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I've heard it said that caves on Earth are roughly 10°C/50°F regardless of the temperature at the surface. Does this hold up if a planet is very hot? How hot could temperatures get above ground while still allowing liquid water underground?

My planet is tidally locked, orbiting a red dwarf. On the hot side water boils, on the cold side there is no energy to sustain life, and the twilight zone is ravaged by extreme winds. I am envisioning caves located on the day side that let in bits of sunlight here and there, which allows plants to grow in meager quantities, and in turn animals to eat them. Nearly all life would congregate just below the point where temperatures are bearable, with a rapid transition to darkness below and boiling above. There would be aquifers in the caves.

Is it possible for bits of sunlight to get into the caves, without making it so hot that water boils, but still allowing plants to grow?

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  • $\begingroup$ There's a related post that may be of interest: How do ocean currents work on a tidally locked planet?. (Disclaimer: I've a vested interest). $\endgroup$ Mar 28 at 18:49
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    $\begingroup$ That sounds like a planet awful for life. To make caves even remotely habitable on the hot side your planet would need to be a considerable distance from the sun. But with BOILING OCEANS i don't think live can survive there really. $\endgroup$ Mar 28 at 18:59
  • $\begingroup$ "i don't think live can survive there really" -- There are microbes that live just fine in boiling water. A famous use case for a protein extracted from one such organism is Taq polymerase (en.wikipedia.org/wiki/Taq_polymerase) -- used in PCR (Polymerase chain reaction). PCR tests became famous for the masses with COVID. $\endgroup$
    – Mörre
    Mar 31 at 9:11

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Unfortunately not. You've heard wrong. Cave temperatures vary depending on where on earth they are. Cave temperatures approximate the average annual temperature of the surface in their geographic region.

https://outdoors.stackexchange.com/questions/5799/what-temperatures-can-i-expect-in-caves-compared-to-surface

The stone acts as a massive heat reservoir, and helps to modulate temperature swings, but if the air in your tidally locked region is hot enough to boil water year round, so will the air inside the caves.

However, caves at and around the terminator will therefore maintain a pretty average temperature, and also avoid the extreme winds! So that might work.

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  • $\begingroup$ Unfortunately, not even the terminator, but slightly behind that area? Temperature builds up, as you stated, as if the whole ground was a giant heat battery. And due t it being tidally locked, it never gets a chance to radiate the heat away. Making the terminator the first area it gets a chance to radiate away heat. $\endgroup$
    – Pica
    Mar 28 at 20:12
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    $\begingroup$ The whole best example that comes to mind for this answer is the en.wikipedia.org/wiki/London_Underground_cooling , which has build up over the centuries and is now barely habitable without active cooling. $\endgroup$
    – Pica
    Mar 28 at 20:13
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    $\begingroup$ @Pica Would the area behind the terminator never have any sunlight? Wouldn't that mean there was no energy source, and therefore life essentially couldn't exist? $\endgroup$
    – Sam Moon
    Mar 28 at 20:26
  • $\begingroup$ It would get very little sunlight, but energy sources could drop in from the green zone- and clouds could moderate the light, something alive gets cooked in the dessert, charcoal-dust blown up into the air and rains down.. $\endgroup$
    – Pica
    Mar 28 at 20:35
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    $\begingroup$ Sunlight is the only energy source for photosynthesis, but not the only energy source for life. Deep-sea thermal vents are a current example, and obviously the first life was neither photosynthetic nor feeding on other life. These models get outcompeted (on Earth, at least) by a photosynthetic food chain where such a chain is possible-- but that competition is not a concern on your world. $\endgroup$
    – Jay McEh
    Mar 29 at 12:35
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It has been suspected for some time that atmospheres of tidally locked worlds can undergo superrotation, which can do a surprisingly good job of evening out temperatures all around the world even with surface pressures as low as 100 millibars (see Simulations of the Atmospheres of Synchronously Rotating Terrestrial Planets Orbiting M Dwarfs: Conditions for Atmospheric Collapse and the Implications for Habitability from way back in 1997 for example). Atmospheres with a lot of CO2 (say, 1000 millibars or more) can even sustain liquid water on the dark side.

I'm also reasonably certain that surface windspeeds need not be nearly as bad as you might have heard, though I'm having difficulty tracking down some good references for that so take my word for it or not, as you like.

Leaving aside surface climate, there's an interesting page I bumped in to, Mercury Temperature vs. Latitude, Depth and Time, which includes a load of math which might be relevant to your interests. Mercury of course isn't locked into a 1:1 spin-orbit resonance, and it doesn't have an atmosphere so it has a better scope for losing heat, but if your world had a sufficiently thin atmosphere, or one that was very low on greehouse gases, you might still be able to make use of the same ideas. The author calculated that there are rings around the north and south of Mercury where, at a depth of less than a meter, the ambient temperature is similar to Earth's surface.

Such shapes will be quite different on your world especially if atmospheric heat transport is as effective as theorists suggests it could be. There's almost certainly a lot of potentially habitable subsurface real estate, if only you can explain how a natural environment could form there.

One problem you might consider is where did these caves come from? Caves on Earth tend to form in limestone, and formation of thick and widespread layers of limestone requires a history that includes oceans of water and a carbon cycle of some kind, which might in turn require volcanism. And that in turn brings up questions of why the sun-side of a formerly quite habitable and quite watery world is now hot... if it were a greenhouse effect, then that means a thick atmosphere and a temperature and probably oceanic night-side. After you've provided suitable materials, how did all these caves form in the first place? Changing sea levels might explain it, but that could leave you quite high and dry caves.

Lava tubes are another good option (and don't require a biological and geological history that produces limestone) but the total volume they can provide you looks more like an island habitat than a country, though that might not matter to you if you're happy to have very small, very scattered life-bearing regions in a dead, dry expanse of planet.

In either case, if enough light gets in to the cave for enough photosynthesizers to grow to feed an ecosystem, the hot dry winds outside are likely to dessicate your water sources, and the growing area might be too small to feed more than a food web of small invertebrates.

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  • $\begingroup$ Interesting did you mean the hypotheticall effect of wind balencing out the hot and cold side @Starfish Prime? $\endgroup$ Mar 28 at 21:51
  • $\begingroup$ @Fallenspacerock pretty much. There's lots of papers on the subject, but the linked one is a good start. $\endgroup$ Mar 28 at 22:21
  • $\begingroup$ Yeah i heard of this topic in some YouTube video once @Starfish Prime. I just wasn't sure if it was this that you meant. $\endgroup$ Mar 28 at 23:00
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If you want to deploy a little bit of handwave physics ...

You're planet is tidally locked with a "hot" side and a "Cold" side. With a zone around the terminator line that has lesser extremes.

You're planet was volcanically active early in the lifetime of the solar but is now largely dormant, so it is has a plethora of lava tubes as suggested by @Starfish Prime. Possibly due to the influence of another planet or moon in the system that was destroyed or had it's orbit changed when the sun became a red dwarf (again, handwave physics).

The difference between day and nigh side acts like a natural refrigeration plant. The heat on the surface of the day side creates a low pressure zone that is drawing cool air and moisture through the caves from the night side to the caves on the day side, with a temperate zone in the middle where life can comfortably exist.

If your planet had a crystalline crust, it could be partially transparent in places, allowing light but not harmful radiation through.

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I will answer second part of your question on top of Aaargh Zombies' answer. Instead of light passing to the cave, your caves is home to plants that stretch from hot surface to the colder regions. These plants then use the heat gradient as the energy source. They also pull any necessary elements from the surface to underground. Entire ecosystem depends on this plant as the bottom layer of your food chain. You might also have lava lakes allowing some bacteria to chemosynthesize.

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