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I have a planet in mind that is highly hostile to basically everything around its equator but is more or less survivable everywhere else(hopefully ^v^). One of the ideas I have in mind for making the equator hell has been to make the hemispheres of the planet rotate in opposite directions. This would have the delightful effect of constant earthquakes, endless volcanic activity, and horrendous winds and storms. Yay! Only, I don't know what could even cause a planet to be like this, and am rather curious if it's at all possible.

What could cause a planet's hemispheres to rotate in opposite directions?

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  • $\begingroup$ Opposite directions is a bit too extreme. Regardless of the initial cause, gravity would press the two halves together and the friction would slow them and stop them very quickly (with massive damage everywhere). It would be like the two plates of the clutch mechanism in a car (and the liquid centre would be like an automatic transmission's torque converter). ¶ Asking about rotating at very slightly different rates would be a more reasonable question. $\endgroup$ Nov 23 at 16:50
  • $\begingroup$ Could you be referring to an artificially created planet? I.e. one with bearings within and a slight but imperceptible gap at the equator? $\endgroup$ Nov 23 at 17:10
  • $\begingroup$ @RayButterworth Then if the north had, say, a 1% difference in rotation speed compared to the south, would that cause any sort of uninhabitability? If earth was taken as example, 1670km/h vs 1653.3km/h? $\endgroup$
    – Rubrikon
    Nov 23 at 19:19
  • $\begingroup$ @ARogueAnt. An artificial/mechanical planet would be the easiest explanation I suppose, even if the only dangerous thing about such a thing's equator would be the winds and the whiplash/firm faceplant/meatpastening from stepping over the equatorial line... the other side would probably just be a blur for a person standing next to the line. $\endgroup$
    – Rubrikon
    Nov 23 at 19:36
  • $\begingroup$ @Rubrikon asks about "a 1% difference in rotation speed compared to the south". No, it would have to be far far less than that. LDutch's answer about tectonic plates is the only credible way of doing it. Tectonic Plate - an overview | ScienceDirect Topics says that they move between 2 and 20 metres per year. $\endgroup$ Nov 23 at 23:57
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For reasonably fast flow rates, it can only happen in a fluid planet, either made of gas or liquid, and it would not happen from the birth of the planet. It could be triggered by a massive impact, locally disrupting the flow of the fluid.

It won't cause quakes, but the friction between the opposing flows would quickly dissipate the energy resulting in a warming up of the medium.

If you accept slow flow rates, that's what we have on our planet with tectonic plates. Ask somebody living in California about the opposite flow solid blocks, they will talk you about the San Andreas fault

Blocks on opposite sides of the San Andreas fault move horizontally. If a person stood on one side of the fault and looked across it, the block on the opposite side would appear to have moved to the right. Geologists refer to this type fault displacement as right-lateral strike-slip.

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Given the appropriate tech, it's actually fairly easy. It's a three step process.

Step 1, obtain a large, thin sheet (slightly larger than the diameter of the planet in question) of (nearly) infinitely strong, frictionless unobtanium. Carve saw teeth into one edge.

Step 2, Using four (or more if desired) spaceships equipped with reactionless drives to hold the corners of the sheet, align it with the planet's equator, and slowly move it through the planet, neatly sawing it into two hemispherical parts. The unobtanium sheet remains between them, keeping them from re-combining due to pressure.

Step 3, use your reactionless drive to slow and gradually reverse the rotation of one of the halves. Fairly simple physics will give you the amount of energy needed.

Of course there are going to be some minor details to work out. For instance, you've disturbed flows in the molten iron core, so the planet's magnetic field is going to be majorly disrupted. And when & if the flows re-establish themselves in the two halves, there's likely to be magnetic coupling between them. This will gradually slow their relative rotation, so you'll have to keep spinning things up every now & then.

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  • $\begingroup$ Also important: do all of this very slowly. $\endgroup$
    – jdunlop
    Nov 25 at 18:10

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