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Water-worlds are becoming increasingly important in the exoplanet/astrobiology community. If plate tectonics create land masses, then can we say that a water-world (a planet with a global ocean and no land) does not have plate tectonics?
Many astrobiologists say that plate tectonics may be a prerequisite for life to form on a given planet. Therefore, if the assumption that water-worlds do not have plate tectonics is true, then can we also say that these water-worlds cannot harbor life?
can we say that a water-world does not have plate tectonics?
No we can't. Take for example Earth, add in a lot of water to cover all of the continents and you get a water-world with active plate tectonics. The driver for plate tectonics is not whether water exist or not on the surface, but rather the internal heat a planet contains.
Or, the plates of plate tectonics could actually be water ice, as is hypothesised for Jupiter's moon Europa. You have to remember that currently our sample size is one. There is only one planet where we know for sure that there are active plate tectonics, and that is Earth. Ruling out plate tectonics on other planets just because we don't know how it will look like or how it will work is not correct.
That said, water plays an important role in plate tectonics on Earth. Be it modifying the composition of the crust, changing the strength and ductility of the rocks, or some other process.
then can we also say that these water-worlds cannot harbor life?
No. Again, our sample size is one. There is only one planet we know with life and that is our Earth. Who knows what may be out there? For starters, we don't even know what fired up life here on Earth in the first place. It could be volcanos or hydrothermal vents in the ocean around hot spots, which are (almost) independent of tectonic activity.
On a personal note, we've all seen how astronomy advanced from "are there any other planets out there?" to actually discovering hundreds of them. What was once the exception is now the rule. Who knows, life might actually exist on just about every solar system out there.
I'm by no means an expert, but this Wikipedia link suggests that oceans make plate tectonics more likely, not less likely:
https://en.wikipedia.org/wiki/Plate_tectonics#Exoplanets
So, a planet with twice the surface water as earth, covered by oceans might have almost no land but I think it's unlikely oceans would stop plate tectonics cause the heat still has to go somewhere, and it either circulates and pulls plates with it or it doesn't and it had the occasional volcano as the only means of releasing heat (Even Mars still has the very occasional volcano, and perhaps the Moon as well)
Now if the oceans are 150 or 200 miles deep like Europa - I don't know what kind of effect that would have but just having oceans over the whole planet, I don't think that would likely prevent plate tectonics. My guess is that mostly water worlds would still have plate tectonics most of the time, provided the planet itself is one that has plate tectonics.
It would be impossible to answer this definitively, of course, but I would have to hypothesize that a water-world could harbor life, not intelligent, tool-using life, but basic stuff. Here's my logic: planet accretion leaves behind a lot of heat, even if it's not from radioactivity. Just the kinetic energy from accretion would take (probably) a billion years to dissipate. Even without plate tectonics, all that heat would have to vent via volcanic activity, and that energy could kick start life. If it hangs on long enough to develop photosynthesis (i.e. take advantage of solar energy), then even if the interior of the planet cools sufficiently to turn off plate tectonics, life would most like continue in one fashion or another. (Just my $0.02 worth...)
Plate tectonics actually seperate land masses resulting into continental drifts, many geologic events can happen at plate boundaries, plates could be crushed into each other(convergent boundaries), a plate could be buried under(subduction zone), new continental crusts are formed(divergent plate boundaries) these geologic events are associated with heat, and heat could result to transformation of matter which could in some way may lead to life. So it is also possible that land subsidence submerged the continents in water. The mid oceanic ridge which is associated with several life forms and it is located at the bottom of the ocean.
