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NB: There is a related question – What kind of planet could have giant sand worms? – but this one is concerned specifically with the sand. Giant sand worms would be awesome incidental.

Assuming a terrestrial planet with a dry atmosphere (the composition is unimportant unless other conditions are dependent), an average surface temperature above freezing, and any other conditions necessary for desertification to progress to the point its status as an arid "desert planet" isn't in question, what conditions are needed to maximize the depth of the sand in the planet-spanning desert?

Here on Earth sand depth has been measured to 43 metres in the Sahara, while dunes in the Namib have been are known to reach as much as 100 metres in height (with a maximum mean depth of 30 metres for surrounding sand). Mesozoic and Paleozoic ergs have reportedly been measured in the hundreds of metres of depth (mean), but it has been suggested these may have been sequences of ergs migrating and stacking over geologic time. (Source: Aeolian Sand and Sand Dunes, pp 155) If hundreds of metres is achievable on a planet with vast liquid oceans and mountains like the Himalayas, would a desert planet be able to boast dune seas tens of kilometres deep?

(There is probably an upper limit to how much sand-on-sand pressure could be endured before the deepest sand begins to undergo lithification or diagenesis and stops being sand, but I have not been able to find what that is.)

Planetary composition, mass, size, surface gravity, stellar parent, age, and so on can be modified as needed within the assumption above. The best answer would be able to specify planetary properties that are likely to produce deserts deeper than other combinations of properties.

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  • $\begingroup$ I think you are right sand on sand pressure will create sandstone at a sufficient depth (and temperature and length of time). Adjusting the planets mass could have a dramatic effect. a 500 mile diameter ball of sand probably doesn't have sufficient gravity to create sandstone, does that count? $\endgroup$
    – Slarty
    Sep 16 '20 at 20:50
  • $\begingroup$ Would a 500 mile ball of sand qualify as a terrestrial planet, support an atmosphere, etc? $\endgroup$
    – rek
    Sep 16 '20 at 20:54
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    $\begingroup$ Perhaps it's worth mentioning that most desert really isn't all that sandy, it's just that windblown sand tends to collect in dunes, instead of being fixed in place by vegetation. Here's an example of a dune that's 600 ft / 180 m high: en.wikipedia.org/wiki/Sand_Mountain_(Nevada) Note that the area surrounding it for many miles/km is desert, but not notably sandy. $\endgroup$
    – jamesqf
    Sep 17 '20 at 5:46
  • $\begingroup$ Does your "sand" need to be sand, or can it be made of lighter/coarser materials? Exotic carbon structures? $\endgroup$
    – DWKraus
    Sep 28 '20 at 14:58
  • $\begingroup$ @jamesqf Real deserts aren't just/all sand, correct, but your typical science fiction desert planet is primarily known for being sandy. That said, if the maximum depth conditions mean x% of the surface won't be sandy, that's within scope. $\endgroup$
    – rek
    Sep 28 '20 at 15:08
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Several steps.

  1. Hydrologically active planet like Earth or ancient Mars. Landforms are worn away and deposited by water into large sedimentary beds of sand. Ancient sedimentary beds on Earth can be many kilometers deep.

  2. Something changes on this planet. Maybe a near miss alters the orbit of the planet to eccentric such that it goes close to its star then far out. Or it becomes tidally locked to its star.

  3. Water goes away. Maybe from heat, driving it into space? Or solar wind stripping it from the atmosphere as happened to Mars? Maybe the water descends into the planet. Without water sand grains will not lock into sand stone.

  4. It gets cold. I thought Venus would have loads of sand because of the dense atmosphere and high winds but apparently not. The heat tends to fuse it. Also Venus has not had water for a very long time. Mars has lots of sand. It is cold. Titan has the most of all. It is seriously cold.

  5. High winds keep sand moving. Wind erosion scours up the great sand deposits from the ancient wet epochs, blasting sand against sandstones to make more sand.

  6. High winds are very intermittent. Very occasionally - perhaps during one part of the eccentric orbit - there are immense winds that stir all the sand up into the atmosphere and generate more from the ancient reservoirs. Then it all settles down again before the sand wears itself down into dust.

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    $\begingroup$ By the time sedimentary rocks get several kilometers deep, they've already consolidated. You can't have the water to cause the erosion and transport and not have the water to prevent sandstone formation. $\endgroup$ Sep 17 '20 at 15:28
  • $\begingroup$ @KeithMorrison - right, just so. But sandstone is more amenable than granite to aeolian weathering back to sand. I wanted big sandstone reservoirs. $\endgroup$
    – Willk
    Sep 17 '20 at 22:30
  • $\begingroup$ Problem is you can look at Mars and see it doesn't happen. $\endgroup$ Sep 17 '20 at 22:50
  • $\begingroup$ @KeithMorrison - interesting stuff on dunes here as re Mars. hou.usra.edu/meetings/dunes2015/pdf/8031.pdf. Mars might not have enough sand and its thin atmosphere makes for low wind kinetic energy. $\endgroup$
    – Willk
    Sep 18 '20 at 0:45
  • $\begingroup$ I thought Venus had lots of water... in the form of face meltingly hot vapour. $\endgroup$
    – Joe Bloggs
    Sep 18 '20 at 9:02
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If you had a column of sand 10 km deep and a meter square it would, ignoring the growing density caused by the weight above compressing the sand within the column itself, have a mass of about 1600 kilos per cubic meter so a total of 16 million kilograms per square meter. Under earth gravity, that's a pressure of about 157 megapascals (deepest part of the ocean gets to 108 MPa).

That's...a lot. You aren't going to get large organisms like sandworms.

Now, realistically, sandstone starts forming at most of a depth of a few hundred meters, depending on water flow and minerals available for cementing. At the depth you're talking about, you're starting into metamorphosis conditions. Thus, no, you're not going to have loose sand a few kilometers deep.

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  • $\begingroup$ So you're saying a lower gravity planet? Or one with much less water? $\endgroup$
    – rek
    Sep 17 '20 at 1:14
  • $\begingroup$ If there's much less water, what's eroding the rock into sand and transporting it into basins? $\endgroup$ Sep 17 '20 at 15:27
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I think that you need a planet that's smaller than Earth. That way, the heat of the core escapes faster and there's less gravity. That way, the sand cannot compact into sandstone, and the planet needs to be sufficiently old. And the low gravity would also allow for sand dunes to stack up higher without collapsing. Oh, and strong winds to wear down stone into sand, and have a watery region nearby, as those will for sure help with erosion. And the sandworms breaking down big boulders would also help erode the sand.

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concerned specifically with the sand. Giant sand worms would be awesome incidental.

You could use the approach that was used on Arrakis(Dune). The sandworms not only live IN the sand, but they make more sand BY living in it.

Very much the same way that earthworms live in fertile soil, but by living there increase the extent if the fertile soil.

Without something to turn over the sand, to bash at underlying rocks, to abrade rock outcrops, the only source of sand is surface weathering. Which is both slow and very limited in extent. And yes, deep sand that is left undisturbed for millenia could transform into sandstone.

The worms provide new sand, and stir up the deep sand, keeping it in particulate form. (actually, in Dune, the junior sandworms also form barriers to prevent water from entering the sand, keeping it nice n dry)

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