boreholes on Earth slowly close. – Loren Pechtel Dec 04 '18 at 02:44
  • @LorenPechtel Well (from answers here & elsewhere) 30 km still gets us below the Armstrong limit (iirc something like -25 km on Mars) so it's not a complete bust. – Pelinore Dec 04 '18 at 02:50
  • @LorenPechtel I don't understand that rock would flow all the way to the surface because of the pressure of rock above it ! When the pressurized liquid rock is getting near the surface there's already no pressure enough from the weight of rock above to keep it liquid. – Cornelis Sep 03 '21 at 09:11
  • @Pelinore Wikipedia says "The martian mantle appears to be solid down to the depth of about 500 km,.." , so why bother about the thickness of the crust ? – Cornelis Sep 03 '21 at 09:17
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    @Cornelis It's not a matter of liquid rock. It's solid rock flowing under pressure. On Earth it starts to become a problem at about 10km down. (Drill a hole, it slowly fills in.) 1/3 of Earth's gravity, it should start to become a problem at 30km. Think of it like a great pile of butter, not as a liquid. – Loren Pechtel Sep 03 '21 at 17:55
  • @LorenPechtel A drilled hole with 90⁰ slopes is quite different then a 100 km wide trench with 30⁰ slopes, isn't it ? The upper 20 km thick rock body of the slopes, that is not butter-like, should have weight enough to withstand the butter-like rock below and beside ? Can you show me a paper or article about solid rock flowing under pressure ? – Cornelis Sep 04 '21 at 09:37
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    @Cornelis The rock under the thick part still flows towards the weak point. It's going to be slower, it's still going to fill in. – Loren Pechtel Sep 05 '21 at 01:49
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    @Cornelis I'd guess that the flow would reduce the hole depth to about 30km below the average terrain height in the vicinity, which determines the rock pressure. I.e. you could reach much deeper digging within the Hellas Planitia than within higher terrains. Nevertheless, on geological time scales, you must expect the total height difference from the lowest point on Mars to its highest point to be reduced back to 30km. Note that the highest mountains on earth just fall short of its 10km limit. (I'm ignoring the deep sea trenches, because they still have all the sea water on top of them.) – cmaster - reinstate monica Sep 05 '21 at 22:42
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    if you put temporary structures at the bottom / movable structures, (and assuming the filling in of rock is at a glacial pace), could you periodically move the entire city off the basin, carve out the rock some more if it has filled in and then plant the city back down? – Sidharth Ghoshal Jun 22 '22 at 20:14