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Question: Could a planet contain all of its (vast) oceans underground?

I imagined a planet with no surface oceans - and no water cycle above ground - but where life was still possible.

My initial misgivings are:

  • The increased temperatures below the crust.
  • Any life must be able to get to the water so the earth must absorb this water.

But then wouldn't these two result in geysers and evaporating surface water, leading to those oceans slowly moving to the surface and creating a above ground oceans?

I'm looking for any ideas on how a planet with underground oceans would stay this way or at least an idea of the timescale for the stability of this. (I.e. I don't want life that evolves to use these conditions to get flooded soon after).

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    $\begingroup$ Not a planet but having a subsurface ocean: en.wikipedia.org/wiki/Europa_(moon)#Subsurface_ocean $\endgroup$
    – Helmar
    Commented May 29, 2017 at 14:27
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    $\begingroup$ Are you looking for a planet that still has a liveable surface? Or are you expecting the surface to be barren rock or ice with any life occurring underground along side the oceans? $\endgroup$
    – adaliabooks
    Commented May 29, 2017 at 18:24
  • $\begingroup$ @adaliabooks I was sort of hoping for life on the surface but I realise, given the answers, that I wasn't very clear about this in my question and thought it was probably unfair to change the question now. $\endgroup$
    – user38754
    Commented May 30, 2017 at 8:42
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    $\begingroup$ @Ludo I'd assumed a you meant a liveable surface but wasn't sure. $\endgroup$
    – adaliabooks
    Commented May 30, 2017 at 9:19

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You mention "oceans", certainly we have huge aquifers in the world (Ogallala in the US is a big one; 174,000 square miles, bigger than the Caspian Sea (at 143,000 sq mi). Here are other aquifers).

A world heavily dotted with a network of such aquifers could have no oceans; but I imagine there would still be a water cycle above ground: Plants could access the water, but evaporate it by normal biology; there would still be some cloud cover and rains due to concentrations caused by planet rotation winds: Just the rain doesn't accumulate, it percolates back down into the aquifers.

I don't think you can have surface plants without them producing water vapor, by drying out (especially upon death) or burning: water is not destroyed by burning, it just becomes steam and escapes (and must go somewhere). If water vapor is produced, an above ground water cycle would exist.

If the plants only grow under ground, the evaporation can be absorbed by the earth or rock ceiling. If they grow under water there is no evaporation, but there could be a cycle of exchange between plant fluids and the water.

added: Animals on such a planet would evolve to get their water from the food they eat; ultimately traceable back to the water the plants are bringing up from the aquifers. IRL we do have animals that only get water from their food, I can't recall which ones off-hand, but it isn't a stretch for evolution. In any case, having gotten their water, they would still eliminate it, by sweat, saliva / panting, tears, urination and defecation. Those in turn would evaporate and dry, contributing vapor to the water cycle, which would collect and become rain. Some animals would evolve (like we have in the deserts, IRL) to get all their water during the rains, and store it within their bodies to be used slowly, in-between the rains.

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    $\begingroup$ Nice point about the animals getting their water from food. Maybe some plants would evolve to have particularly watery fruits? Then the animals will want to eat these and then spread the seeds. $\endgroup$
    – user38754
    Commented May 30, 2017 at 8:44
  • $\begingroup$ @Ludo Sure; plus in our deserts, we have succulents; cacti and other "fat leaf" plants (like Aloe Vera) that store large amounts of water and have evolved tough skins to prevent water loss; and heavy thorns to protect them against thirsty animals. $\endgroup$
    – Amadeus
    Commented May 30, 2017 at 9:13
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Such a body is probably already existing... in our solar system !

I'll suggest that you take a look at Europa.

It is estimated that Europa has an outer layer of water around 100 km (62 mi) thick; a part frozen as its crust, and a part as a liquid ocean underneath the ice.

Basically, you can absolutely have an inner ocean entirely surrounded by a thick ice crust. Ice being less dense than water, it will always stay on top, thus preventing the need of a "earth-absorbing" water. As opposed to a rocky crust than would inevitably sink down.

As for the heat, you have internal volcanism. Remember that Earth's life probably arose around deep oceanic thermal vents. This volcanism could be sustained by tidal forces created by a large moon or if your "planet" is actually a moon of a bigger planet (like... Europa around Jupiter). Plus, the ice crust would act as a pretty good insulator.

In fact, it is considered possible that life has developped in Europa's deep ocean. Maybe only microscopic, single-celled life, but one can always dream of Europa's fish-like denizens...

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    $\begingroup$ Would Europa have a strong enough gravity to keep any water that did ecape through the vents? Or would the vapour just keep on drifting out? $\endgroup$
    – user38754
    Commented May 29, 2017 at 15:06
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    $\begingroup$ The estimated eruption rate at Europa is about 7000 kg/s (source). Given that and its mass of 4.8x10^22kg, it would take billions of years to make a serious dent in its ocean, and that's assuming none of that 7 tons per second falls back on Europa or isn't replaced by various cometary impacts. Furthermore, Europa is just an example, you can twitch the surface gravity and geyser frequency to fit your needs ;). $\endgroup$
    – Keelhaul
    Commented May 29, 2017 at 15:17
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    $\begingroup$ Also Enceladus! It's like Europa but with better geysers, less gravity, and less ionizing radiation! $\endgroup$
    – foobarbecue
    Commented May 29, 2017 at 23:49
  • $\begingroup$ And I'll just mention that I was in the room when John Culberson (R-Tx) promised a couple thousand fellow JPLers that we will find krill on Europa :-) $\endgroup$
    – foobarbecue
    Commented May 29, 2017 at 23:51
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    $\begingroup$ Nitpicking here. The question says 'underground'. In Europa, the liquid water is not actually underground, its under ice. $\endgroup$
    – Nitish
    Commented May 30, 2017 at 5:23
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If the body surface is too cold to keep liquid water but the core is warm enough to melt the ice, you can have a thick ice crust "sealing" the oceans.

There could be geysers, yes, but the ejected water would then freeze and precipitate as ice on the crust.

Possible mechanisms to have a warm core are:

  • intense radioactive decay
  • tidally induced heating (think of satellite orbiting a gaseous giant)
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