Categories
Issue 18 Science (of) Fiction

Dune: how sand takes form

🕒 8 min

“Dune” is a powerful word, and a fitting title for the Frank Herbert book which has been all the rage recently due to its long-awaited new film adaptation being released. Dune is really an entire franchise set in a politically, socially and scientifically intricate universe thousands of years in the future. The eponymous “Dune” is a planet also known as Arrakis, covered in sand and wildly alien creatures, which plays a key role in the Duniverse. In fact, it happens to be where most of the new movie is set. There is something not quite so alien, though, that is related to Dune (the planet and the book) very intimately, but also happens to be one of its rare phenomena you can witness first hand here on Earth, without much of a stretch of imagination. The thing in question would be Dune‘s other namesakes – sand dunes.

Strangely familiar

Dunes occupy many places on Earth, not just deserts, which most of us probably associate them with. They are very much at-home in most regions where sand and wind get the chance to mix, though water can form them too. Sand is actually a very broad category of materials, classified as such based on grain size and not much more. Any fine-grain rocky particles within certain size limits, which are defined by international scales (or, more bluntly, convention), fit the name just fine, regardless of their chemical composition. This is a fine categorization, though, because grain size is exactly what makes sand behave the way it does – super weirdly. Large enough heaps of sand can be compact enough to sit or lie down on. Yet still, you can blow a small amount of sand from the palm of your hand and it will diffuse, almost how a liquid or even a gas would. When wind blows over sand, it can pick it up, but the particles quickly drop back down and bounce when they fall. This unique behavior is what allows sand to form dunes, and its various origins – from eroded rocks to ground-up shells – make it a frequent dweller of almost any corner of the globe.

However, while we may not be strangers to dunes, or they to us, they provoke a special kind of interest in people. They are strangely captivating in many ways, so much so that researching them inspired Herbert to write Dune in the first place. For a start, large fields of sand form dune oceans of sorts, which are just plain beautiful. Then there is the fact that dunes have the ability to migrate – not dying as the wind hits them, but rather evolving. Even though dunes resemble hills and cliffs, they are not mappable terrain because they are ever-changing. Dunes form as wind accelerates over a large heap of sand (which was probably caused by a strong gust of wind itself), taking some surface sand with it and depositing it at the top, as well as farther away. Sand is not fine enough to withstand a steep incline, so the top of a dune soon falls apart in an avalanche, leaving a cliff-like shape behind. This causes that characteristic crest, an asymmetry stemming from the inherent asymmetry of wind flow. If the winds are consistent, this depositing and avalanching has a preferred direction, which means the dune moves with the wind.

Migrating dune crests. Credit: Carlos H. Grohmann, Dune migration and volume change from airborne LiDAR
Upper left: Ridge formation in small heaps of sand.
Lower left: Sand circulation over and through a dune advancing without changing shape or size.
Right: Dune avalanching to form a crest and a slip face.
Credit: R. A. Bagnold

Dunes on other worlds

It would be curious to think about what sorts of dunes the winds on Arrakis would really form, or how the landscape would change over time. Even on Earth there are many varieties of dunes, usually caused by their orientation relative to the wind and how frequently the direction or speed of wind changes. Dunes can also interact with each other, as wind passing over a large dune clearly behaves differently than air flowing over an uninterrupted straight surface. If two dunes are close enough, the turbulence caused by the first will cause the second one to migrate faster until they’ve spread apart, almost as if they are repelling each other.

Due to Arrakis’ uniquely sandy surface, its prevailing winds might even cause some large-scale pattern. Even on smaller scales, the dunes of Dune would likely behave in strange ways. Dunes on Mars also behave differently from those on Earth, partly because the atmosphere is much sparser and its winds don’t have the same kick that those on Earth do. Martian dunes thus evolve significantly less dramatically, except in some areas, which allow for migration that could never be seen on Earth. This is because the surface is free of any obstacles and the winds change drastically because of the terrain and temperatures. Since Arrakis is, again, desert all around, its dunes could probably move quickly and without much interruption in most places, in ways that are unimaginable for their terrestrial counterparts.

Sandworm activity on Arrakis would probably result in a whole lot of static electricity, which adds another layer of interest. Electrified sand has been proposed as an explanation for the distinctive shapes of dunes on Saturn’s moon Titan, which seemingly form in reverse, against the wind. The idea is that Titan’s sands are too electrically sticky to get moved by its common, weak winds, but could have gotten picked up in a more powerful gust some time ago. In Dune, sandworms are even known to attract dry lightning because of how much static electricity they produce, so the effect might not be negligible.

Surfacing sandworm causing electric discharge on Arrakis. Credit: David Lynch, Dune (1984)
The reverse dunes of Titan. Credit: NASA

Fractal-like

Upper: Different behaviors of particles getting picked up by wind (so-called saltation, or bouncing).
Lower: The wavelength of ripple marks coincides with the range of a bouncing grain of sand.
Credit: R. A. Bagnold

Another reason for fascination is that, while dunes may look fairly smooth and elegant from afar, zooming in closer unveils intricate patterns, tiny waves of sand that cover the slopes. Wind blowing across a dune’s face does not smooth it out because sand particles keep getting picked up and dropped off elsewhere. They are so light, though, that they bounce and splash around when they fall – in a regular pattern, too. Their bouncing has a wavelength of sorts, dependent on the wind speed and the size and weight of the sand particles. As soon as more sand starts to accumulate in one spot, completely randomly, the sand flowing over it gets stuck in a feedback loop as it catches on the newly formed heap. More sand lands where there is already sand, and the regular bouncing pattern causes a shadow on the sheltered side and a higher chance of another heap forming an even distance away. The ripples thus spontaneously form surprisingly regular waves, in a process that mimics the formation of dunes.

Sand dunes and ripple marks. Credit: Gerhard Huber

Another beautiful thing to gain from those similarly pretty ripple marks is an example of how gases and liquids are both fluids. As we mentioned earlier, water can cause dunes to form too. Basically the same mechanism that happens on the faces of air-formed sand dunes goes on underwater too. You may have even seen ripples in the shallows of sandy beaches. Flowing water picks up particles from a bed of sand wherever it is especially shallow, and deposits it in much the same way that wind does. Underwater dunes and ripples thus move downstream, making them a frequent feature of estuaries, where rivers are at their shallowest and least powerful. Sometimes these can set and become much more permanent, especially if the flow of water starts decreasing for some reason. Because the shape and spacing of ripple marks also depends on the properties of the medium that caused them, one can learn a lot by simply measuring them. This is one of the ways we can learn how strong a river that used to flow along a long dried up river bed was, or in which direction.

Ripple marks on exposed sand bank at Camel estuary. Credit: Christopher Hilton

Sand dunes are seemingly as complex and fascinating as the book they inspired. There is so much to be said about Dune, the franchise, and its various components, enough to make a promising series (which I feel inclined to start at some point). From the question of how a substance like spice could enable not only interstellar travel but superluminal (or faster-than-light) travel, over the numerous types of sand found on Arrakis, all the way to whether such ornithopters could even fly or how paracompasses would function. Straying further from mechanics, biology could have a field day with the many challenges Arrakis poses for life. In fact, that potential for scientific exploration hasn’t been lost on people – there are entire encyclopedias on the science of Dune. I do hope that the new movie will inspire more people to give this fascinating universe a slice of their time, just like it inspired this post. If you do find yourself eager to explore, while it may be difficult to find your footing when becoming part of such a vast community of specially curious people, you can always take a step back and admire the dunes of your own planet – the memory of winds long gone, and the future of many to come.

Yes, you just read a post (mostly) about sand. Good job! What did you think? Tell us in the comments.


Sources

  1. Bagnold, R. A. The Physics of Blown Sand and Desert Dunes (1973)
  2. International scale for the grain size of granular materials
  3. Sand dunes repel each other
  4. Electric sand on Titan
  5. Martian dunes are weird
  6. Arrakis
  7. Sandworms

Word count

  • dune: 47
  • sand: 37
  • wind: 22

By Laura Busak

Laura is a physics student with a love for all things cosmic. She enjoys making and listening to music, reading books that make her think, and generally doing whatever random things she thinks of, often until 2 am.

Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.