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I want to build a habitable desert gas-giant moon and I´m not sure how little water I can give it and still get plate-tectonics instead of an Io-like lid-tectonic setup. I want plate tectonics because continental-continental divergent (East-African rift valley) and convergent (Himalayas) plate boundaries create way more interesting geological features than the uniform mountains and plains of Io-like lid-tectonics. Additionally, plate tectonics might be necessary to keep the planet habitable. Since no two papers, I read on the subject of what mass a planet needs to sustain plate-tectonics agreed on a mass range(either Earth is at the upper or lower boundary...), I settled on 0.2 to 5 earth masses and decided to call it a day. While even the role of water as a lubricant and thus its role in enabling plate-tectonics has been challenged, most sources consider it necessary. But how much water, specifically surface water, do I need on my planet in order to maintain plate tectonics? Earths oceans contain a lot of water, but the mantle and the crust might hold between 2 to 25 times the amount of water found in the oceans. So is ocean water even necessary?

My planet has the following, more or less fixed parameters.

  • 10 to 20% global ocean cover

  • ca. 0.3 Earth masses

  • heavy volcanism due to tidal heating ( not Io-like but close)

  • a similar composition to Earth

  • first and biggest of the three moons of a super-jovian ( 12,6 Jupiter masses)

  • super-jovian orbits an early F-Type star (1,15 solar masses) near the end of its 7 byr lifetime

  • desert world with habitable poles

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    $\begingroup$ I'm confused: as far as I'm aware, plate tectonics aren't affected by surface water. They're a function of a relatively brittle crust moving on top of a relatively viscous liquidy mantle layer. $\endgroup$ – elemtilas Jun 28 at 22:08
  • $\begingroup$ @elemtilas Really? That would make things very easy for me. I remember reading that water has a crucial role as a lubricant because the plates would otherwise be to non-flexible to allow for subduction to happen. Would you mind sharing your source with me? $\endgroup$ – TheDyingOfLight Jun 28 at 22:13
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    $\begingroup$ It is most likely (not 100% certain) that Mars has plate tectonics, and yet it has almost no water. $\endgroup$ – Bilbo Baggins Jun 28 at 22:23
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    $\begingroup$ If you look at Earth, the mantle (with mountains, rivers, oceans ETC) is 1800 Km deep, the ocean is much much less than that. The forces involved in the creation of plate tectonics are vastly more huge than can be applied by an ocean's tidal forces or thermodynamics from water's influence. $\endgroup$ – 011358 smell Jun 28 at 22:23
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    $\begingroup$ As far as we can tell Mars does not have plate tectonics. It's possible it had it briefly in the deep past, but certainly not recently. Look at geological maps of Mars -- there are no typical plate tectonic features. $\endgroup$ – Mark Olson Jun 29 at 1:09
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None

The prevailing theory is that plate tectonics are made possible, in part, by the relative density of the ocean crust on oceanic plates. However, there is no requirement that the denser oceanic plates be overlain by actual water. Consider that an oceanic plate is about 10km thick, with a density three times that of sea water; you get an order of magnitude more mass from the crust than you do from the ocean on top of it. You could do away with ocean, and still end up with the same geological effects.

Also, it is possible that the theory about ocean crust density as a driving force for tectonic motion is wrong, so then it wouldn't matter anyways.

So, in either case, you don't need any surface water on the planet.

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  • $\begingroup$ Iceland sits atop the Mid-Atlantic ridge. The divergent boundary between the North American and Eurasian plates goes right through the island. The East African Rift shows how an ocean is born when a continental plate splits in two parts. $\endgroup$ – AlexP Jun 29 at 1:01
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    $\begingroup$ It appears that on Earth the thick, wet sediment overlaying the oceanic basalt is essential in lubricating the subduction of plates, so "None" is probably incorrect. $\endgroup$ – Mark Olson Jun 29 at 1:07
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    $\begingroup$ That is disputed: m.phys.org/news/… $\endgroup$ – rek Jun 29 at 6:44
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    $\begingroup$ More than disputed there was no evidence to support the claim in the first place. Dry faults exist all over hte planet. $\endgroup$ – John Jun 29 at 14:43

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