How does composition change if you extend an Earthlike atmosphere 50km below sea level?

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

posted about 4 years ago

Lawton‭

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