I'm working to design a fictional planet inspired by Venus (let's call it Cael). At an altitude of 50 km, Cael's atmosphere is essentially identical to Earth's atmosphere at sea level, and parallels Earth's atmosphere as altitude increases beyond that. 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. I'll call this the Sea-Level Equivalent surface, 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.
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
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 while the lowest natural temperature ever recorded on the ground on Earth was -89.2°C, measured Antarctica's Vostok Station located 3.5 km above sea level.
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.