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I would like to have giant sand worms similar to the ones in the 'Dune' novel. What should the planet be like so that they are realistic? Giant sand worms are roughly 100m long and 5 meters in diameter. They can burrow/ dig through sand similar to earth worms, they don't need to create permanent tunnels, their tunnels can collapse after them. Their diet consists of some tiny critter in the sand, so they digest the sand and filter out the critters, similar to how whales filter tiny krill out of the water.

On earth a creature like that can't exist because sand would just crush them and digging through harder materials takes way to much energy (there was a question about that here recently). So let's change the planet to make them realistic. I don't care about whether this species could have evolved, it is sufficient that they can exist as is. The planet doesn't need to be suitable for humans but that would be a bonus.

Some parameters that could be changed:

  • Lower gravity should help a lot because sand becomes less heavy but I can't put numbers on it.
  • The atmosphere can be denser or thinner, atmosphere composition and temperature can be adjusted but I don't know whether that would change anything.
  • The sand: Sand grains on mars are about 1000 times smaller than on earth (by diameter), even gravel with a diameter of 10cm on average could still be sand. Not sure what works best for digging.
  • Shape of the sand grains: polished round almost spherical for low friction or complicated shapes for lower density?
  • Chemical composition of the sand: sand is usually some silicate but if nitrogen snow at -210°C is the only way to go then so be it.
  • Whatever else needs to be changed to make giant sand worms.
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    $\begingroup$ just add water and make the sand substance a similar density so its more of a suspension $\endgroup$ – Ewan Nov 16 at 7:27
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    $\begingroup$ Before you go for giant sand worms: Does a small animal exist on earth that lives as a worm in dry sand, and if so, how? $\endgroup$ – Karl Nov 16 at 11:41
  • $\begingroup$ Why not just wave away the problem? After all, _Frank Herbert did, and his story didn't suffer from it. $\endgroup$ – RonJohn Nov 16 at 16:45
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Fluidized bed world.

fluidized bed hot tub

https://www.youtube.com/watch?v=My4RA5I0FKs

This guy was sitting on a hot tub full of sand. When gas jets underneath were turned on, he sank into the sand. This is called a fluidized bed.

https://en.wikipedia.org/wiki/Fluidized_bed

A fluidised bed is a physical phenomenon occurring when a quantity of a solid particulate substance .. is placed under appropriate conditions to cause a solid/fluid mixture to behave as a fluid. This is usually achieved by the introduction of pressurized fluid through the particulate medium. This results in the medium then having many properties and characteristics of normal fluids, such as the ability to free-flow under gravity, or to be pumped using fluid type technologies. The resulting phenomenon is called fluidisation.

When the sun comes up on your world (or some other form of heat appears - tides? geothermal?), beds of liquid (or solid) materials under the sand volatilize and the sand becomes fluid. Methane clathrates would work, or CO2, or a number of other substances depending on the temperature and pressure of your world. I think phase change from gas to liquid would be best. When the bubbles start, the sand becomes fluidized and the sandworts start moving.

At night all of the gas that has bubbled thru condenses and rains down onto the sand, traveling back down as the fluid to the reservoirs below. The sandworms are stuck where they are, and they wait for morning.

This percolation of stuff thru the sand also offers a possible energy/carbon source for the primary producers that the worms eat.

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    $\begingroup$ That sounds awesome. I just wonder whether one could make a stable cycle out of that. How long does it take for the gas to evaporate during the day? The same amount of gas need to rain down at night and sink sufficiently deep into the sand. I'm not sure whether one could balance these effects properly. $\endgroup$ – quarague Nov 15 at 13:29
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    $\begingroup$ This sounds great, but the timescales involved would be centuries per cycle, to cause that level of percolation through the super-stratum back into the substratum. ... Elliptical orbit? $\endgroup$ – Archerj Nov 15 at 13:34
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    $\begingroup$ @Archerj - I am digging the long cycle. During the solid phase the growers would grown Then the arrival of a moon with an elliptical orbit. It would shine weirdly and its proximity would cause tidal heating. Get to high ground before the worms awaken. $\endgroup$ – Willk Nov 15 at 13:59
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    $\begingroup$ Why would the worms come up if most of their prey is probably sinking through the sand down into their open mouths? $\endgroup$ – Muuski Nov 15 at 20:40
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    $\begingroup$ @Muuski - OP states that some "tiny critter" is the sandworm prey. These would be primary producers like algae and would probably not sink. They would be in the upper strata in the light. The worms would move so that their mouths maximally swept thru this area. $\endgroup$ – Willk Nov 16 at 19:01
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  • low gravity would certainly help. The less the better. But then comes the question of atmosphere: do not count for less than 1/3 of g to have atmosphere more than 1/10 of Earth. Mars with a magnetic field is a good candidate

  • Atmosphere would matter a lot since worms need to breathe. You want low CO2 (<2%), much O2 (15-30%) and heavy atmosphere (due to low gravity), so "filler gas" (85-70%) should be of some heavier compound (argon, or some organic gas? - I can't give advice here)

  • Sand on Mars is the same as on Earth, but sand in air there much thinner due to low atmosphere pressure (it can't lift heavier particles). So there would be a surface layer of denser and finer sand few meters deep and then "normal" sand. For digging is better have clay - it can support itself. As for sand, any would be bad for digging, but bigger-grain sand would allow easier breathing and slightly deeper digging. Smaller-grain sand is better for "filtering" food. So middle-size grain is preferable.

  • Shape of sand doesn't matter much. Rough sand is better it would better hold and distribute weight.

  • Good old silicon sand is good enough. All exotics would be too speculative. Worms need a biosphere to live in, and the only one we know is based on a rocky planet.

  • Main problems of giant sandworms is the weight of the sand above, water, food and air (in that order). They should have very slow metabolism (like move 1 meter/hour) in "standard mode" to preserve energy and avoid places, that can be, say, flooded with water or more sand. They also would tend to be narrow and long. No more than a meter across at max (but some exaggeration is ok). Dune worms are completely unrealistic without the magic they had.

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  • $\begingroup$ I was also thinking 'mars'. $\endgroup$ – user70311 Nov 15 at 11:40
  • $\begingroup$ Good way to do this is to have younger mars sized planet (~ 1-2 billion years old) that was terraformed and seeded with life. Shorter age gives less time for planet to lose its atmosphere. Especially, if the planet was terraformed and seeded with life by some ancient civilization, but was never colonized and left for ~ 100 million years, that might be setting that gives us a good enough planet with both thick atmosphere and weak gravity. $\endgroup$ – Failus Maximus Nov 15 at 13:35
  • $\begingroup$ It's probably not a good idea to assume things based on the exact breathing conditions of earth critters. The idea that a sci-fi worm has to have low CO2, has to process oxygen, and has to have an inert filler gas is needlessly restrictive. $\endgroup$ – Kevin Nov 15 at 21:04
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Here is a link to a post asking about the largest possible size for a giant serpent or worm:

What theoretically is the limit or max size and length for a serpent/worm before they get crushed by their own body?1

You might want to consider how big you want your sandworms to be, because I can believe there should be a size range where they could move through the sand or sand like substance in your world, and another size range where those sandworms could get enough food to eat, and another size range where those sandworms would not be crushed by their own weight like whales out of water. And you should hope that all three of those size ranges overlap to form a somewhat more restircted size range, and that it includes a size that you consider sufficiently impressive for your sandworms.

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