Landable (Semi) Habitable Planets... Are A Thing?

[Markov Chaney]

IFF you made the bold assumption that you knew the temperature vs altitude curve as well...

This does bug me, but last night it occurred to me that all these "Hycean" worlds have surface gravity of 1.5g to 2.5g and those have hydrogen atmospheres. Water vapour atmosphere gonna be a lot easier to hang on to.

When you said "nope" in reply to me mentioning that something was broken (I was referring to the "water worlds that just don’t have an atmosphere to begin with" and I thought you were too), I thought you were saying that water worlds don't need to have an atmosphere - it's not clear now that you were saying that at all.

I was saying "surprisingly they only need a very thin one" but then confused things by talking about the whole of the phase space instead of just picking an example like you did. I blame the wine.

Meanwhile, the 10 mbar threshold would explain the whole thing, since a cold surface (anything from a few degrees above freezing) would easily slip below the threshold.

Yep!

Ta! Which bit of that paper convinced you about which thing, though? Sorry, losing the thread here

My priors were "OK it could happen but it will be at best metastable and really not very likely in what is basically a random problem space; we've only found hundreds of exoplanets but these are cropping up all the time already. Weird."

But actually it is stable and there's a BIG set of conditions that can converge to it, because the various factors all work together in many situations.

 

[Neilski]

But actually it is stable and there's a BIG set of conditions that can converge to it, because the various factors all work together in many situations.

And what's the "it" that is stable in this sentence? (Please forgive me if that should be obvious from the context.)

This does bug me, but last night it occurred to me that all these "Hycean" worlds have surface gravity of 1.5g to 2.5g and those have hydrogen atmospheres. Water vapour atmosphere gonna be a lot easier to hang on to.

Yes, I had been wondering what fraction of the atmospheric losses would be driven by a simple thermal process (escape velocity) vs. say the influence of the solar wind. But of course there turns out to be a whole Wikipedia page about it - https://en.wikipedia.org/wiki/Atmospheric_escape

For Earth, the page says that 10-40% of hydrogen loss is by the thermal path, with the rest being linked to the solar wind. I have no clue what amount of oxygen/nitrogen/water ever makes it up high enough to have a chance at escape by any route - maybe hardly any. (Ah, I just found another page which shows that a surprisingly large amount of atomic O makes it up very high, where it's really the sole competitor with the He and H. I guess O2 must somehow be much more effectively broken up in the upper atmosphere than N2 This perhaps also means that a "pure" H2O atmosphere would have a layer of atomic O and H at the top, depending on the star it's around.)

One other thought: since gravity around a spherical mass is GM/r^2 and escape velocity is sqrt(2GM/r), worlds with the same surface gravity can have different escape velocities. For example, if planet B had half the density of planet A but double the radius, it would have 8x the volume, 4x the mass and the same surface gravity. It would also have 1.4x the escape velocity (for the same atmospheric temperature) and therefore potentially hang onto stuff better, since I guess both the thermal and non-thermal routes should (maybe?) be penalised by a higher escape velocity.

I wonder if the Stellar Forge has much of this stuff built into it... Probably yes?
(Gonna grab my climbing gear and start trying to haul myself back up out of this rabbit hole now! )

 

[Greasetrap42]

Possibly already discussed in this thread, wide swings in temperature would also be a big factor in making low atmosphere planets semi-habitale for humans.

Full atmosphere planets obviously have atmospheric shielding from various radiation. The atmosphere also provides a significant thermal buffer for stabilizing the temperatures, reducing the massive swings. Liquid oceans and lakes also have a big buffering effect.

Perhaps if a deep canyon was located on a tidally locked planet a semi-habitable solution would work.

 

[Starsong]

Possibly already discussed in this thread, wide swings in temperature would also be a big factor in making low atmosphere planets semi-habitale for humans.

Full atmosphere planets obviously have atmospheric shielding from various radiation. The atmosphere also provides a significant thermal buffer for stabilizing the temperatures, reducing the massive swings. Liquid oceans and lakes also have a big buffering effect.

Perhaps if a deep canyon was located on a tidally locked planet a semi-habitable solution would work.

Yeah. If temperatures are a concern you need a tidally locked planet. That's the reason I picked Swoilz, I'd thought of everything EXCEPT the sulphur dioxide...

 

[Starsong]

Swoilz might have failed, but I might have found a better candidate anyway. Col 285 Sector ZX-R b5-0 A 4 has an atmosphere of 0.08 ATM pure O2 (no sulphur dioxide this time) and contains a crater a whopping 17.5km deep and 420km across at the coordinates (-20 -143), which gives a lot more space to put a proper colony in. And with a consistent temperature of 16C/289K, it's quite temperate too (despite being a Icy Body. Weird).

Atmosphere calculations:
Atmosphere temp = 213 - (38 x (213/288)) = 185K
Scale height = (1.381x10^-23 x 184)/(6.022x10^-23 x (0.34x9.81)) = 12.72km
Crater pressure = 0.0802 x e^(17.5/12.72) = 0.317 ATM

So, 0.317 ATM of pure oxygen at the bottom of that thing. Still well within oxygen toxicity safety limits, but my new colonists will need to have some damn good fire safety precautions. They do, however, have the benefit of 34th century tech to do so.

Although... is the overall atmospheric pressure going to be constant, given it's tidally locked and therefore has wildly variable temperatures that are constant for each area? Maybe I need to calculate the pressure here with the temp over the crater, instead.

Atmosphere calculations MK 2:
Atmosphere temp = 289 - (38 x (289/288)) = 251K
Scale height = (1.381x10^-23 x 251)/(6.022x10^-23 x (0.34x9.81)) = 17.26km
Crater pressure = 0.0802 x e^(17.5/17.26) = 0.221 ATM

Still 0.221 ATM. So the planet's atmosphere is breathable in either case (or anything in between). Good to know.

 

[Neilski]

Col 285 Sector ZX-R b5-0 A 4 has an atmosphere of 0.08 ATM pure O2 (no sulphur dioxide this time) and contains a crater a whopping 17.5km deep and 420km across at the coordinates (-20 -143), which gives a lot more space to put a proper colony in. And with a consistent temperature of 16C/289K, it's quite temperate too (despite being a Icy Body. Weird).

Nice!

Although... is the overall atmospheric pressure going to be constant, given it's tidally locked and therefore has wildly variable temperatures that are constant for each area? Maybe I need to calculate the pressure here with the temp over the crater, instead.

Hmm, that gets interesting, because if the scale height were 13 km at one part of the planet (average? or maybe cool side?) and 17 km at another (the warm side? or just a warmer than average spot?), you'd have very different pressures at the same altitude for different regions. This would drive winds and heat transfer, and then I think (I can't say I've thought it through very carefully! ) that since this planet is tidally locked, the process might reach a quasi-equilibrium with pressure (but not temperature) having the same variation with altitude around the planet. I found an article while thinking about this (https://agupubs.onlinelibrary.wiley.com/doi/full/10.3894/JAMES.2010.2.13) which isn't directly relevant, and doesn't really answer the question but is pretty interesting all the same (It's not directly relevant for various reasons including that it's about Earth-like worlds, but they are at least tidally locked.) NB: the article kinda suggests that the winds would be a constant thing, so my guess was probably wrong.

Anyway, having done that extra bit of thinking, I've now concluded that to work out the variation of the pressure with depth in your crater, you only need to worry about the local temperature (between the local surface and the crater floor). This is because it seems to me that the vertical variation of the pressure within any given column of air should only be influenced by the temperature of that same air, whether the temperature is constant with altitude or not. This makes the calculation much simpler, I reckon.
Bottom line though: yeah, sounds livable

 

[Markov Chaney]

That's a very cool paper you dug out there. What's particularly interesting is that they say the winds do serve to even out temperature between the sun side and the dark side, whether the orbital period is one day or one year.

And they're not particularly hellacious either, tens of metres per second, and they don't vary wildly from hour to hour or even week to week (says something about 80 days?). I honestly thought it would be a total hellstorm from pole to pole and the storms would move around at random.

So yes, find yourself a nice big crater at one of the quieter latitudes (to avoid the worst of the wind) and longitudes (to get a nice temperature) and away you go.

 

[Markov Chaney]

Anyway, having done that extra bit of thinking, I've now concluded that to work out the variation of the pressure with depth in your crater, you only need to worry about the local temperature (between the local surface and the crater floor).

Yep, because all you're really doing is using temperature as a proxy for figuring out the effects of atomic motion in an ideal gas, so you're only concerned with the chunk of ideal gas you're actually looking at... it's just a mild mind-bender because pressure influences temperature and vice-versa.

... but still, if there's a wind of a lower temperature crossing the top of your nice crater, that cold air will sink into it, which will shift the equilibrium. Provided the wind effect is more or less constant, it is only shifting the equilibrium though, so you still have an equilibrium.

This is because it seems to me that the vertical variation of the pressure within any given column of air should only be influenced by the temperature of that same air,

Yeah if it's the same air. But if you have other air blowing in...