Sunday 20 February 2011

Swimming in Sand II: some facts, more fiction ('Tremors' & 'Primeval')

The previous post and the appendix dealt with Dune's sandworms, and how difficult it would be to getting millions of tons of sand out of the way to make such animals move. While the hard-boiled scientist in me rejected them as utterly impossible, the somewhat more romantic SF fan, also inside me, decided not to care one bit.

The size argument probably means that any real sandswimmers, by which I mean those on earth and in serious speculative biology projects, had better be small. Are there any other limitations on what they can do? Can sandswimmers dive as deep as they like?

I searched for an answer by looking at the physics of soils. Not surprisingly, the pressure in sand or another type of soil gets deeper the deeper you get, the same as pressure increases with depth in water. Typical soils contain not just grains of sand, but water as well. That complicates matters because there are separate pressures of the grains and the water between them. Luckily we can forget the water component as we are dealing with pure, dry sand. Comparing the effects of depth in water and sand reveals that sand isn't really a fluid.. They have in common that pressures increases with depth, and more so in sand, as it is heavier. But in water the extreme pressure at the bottom of an ocean trench does not stop fish from moving at all, whereas I imagine that movement is impossible under even two (maybe even one) meters of sand. While this seems intuitive, I found no readily available physical explanation. I suppose it has to do with the internal friction between grains of sand.

Click to enlarge. From: Baumgartner et al. Investigating the Locomotion of the Sandfish in Desert Sand Using NMR-Imaging. PLoS ONE 3(10): e3309. doi:10.1371/journal.pone.0003309

Loosely packed sand can be shaken, and then it sort of acts like a fluid. The image above is in fact from a study on sandswimming lizards, in which the authors speculated that decompacting sand might make it more fluid.

Click to enlarge. Copyright Gert van Dijk

But simply adding more sand on top would cause the sand to become impacted, and then no amount of shaking is going to create spare room. That is what is shown here. If something (a sandswimmer!) starts pushing a number of grains upwards, an entire column of sand above needs needs to be shifted. In fact, initially that column might look more like a cone, until the grains start shifting. At any rate this is real work.

What all this suggests is that pressure limits swimming in sand to superficial layers. A consequence of that would be that you would see the surface move when the swimmer is passing. It does not seem likely that sandswimmers can swim up to a potential victim without that victim being aware of the sand apparently moving of its own accord. Obviously, if the sandswimmer is a herbivore, any plants it is creeping up on wouldn't notice that. But sandswimming predators that wish to devour surface-walking prey had better do so from an ambush. Having said that, let's have a look at more fictional sandswimmers.

'Tremors' is a film about monsters ('graboids') that beleaguer a small community in a desert environment. The graboids swim under the sand at an amazing speed. As you can see from the video clip, they do break the soil above them, so the filmmakers gain some points for that. The speed at which they move is ludicrous, I am sorry to say. I couldn't see how the animals move: no bristles or legs or anything else I could find. The scene in which the graboid crashes into a concrete wall is wonderful from a story-telling point of view, but less so from a biomechanical point of view. It's not that the animal wouldn't get hurt, hitting concrete at that speed, but that the packed sand it moves through with ease would in reality provide a similarly unyielding obstacle. Anyone who ever took a failed dive and belly-flopped on water knows how hard water can be when struck at some speed. Now take the same dive on sand: there wouldn't even be a dent in it. The same principle is at work with sand sacks: they stop bullets. So I am sorry once again, but graboids are not feasible as shown. As was the case for Dune, I do not think that this harms the enjoyment of the film in any way. Apparently, the needs of fiction outweigh the needs of facts (Mr. Spock might have said that, but I did). Luckily 'Tremors' has a lot of humour in it, so I feel the same about it as I did with Dune.

The scene above is from a British television series called 'Primeval' in which time portals come into life now and again, allowing past and future wildlife to blunder through and run amok in England. In this scene a villain has captured some Silurian scorpions and uses them to terrorise a beach. The scorpions are a lot bigger than they were in reality and are supposed to be excellent sandswimmers, with lots of long limbs sticking sideways. Hmm; that can't really help their sandswimming abilities. When they approach, sometimes you see the sand move, particularly when the scene require drama to be built up, while at other times the scorpions strike without causing as much as a ripple on the sand.

There is a delectable irony in the Primeval scene. A man has been buried in the sand by his children. This is what people do at beaches, and if you search Google or YouTube, you will soon find examples of people being unable to free themselves, especially when the tide comes in: one wave of water can undo a minute or two of frantic digging by bystanders. In the Primeval scene, the buried man cannot free himself, and through this nicely illustrates that humans are very poor at sandswimming. And then the scorpion swims in, from deep beneath the ripples, and sucks the poor man under. Unfortunately, there is nothing about a man-sized scorpion that would make it a better sandswimmer than its victim. I guess the reasoning was once again that "The needs of fiction outweigh the needs of facts".

Saturday 12 February 2011

Swimming in Sand 1a: Dune appendix

Strictly speaking this post has no exobiology in it, so has no place here. But I could not resist digging up the only painting I ever did of an existing work of SF, and that was Dune, the subject of the previous post. So here it is. The painting is (shockingly) old, but the lettering is all new.

(Until now New Hades Publishers specialised in nature books -have a look at the Furaha site-, so this must be a new approach for them.)

Click to enlarge; copyright Gert van Dijk

Why no sandworms? As I said before, for me Dune was about people foremost, and sandworms second.

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...