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
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    $\begingroup$ Unfortunately the geosciences only have models to describe what they find in nature. If you say you create a fantasy planet, than just go for it, have it behave so that it fits your story and don't care so much about realism. I am not even sure whether earth could hold such a dense nitrogen/oxygen atmopshere. Or if the berometric step can simply be projected down. Compressibility of gases changes, their phases, interactions between atmo- and the other spheres, would it still support a biologic evolution ? Who knows :-) $\endgroup$
    – user18607
    Jan 23 '20 at 19:11
  • $\begingroup$ Hi Lawton, welcome to Earth Science. I'm not sure if we'd be able to answer your question here and we're not sure if it's on topic. Are you familiar to our Worldbuilding sister site? If you want, I could migrate your question there. The answers you should expect there will be more speculative in nature (but some of them can be very good), which may be suitable for a work of fiction. I'm afraid the best answer science can give here is "we don't know". $\endgroup$
    – gerrit
    Jan 24 '20 at 9:46
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    $\begingroup$ @gerrit I was hoping to get hard-science information, like some way to model the situation or details on the high-pressure behavior of Earth's atmospheric mix. I've had two other people suggest asking the question on Worldbuilding, so I did go ahead and post it there as well. $\endgroup$
    – Lawton
    Jan 24 '20 at 13:24
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    $\begingroup$ @Lawton I doubt you will get any hard science answers, unless some scientists have modelled exactly this, which would appear unlikely, since it would take significant work to build a model that can describe this situation accurately. $\endgroup$
    – gerrit
    Jan 24 '20 at 13:30

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.

  • 1
    $\begingroup$ That scenario isn't quite what I meant, but it might be an acceptable approximation. The entire surface of Cael is (on average) 50 kilometers below the altitude where conditions match Earth's sea-level atmosphere. At an altitude of 50 kilometers on Venus, the temperature and pressure are similar to Earth's sea-level atmosphere, but the composition is very different. $\endgroup$
    – Lawton
    Jan 23 '20 at 16:11
  • $\begingroup$ Why use an imaginary planet then? Why not use Venus? Then there would be no dispute about conditions at the bottom of the atmosphere, because conditions on Venus are known. Composition is very different at the bottom as well as at the top. $\endgroup$ Jan 23 '20 at 16:16
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    $\begingroup$ Putting it in terms of Venus only, then, I want to know what conditions would exist below the 50 km altitude if Venus's atmosphere were adjusted such that the temperature, pressure, and composition at the 50 km altitude were equivalent to those same metrics on Earth at sea level. $\endgroup$
    – Lawton
    Jan 23 '20 at 16:29
  • $\begingroup$ Conditions at various depths in the Venusian atmosphere are not readily available Conditions at or near the surface are, and conditions near the top, but I don't know where you could find data for every 1000 feet of descent. It's the same for pressure and temperature in the depths of ocean trenches. There is a better chance of finding the latter, as ocean trenches are more accessible than the depths of the Venusian atmosphere. The Russians have had by far the most success in landing a probe on the Venusian surface. Perhaps an enquiry at the Russian embassy would produce the data you ask for. $\endgroup$ Jan 23 '20 at 16:58
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    $\begingroup$ Yes, that is why I was saying that Cael is inspired by Venus but with an Earth-like atmosphere. Like I say in the question, I have researched Venus's atmosphere. What I can't find is information on how Earth's atmosphere would behave with 50 kilometers of extra depth. $\endgroup$
    – Lawton
    Jan 23 '20 at 18:45

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