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I have a planet with homogeneous core and mantle, but a thick crust separated in two parts: the north half is composed of water and the south part is an alloy of heavy metals, making the north lighter than the south.

What would be consequences on planet shape? The usual - not perfect - spherical form is generated because of gravity, but here, the gravity center would be slightly offbeat because of surface height heavier on a side compared to another.

I know that an effect (if any) would be very slight considering the implication of the crust in global planet weight, but I'm still curious of details.

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  • $\begingroup$ I have the impression that if the southern half would be substantially heavier, your planet would not have formed the way it did in the first place. The pressure of the heavier elements in the mantle will push down on the liquid (part of the) core, pushing the southern half deeper in, pushing the rest further out, melting in the liquid core, creating massive earth quakes and by the time it is at least remotely habitable, things will have evened out. $\endgroup$ – Burki Oct 2 '15 at 14:28
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    $\begingroup$ Earth's gravity is already pretty variable... $\endgroup$ – Scott Downey Oct 2 '15 at 14:29
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Regardless of what the crust looks like, the planet will be spherical, or rather, it will be an oblate spheroid similar to other planets.

The reason for this is that the crust isn't thick enough to significantly affect the shape of the planet. The crust, at least on Earth, is at most 25 miles thick. The radius of the Earth, on the other hand, is almost four thousand miles. Even removing the crust from half of the Earth would have about as much affect on its shape as removing the skin from half an apple would change the shape of the apple.

On a very small scale, the planet would be a bit pear-shaped, similar to the Earth, which also has land masses concentrated in the North. That is to say, the 'sea level' at the North pole would be a bit further from the equatorial plane than would be the sea level at the South pole. This difference would be very slight. On Earth, the difference is about 40 m. Even if our planet is more lopsided than the Earth, which it probably will be, having a more lopsided distribution of land masses, this deviation will be significantly less than a kilometer. It will not be enough to be noticeable without satellite mounted instruments, and will not affect things like map making on the surface.

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  • $\begingroup$ @Aracthor It will be slightly pear shaped. Added that into my answer. $\endgroup$ – ckersch Oct 2 '15 at 15:19
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Apart from the fact (already expressed by others) that the crust is usually considered a thin layer (40-50 km) compared with the rest of the planet, the issue is that such a configuration would not be natural.

If it was present at the moment of the planet formation, the layer of heavy alloys would have been melted, fused with other elements and most of it would be in the inner layers of the planet. If it was naturally added later, it would come from a massive meteorite impact with such a power that, again, would have melted most of the planet and buried most of the material, with the leftovers distributed all over the planet.

So, this design shows an artificial origin, and can be as un-spherical as the materials allow it. Which would not be that much, IMHO.

First, think only of the "half a crust" with the metal alloy part. A metal may seem to be very hard to bend, but when you put in under the pressure of kilometers and kilometers of the same heavy material, it will begin bending like soft wax.

The thicker/more desigual your crust is, the more plastic the base of the crust will be, invading the other part of the planet and making the metal crust thinner and thinner, until the metal has spread enough. To put an example, Earth mantle is highly viscous, but its composition is not that different from "rock-solid" Earth crust (if anything, it has more metal in it).

Additionally, the weight of all of that metal would depress the mantle itself (that, if it is Earth-like, it is not solid), sinking your metal layer deeper.

So, your metal crust will not be very tall (I bet something like 20-22 km tops). If you take into account that Earth radius is 6700 km, it would not make your planet distinctly un-spherical.

More thoughts about this, by someone who has done actual research

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  • $\begingroup$ Additionally, note that the definition of planet (en.wikipedia.org/wiki/Planet) includes is massive enough to be rounded by its own gravity so, if it is not spherical enough, then it is not a planet. $\endgroup$ – SJuan76 Oct 2 '15 at 17:23

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