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).

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 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.

More information about Cael:


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
Average air pressure at SLE: 1 atm
Solar intensity and spectral makeup at SLE is the same as on Earth
This seems like a World Building question. I presume that on your imaginary planet something, perhaps an asteroid strike, has created a huge bowl 50 km deep. On the rim of the bowl, the atmospheric pressure and composition are the same as at sea level on Earth. You want to know what conditions would be like at the bottom.

In some respects a bit like Venus. Very hot, but exactly how hot would depend on whereabouts on the planet this deep bowl had formed. The composition of the atmosphere would be the same as at sea level (I assume the lip of the bowl is not close to a sea). The pressure at the bottom would be many atmospheres, but not as many as the 90 atmospheres on Venus. It would seldom rain down there because of the heat and high pressure. Rain falling in from high altitude would usually evaporate before reaching the bottom. As explorers ascended the sides, conditions would become more normal 

Would it be habitable? Yes, but how habitable depends on where on the planet this bowl occurred. There would be habitats on the slopes, where conditions were less extreme than at the bottom. If this bowl or basin were in the tropics, the bottom would probably be too hot to sustain life. A water supply would also be problematic. Pressure is less of a problem; there are life forms 12 km down or deeper in Earth's ocean trenches.