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I'm working to design the atmosphere of a fictional planet inspired by Venus (let's call it Cael).

Cael's atmosphere at an altitude of 50 km is essentially identical to Earth's atmosphere at sea level, and parallels Earth's atmosphere as altitude increases beyond that. I want to figure out what needs to happen in the lower 50 kilometers in order to keep the Earth-like atmosphere where it is. My problem is that I can't find resources on what happens when an Earth-like atmosphere is extended downward by any significant distance.

The atmospheres of Venus, Jupiter, and Saturn all contain distinct layers of varying composition caused by the changes in temperature and pressure with increasing depth. While none of them have a layer of Earth-like composition to use as a convenient reference, it seems logical that this would hold also hold true in the case of Cael. So my question is,

What kind of layers would form beneath a complete Earth-like atmosphere?

For the purposes of this question, the Earth-like atmosphere starts at the imaginary surface where the temperature and pressure of Cael's atmosphere are functionally identical to Earth's atmosphere at sea level, 50 km above the true rocky surface. I'll call this the Sea-Level Equivalent altitude, or SLE.

Just like on Earth, Cael has a tropopause roughly 10-20 km above the SLE that marks the beginning of the stratosphere. Above that is the mesosphere, thermosphere, and exosphere. As on Earth, atmospheric composition is effectively constant all the way up to the lowest part of the thermosphere due to turbulent mixing dominating its molecular interactions.

A very rough estimate for the air pressure at Cael's surface is 50 atm, according to this "Air Pressure at Altitude Calculator" from Mide Technology Corp. That pressure is well above the critical pressure for nitrogen (33.5 atm) and right around the critical pressure for oxygen (49.8).

Based on my research on other planets, I believe temperature is likely to increase with depth to somewhere between 100°C and 500°C. Even if we assume that temperature remains constant rather than increase as you descend beneath the Earth-temperature SLE, the critical temperatures of both gasses are below -100°C, a temperature that has never been recorded at Earth's surface.

Thus, I would expect to find a very high volume of supercritical nitrogen as well as a bit of supercritical oxygen at Cael's rocky surface. Argon, neon, and methane would all be supercritical under those conditions as well.

I also somewhat expect liquid water oceans, because Cael needs to have enough water to experience water clouds and precipitation above the SLE, and my guesstimates for temperature and pressure are within the liquid section of water's phase diagram.

The true surface of Cael is almost certainly devoid of any kind of organic life except for the most hardy extremophiles. Unless another gas or process keeps oxygen limited to 40+ km, the extreme pressure and (presumed) high temperature should make even low percentages of oxygen quite dangerous.

Cael's biosphere is made up of floating and flying lifeforms living around the Earth-like altitudes. These organisms maintain the high oxygen concentration.

More information about Cael (bold items are fixed, others can be altered):

  • Mass: 6 × 10²⁴ kg
  • Average radius of planet surface: 6,450 km
  • Average gravity at planet surface: 9.65 m/s²
  • Average altitude of SLE: 50 km
  • Average gravity at SLE: 9.5 m/s²
  • Solar intensity and spectral makeup at SLE is the same as on Earth
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  • $\begingroup$ The deeper atmosphere could be almost anything. Venus has Earthlike temp/pressure at around, IIRC, 70 km (just above the cloud tops, at any rate), yet the clouds are sulfuric acid rather than water. $\endgroup$ – Zeiss Ikon Jan 23 at 19:25
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    $\begingroup$ Yep, Venus's atmosphere was the first thing I researched. Whatever the composition of the deep atmosphere is, it can't be anything that interferes with having shirtsleeve weather at the SLE altitude. Sulfuric acid clouds, if present, are banned from rising above 45 km. 😁 $\endgroup$ – Lawton Jan 23 at 19:29
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    $\begingroup$ Note there would be no phase boundary between any supercritical layer near the surface and the high pressure gas layer above it one would just merge into the other.. $\endgroup$ – Slarty Jan 23 at 19:37
  • $\begingroup$ @Slarty Yes, but there might be clearly defined atmospheric layers in the same way the Earth has atmospheric layers (troposphere, mesosphere, et cetera). Or there might be no changes in behavior all the way down to the surface. That's what I want to find out. $\endgroup$ – Lawton Jan 23 at 19:52
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    $\begingroup$ Somewhat related to what you want: To'ul'hian Worlds from Orions Arm $\endgroup$ – TheDyingOfLight Jan 24 at 14:23
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I would suggest that the result would be something like Venus. You might start with a planet with 1atm Earth like atmosphere at 50km altitude and a similar composition down to the surface but that would not be stable.

A much deeper atmosphere would absorb more heat and at such a high pressure and oxygen concentration, organic material on the surface would combust generating a lot of Carbon dioxide also increasing the temperature and warming the surface. This would lead to greater evaporation of water from the oceans and greater release of carbon dioxide from the oceans increasing the greenhouse effect.

The temperature would increase dramatically, if it reached around 270-300 degrees C it would start to boil the oceans away, but it might well not get that high, but it would leave a dead boiled world with the oxygen concentration decreasing as surface materials were oxidised and the oxygen was not replaced.

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  • $\begingroup$ I want a stable configuration with the Earth-like atmosphere on top, and whatever is needed to support that beneath it. The only living matter that would be on Cael's surface would be life that evolved under whatever conditions exist there, if that's even possible; and since this is a stable environment rather than one that had a sudden increase in pressure, there shouldn't be any conflagrations. $\endgroup$ – Lawton Jan 23 at 19:49
  • $\begingroup$ Well you need a much lower oxygen concentration on the surface to prevent fires and you can't sensibly have an oxygen gradient increasing into the upper atmosphere. I fear that what you are after is not possible, but someone might come up with something. People here are quite inventive... $\endgroup$ – Slarty Jan 23 at 20:00
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    $\begingroup$ I'm perfectly fine if the oxygen concentration on the surface instantly incinerates anything that falls down there, as long as I have an Earth-like atmosphere 50 km up. $\endgroup$ – Lawton Jan 23 at 20:03
  • $\begingroup$ Yes that is a problem. It would need something that produces prodigious amounts of oxygen on the surface, that can itself withstand oxidation in a high oxygen pressure environment at high temperature. Maybe some alien artefact or geoengineering machine gone wrong? Or is that too outlandish? Such a device might well produce sufficient oxygen to burn and oxidise everything that could be burnt or oxidised eventually leading to a high temperature high pressure oxygen environment that would burn anything that fell in. $\endgroup$ – Slarty Jan 23 at 20:15
  • $\begingroup$ There is an extensive airborne biosphere with plant-analogues that produce the oxygen, kept aloft by buoyancy and wind. $\endgroup$ – Lawton Jan 23 at 20:23
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If there is a floating biome high up in the atmosphere that might produce a lot of oxygen and if there were limited numbers or no species of animals that oxygen might build up. At first anything that could be oxidized on the surface would be but eventually there would be little left to burn and high pressure oxygen would accumulate.

Any falling matter would descend to the ground and be burnt up releasing CO2. CO2 levels would be a little higher nearer to the surface as CO2 is heavier than O2 but a lot would defuse back up to the biome and would be reabsorbed by the plant layer(s) leaving an Earth like high O2 low CO2 1bar pressure atmosphere at 50km. The bulk of the planets accessable carbon would be either locked up under ground in carbonate rocks or in the floating biome.

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