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...