Dec. 16th, 2015

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A writer friend who has mostly focused on heroic fantasies is switching focus to write a space opera and wondered whether it’s possible to create plausible, planetary-scale dust storms, and what the fictional consequences might be. That inspired a few thoughts on the subject based on my academic background in ecological science and the research papers I’ve been editing for the past couple decades for Chinese desertification researchers. There will be obvious echoes of Frank Herbert’s Dune in this essay, some implicit and some explicit.

For what it’s worth, if your only experience with Dune is the David Lynch movie -- something so appallingly bad and distasteful I feel the need for mental mouthwash even thinking of it -- you owe it to yourself to read the actual novel; it’s good, and in places, very good indeed. Possibly stop after the first novel; the quality slides rapidly downhill from there. Haven’t read his son’s subsequent novelizations, but a few excerpts I’ve seen don’t bode at all well for their quality.

A caveat before I begin: First, I will not even come close to touching on all the potentially important aspects in this essay. Thus, consider it woefully incomplete, and it will remain so until it’s expanded to textbook length. Someday. Second, although I have tried to stay within the limits of what I know, I am not an expert on many of the subjects that I’ll touch on. Thus, treat what I’ve written as a decent first draft of a fuller and more accurate description. The final draft will have to wait for the aforementioned textbook. If you need the *ahem* nitty-gritty details of these subjects, you’ll need to hunt down a real expert to obtain those details.

Nothing to fear but dust itself... and sand

A couple brief etymological notes before I begin: “Dust” (the really small stuff) and “sand” (the less-small stuff) are often used interchangeably in words such as “sandstorm”. This is because a significant storm will transport large quantities of both. Lighter particles will be transported by the first significant winds, and as the wind intensity increases, larger particles will be transported. I’ll use “sandstorm” promiscuously to describe all such events, but will try to use “dust” and “sand” appropriately for the specific particle sizes I’m discussing at any point without recomplicating things. The process of transforming quiet surface particles into windblown particles is called “entrainment”; the heavier the object, the stronger the wind that is required to entrain it (i.e., to carry it along for the ride).

First, let’s consider the overall special effect one might be striving for. Mars is famous for its planet-covering sandstorms, and you can’t do better than NASA’s The Fact and Fiction of Martian Dust Storms Web page for a primer on the subject. These sandstorms are known to cover continent-sized areas quite frequently, and to cover most of the planet occasionally (about every 3 Martian years according to NASA). One unique thing about Martian sandstorms is that the atmospheric density is only about 1% of that on Earth, so the wind doesn’t exert much pressure; pressure results from the joint action of many atoms or molecules, and this means there’s not much pressure in a Martian wind. Furthermore, Martian winds seem not to reach the extreme velocities we see here on Earth. Taken together, these two bits of trivia mean that Martian winds are unlikely to be strong enough to propel particles that are big enough to cause significant damage to hardened structures or even to soft humans in vacuum suits. Even when a Martian wind is moving fast, there aren’t enough air molecules to generate significant thrust.

A consequence of this problem is that most of the material transported by Martian sandstorms will be dust rather than sand or gravel. That dust won’t carry much of an impact, but it will work its way into every exposed nook and cranny of things like a space suit or vehicle. Worse, it is likely to adhere electrostatically to every surface. When you break sand and larger things (gravel to rocks) into fine dust particles, you break a lot of bonds between particles. The surfaces of the particles are likely to develop negative or positive electrostatic charges*, depending on their mineral composition. Removing the adhered particles from a surface is likely to require more than just brushing the particles with a broom; without first neutralizing that charge, the particles are likely to spring back into place. Water works, but on the kind of desert planet most likely to generate ongoing sandstorms, you probably won’t have a lot of water to spare. In any event, a lot of cleaning will be required.

* This is related to a property called the “cation exchange capacity”, which represents the number of sites on a particle’s surface that can bind to free-floating positively charged atoms or molecules. All broken surfaces have such sites, but because there may be hundreds or thousands of dust particles for each grain of sand, the finer particles have a correspondingly higher capacity per unit volume. This capacity is important for soils used to cultivate crops, since it determines the quantity of nutrients such as potassium, calcium, and magnesium the soil can retain until plants take them up.

In some cases, various chemical properties of the dust will create additional problems. For example, Martian regolith (immature soil) seems to have a high perchlorate content. Perchlorates are chemical groups made from a single chlorine atom and four oxygen atoms. They don’t mix well with living things, since they become perchloric acid (a strong acid) when dissolved in water. Even in the absence of water, perchlorate is a strong oxidizer (i.e., it’s eager to donate its oxygen to or steal electrons from other substances), and will therefore be highly corrosive to many substances. If you want to simplify your description of sandstorms, best avoid planets with soils that have high contents of perchlorate or other substances that would not usually be present in large quantities unless you’re willing to explore the consequences.

If you want a planet where people can walk around in normal clothing, yet where sandstorms can be problematic, we can turn to Earth for an example. Earth has many dry regions that are likely to create sandstorms. Particles from African desert sandstorms blow annually across the Atlantic in clouds dense enough that they sometimes block the sky, and parts of these clouds reach as far as North America and South America. You can see a fairly spectacular picture at NASA’s Dust storm sweeps from Africa into Atlantic Web page. Similarly, dust from sandstorms in northern China sweeps across the Pacific annually during the prime sandstorm season, as described in NASA’s The Pacific dust express Web page. The quantities of dust transported annually are huge, and they have important consequences for air quality (e.g., pollutant transport). They also increase the fertility of large areas of the ocean (which is relatively nutrient-poor compared to land) and of land where the dust falls to earth -- or is washed out of the atmosphere by rain.

An important difference from Mars is that Earth’s atmospheric density is high enough that significantly large particles can be “entrained” by the wind and carried at high velocities. If you’ve watched the annual hurricane reports on TV or the Web during the Caribbean hurricane season, you’ve seen just how powerful terrestrial winds can become. My brother lives just north of Miami, and every year he has to put up massive storm shutters to protect the windows; one year, a window was shattered through the shutter when a windblown coconut struck the shutter with the force of a cannon shot. There are stories of pieces of straw (dried grass stems) being embedded in a tree by storm winds; the cheerfully nihilistic investigators at Mythbusters confirm this to be true, though perhaps not as exaggerated an effect as reported in some tales. This means that both humans and human structures will need to be armored against windblown projectiles, and if there’s a significant sand source upwind, they should be highly resistant to abrasion. Imagine having several miles of coarse sandpaper run rapidly across your skin for an hour or more and you’ll understand why you really don’t want to be caught out in a severe sandstorm. After all, there’s a reason they use sandblasting to clean building surfaces down to the bone.

Even leaving aside the issue of abrasion or impacts, significant sandstorms can bury fields of crops and other human structures. China invests remarkable amounts of money annually on engineering to protect people and infrastructure from sandstorms. People living on a sandstormy planet would have little choice but to learn from the Chinese. The Dust Bowl in North America during the 1930s is a good period to study if you want to learn about the consequences when ill-considered agriculture meets climate change and to mine those accounts for story material. Or perhaps just wait a few more years and see what happens to the American Midwest if current farming and climate trends continue.

Atmospheric circulation patterns

To provide a realistic description of how wind interacts with surface sediments to create sandstorms, you first need to know a little bit about the effects of wind. This is not a trivial subject; some of my Chinese clients have been studying these phenomena for decades, and each new study reveals how much more they and other scientists have yet to learn. Since this article is written from the perspective of supporting fiction, I’ll keep to the essentials. But if the subject fascinates you as much as it fascinates me, you’ll want to pick up a good book on geology to learn more details about the interactions between winds and surface materials.

First, let’s start with global-scale effects, since the subject that inspired this essay was global-scale sandstorms, and use familiar examples from Earth. The first thing to understand is that on a planet with a reasonably dense atmosphere, convection becomes a highly significant process. Convection occurs when air at ground level is heated by the sun, becomes less dense, and rises above the more dense surrounding air; as it rises, it cools, becomes more dense, and falls again to the ground. At a local scale, this is part of the process that leads to thunderstorms. At a global scale, heating is greatest at the equator, so that’s where convection is most intense. This convection creates three large latitudinal bands between the equator and the poles, each with its own convection patterns, and the boundaries between these bands are defined by the positions of the rising and falling convected air. Although large bodies of air can certainly cross these boundaries, much of the circulation occurs within (rather than between) the bands. Wikipedia’s Atmospheric Circulation article provides a good explanation and illustration of this phenomenon.

At the same time, the planet is spinning beneath these bands. You can imagine the effect if you go to the local playground, climb onto the merry-go-round, start it spinning, attach yourself to a rope, loosely wrap the rope around one of the bars, and then jump into the air; the surface continues rotating, carrying the rope with it, whereas your momentum is perpendicular to the rotation direction, causing you to fly off that surface and possibly collide with something. In effect, the part of the rope attached to the merry-go-round keeps traveling in a circle, but you don’t, and this causes the rope to twist. In a similar manner, at least by analogy, air masses still touch the surface of the rotating Earth, so their bottom gets dragged along with the surface due to friction while their top gets left somewhat behind, causing the air mass to twist.

The latitudinal bands of air circulation dominate global air circulation patterns, and therefore affect wind patterns and how they transport dust on a global scale. These patterns affect all kinds of story-relevant things, including prevailing wind directions and the strength and frequency of major storms, such as hurricanes. The Wikipedia article on Hadley cells will provide additional grounding on these effects. But winds and climate are more complex than this simplistic description; for example, local obstacles such as mountains and local heat sources such as cities and large bodies of water can act on the patterns at smaller scales. Thus, treat my description as only the starting point for your explorations.

Next, let’s look at smaller-scale surface effects. First and most obvious, there’s the law of physics that says “what goes up must come down again”. As I noted earlier, sand and dust transported by the wind eventually get dumped back on the surface. This means we can’t ignore the final destination of the materials transported by sandstorms. For example, unless the storm is eternal, it will eventually deposit its burden to form sand sheets and sand dunes, which have bewilderingly diverse and beautiful morphologies. Moreover, each evolves in its own way -- yet another of those “infinite diversity in infinite combinations” things that Nature does so well. From a story perspective, several factors must align before you create sandy deserts, their dunes, and sandstorms: wind strong enough to cause erosion, a supply of erodible sediments (river sediments, dried lake bottoms, exposed soil, etc.), and factors that make those sediments vulnerable to erosion (e.g., low precipitation, high evaporation, and low coverage of the ground by vegetation*). In northern China, a combination of climate change (a warming and drying climate) and unsustainably intense human activities (agriculture and grazing animals) have combined to create one of the most severe desertification crises in the world. China’s not alone in this problem, and when you consider your story world, don’t neglect the effects of human activities on the sandstorms.

* Water creates cohesion between particles, helping them to resist the wind. This is particularly true if it dissolves salts that form a glue of sorts that binds the particles together, forming what’s called an inorganic or salt crust. Evaporation removes water from the soil, either weakening cohesion between particles or creating a salt crust. Vegetation stabilizes soil by breaking the force of the wind, thereby reducing its ability to entrain particles, but also captures blowing particles. The net effect of all these factors acting simultaneously is, predictably, complex.

Plants and animals

Here’s our first explicit mention of what’s been called the “Dune problem”. First, some context: For a planet to have significant amounts of oxygen in its atmosphere, current consensus suggests that it will need large amounts of plants or comparable organisms capable of releasing oxygen.* On a hypothetical alien world, you can choose something more exotic to produce the oxygen, but whatever that may be, it will most likely need involve extraction of oxygen from water, which is likely to be the most readily available oxygen source in most habitable worlds**. These organisms can range from extensive areas of forest and grassland (an easy, familiar solution) to seemingly barren areas that nonetheless contain a surprisingly high level of cryptophytes, which are tiny, inconspicuous plants that hide in places you’d never expect to find plants. Such places include the biological crusts that form in the soil surface when microorganisms bind the particles together with their metabolic byproducts. They also include -- surprisingly -- rocks, as in the case of endolithic species that survive beneath the surface of rocks, between the grains. Endoliths are particularly interesting because they must live deep enough in the rock to be protected from harsh environmental conditions that would otherwise kill them, but close enough to the surface that light will penetrate to allow them to photosynthesize.

* Physical processes that liberate oxygen undoubtedly exist, but the problem is that free oxygen is highly reactive; it combines with just about anything it touches, becoming chemically unavailable to support life. Thus, it must be renewed on an ongoing basis to prevent the atmosphere from becoming oxygen-depleted. Purely nonbiological processes seem less plausible as a source of oxygen. You’d probably have to invoke some reasonably whacky geochemistry. For example, large deposits of exposed platinum (a powerful catalyst) might do the trick if you could create a mechanism for moving inorganic oxygen across the catalyst. In fiction, if not in reality, this could perhaps be done through sandstorms that are a plague upon the land, but that nonetheless are essential to provide oxygen for the atmosphere. Photolysis (the breakdown of water and nitrogen oxides by intense light) is another possibility, but it’s a slow process and not particularly important to the big picture. For it to be important, you’d need to propose an alien race who tuned the emissions of their sun to optimize photolysis. Which would be interesting, albeit implausible.

** Here, I’m assuming plain vanilla biology, which (to the best of our knowledge) requires oxygen and water. That system is simply the most efficient we know of. Anaerobes, of which there are many, can live without free oxygen and sometimes without any oxygen, but they are not energetically efficient for complicated reasons related to chemistry. To the best of our knowledge, there are unlikely to be environments populated by thriving communities of higher organisms that have based themselves on biologies other than oxygen-fueled carbon-based aerobes. The problem is rooted deeply in the laws of thermodynamics and the chemical properties of the only elements sufficiently ubiquitous to support life. Metabolic reactions and biosynthetic reactions simply wouldn’t be very efficient for breathers of (say) methane or ammonia that rely on a silicon-based metabolism (rather than carbon-based).

The aforementioned problem with Dune is that Frank Herbert’s description suggests there were essentially no plants on the planet’s surface; thus, there would be no organisms to produce oxygen and no inputs of organic matter into the soil to sustain organisms such as soil bacteria and fungi, let alone the giant sandworms the planet is famous for. Since Herbert would not have known about endoliths and probably did not know about biological soil crusts, his planetary ecology simply couldn’t work as described. (You could, perhaps, speculate that the giant sandworms processed oxygen-rich minerals into forms that provided sustenance, releasing oxygen as a byproduct. Possible, but still a large logical leap of faith because such minerals would not provide any usable energy to power the sandworms. Like Earth whales, they'd need to consume large quantities of smaller organisms.)

Ecological consequences

Let’s make the literary assumption that sandstorms are sufficiently frequent to be natural parts of a planet’s ecology rather than rare phenomena invoked solely for the purpose of a momentary plot diversion. This has many consequences for the planetary ecology.

First and foremost, all living organisms must have evolved a means of surviving these recurring natural disasters; indeed, what we know of terrestrial ecology suggests that many species will evolve in ways that let them actively benefit from these events at the expense of other species. This adaptation is most important for the plants, since these are the primary producers in most ecosystems; that is, they’re the organisms that capture energy that is then distributed throughout the rest of the food web. Thus, a routinely sandstormy planet will require highly sand and dust-tolerant vegetation, which is likely to have some or all of the following features:

  • It will need to be sufficiently robust that it can shed the heavier sand particles before it is crushed. A good Earth example would be something like Norway spruce, a tree whose branchlets trail downwards from the branches to help them shed snow before the weight snaps the branches.

  • It will need to be self-cleaning to shed the dust that will block the light it requires to survive; if not, it will need to store sufficient nutrient and energy reserves that allow it to stop photosynthesizing (or switch to an entirely different metabolic pathway) for days, weeks, or even months until full light intensity is restored. Sandstorms are typically not followed by large rainfall, since the climate that produces sandstorms tends to be dry. An Earth example might be something like the mimosa “sensitive plant”, whose leaves collapse when touched. Alternatively, the plants might have leaves that tremble easily in the wind, and thereby shed the dust.

  • Dust particles are, by definition, small, and therefore have a high surface area per unit volume. The minerals that comprise the dust have certain chemical properties, including the fact that most develop a surface electrostatic charge that causes them to cling to a surface. To shed such dust, plants would need to develop a mechanism to cancel that charge, unless they can wait long enough for rain to wash away the dust.

  • If the dust is at all acidic or a powerful oxidizer, as it’s likely to be on Mars, the plant will need a powerful acid neutralization mechanism. The problem isn’t just that acids and oxidants are caustic; they also strongly affect nutrient dynamics, cell membranes, and the chemical reactions involved in normal metabolism.

  • Plants that survive will have fast growth or other mechanisms to survive burial. (As noted above, plants block the wind; this tends to cause deposition of windblown particles around the plant.) A plant that is buried is cut off from the sunlight that it needs to survive. As a fun survival mechanism that doesn’t involve fast growth, consider the possibility of alien plants that have a mechanism for retracting their roots and squirming upwards through the dust until they reach the light and can re-extend their roots.

  • Dissemination of seeds during sandstorms will also be ecologically important. Since sandstorms require a large source of wind energy to move dust and larger particles, they will also provide enough energy to move seeds. Thus, it would be reasonable to expect that many plants will have evolved to harness this energy; think of the familiar dandelion seeds or the less familiar “tumbleweeds”, for instance.

  • Plants that can survive burial by shedding the sand and dust and by outgrowing the rising soil are likely to find themselves in highly rich soil, as in the case of China’s extensive loess deposits. The soils of such regions are deposited over the millennia, sometimes reaching depths of tens of metres. Because the particles are so fine, the soils are chock full of important nutrients. This is because the soil particles often have a high cation exchange capacity, as I noted earlier. The flip side is that such soils are highly vulnerable to erosion, and if the plant cover is lost, they can disappear rapidly, leading to severe sandstorms downwind. This commonly happens in agricultural areas; during the fallow season, farmers who aren’t aware of the problem of wind erosion may fail to plant species that can protect the soil during the fallow.

    Animals that live in sandstorm-prone environments face many challenges. First, they need plants to survive, and if the environment is as dry as it’s likely to be in most sandstormy regions, there may not be a lot of (or any) potable surface water. Most desert animals have evolved to obtain their water entirely from vegetation, or to strongly conserve water until the next rainy season; examples include the Arabian oryx and many desert rodents and reptiles. It’s also true of insects, though some insects have evolved to harvest moisture from the air, such as the Namib desert beetle. Second, conditions must permit the survival of some food source for the animals; that means plants, and most desert plants are tough (they must survive the occasional animal that wants to make a meal of them) and may not be very nutritious (because they live in nutrient-impoverished environments). Finally, you won’t see many herbivores (there won’t be enough vegetation to support large populations), and you’ll see even fewer predators, since predators must consume relatively large numbers of herbivores to survive.

    As a lagniappe, I feel obliged to throw in a mention of something non-ecological for the human animals: sand yachts. What adventure yarn would be complete without thrilling chase scenes across Lawrence-of-Arabian expanses of dunes and that final climactic battle between the sandblasted pirates and our intrepid adventurers? (I someday hope to try sand yachting; as a teen, I occasionally sailed iceboats on a frozen local lake, and because the drag is far less than that created by water, the acceleration and speed have to be experienced to be believed. It’s exhilarating.)

    A final factor to consider is the nature of the sand or dust being transported by the wind; apart from the externally abrasive characteristics I’ve described above, the animals must have a way to cope with the finest particle-size classes. Particles in the PM2.5 and PM10 classes (smaller than 2.5 µm and 10 µm, respectively) are highly damaging when inhaled. Thus, humans will need high-efficiency masks to exclude these particles. PM10 particles are unpleasant enough, since they can penetrate into the deepest parts of the lungs, where they abrade tissues and cause diseases such as silicosis. PM2.5 particles are worse, since they can penetrate directly into the blood; details of the consequences are still being discovered, but once transported throughout the body, these tiniest particles can wreak havoc throughout the body.

    And in a fiction context?

    Deciding how to deal with these matters in a fictional context requires you to start by answering a crucial question: Am I trying to write a story sufficiently convincing that the physicists in the audience will write me glowing fan letters, or am I just trying to tell a cracking good yarn? In most cases, you’re going to want to aim somewhere in between: carefully think about what I’ve written so you can retain enough veracity to prevent amateur physicists like me from flinging your book across the room, but not so much that you sacrifice a good story on the altar of science. (I’ve been wanting to use that latter phrase for years. Thanks for the opportunity, Martin!)

    Your hardest job, as in any case that involves writing in an ecological context, is to think through the consequences of your authorial choices. In ecology, everything is linked and every change has consequences for other parts of the system. For example, a hypothetical world with frequent, severe, recurring sandstorms will have severe effects on plants, which will in turn affect the herbivores that consume those plants and the carnivores that consume those herbivores. The sandstorms will also affect the humans on that planet, and in turn the humans will affect the sandstorms, whether by exacerbating their severity through unsustainable agriculture or mitigating the damage through preventative activities such as planting windbreaks to break the force of the wind or building artificial sand barriers. This may even extend to large-scale efforts to reduce the severity through geo-engineering. Of course, we have little hope of successful planetary-scale geo-engineering based on present knowledge and technology; the ecological system is far too large to be easily changed and far too complex for us to predict the likely consequences of such changes (not to mention the unexpected ones) with any confidence. Of course, therein lies fertile ground for fiction.


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