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I have, for a while now, been intrigued by the planet Canyon in Larry Niven’s Known Space universe:

Canyon was once an uninhabitable Mars-like world known as Warhead. It is the second of seven planets around p Eridani A, 22 light-years from Earth.[2] It was used as a military outpost by the Kzinti, until the planet was hit by a weapon called the "Wunderland Treatymaker" during the Third War. The attack tore a long, narrow, kilometers-deep crater into the crust approximately the size of the Baja Peninsula. The air and moisture in the thin atmosphere gathered at the bottom of this artificial canyon, creating a breathable environment, complete with a sea at the bottom.

The idea is basically that the air pressure (like water in the Mariana trench) increases the farther down you dig a hole, so if, on a planet with a thin atmosphere, you dug a deep enough hole, the atmospheric pressure would be Earth-like at the bottom.

I was wondering; let’s say I had a similar planet to Canyon, with an atmosphere of 7 millibars or 0.007 bars. How deep a hole would I have to dig for the air at the bottom to be at a pressure of 0.5-1 bars? (Assume an Earth-like atmospheric composition).

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    $\begingroup$ Not enough information: how strong the is gravity? Stronger gravity means a shorter atmospheric scale height, which means you can get away with a shallower canyon. $\endgroup$ Mar 31, 2023 at 19:02

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Given a similar atmospheric composition to Earth (impure nitrogen, from a pressure/density standpoint), there would need to be in excess of 140 km of total atmospheric depth from "sea level" to effectively zero pressure (low enough for a reasonably durable orbit, say several months at least) to get 1 bar at the bottom.

If you started with 7 millibar, you'd need to dig roundly 100 km (possibly a little more) -- and then there would be a question of whether there was enough total atmosphere to fill the canyon. Beyond that, of course, you need to wonder if your planetary crust is thick enough to even do so without hitting mantle (on Mars, apparently, it is, now, but there's nowhere on Earth you could dig that deep and not wind up making an artificial volcano).

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Deeper than it's possible to dig a hole on a Mars-like planet.

Pretty much asked and answered already on Space Exploration Stack Exchange.

If the planet really is "Mars-like" then you're probably looking at a touch over 40km deep just to reach the same atmospheric pressure found at 6km above sea level on Earth.

Less than that pressure and plants are unlikely to be a thing, and no plants = no life as we know it.

The problem is the rock will begin to flow like putty under the weight of all the rock above it before you reach that depth .. so you can't dig one that deep as the hole just keeps filling itself back up 🤗

Which means (if you can't dig one deep enough to get the air pressure at a 6km elevation on earth) you definitely can't dig a hole deep enough to get an Earth-normal sea level air pressure.

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  • $\begingroup$ Just to note -- rocks "flowing like putty" is on a millennial time scale. What you might get is stress explosions (these sometimes happen in very deep mines) due to the pressure differential between rocks a few tens of meters from the new surface and that newly excavated surface -- and once those settle down (and they do, at least in mines) you get a geologically short-lived feature (which will still last much longer than human lifetime). In the case of Canyon, Ringworld Engineers was set several centuries after the war in which that weapon was used. $\endgroup$
    – Zeiss Ikon
    Apr 2, 2023 at 0:57
  • $\begingroup$ @ZeissIkon at the depths and pressures were talking about here? the Russians with their hole described the rocks as behaving more like plastic at only 7km down .. granted this is a Mars sized planet with less gravity but it needs by your estimate a 100km hole .. you don't think at that depth the pressure won't have the rock behaving more like treacle than putty under a Mars gravity? .. the deepest existing features on Mars are likely reasonable indicators of the deepest you can expect a hole on Mars to remain long-term and they're far too shallow for the OPs requirements 🤗 $\endgroup$
    – Pelinore
    Apr 2, 2023 at 2:10

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