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The following existing questions Shallow sea world - plausable geology? Would a shallow ocean planet be possible? both address shallow seas on Earthlike planets but don't touch on global oceans of reasonably uniform depth without any dry land. I'm interested not so much in the formation of such bodies of water but rather the ability of planets to retain liquid water over geological time in the absence of tectonic activity that would otherwise create above water topography.

So my question is can a planet with earthlike gravity and composition have a shallow (in the range of 2-5 metres (6-16ft) deep) global sea, that is stable over geological time?

Particular attention needs to be given to the mechanisms and parameters that need to be met in order to allow liquid water to persist on a planet that I expect to be geologically quiescent.

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  • $\begingroup$ @ARogueAnt. That's kind of what I'm asking "Particular attention needs to be given to the mechanisms and parameters that need to be met in order to allow liquid water to persist on a planet that I expect to be geologically quiescent." $\endgroup$
    – Ash
    Aug 31 at 4:37
  • $\begingroup$ Oh, fair enough, it was sort of implicit in the question then. $\endgroup$ Aug 31 at 4:37
  • $\begingroup$ @ARogueAnt. Yeah, it's a big question, the implications of the problem are too big for me to get my head around by myself. $\endgroup$
    – Ash
    Aug 31 at 4:40
  • $\begingroup$ If it were engineered, terraformed, then maybe there are conditions that would allow it to continue in that state for geological time - trying to think what you'd need beyond no tidal stresses above a certain limit and that limit might be minuscule, precluding day/night cycles and moons and the like. $\endgroup$ Aug 31 at 4:56
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    $\begingroup$ This isn’t an answer exactly but what you’re describing reminds me of the “superhabitable” archipelago planets described in this 2015 Scientific American article. These theoretical exoplanets orbiting red dwarfs are covered by warm, shallow seas. Granted, these are “super Earths,” so they’d likely have higher surface gravity, but perhaps only slightly higher: scientificamerican.com/article/… $\endgroup$ Sep 2 at 4:28
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It is the moon of a gas giant.

The planet itself is nearly all light materials, typical of the Earths crust and Luna. It has a minimal metal core.

But it has 2 magnetic fields! The gas giant has an ample magnetic field which protects your world.

Also, this cool wet moon is wet inside as well. Water extends deeply and throughout the crust. This is salt water, and a conductor of electricity. As your planet traverses the field of its giant, the external field will induce a current in the salty heart of your planet. This in turn produces a magnetic field. This happens on Jupiter's watery moon, Europa.

The giant's field protects against solar wind and your planet's field protects it against charged particles around the giant.


There are not tectonics on this planet. The crust does get some tidal flexing. More importantly, the oceans have serious tides. Energy from the tides moving the seas is what has scoured this moon smooth over the millenia. Silky smooth.

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  • $\begingroup$ But does what you're describing meet the OP's Earthlike criterion? $\endgroup$
    – jamesqf
    Aug 31 at 18:49
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    $\begingroup$ @jamesqf - in that earth has a molten metal core and orbits a star and has tectonic activity, no. But you can have an earthsized moon of a gas giant and other earth things (life, oxygen atmosphere, Tuesday 2-for-1 drink specials) can all be on this shallow sea world. $\endgroup$
    – Willk
    Aug 31 at 19:03
  • $\begingroup$ This works, it can even be a little outside what would normally be considered the Goldilocks Zone if the planet it orbits is big enough to produce some of it's own heat. $\endgroup$
    – Ash
    Aug 31 at 23:23
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    $\begingroup$ I'd say this would work nicely as is, also if the gas giant were at the mutual L3 point of the two stars of a binary system - to make sure your planet got lighting at every point of its orbit (a requirement for liquid ocean rather than ice?) - if that's not a bit too contrived/unstable in orbit. @Ash $\endgroup$ Sep 1 at 3:19
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Im trying to approach this systematically.

Not having surface features (or the few it has being geological time stable, few, and/or eroded), all suggest a very old planet. The main driver is internal heat, both from formation and from radioactives. Earthlike planets as far as we know, form by aggregation and collision - meaning immense heat. Extreme age - or perhaps unusual composition - could maybe explain that. Also probably explains its flatness a lot (time for erosion, moonlike "seas" from impacts, no or minimal "rebuilding" dynamics like tectonics, water erosion when those re-melt and begin to move), which is helpful.

Second problem. Earth's magnetic field protects it from the solar wind. If its that cool, there probably isnt a rotating metallic core. (There could be a small one, but if so, not magnetically significant, or else I guess its hot and moving enough for tectonics). So no meaningful magnetic field.

So then the next issue, is, explaining an earthlike planet being "that old". Old enough to lose most internal heat and have a solid core (probably or for practical purposes) yet still support an earthlike environment over geological time.

My guess is it formed long long ago, long enough its been ejected into interstellar space when its star died or by some near collision. By some handwave it ends up in a near circular ideal orbit around another star some 10 billion years later. (woooo!). Perhaps its much smaller than earth - hence denser. Iron higher percentage maybe? Could help it lose heat.

Now in orbit, it slowly warms - but from the top down. Its still essentially solid and cool inside. (Note that cool just means "cool enough not to have moving liquid internally, even with the immense pressure". Not absolute-zero cold). Its orbit means heat is constant hence with no features or tectonics, so is climate over time. Pretty much. Within handwaving distance.

Bear in mind its already done a full evolutionary cycle. Its oxidised its iron the mandatory 2 billion years, got water and oxygen, got complex molecules etc. Theyre all there. Since complex chemistry can occur in space, these wont break down to the point that everything has to start from scratch. As son as it warms a bit, it'll have its liquid water, atmospheric oxygen etc. Or could have.

Now you have some much easier problems left. The (re?) evolution of life without magnetic fields, but with that helpful starting point. Maybe it never quite died out. After all, life can handle 0 - 400 C (ice water to deep sea vents) on earth. And a cooled planet only means under 1000 C (liquid iron/rock) or some such. So perhaps as the planet cooled, theres going to be a band within the planet that is a suitable temperature for simple life. As that shrinks, life retreats. Tricky, if it retreats too much, surface warming cant restore it. Maybe it didnt need to and it re evolved or came from elsewhere.

But id say something like that is your probable best bet.

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No. If the planet has Earthlike gravity & composition, it is going to have plate tectonics. That means you will get continents, and mountain ranges where plates collide.

Even if you have sufficiently different conditions to avoid having active plate tectonics (for instance, if the amount of water isn't sufficient), you'll still get considerable relief, from things like volcanos and meteors.

Consider that Mars doesn't have plate tectonics or much water, yet the planet still has considerable relief. Similarly with Venus: no liquid water at all, yet mountains and "continents".

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  • $\begingroup$ Drat, that's what I thought, I'm still hoping someone finds a way to disagree with us both. $\endgroup$
    – Ash
    Aug 31 at 4:54
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A shallow global sea with 2-3 meter depth implies that there are no extended surface features higher or deeper than that.

2-3 meter surface roughness on a radius of 6300 km is an astonishingly smooth surface roughness, challenging to achieve even with modern machining technologies. For a reference, Earth is as smooth as 320 grit sandpaper.

Considering that for an Earth like planet the forces shaping into a sphere tolerate a mountain high about as much as mount Everest, there is still a long way to go achieve it.

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