Saturday 5 February 2011

Swimming in Sand 1: the Sandworms of Dune

Sand is an odd material. In some ways it is like a fluid, as you can scoop up a handful of it and let it run through your fingers. Still, hills and dunes of sand will hold their shape, which water cannot do. You also cannot stand on it, and if you are buried underneath even a few decimetres of it on a beach, you will find it is much more resistant to movement than water is. Some animals 'swim' in it though, both real and fictional ones. There seems to be enough material for more than post, so this one is just the first. To start, let's begin with SF's biggest sand swimmer.

Click to enlarge; painting by John Schoenherr

Click to enlarge; painting by John Schoenherr

This is Shai-Hulud, the sandworm of Arrakis, the spice planet! These paintings are by John Schoenherr, a brilliant SF illustrator who died in 2010. Sandworms started life in Frank Herberts's Dune, an SF classic. I loved it when I first read it, several decades ago. I think I read the original novel more than once, but cannot say the same for the sequels, of which I may have read two, or perhaps even three (I tend to dislike trilogies with more than three volumes).

Much has been written about just about any aspect of the Dune books, and the sandworms are no exception. In fact, there is an entire book devoted to 'the Science of Dune', and one chapter was devoted to 'The Biology of the Sandworm'. You can download that chapter for 1 US $ here. I liked it, as it explored sandworm biology by comparing it with Earth biology. Sybille Hechtel, the author of the essay, had this to say about sandworm movement: "Herbert never describes precisely how the worm moves, only that it looks like a fish that 'swims' just under the surface. He frequently describes the worm’s motion in sand as 'a cresting of sand,' or mentions the 'burrow mound of a worm'. The worm primarily comes above the surface when it’s eating a ’thopter or crawler, or when the Fremen catch one and put their hooks in its scales to drive it up out of the sand." Well, that means there is no information whatsoever in Dune about how sandworms move. Starting with the word 'worm' there seem to be two major Earth modes of movement to compare it with.

Earthworm movement; click to enlarge; copyright Gert van Dijk

The first are earthworms. These move in an ingenious way, as the segments of their bodies can change shape: when they are short and wide, they press against the soil and anchor themselves to the soil. Any segments behind them that are long but narrow can be pulled up by shortening them in turn. The beauty of the system is that any segments in front can be pushed forwards by narrowing them. In this way, shortened segments function as anchors for thinner segments in front and behind. I have no idea whether or not sandworms are supposed to move like this, but one thing is certain: to move forward, earthworms have to force the soil aside to make room for their body.

Undulation; click to enlarge; copyright Gert van Dijk

The second way is by undulation, that way snakes, eels, and lots of Earth fishes move. the body moves in bends, and each bend pushes against the material around it. Those forces have sideways (blue) and backwards (red) components. The reaction forces of the material push the animal forward. Again, one thing is certain: whether in water or soil, the animal has to push that material aside to make room for its body in front of it.

So however sandworms move, they have to make room for their bodies. There are only two ways to do that: if the stuff around you is compressible, you can compress it, and otherwise you have to shift it. Water is almost not compressible at all, and sand doesn't compress well either. That leaves shifting it, and the only direction to shift it in is upwards, which sounds like a lot of work. Moving aside water is fairly easy; the fact that the largest animals on Earth, whales, live in water proves this. But the energy costs of shifting sand upwards are probably much higher, something that will probably recur in future posts.

Sandworm scale; click to enlarge

One thing is certain: more sand means more work. So how much sand must a sandworm shift? Well, the biggest sandworms are supposed to be 400 m long with a 80 m diameter. That is big. Very big. In fact, that is longer than a US aircraft carrier (only 333 m) and in the same league as the biggest super oil tankers (458 m). The Wikipedia page on tankers had a very nice scale drawing showing that tanker as well as some of the world's largest buildings, to which I added two sandworms: a puny 200m one and a fully grown 400 m one.

The clip above (taken from YouTube) is from the 1984 film adaptation, and nicely conveys the scale of the worm. Also look back at the paintings and look at the humans, giving a sense of scale. If such an animal is to move one body length forward, it will have to displace the volume of its body in sand. Assuming a completely cylindrical body, the volume of a 400 m sandworm comes out at a bit over 2 million cubic meters. I found figures for the mass of various forms of sand. The lowest density concerned loose sand, so let's use that. One cubic m of loose sand has a mass of 1442 kg, so the mass of the sandworm's volume in sand is 2884 million kg.

I hope these numbers convey the nerve Herbert had in thinking up such a monster. I must say that they impressed me. Regardless of whether a sandworm moves like an earthworm or undulates, displacing all that sand is serious work! Aircraft carriers run on nuclear reactors, and supertankers take ages to turn around. That is in water, a much more forgiving medium than sand. Simply pushing against sand with big muscles isn't going to do the trick. In this respect it is intriguing than no illustration of a sandworm I've ever seen shows anything in the way of a propulsion system. No limbs, no bristles, no segmental thinning, nothing. That goes for paintings well as the film and TV adaptations. Perhaps there are workarounds...

  • One solution would be to give Arrakis, the sandworm's planet, a very low gravity: moving the sand would still require overcoming its inertia, but at least it would weigh a lot less. But reducing gravity has other effects: humans on Arrakis could jump around a lot, but the story doesn't mention that, so that's off.
  • Perhaps the sand could be made lighter? The website I mentioned above states the density of lots of materials. Powdered carbon only masses 80 kg per cubic meter, and pulverised kaolin (china clay) weighs in at only 352 kg. Better, but still...
  • Perhaps the friction between grains of sand can be reduced, but you wonder whether that is possible while still allowing people to walk around on sand.
  • Are sandworms hollow cylinders? If so, then the sand inside their bodies could more or less stay in place. That would reduce the volume needed to shove aside. Perhaps their large size is a way to enlarge area without necessitating a large volume; who knows. It would not do wonders for friction though.
I am afraid that the only way out is to use a lot of 'handwavium', which basically means not worrying about the impossibility of sandworm movement. After all, suspension of disbelief is what keeps SF going. Does this make Dune a silly book? Not really, I think. I liked it very much when I first read it, and countless others did and still do. I only get irritated when authors or film makers pretend to be accurate and then blatantly are not. It is like the difference between someone who says he can bend spoons with psychic powers and someone who won't tell you how he does his tricks but acknowledges that they are tricks. Sandworm movement is such a trick...


j. w. bjerk said...

Some of the later books mention that sand is burned up by the worms internal (and very hot) metabolism -- possibly implying that ingesting a certain amount of sand is a normal part of it's movement. That lessens the displacement problem a little, but still as you say, massive amounts of handwaving are needed.

Anonymous said...

I like in the _Science of Dune_ it said, "if i had to describe how such a creature moved, I too would say as little as possible"...or words to that effect.


Evan Black said...

It's an important thing to note in literature that particular stories have focus on particular subject matter, often at the detriment to incidentally related plot elements. I'm sure some of you have read thrilling spy novels that have several technical inaccuracies, or historical fiction that just plain gets some things wrong. As you mentioned here, suspension of disbelief is necessary with much of fiction, and with SF in particular. Dune is an excellent book, not for its clever but "hand-wavey" ecology but for the political intrigue of the story and the abstraction-- and subsequent analysis-- of familiar historical tropes. When a Sci-Fi story misses out on a few scientific details but tells a thrilling good yarn, it's forgivable; when a story slaps a cliche plot around an idea they want to showcase that contradicts confirmed scientific findings in the first place, that's what we call "bad Sci-Fi." I'm glad to see the distinction made here, and the plausibility of Herbert's sandworms critically explored while still doling out respect to the man and his work.

This may be somewhat tangential, but I would feel remiss if I didn't point out that there are some trilogies or franchises labeled as Sci-Fi for the sake of the masses, but in fact fall under another-- and less scientifically stringent-- category of science fiction. We should be willing to recognize such works for what they are and apply appropriate levels of "handwavium."

Sigmund Nastrazzurro said...

J.W, Rodlox and Evan,

You all seem to agree that sandworms are physically impossible. I wondered whether people would take offence at me attacking this aspect of Dune. You did not, which may mean that you did not hold the improbability of sandworms against the credibility of the story. Evan describes that aspect of SF very well, I think.

I deleted a few sentences in my post; they described the possible cover for Dune I painted ages ago. It did not show sandworms at all, just a man (Paul Atreides) on a sand dune. That reflected my sentiment that the novel was primarily about people. Perhaps I should dig it out, photograph it and show it here.

j. w. bjerk said...

That a sandworm couldn't remotely work under our current understanding of physics is pretty clear. But i don't think Sigmund is criticizing Dune as a story for including biologically impossible things.

This blog is about the mechanic of biology, and applying that understanding to theoretical creatures. It is often fun to use well-known creatures from fiction as examples to test against the science. But i don't think anyone is trying to make the point that books or movies that play fast and loose with biological science are necessarily inferior or bad.

Anonymous said...

i ask my self
must the Sandworm work like Earthworm true the sand ?
wat if it drills true it, or even eat true the sand !
in last case the worm push roughly 2884 million kg sand
true his digestive tract...

Michel Van

Sigmund Nastrazzurro said...

About the 'health quotes' comment:

This is an example of 'spam blogging': someone copied part of Evan's commentary to make it appear as if he or she had something worthwhile to contribute. Such 'contributors' aim to direct traffic to their own sites. With Google blogger that does not really work, but people do it anyway.
I will delete all posts that contain links to irrelevant sites; I will keep this one in place for a day or so.

I understand that you wish to circumvent the problem of shifting immense volume of sands, but drilling won't help: when drilling, you create a hole by displacing the material there was before. As for eating sand, well, that would probably be eating something inside the sand, not the sand itself, I guess. But as you wrote, it still involves millions of kg of sand...

Evan Black said...

"Eating the sand" seems a lot like Sigmund's "hollow body" theories presented in the original post. Rather than shove the entirety of the sand out of its way it allows a percentage of the sand to pass through it. I don't want to think about what so much abrasive sand would do to an unprotected gastrointestinal tract, and the adaptation to protect against such abrasion and friction would no doubt lower the (for lack of a better word) absorbancy of the digestive system.

Even if the worm could allow great quantities of sand to rush through its gut it still doesn't solve the problem completely, as there is still a great deal of mass to move, not only the worm itself but the sand in front of it. The energy required is mindboggling, as Sigmund has illustrated. We're best just lacing the sand with density-lowering handwavium and just suspending our disbelief in watching the majestic Shai-Hulud tunnel its way through our imagination.

Anonymous said...

So far i remember the Book "DUNE"
Has Arrakis desert a complex ecosystem
with sandplankton, Sand Fish and Sand worms
who froms tha Wale Analog for this
so its look logical that Worm eats true the sand
to nourish from sandplankton

-Michel Van
p.s. I refuse to use OpenID or Google Account
or Name/url stuff, wat i consider as misapplication.
I prefer the Anonymous function.

Anonymous said...

I do not know if something like sandworm is possible, but I hope that next post will say more about it. I think that there are more than one way how to move through sand (some kind of cilia?) and nothing is known about the metabolism of sandworm so... how close could we go to the realistic shai-hulud?

Dominic Brown said...

Everyone may wish to have a look at some recent work on swimming in sand:

Seems that snake-like sine-wave swimming is perfectly workable, and of course a larger creature would have greater strength in proportion to the greater amount of sand it has to move. So sandworms are not obviously impossible.

At a glance, though, the sand-swimming lizard is disturbing a volume of sand-analogue out to several times the radius of its body, and about three times the depth of the sine-wave its body makes. That suggests a huge energy cost for something the size of a sandworm, and it’s by no means clear to me just where shai-hulud is supposed to get that much energy—what exactly does it eat, that supplies so many calories, and is found in sand at a high enough concentration?

In fact you could do some quick calculations based on the energy dissipated by pushing each kg of sand around, and work out whether the sand would need to be pure sugar to supply enough energy. I’m doubtful.

As others have said, the sandworm is a bit of fictional hand-waving easily worth it for the symbolism and imagery. But a novel of ecology should at least have hinted at its diet, surely?

Sigmund Nastrazzurro said...

Hi Dominic, part III will be about living and robotic real sandswimmers...

Alex said...

Just a quick note that if you read further in the series Arrakis is Earth, so same gravity, at least. But this is far into the future, and there's no point in comparing current concepts of science and biology to whatever the sandworms are. There's plenty about them which are so different from what we know about worms (like it's incredible internal heat). In addition, the label "worm" can't be taken for whatever we use the term "worm" for now. Lots and lots of "worms" aren't worms at all, just like the Mantis Shrimp is neither a mantis nor a shrimp.

As far as I can recall (been a while since my last run through the series) the worms are both evolutionary and genetically modified things. Electrical storms follow them, so why not their hull having a strong electrical force field that simply force sand off it in all directions, making swimming through sand much more possible?

Also, I think I recall Leto II talking about electric fields running along its thick skin in addition to some extraordinary strength. Combine them, and the shifting of the sand through "swimming" (however that is defined) is at least plausible.

Sigmund Nastrazzurro said...

Alexander: well, I stopped reading the novels after the first three or s, so I did not even knew that Arrakis was Earth. But I do not understand how a strong electrical charge would help the sandworms shift all that sand. Should the sand also be charged?

Alex said...

Mayhaps, but the main point is simply that the electrical field around the worm will shift it free from the sand; sand (as in silica) is conductive and will react to the electro-magnetic field. As to the details of how that would work, I don't know, but that the worms have intense heat inside and an electrical field is certainly canon.

Unknown said...

First I just feel I should point out that Earth is not Arrakis. In the book Dune: Butlerian Jihad, Earth and Arrakis exist at the same time as 2 very different planets.

Second I don't remember any mention of the worms themselves having an electrical field, rather the electrical storm/worm sign were caused by the shifting sands displaced by the movements of the worm. I could be mistaken. However I do remember there was an intense heat within the worms.

My thoughts on the movements of the worms are that they move similar to earthworms rather than undulating because they are segmented. In the books worm riders use "maker hooks" to hold open segments to prevent the worm from submerging beneath the sand.

As far as the effort needed to displace the enormous volume of sand more than just the density needs to be considered. Granule shape has a significant impact on resistance. Sand with a more cubic/sharp angle shaped granules gives much more resistant than more round/smooth shaped granules. Considering the massive sandstorms, monstrous "Coriolis storms", and explosive spice blows, I think its safe to assume the sands of Arrakis are weathered smooth providing less resistance to the sandworms.
The approach of sandworms was accompanied by the hissing sound of shifting sands as well as a deep rumbling tone. Intense low frequency sounds can disrupt the density of tightly packed granulated solids, making them flow like water. Sandworms could possibly be emitting a low frequency sound to lower the resistance against their movements. When their heads crested the surface it was said they roared, producing a deafening sound.

Great article.

Sigmund Nastrazzurro said...


Thank you. On whether or not Earth and Arrakis are the same, that is a matter I will leave to Dune saga experts.

As for the effect of granule shape, I had wondered about that too. But is the friction is decreased to let sand act more like a liquid, it might become difficult to walk on it. I suppose artificial 'sands' with perfectly globular grains have been made; those might be worth looking in to.

Sovrin said...

Water is 1000 kg per cubic meter. The sand is essentially a low friction ballpit, rounded from the constant friction of wind and animals. A decent chunk of the total volume of the sand is air since the sand is mostly dry. Both undulating movement and segmented movement make sense here. Yes it is very intense as far as muscles go, but sandworms are huge and clearly very muscular. Smaller ones are smaller so less sand needs to be moved. What confuses me is energy. Where does the ecosystem get so much energy? Solar power is out. Everything is underground. Perhaps infrared photosynthesis? The planet has a moon which means tidal heating.