We know the Earth's interior is differentially rotating -- the core is rotating much more quickly than the crust. This is possible because the outer core is liquid and is presumably low enough viscosity that it doesn't transfer angular momentum outward very quickly.

Some other planets/moons have solid (or near-solid) interiors that rotate at the same rate throughout.

What would happen as a planet -- let's say Mars -- cooled enough that the outer core solidified and (like a gigantic planetary clutch) connected the inner core angularly to the mantle/crust? Would it happen gradually enough that it would unnoticeable to any surface dwellers? (other than eventual extinction due to cosmic radiation as the magnetic dynamo faded)

My question is inspired by a recent article about Mars' ancient dynamo possibly still recorded in surface ferromagnetism. http://m.phys.org/news/2016-11-mars-ionosphere-crustal-magnetic-fields.html

Update: as JohnRennie points out below, this question is based on a faulty assumption that the Earth's core rotates much faster than the crust. In fact it takes about 400 years(!) for the core to rotate one more time than the outer portions of the planet. Instead, the dynamo is caused by convection cells in the outer core.

  • $\begingroup$ The solidification process would require removal of a substantial amount of heat (enthalpy of solidification) from the core, so by necessity will be a slow process. $\endgroup$
    – Jon Custer
    Nov 7, 2016 at 17:38
  • 3
    $\begingroup$ You say the core is rotating much more quickly than the crust but the Earth's core rotates about 0.001% faster than the rest of the Earth. I'd be surprised if the small change required to slow it down by 0.001% was noticable to anyone. $\endgroup$ Nov 7, 2016 at 17:42
  • $\begingroup$ @JohnRennie - Wow, really?? I have a PhD in astronomy and I never knew that. I was under the mistaken impression that a large rotation difference was needed to generate the dynamo. For that correction alone, I'm super-happy that I posted this question! $\endgroup$ Nov 7, 2016 at 18:11


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