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I hope this is "specific" enough, and sorry I don't really know formatting rules yet.

I'm trying to build a habitable desert super-Earth, where water can only exist seasonally around the poles (mostly concentrated in a South Polar ocean, if that makes any difference). The rest of the surface would be dry year-round, save for transient seasonal atmospheric moisture. I'm guessing water wouldn't extend lower than 45° latitude, and that's being generous.

Now, I've read that plate tectonics are possible without water. People seem to have some conflicting opinions about how likely plate tectonics are on super-Earth sized planets, but some suggest that they would be more dynamic due to thinner crust. My planet has roughly 2x the mass of Earth and a 15% larger diameter, if that makes any difference. That works out to about 50% stronger gravity. It's pretty dense and rich in iron and other heavier elements.

Assuming plate tectonics is possible on a largely dry planet, my most major question is this: What exactly might that look like? What would its appearance be from the ground and from space?

My best guess was that tectonic rift valleys would be lava-filled hellscapes covered in black basalt and other volcanic minerals. After that, the surface rock would begin to weather and oxidize on exposure to the atmosphere, slowly lightening in color and becoming familiar shades of rust red like the Colorado desert as you moved further away from the rift. I then thought it might be possible that as weathering continued, the surface could become scoured and bleached a lighter shade, especially at higher altitudes, and this would mostly be seen at the "leading edge" of a given tectonic plate (relative to its direction of motion, furthest from the rift).

I wanted to run this by everyone and see if anyone thought it sounded realistic, or had any better interpretations of how exactly a tectonically active world without a global ocean might appear. I am not an expert on any of this by any stretch, and any input anyone has is greatly appreciated.

Edit to add details in response to some answers: yes, it DOES have water, concentrated in an impact basin ocean at the south pole and lakes at the north pole (I did some tectonic hand-waving about the long-term stability of such an ocean on a tectonically active world.) This world is something like 6.5-7 billion years old. In addition, there's more hand-waving about why it has such abundant oxygen that it developed complex animal life; a photosynthetic aerial biosphere. Sky plankton, basically. The planet is a super-rotator, meaning constant winds, constantly kicking surface matter and nutrients into the air, along with enough water vapor that such a biosphere can (seasonally) survive. They're able to stay aloft thanks to the wind and some strong thermal gradients, as well as aerodynamic body shape, and in some cases systems similar to a hot air balloon, using pigmentation to absorb sunlight and heat internal voids filled with gas, increasing their buoyancy.

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  • $\begingroup$ The reason there are conflicting opinions is because you seem to be expecting a scientifically viable answer to a question about fiction. To date, we have not found a "super earth." We have no idea what it would look like. We have plenty of tectonically active planets and moons here in our solar system, though (Venus, Io...) but you're trying to build an oxymoron: a habitable planet that isn't habitable. Keep in mind, Arrakis is a fake. Tectonics means mountains and volcanoes. No water means skies full of ash and sulfur (at least). Otherwise, it's what you'd see in a terrestrial desert. $\endgroup$
    – JBH
    Nov 13, 2023 at 7:15
  • $\begingroup$ BTW, why do I say uninhabitable? A desert-wide planet with an atmosphere full of oxygen must have water because hydrogen is the single most common element in the universe. You can't have a planet without hydrogen. You could have a planet low on oxygen... but then it's not habitable. $\endgroup$
    – JBH
    Nov 13, 2023 at 7:17
  • $\begingroup$ FWIW, tectonics isn't obviously possible without water to help lubricate stuff. That aside, you should defined "habitable", because "breathable atmosphere" requires some considerable handwaving, but a nitrogen/argon/co2 atmosphere would be achievable, which combined with benign pressures and temperatures make for something an awful lot more habitable than basically anywhere we're discovered in space so far. $\endgroup$ Nov 13, 2023 at 20:40
  • $\begingroup$ Though now I look at the old atmospheric escape chart (which might have been removed from wikipedia, which seems sad) and have a think about your planet's escape velocity (~14-15km/s?) It looks like it would potentially retain a helium atmosphere, and I wonder if that risks your world turning into a gas dwarf with a crushlingly thick atmosphere... $\endgroup$ Nov 13, 2023 at 20:49
  • $\begingroup$ Thank you for that. It's dense and iron rich with a thin atmosphere because it was initially an innermost rocky planet; orbital resonances in a compact planetary system flung it out into an orbit with survivable temperatures, and volatiles which would form the oceans and atmosphere were delivered by events similar to the Late Heavy Bombardment. And yes, it's an oxygen-nitrogen atmosphere $\endgroup$ Nov 14, 2023 at 9:20

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Asking questions here isn't simple

I left a few comments. Nevertheless, I'm going to take a crack at answering your question. However, you're about to realize that "specific" means both focused (which your question is) and detailed (which your question isn't).

Where will I start?

If you have any visions of Frank Herbert's Arrakis, put them out of your mind right now. Herbert rationalized the ecology of his desert planet (as you find out later in the book series) with the sandworm life cycle, which included a "vector" (the sand trout) that imprisoned the aquifers. If you're into geology and hydrology, what he had was a a verdant planet that had nearly all its water locked up in constrained aquifers. He could therefore justify a fairly complex ecology because life existed before the sandworms remade the planet and that life either died or evolved to live on the resulting world. It's a wonderful and amazing fiction. So...

Assumption #1 I'm going to assume that for the purpose of this question we're going to focus on more-or-less "the truth." You can spice (ha ha) it up later with fanciful worldbuilding.

Assumption #2 Then I'm going to ignore the super earth condition (with the exception of a note, below). We've never seen a super earth. We have no science predicting beyond wild speculation what might be found on a super earth. Unless we consider the comments made by @Sphennings about what would be most likely retained in the atmosphere of a higher gravity planet, the "super earth" condition isn't relevant to the question. A super earth is no more or less realistic than the idea of terraforming Mars such that humans could live on the surface without oxygen masks.

Assumption #3 I'm also going to ignore the habitability question in favor of habitability. In other words, we're going to assume an oxygen-rich atmosphere despite not having the one thing that allows an oxygen-rich atmosphere: water (and the substantial plant life that comes with it). I have no idea where your planet is getting an oxygen atmosphere but, poof, there it is.

So, no global oceans, what does the planet look like?

We need to make some more assumptions:

Assumption #4 The planet is tectonically active, which means there are volcanoes and earthquakes, but since we're assuming habitability, we're going to assume the sulfur and ash from the volcanoes doesn't exist. This is the single most difficult to believe assumption on the list. Volcanoes would periodicaly dump ash and sulfur into the atmosphere, which would mix with the sand storms and never be pulled down into the "soil." The atmosphere should be toxic to humans but, poof, there it is.

Assumption #5 Tectonically active also means the planet has periodic mountain-building events.

Assumption #6 No global ocean means no substantial water in the air to bring particulates down out of the air. Which means sandstorms. Big honking sandstorms. Why so big and honking? No water on the night side means less heat retention which means a greater disparity of temperature between night side and day side — which means big honking sandstorms.

Assumption #7 How old is the planet? Even if we only count from the last substantial mountain-building event, it matters how long we wait. On Earth sand and wind are the tools that whittle mountains into hills, and then valleys. On your world, it's sandstorms. And yes, they'll whittle the mountains down, too, and sand will do it faster than water (with the exception of the freeze/thaw cycle). I'm going to assume old but not venerable age.

So, what do all those assumptions give us?

  1. Without significant water there isn't an easy way for beneficial bacteria to grow and nowhere for more complex life to form. The more plants you rationalize, the more you're either handwaving or ignoring your planet. Where you have life, if your planet can evolve life, it will exist mostly on the border of the polar regions where what moisture exists will wax and wane with the seasons. Standing water is great for mosquitoes, but it's lousy for life in general. Water needs to move a little for life to be happy. It's also where you will have the only temperate regions on your planet. Any further toward the poles and it's too cold. Any further away from the poles and it's too dry. And under no circumstances would you have kilometer-long sandworms.

  2. A lot of sand. A whomping lot of sand. No moisture means no soil or loam which means sand. Lots and lots of sand. When I think of sand... I think of sand paper. So...

  3. There are few mountains. Unless your mountain-building events are (a) rapid and (b) involve really hard rock (see note below), your mountains will be eaten away and covered by sand fairly quickly. What mountains you do have will have sharp edges and be more vertical than here on Earth because there's nothing (moisture, plants) to slow erosion to create gentle angles. Dry dirt (sand) is blown away in the wind. You need moisture and plants to lock it down.

  4. You will have a lot of rock shelves just under the sand. Once the mountains are out of the way of the sandstorms, they'll wear down much more slowly. Consequently, just as Earth has beaches, shallows, and deep water surrounding its oceans, your planet will have beaches, shallows, and deep sand surrounding your global sand oceans. Those shallows will be the rock shelves.

NOTE: You mention that your world is dense and has high concentrations of iron. The density increase is necessary because increasing the diameter to 115% of Earth's wouldn't otherwise create twice the gravity. You'll still have sand, but you'll have a few more mountains and they'll look a lot like Utah's Goblin Valley because even the smallest change in density would give way to the sand. Iron cannot be realistically universal, so you'll have exposed blocks of iron ore. Will it be red? Yes, but only because we're ignoring reality and assuming an oxygen-rich atmosphere. In reality it wouldn't be red because there wouldn't be enough oxygen in the atmosphere to significantly oxidize the iron. Of course, other chemicals (like fluorine) could oxidize the iron... I don't know if iron fluoride is red. If the tablets I chewed in grade school to prove I had plaque and needed to brush my teeth more are any indication, it'd still be red.

  1. from space your planet will have no clouds and look yellow and orange with a little mottled black and brown around surviving mountains as one approaches (those features will likely be too small to be seen from space).

Now, having established "the truth" insofar as I understand it, which could be completely and entirely wrong... take only what's useful to you and run like the wind with the rest. If you want kilometer-long sandworms, have them.

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Earth has plenty of tectonic features above sea level. I would assume that the features on your planet would resemble them. No doubt parts of rift valleys would become hellscapes from time to time, but there is no reason to believe it would be continuous any more than it is on Earth as plate tectonics generally involve slow processes from a human perspective.

The color of the rocks could be as you suggest but it would depend on the composition and distribution of the minerals of the crust. Given these variables and the lack of water it is very difficult to predict.

General note about your world: With so little water and vegetation it is hard to believe that the planet could maintain high levels of oxygen for an extended period due to oxidation. In addition (incidentally and in case it has an effect on any story) chemical rockets will not be able to lift from the surface in 2g.

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  • $\begingroup$ For visual examples, one can start with the Afar triple junction, work though the involved plate boundaries, and look for pictures. $\endgroup$
    – AlexP
    Nov 13, 2023 at 9:51

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