One of the common uses of science fiction is to imagine the habitability of worlds very different from our own. Some famous science fiction narratives consider a desert as their setting. In some cases, such as the world Abydos seen in the Stargate universe, it is unclear whether we are simply seeing a desert region of an otherwise temperate world, but in others, such as the planet Arrakis in Dune, there is no doubt that we are seeing true desert worlds. But how is human habitability envisaged in these worlds, and how plausible are they?

The Spice World

Amongst the most famous of desert worlds is one named (unofficially at least) after a sand dune. The world Arrakis, nicknamed Dune by its inhabitants and visitors, first appeared in James Herbert’s Dune (novel,1965) and then in its many sequels. This militaristic and political science fiction drama begins with the planet and its valuable drug resource (a psychotropic which bestows paranormal perception, known as melange or spice) being granted to one noble house within an imperium. The house displaced, an order of psychic religious women with their own objectives, and the emperor himself, all act to undermine and ultimately destroy the newcomers, leaving a son of the house to act as a rebel leader and “promised-one” amongst the resident desert dwellers, the Fremen.

Life on Arrakis, which orbits the white supergiant star Canopus, is undoubtedly hard. While the occupying imperials dwell in comparative luxury in robust cities imposed on the landscape, and carefully nurture imported plants, the Fremen instead occupy settlements built into natural or modified cave systems in the desert and exercise extreme water conservation. This involves wearing stillsuits – complex skin-tight environment suits which condense, purify and recirculate perspiration, respiratory moisture and all other liquid wastes as drinking water. These reduce water losses to “a thimbleful per day”. Moreover the remains of any dead individual are themselves rendered to return their water to a communal reserve. A Fremen greeting involves spitting at the feet of a visitor as “a gift of moisture” – one easily misinterpreted by those unfamiliar with the convention.

In addition to the water conservation issue and squabbling between noble houses, the residents of Arrakis face two other significant threats which are rooted in the dry nature of their world: sandstorms and sandworms – both of which are lethal, and both are somewhat mitigated by the Fremen’s rock-buried “sietch” settlements.

Sandstorms are a natural consequence of an arid and hot environment, with a strong solar or geothermal heating regime. Differential thermal heating of the atmosphere, rock formations and sand dune systems can lead to strong pressure gradients, and hence to storms. In an environment with a loose and light surface covering, these rapidly become highly abrasive and also potentially suffocating to anyone caught outside in such an event.

The sandworms – vast burrowing creatures, extending hundreds of metres and with a huge maw at one end - are a bit more of a unique problem to Arrakis, but represent an interesting visualisation of the kind of life that could occupy such worlds. The loose surface material of a desert world is well suited to burrowing creatures, which also secure protection from the sun (behaviour seen in desert animals on Earth). At the same time, a large size yields a small area-to-volume ratio, reducing moisture loss from the surface, and the sandworms in Dune also show an ability to detect vibrations carried through the sand across large distances. These highly carnivorous predators present a constant problem for all the surface residents, but also demonstrate how useful well-adapted native life can be to those who learn to interact with it. Curiously, there is no evidence for any native life beyond the sandworms in the Dune series. They are described as feeding on smaller sandworms or on sand plankton, small organisms which develop into sandtrout, and then into sandworms themselves. Indeed the books imply that the sandworms/trout may themselves have been largely responsible for desertifying the planet. An example of such discussions can be found in an exchange between Lady Jessica Atreides and Doctor Yueh early in the first book:

"Water!" she snapped. "Everywhere you turn here, you're involved with the lack of water!"
"It's the precious mystery of Arrakis," he said.
"Why is there so little of it? There's volcanic rock here. There're a dozen power sources I could name. There's polar ice. They say you can't drill in the desert - storms and sand tides destroy equipment faster than it can be installed, if the worms don't get you first. They never found water traces there, anyway. But the mystery, Wellington, the real mystery is the wells that have been drilled up here in the sinks and basins. Have you read about those?"
"First a trickle, then nothing," he said.
"But, Wellington, that's the mystery. The water was there. It dries up. And never again is there water. Yet another hole near produces the same result: a trickle that stops." (Berkley Medallion edition, 1977, pg 61)

In other words, the planet has water but it has simply been rendered inaccessible by the indigenous life.

In terms of astrophysical plausibility, Dune’s host star, Canopus, is certainly substantially hotter than our Sun, although the planet’s orbit must also be rather more distant than Earth’s (otherwise the surface temperatures would not be survivable even for seconds), which suggests that the year is relatively long. More worryingly, as a massive (class A9) star, Canopus is very short-lived – probably only 25 million years old (as opposed to our Sun’s 5 billion) and so we might question whether planets such as Arrakis (let alone complex life like Sandworms) would have had time to form and cool. The third book in the series, Children of Dune (1976), in fact suggests that the sandtrout were originally introduced from elsewhere.

Binary Sunsets and Water Farms

“If there’s a bright centre to the Universe, you’re on the planet that it’s farthest from!” – Luke Skywalker, Star Wars, 1977.

Probably matching Dune for its fame is the desert world Tatooine, home-world of Anakin and Luke Skywalker in the Star Wars saga (film, 1977-present). Tatooine is described as lying on the outskirts of the galaxy, in an orbit around two suns – one white and one red (i.e one hotter and one cooler than our Sun). This double irradiation is usually described as being responsible for the planet’s high surface temperatures and low free-water content, although the extended Star Wars universe meta-texts (e.g. encyclopaedias, games, annuals or other supporting material) suggest that it was originally more temperate and the climactic shift to global desert is relatively recent, with an unknown cause.

Despite this, the Star Wars universe shows Tatooine as having a fairly well-developed desert ecology. This includes two native intelligent lifeforms – both shown as tribal, territorial and routinely completely covered up (presumably to prevent sun exposure and perspiration) - as well as numerous forms of desert-adapted animal life. These include both large mammals such as Bantha (with a small surface area) and small ones such as Womp Rats (capable of tunnelling to avoid the suns), as well as reptilian species such as Dewbacks and Krayt Dragons. All are capable of surviving in the open desert, and through the planet’s violent sand storms.

Human (and other) settlers on the planet rely on moisture farms – mechanisms that slowly draw moisture from large regions of the atmosphere and condense it into usable liquid. Such farms were remote settlements, with few humans, but routinely used droids (robots), due to their low water requirement and ability to interact with the essential machinery. However while life may be hard, it is nonetheless routine and abundant.

In most respects, then, Tatooine is a less extreme and more habitable world than Arrakis. While water is still extremely valuable, the measures required to secure and preserve it are less dramatic, and the planet’s native biosphere is richer and more abundant.

Vulcan’s Forge

Less commonly discussed but also important as a desert world in science fiction is the planet Vulcan in the Star Trek universe, seen originally in the original series episode “Amok Time” and then in many other episodes and spin-off media across the years since. Canonically, Vulcan orbits the star 40 Eridani A, a K class star (cooler than our Sun) in a trinary system with a white dwarf-red dwarf binary. It is described as being typically arid, with a shorter year than Earth’s, thinner atmosphere and a higher surface gravity as well as temperature. None of these things are unreasonable in terms of properties of the 40 Eridani system.

As shown on screen, Vulcan has a few small, scattered oceans, with most of the surface occupied by large deserts or mountain ranges. It has a fairly abundant biosphere, with large mammal-analogues such as the sabretooth sehlat, as well as other Earth-like animal and plant types. Notably, this includes the native Vulcans (and Romulans) who are similar in both physiognomy and habitability requirements to Earth. An interesting insight into life on Vulcan is actually attributable to the seldom-discussed Star Trek: The Animated Series (1973).

In the episode, “Yesteryear”, we learn that young Vulcans undergo a coming-of-age ritual involving surviving alone in the desert, amidst hostile life and the heat. As Sarek told his seven-year-old son Spock: “Soon you will undergo your test of adulthood in the desert. To survive for ten days without food, water or weapon on Vulcan’s Forge will demand more of you than anything ever has.”

Script writer D. C. Fontana (who was also responsible for 10 episodes of the original series, 5 in ST:TNG and 1 in ST:DS9) noted in interviews that the animated format permitted more detail to be shown of the desert world and its life than was possible in live action TV at the time. The episode shows the landscape of Vulcan as rocky and barren, with the city of Shi-Kahr forming an isolated island amidst the desert. Although Sarek’s family home boasts a water fountain, it is clear that this is an affluent household and no open water is seen outside of the house. D. C. Fontana returned to some of these themes in her Star Trek novel Vulcan’s Glory (1989), which begins with a recap of the events of "Yesteryear" and a brief description of Vulcan itself:

Much of his planet was desert, relieved only by the ragged hulks of mountain ranges, and the great blood-red oceans. Hardy succulents, gnarled and tiny-leaved isuke bushes, and karanji - similar to Earth's barrel cactus - constituted much of the wild flora of Vulcan. The flame-leaved induce trees clustered in the cases that had originally dotted the deserts - except, of course, on Vulcan's Forge. Nothing grew on the Forge, that immense blistering range of hellish sand and rock into which no one - not even the most toughened and experienced Vulcan - ventured willingly, or for long. (Vulcan's Glory, Fontana, pg 10)

This representation of the planet also influenced the trilogy of Star Trek: Enterprise episodes starting with “The Forge” (2004) which explored a rebel faction of Vulcans living in the desert. This trilogy of episodes also establishes that the desert experiences “sandfire storms” – blinding sandstorms with strong winds and intense lightning discharges, similar to those known to occur on Mars.

The Red Planet

Arrakis, Tatooine and Vulcan represent a spectrum of desert worlds in science fiction, ranging from the most arid and hostile Arrakis to the far more habitable Vulcan. In fact, this range of planetary conditions probably represents a very narrow part of the full range of conditions that might be found on planets, all lying broadly in their stars’ habitable zone where surface temperatures are survivable by humans. Indeed, they could plausibly represent a progression from the inner edge of the habitable zone (e.g. a Venus-like irradiation [1]) towards its centre (e.g. a slightly hotter than Earth-like temperature and solar energy input).

The planet Mars, on the other hand, lies on the outer edge of the Sun’s habitable zone. Its surface temperatures and conditions are more akin to those of Antarctica than the Sahara. Despite this, its red colour and the frequent dust storms visible from Earth have led to the Martian environment being described primarily in desert terms in science fiction.

This was the case, for example, in Stanley Weinbaum’s influential short story "A Martian Odyssey" (1934), which described the trek of a human through the Martian landscape, in order to rejoin his team after being stranded by a vehicle crash. Along the way he encountered an intelligent native which he named Tweel, who joined him on the cold and barren journey:

"Just as we sighted Xanthus, or the cliffs that bounded it, one of those nasty sand clouds blew along, not as bad as the one we had here, but mean to travel against. I pulled the transparent flap of my thermo-skin bag across my face and managed pretty well, and I noticed that Tweel used some feathery appendages growing like a mustache at the base of his beak to cover his nostrils, and some similar fuzz to shield his eyes."
"He is a desert creature!" ejaculated the little biologist, Leroy.
"Huh? Why?"
"He drink no water—he is adapt' for sand storm—"
"Proves nothing! There's not enough water to waste any where on this desiccated pill called Mars. We'd call all of it desert on earth, you know."

Another relatively early example of a plausible Martian desert can be found in Arthur C Clarke’s 1940 novel The Sands of Mars [2]. This correctly envisaged Mars as frigid, oxygen-deficient and with a low air pressure, although it clung to an earlier picture of fairly abundant, oxygen-sequestering vegetation and even some animal life. As Clarke admitted in his preface to later editions, conditions on Mars are even more extreme in terms of temperature and pressure than he appreciated at the time of writing, but even so this is an interesting study of human settlement in a hostile and arid environment which is visualised as a cold desert. The settlers make use of all-terrain vehicles known as Sand Fleas, and the plot is driven forward when the protagonist and his companions crash land as a result of a Martian sandstorm.

[Image: Cover Art for Arthur C Clarke's novel The Sands of Mars showing Sand Flea vehicle, illustrated by Gordon C Davies. Source: 70s Sci Fi Art on Tumblr]

These images of desert life were very different from the lush planetary romances of earlier decades, although these lingered in some less serious fiction. In many respects, the portrayal of Mars as a cold desert hasn’t changed significantly over the decades since, except to emphasise its harshness and the thinness of its atmosphere. Robinson Crusoe on Mars (movie, 1964), for example, was filmed in California’s Death Valley. Sandstorms (or more correctly dust storms) remain a feature of Mars in literature to the current day, also appearing as plot devices in Greg Bear’s Moving Mars (1988) and Andy Weir’s The Martian (2013) – perhaps unsurprisingly as the actuality of such storms is now well established both through remote observations and in the experience of Earth-built robot landers. However storms as severe as many of those depicted in fiction require a significantly denser atmosphere than Mars actually possesses and the wind speeds and dust densities weathered by the Martian landers are much lower. They are still hazardous enough to block solar panels and cause some abrasion, but not to the extent seen in the fictional accounts mentioned above.

The Spectrum of Deserts

As we’ve seen, desert worlds in science fiction represent a continuum in properties from barren cold words akin to our Antarctic deserts (like Mars), through near-Earth-like arid worlds (like Vulcan), and desolate but ecologically-rich environments (like Tatooine), all the way to the parched world of Dune’s Arrakis, in which the ecosystem is reduced to a single (possibly-introduced) lifeform and human survival requires extreme measures. In terms of solar irradiation and chemical properties, these worlds are likely plausible, and their sequence represents a journey through their star’s respective habitable zones (simply defined as the region around a star in which liquid water is theoretically possible). All other things being equal, worlds with marginal habitability (slightly too hot as in desert worlds, or slightly too cold as in ice worlds) are likely to be more common than those sitting perfectly in the centre of their habitable zones.

However, as we know very well from the properties of Venus and Mars in our own Solar System, the actual habitability of a planet is moderated at least as much by its atmosphere as its position in the habitable zone. Venus’s thick atmosphere and greenhouse effect lead to a temperature far higher than might be expected at its location, while Mars’ thin atmosphere makes it significantly cooler than a planet with an Earth-like atmosphere might be in the same distance from the Sun. In this regard, it’s worth considering that all of the exoworlds described above have surface pressures and oxygen contents survivable relatively comfortably by humans without pressure suits. Is that entirely plausible? The pressure (i.e. gas density) of the atmosphere is determined (to a first approximation) by the surface gravity – and again, this appears to be comfortable for humans on Vulcan, Tatooine and Arrakis alike. A hotter, more irradiated atmosphere is more likely to lose gas rapidly through evaporation into space, though, suggesting that Arrakis’s atmosphere is probably the least stable of these worlds.

However Arrakis in particular presents another problem. On Earth, the free oxygen content of our atmosphere is determined to a large extent by bacteria and plankton in our extensive oceans, and then moderated again by the vast mass of land-based vegetation. In the absence of these bio-processes, most of Earth’s oxygen would likely be locked into chemical reactions with the crust (as is the case on Mars) or have escaped into space long before. Tatooine and Vulcan are arid worlds, but do appear to have at least some functional ecology, so could plausibly have some form of oxygen-generating life. By contrast, Arrakis is essentially barren, with no sign of sufficient organic activity to generate an oxygen-rich atmosphere. While it’s vaguely plausible the sand-plankton might contribute here, there’s little evidence for this.

Either way, if we were to discover a desert world in our observations of the universe, how easy would it be to tell what we were looking at? As is the case with the search for biosignatures, we’d be looking primarily for evidence in the spectrum of light – changes to the way light is reflected with colour due to the surface composition. Studies of Earth desert environments (including the Tunisian desert used to film Tatooine) have indicated a number of spectral characteristics indicative of the presence of gypsum or quartz grains on the surface, and also confirmed that these are highly reflective, leading a high albedo (reflected light fraction) from deserts. This characteristic is widely used in Earth-monitoring observations to measure and track desertification. However the signatures of desert environments in observations of remote objects are likely less distinctive than those of vegetation, and most worlds would be expected to show evidence of the chemical signatures involved. On the plus side, the high reflectance might make direct imaging observation of these worlds (slightly) more plausible than those of fainter planets.

But why are desert worlds so lastingly popular in fiction? Well, to most consumers of science fiction, the environment is sufficiently alien to invoke otherness and hence novelty interest, while being sufficiently familiar not to cause cognitive dissonance. As environments with marginal habitability for humans, they also present their inhabitants or visitors with a struggle for survival which often provides a driver for strong narratives. Characters faced with such challenges pass through a crucible which forces them to show their true character.

Ultimately desert worlds enable narratives which explore what we are prepared to endure in order to survive, and how far our environment modifies what it means to be human.