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I have a planet that is tidally locked to its sun.

The ocean is shallow (a few hundred meters) and was artificially created in the past by harvesting the system's Oort cloud or outer-system ice asteroids (machines that autonomously operated over centuries to re-direct ice bodies at the planet) – a terraforming process I'll call "ice bombardment".

Backstory: This world was created for a utility purpose (1 of hundreds), and over the centuries has been abandoned (political or economic powers have long-since shifted). The pretext is that large interstellar ships were sea-vessels, as opposed to "parking" in orbit. It is not hard sci-fi, but the biggest "handwavium" in the story is economics (the Great Wall and the Pyramids were built even though the economics don't make sense). None of this backstory will be discussed.

I realize a tidally locked planet will have a low magnetic field (if any), and this ocean will eventually evaporate. I need water covering the planet, except for a glacier on the "north" pole. The glacier is large enough that its weight has uplifted a scattered ring of small islands.

Is a glacier possible on a tidally-locked, shallow-sea planet? What kind of atmosphere/weather conditions would the planet allow on the sunny side? I have researched tidally-locked, ice, shallow-sea, and "eyeball" planets, but I'm not sure I can have mostly ocean and a glacier too (it seems logical to me but I'd rather be sure).

Bonus question: could I allow for bio-engineered oxygenation? I don't want a perfect atmosphere, it is intended to be an artificial world in decline.

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    $\begingroup$ Wait, “the glacier is large enough that it’s weight has uplifted a scattered ring of small islands”? How’s that work? $\endgroup$ – Dubukay Oct 28 '18 at 15:20
  • $\begingroup$ @Dubukay, It's called "glacial isostatic adjustment". The weight of a glacier causes a depression, but some of the land around the glacier is uplifted, called a "forebulge". Layman's explanation can be found by searching for "glacial rebound" which is the current process as our glaciers are receding. a good layman's article is here: smithsonianmag.com/science-nature/… $\endgroup$ – wetcircuit Oct 28 '18 at 17:02
  • $\begingroup$ There is no reason to believe that a tidally-locked world will lack a magnetic field, just because it's tidally locked. Such worlds still rotate. $\endgroup$ – Logan R. Kearsley Oct 29 '18 at 0:21
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The problem is that the planet is tidally-locked. So you have a perpetually-lit (and hot) side and a perpetually-dark (and frigid) side.

The atmosphere on the perpetually-sunlit-and-hot side will be continuously evaporating water from the liquid sea...then depositing that water as snow on the perpetually-dark-and-cold side.

That cycle does not seem sustainable, since there is no mechanism to return that frozen water to the daylight side to replenish the liquid sea. Eventually, the planet will have hot desert on the sunlit side and vast frigid glaciers on the dark side.

Some glaciers (or, earlier, sea ice) on the dark side will push into the twilight --very slowly--, producing a trickle of meltwater, but not nearly enough to replenish the sea. However, that depends upon the local topology -- in many areas, glaciers (or sea ice) may instead flow farther into the dark side.

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    $\begingroup$ I think this makes the unrealistic part the sea, and not the glaciers. In such a climate system, half of the planet becomes essentially one solid ice-pack if there is enough water there. The weight of the ice would eventually smooth everything beneath the glacier into one median elevation. If that elevation we below sea level on the light side, no sea, if above, a sea. All you need is enough water. $\endgroup$ – Iron Gremlin Oct 27 '18 at 0:19
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from OP: /a terraforming process I'll call "ice bombardment"./

Ice bombardment continues.

If this method could create your ocean, it could maintain your ocean. The terraforming process accumulated many more ice asteroids than were needed at the time the planet was built. The surplus were left along the orbit of the planet in a rough ring. The terraformers distributed them so they would come down through the atmosphere at predictable intervals, replenishing the planet with space rain.

I do like @user535733 idea of glaciers forming on the dark side and pushing their way into twilight. Have that too, for sure.

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Yes, in fact it is unavoidable.

If you have snow and land you can have glaciers. With one side perpetually cold you will have some very large glaciers probably as ice sheets (many many linked glaciers on the same land mass. Remember that glaciers move they are constantly flowing downhill, so if all your highlands are on the dark side the glaciers will constantly be flowing back into the ocean, albeit slowly. keep in mind your ocean will probably freeze on the dark side so much of your ice will be frozen ocean as much as glacier. Ice sheets don't even need mountains to flow, just weight will keep them constantly flowing outwards.

If your planet starts with a LOT of water most of the cold side will be covered in a huge ring shaped ice sheet (the center of the cold side will not have large glaciers as there is no moisture left in the air by the time it gets there). The ring will slowly be flowing outward as the snow builds up. Funnily enough isostatic uplift will keep the center of your cold side "continent" high since there is far less ice on it so the process should be close to self perpetuating.

Depending on the average temprature of your planet from a distance your planet may look disturbingly like the image or the negative image of an eyeball. Warmer worlds will like like a negative eye looking out while a colder one will look like a normal eye looking in.

the picture below would be close to a cross section of the ring. Although do note the vertical exaggeration.

enter image description here

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  • $\begingroup$ A glacial wall is interesting. Could it be tall enough to be a moisture block, like a mountain range? That would put a gloomy atmosphere right over the ring of islands…. $\endgroup$ – wetcircuit Oct 28 '18 at 23:08
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    $\begingroup$ depends on what you mean mountains need to be very tall to block moisture effectively, and ice will flow out as it builds up, but moisture is not going to make it that far inland anyway, we see this in antarctica $\endgroup$ – John Oct 28 '18 at 23:51
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yes, the glaciers are the easy part, the oceans or simply minimal water supply the hard part.

reservoirs & aquaducts could be established near the glacier front which, given tidal locking, should be fairly static.

using nuclear or geothermal power or a stirling engine variant (driven by the temperature gradient), drill & transport ice into the reservoirs, where they melt & run off into the aquaduct

the drilling should take place at the top of the glacier to minimize complexities.

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  • $\begingroup$ Interesting idea. If they can transport ice from the outer system, I suppose they can deliberately melt their glacier. Thanks! $\endgroup$ – wetcircuit Oct 28 '18 at 17:05
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Can I have a glacier on a tidally-locked shallow-sea planet?

Yes.

One of the outer system planets, somewhat like a large Jupiter could be a small star:

It takes a little over .08 of a solar mass for a hydrogen-burning star to form

That's roughly 80 times the mass of Our Jupiter, which itself is 320 or so Earth masses. It has an orbital perion of just on 12 Earth years see here.

If every year (or so) your planet was in a position to be slightly warmed on it's dark side for a few months by enough to kick off the cycle of evaporation and re-deposition - then yes it would seem possible by this means.

As to (Bio-) engineered oxygenation - I'd add the possibility of harnessing geothermal energy on the dark or light side - wherever it's available, cracking the water, using the hydrogen as a fuel or locking it up in plastics production - freeing the oxygen.

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First problem is with location of glaciers on north and not south poll. In reality they would be on both, unless significant permanent axial tilt exists on your world. Suggest you're planet, if tidally locked, would have glacier on dark side. Only exception would be your plant is spinning still, but one pole (South?) is facing the systems star for what ever reason.

If Earth were to be tidally locked to Solis, the ocean at the polls would become very deep and the sea level at equator would be shallow or even nonexistent. (continents would emerge here and the seas divide) It's the planets rotation that keeps the seas deep at the equator.

Maybe your planet needs to be a moon so there would be more seasonal variation rather than just constant convection of heat from one side of the planet to the other. This would only create and drive a constant global wind. It would make more sense therefore to make your planet the smaller of a binary planetary system, then there would be the possibility for tides, eclipses and freezing of the waters at the southern poll, thus regular ice formation, storage and melting. Also you can then create a classic Io/Jupiter dynamic with millions of amps exchanging between planetary bodies, driving differentiation/ plate tectonics and volcanism.

introduce a seaonal based algae bloom in the southern hemisphere and you have your bioengineered oxygen generation system.

your welcome 🖖🏼

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    $\begingroup$ I have read "north" used to describe the "night side" pole, and "south" to describe the sunward pole. I'm not aware of any common terms for the concept – hence my use of quotation marks. The planet is tidally locked so concepts of "north" and "night side" are stand-ins. Sorry for the confusion. I am considering re-working the planet so rotation is a possibility, Thank you. $\endgroup$ – wetcircuit Oct 28 '18 at 23:02

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