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Background

I just read this question about how to get a planet to have one hemisphere with tropical climate and one with polar climate. The (not yet accepted) answer with the highest score recommends placing large enough mountains on one hemisphere, so that simply by altitude, it would be cold on the ground.

Question

Now I ask myself, what would have to happen to a planet so that exactly one hemisphere is (exclusively) full of high mountains whereas the other hemisphere is entirely flat?

Related

This question might be the same as mine, just inverted. I do not know if this is the case though, and I also do not find the higher-scored answers to be answering my question.

Edit

Since it came up in the comments: I'm only asking about landmass. The underwater topology does not factor in here, since the question linked above is framed in terms of human habitat.

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    $\begingroup$ Pluto's moon Charon demonstrates something like this, with one hemisphere appearing much smoother than the other. Current hypothesis seems to be that the smoothness is the cryovolcano equivalent of lava-planes, and the weird geology (cryology?) is due to Charon expanding as more of its internal water freezes. I'm not sure if there's anything useful to be taken from that to be applied to a rocky planet, though. $\endgroup$ – CAE Jones Sep 5 '19 at 10:58
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    $\begingroup$ Also Earth's moon. While not perfect, the near side contains many near-flat maria, while the far side has almost none. $\endgroup$ – jamesqf Sep 5 '19 at 16:51
  • $\begingroup$ Do you want the planet to be habitable or have any other properties aside from one smooth side? $\endgroup$ – Iron Gremlin Sep 5 '19 at 20:45
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    $\begingroup$ Do you mean the land on a different parts of the planet, or are you including the ocean(s)? Are there oceans on your planet? Have you looked at simulations of what the arrangement of the Earth's continents looked like over millions of years? $\endgroup$ – Makyen Sep 6 '19 at 1:08
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    $\begingroup$ if your planet has water the flat portion will be nothing but ocean. $\endgroup$ – John Sep 6 '19 at 3:28
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It turns out there's something very much like this, right next door to Earth. Observe the topography of Mars:

enter image description here

(Image courtesy Wikipedia; larger version here.)

The large blue areas in the northern hemisphere, making up about a third of the surface, are about 4 to 6 km lower in altitude than the yellow/orange highlands to the south. This dramatic discrepancy is known as the Martian dichotomy and there are two major schools of thought for how it came about.

The first is that the dichotomy represents one or more colossal impact craters. If it was from a single impactor, it would be the largest impact crater in the solar system. Opinions are divided as to whether a single impact can explain the resulting geography, or whether it's better modeled as multiple (still massive) impacts. Of course you could do either.

The other theory is related to long-ago Martian tectonics. The theory goes that for reasons unknown (and this is still an active subject of research on Earth, let alone Mars), one hemisphere featured one or more huge upwellings of material from the mantle into the crust, and the other featured downwellings. Over an extremely long period of time, the result is that crustal material migrates from one hemisphere to the other. (The average thickness of the southern crust is twice that of the north.)

In the case of Mars, the dichotomous hemispheres are north and south, but this may just be coincidence. Neither process has any particular reason to favor one orientation over another as far as I know, so having a dichotomy between east and west hemispheres should be possible.

Note that there are still local variations: there are craters and mountains in the north part of Mars, valleys in the south part. However, the difference should be enough to establish the broad climate dichotomy that you're looking for.

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    $\begingroup$ You don't even have to go that far. Look at Earth. Center a globe (or Google Earth, etc.) on the Pacific Ocean and you'll barely see any land above sea level. Far more than half of the land (mountainous or otherwise) is on the other side. (The Pacific doesn't line up with where we arbitrarily chose to divide the east/west hemispheres, but it is basically about half of the planet.) $\endgroup$ – Darrel Hoffman Sep 5 '19 at 18:45
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    $\begingroup$ And in between, the Moon also has one fairly flat side (the side we can see from the Earth), and one rough and mountainous side. space.stackexchange.com/questions/2631/… $\endgroup$ – Mike Scott Sep 5 '19 at 19:59
  • $\begingroup$ Here is an interesting comparison of these three bodies. radicalcartography.net/index.html?histplanets $\endgroup$ – Anton Sherwood Sep 6 '19 at 16:42
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Here is a more speculative explanation.

  1. The planet is tidally locked to its star. https://en.wikipedia.org/wiki/Tidal_locking

The result: one side is always bright and the other dark. The bright side is a lot warmer than the dark side.

  1. The planet's surface is made largely of volatiles. Water would work. Other ices like methane or ammonia would work.

  2. On the hot side, the surface material sublimates or evaporates in the perpetual day. When the gaseous material gets to the dark side, it precipitates out as rain or snow. Once it is over on the dark side it is never going to melt again. It piles up.

This results in the gradual transfer of crust from the hot side to the cold side. The hot side would flatten out as it was stripped away. The mountains on the cold side would not be rocks, but more like glaciers.

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Quickest solution - one hemisphere is where all the tectonic plates meet. Or if there are some on the other one it's under the ocean so the height of the "mountains" don't have influence on the weather above.

Other solution could be - mountains from rocks soft enough to be levelled with glacier and erosion.

Fun and make for good storytelling solution - there was a time when the moon of the planet didn't covered planet during meteorite shower that destroyed the mountains (assumption must be needed that the both hemispheres are just mountains and valleys so there is no planes where meteorite could create mountain range by making a very big hole)

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The moon's center of mass is about 2km from it's geometric center. If you covered it with water, sea level would be more-or-less centered on the center of mass, so the ocean would be much deeper on one side than the other. And if you covered it with an atmosphere, the atmosphere would be much thicker -- i.e., denser at the surface -- on one side than the other.

As I understand it, this discrepancy in centers is thought to be related to the moon's mode-locking to the earth. So, to have a planet with a normal day-night cycle that forms this way, you would have to re-spin it, possibly about a different axis than the orbit. What's harder, you'd have to re-spin it without breaking up or melting the crust. And you might have to remove the body it was mode-locked to.

One problem with your question shows up if you want an earth-like percentage of ocean. The low terrain would mostly flood and be ocean floor. If you want lots of ocean, and land masses in both hemispheres, and a high / low distinction between the hemispheres, you need a more complicated situation.

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