In a story I'm writing, the setting is a tidally locked planet with a moon a fifth its size in a binary system of a mature red dwarf and much smaller second sun. I haven't decided the planet's exact orbital period (30-40 earth days) at 0.5 AU from the larger sun, but its inhabitants experience slightly stronger gravity than on earth, and it is larger than earth with a large molten iron core for a protective magnetic field and tectonic activity.

While designing a map of the planet's surface, I've hit a point in my internet research where I can't decide how to go about placing mountains, which impacts flow of rivers, ice build up, terrain lighting, wind, etc. I've been looking up plate tectonics and found that the mantle could flow away from the sunny apex, sliding the crust along through convection toward the sub stellar hemisphere, and the crust might buckle and fold along the way. I thought this would make the mountains on the planet look like they were leaning away from the sun but also wondered whether this scenario creates so many mountain ranges parallel to the twilight band so as to interfere with the water cycle and air currents.

So I dug farther, looking for a planet tectonic simulator I could enter valid information into. If there is one out there, please link me to it. I found one, but the poles are set like Earth's, so I improvised and attempted to slide the equatorial region to the base and the poles to the top...which is difficult to describe, so I'm including a drawing of this and the first model I found. enter image description here

Up to this point I've had a pretty good feeling about all my research in planet building where the laws of physics are at least fairly realistic. But I'm at a loss as to how to plop mountains about the planet surface, unless I should just pick any ol' environmentally ideal, plot strategic or even scenic location. Usually I stem my creativity from factual research conclusions.

  • $\begingroup$ Could you tell me the name of the tectonic simulator you found? I may be looking for one at some point. (Also, very interesting question, thanks for posting it!) $\endgroup$ Commented Feb 9, 2020 at 9:07
  • $\begingroup$ Just a nitpick.. A binary planet around a binary sun is not likely to be tidally locked to anything. Your statement seems to be that the planet and its moon are locked, but your tectonic diagram seems to show a constant direction to the sun, implying the planet is tidally locked to the sun? But That is highly unlikely, as the 1/5th mass close moon (has to be close, to be stable in a binary sun system), will dominate gravitationally! (but not enough to cause tidal locking of the planet primary to the moon, either) $\endgroup$
    – user79911
    Commented Nov 16, 2020 at 16:13
  • $\begingroup$ @user79911 So the sun and moon would sort of fight, but neither would win? $\endgroup$ Commented Nov 28, 2021 at 19:25

3 Answers 3


As of 2016, we have evidence that Mercury is tectonically active (http://www.nature.com/ngeo/journal/v9/n10/full/ngeo2814.html) and it is also in a spin/orbit resonance. While it's not a perfect analog to your scenario, it's probably the closest example currently available.

The map in Figure 3 shows the locations of (known) tectonic scarps, which do not seem to be clustered around the twilight band.

My general intuition about such a situation is that the day/night disparity would not be a major factor in tectonic activity, because the Earth's core is hotter than the surface of the sun. Any difference in external solar heating would be dwarfed by the temperature differences within the planet itself which drive tectonic activity here on Earth. Nature's Figure 3 map is consistent with this intuition.

Based on this, I would tend to just run tectonic simulations normally and not apply any modifications to that stage of the simulation related to the planet's tide-locked condition.

  • 2
    $\begingroup$ I can only add that maybe some tectonic activity could be enhanced by stellar tides, if the planet is tidally locked but in an eccentric orbit. We know Io's vulcanism is driven by tidal flexing. I don't know about Mercury but with a 3:2 lock and a very high eccentricity, the effect must be important as well. $\endgroup$
    – pablodf76
    Commented May 3, 2017 at 12:27
  • $\begingroup$ Thanks for the addition, @pablodf76! I wanted to say something about tidal flexing, but wasn't sure whether it would be a factor or not. Eccentric orbits completely slipped my mind and, as you say, it would definitely be relevant in such a case. $\endgroup$ Commented May 4, 2017 at 6:33

The heat of your planet's core is (sort of) independent of the sun

Your planet is tidally locked to the sun. You do not specify what the temperature regions are (is the sunny side habitable? the twilight zone?), but I think it is safe to assume parts of your planet have Earth like temperatures, and therefore no part of the planet is as hot as Venus.

The temperature of the Earth's surface is about 290 K, while the inferno on Venus is a sintering 750 K. However, the core of the Earth is in the 6000-7000 K range, while the mantle is around 900-1200 K.

Considering the basic laws of thermodynamics, it should be apparent now that the mantle is heated by the core of the Earth, not by the sun; the radiant energy that the sun deposits in the oceans, atmosphere, and lithosphere does not transfer to the hotter mantle.

The conclusion is, as long as your planet's core is similar to Earth's, the convective forces of the mantle are (mostly) driven by the core, not the sun. This means that plate tectonics will work as they do on Earth.

The reason I say sort of and mostly is that there will be a gradient in temperature loss form the mantle, where more energy will be lost on the dark side of the planet than the light side. However, if you are planning on an Earth-like planet with a thick atmosphere and lots of water, the temperature delta will be relatively small compared to the temperature delta between the mantle and the surface . For example, even a 100 K difference between light side and dark side is less significant compared to a 500 K difference between the mantle and the lithosphere.


I'd simply add a massive mountain range on the dark side, and an elevated mesa on the sunlight side (because of tidal force) and be ok with that. We know so little about exogeology - hell, we almost know nothing about our own planet's core - that you are entitled to almost any amount of handwavium here.


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