Volume of oceans 1335 million cubic kilometers
Surface area of Earth 510.082 million square kilometers
So average depth of ocean on a smooth Earth with the land in perfect hydrostatic equilibrium would be 1335/510.02 = 2.6175 kilometers = 1.626 miles
Volumes of the World's Oceans from ETOPO1
Just to add a bit to this answer. Though it seems a lot, and indeed in our solar system it is a lot, for a terrestrial world like Mars, Venus, Earth, Mercury, Earth is the only one with such a large amount of water - but 2.6175 kilometers of water is not really a lot compared to the diameter of the Earth of 12,742 km.
This graphic compares the amount of liquid water on Earth, Europa and Titan
Europa has twice as much water as Earth, for a moon that is much smaller than our Moon.
And that's just the liquid water. Europa also has huge amounts of ice, about 10 - 30 km thickness of ice on top of an ocean about 100 km in depth. Same also for Titan - its ocean if it exists is deep below the surface - and the surface of Titan consists of ice as the "rock" with shallow seas and lakes of methane and ethane, mainly at its poles.
Other moons of Jupiter, apart from Io, have similar amounts of ice or water. And are now thought all to have subsurface oceans.
So - it's very likely that many exoplanets around other stars are "water worlds" - like Earth but with maybe tens or hundreds of kilometers depth of water.
The water worlds with the deepest oceans may not be very habitable. Because ice normally floats, but below a certain depth, it takes up a different structure, and sinks. So their ocean floors would probably be coated in ice creating a barrier between the hydrothermal vents and the rock floors of the ocean and the ocean itself. That would lead to a rather static ocean with little by way of "redox potentials" for life to make use of.
Or their oceans may be similar to this model of Ganymede
Where you have different layers of different types of ice, some float, some sink, and if you have salty water some will float or sink in that also, and depending on the pressure also, so once you combine with salty water as well the whole thing gets very complicated and you can get multiple layers like this. See Ganymede May Harbor 'Club Sandwich' of Oceans and Ice
Surface area of Earth 510.082 million square kilometers
So average depth of ocean on a smooth Earth with the land in perfect hydrostatic equilibrium would be 1335/510.02 = 2.6175 kilometers = 1.626 miles
Volumes of the World's Oceans from ETOPO1
Just to add a bit to this answer. Though it seems a lot, and indeed in our solar system it is a lot, for a terrestrial world like Mars, Venus, Earth, Mercury, Earth is the only one with such a large amount of water - but 2.6175 kilometers of water is not really a lot compared to the diameter of the Earth of 12,742 km.
This graphic compares the amount of liquid water on Earth, Europa and Titan
And that's just the liquid water. Europa also has huge amounts of ice, about 10 - 30 km thickness of ice on top of an ocean about 100 km in depth. Same also for Titan - its ocean if it exists is deep below the surface - and the surface of Titan consists of ice as the "rock" with shallow seas and lakes of methane and ethane, mainly at its poles.
Other moons of Jupiter, apart from Io, have similar amounts of ice or water. And are now thought all to have subsurface oceans.
So - it's very likely that many exoplanets around other stars are "water worlds" - like Earth but with maybe tens or hundreds of kilometers depth of water.
The water worlds with the deepest oceans may not be very habitable. Because ice normally floats, but below a certain depth, it takes up a different structure, and sinks. So their ocean floors would probably be coated in ice creating a barrier between the hydrothermal vents and the rock floors of the ocean and the ocean itself. That would lead to a rather static ocean with little by way of "redox potentials" for life to make use of.
Or their oceans may be similar to this model of Ganymede
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
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