The planet we've had the most time to visit is Tatooine. It feels so familiar now, I almost feel like I've been there.

Home of Anakin and Luke Skywalker, Tatooine is a harsh planet and appears to have no open bodies of water. Violent sand-storms sweep a landscape of desolate sand dunes, sharp mountains, narrow canyons, and barren rocky wastes. Water apparently exists beneath the surface of the planet and as water vapor in the air.

While we don't know all the details of Tatooine's environment, we do have similar environments on Earth: deserts. The defining characteristic of a desert is the small amount of precipitation it receives: less than ten inches per year. The area of southern Tunisia where parts of Star Wars were filmed receives on average only six inches of rain per year. Tropical rain forests average 80 inches per year. Arabs call the desert the "sea without water." This is echoed in the name of Tatooine's "Dune Sea", the undulating field of sand dunes where Return of the Jedi's Sarlacc lives.

This lack of water provides the main challenge to life. In some parts of the Sahara, no rain has fallen in more than twenty years.

Then suddenly rain falls in a torrent, flooding across the landscape, plowing boulders ahead of it and cutting channels into the ground before it finally sinks into the subsoil. We see signs of similar rain erosion on Tatooine, which has several dry riverbeds: one where Artoo is captured by the Jawas, and another by Jabba's palace.

In the desert, the land is so dry that the air is often parched, with only 2 to 5 percent humidity. Clouds, then, are rare, and sunlight beats directly down, with up to 95 percent of it reaching the ground. In midday, heat-absorbing black rock can reach 185 degrees, way too hot to walk on. Since water serves as a moderating influence on temperature, areas without water heat and cool much more quickly. At night, the temperature can plummet to only 50 degrees. Such extreme temperature variations actually cause rock to crack and burst. As Luke Skywalker stands in the heat of the day on a mountain cliff above Mos Eisley, he may hear a series of loud cracks like cannon fire as the mountain rock fractures. As rocks break off and fall down the mountains, they shatter into smaller pieces. Weathering reduces these fragments to sand and even smaller dust.

Another phenomenon Anakin and Luke may be familiar with is ghost rain. In the desert, a hot layer of air often forms just above the ground. Rain may begin to fall, but when it hits that heated layer, it evaporates, never reaching the ground.

Contrary to the vision you may have of it, a desert is not all sand. Sand and dust are simply the endpoints of erosion of rock by heat and wind. The Sahara is only 20 percent sand. The remainder includes mountains, plateaus, piles of slag, gravel-covered wastes, ravines, and canyons. We see many of these features on Tatooine.

The Jawas hide in caves in a rocky cliff face. Luke catches up with the renegade Artoo in a canyon, and Threepio falls off a plateau when the Sand People attack.

In The Phantom Menace, more of Tatooine's topography is revealed in a white-knuckle pod race. The race course seems to be part of a network of deep twisting ravines and crevasses that includes some bizarre stone formations, such as a series of natural stone arches. The Sahara's Tassili n'Ajjer Plateau, an area of southern Algeria, provides a striking parallel to this part of Tatooine.

The Tassili Plateau is made up of hundreds of huge blocks of sandstone that split apart long ago. Narrow steep-sided ravines, up to 2,000 feet deep, separate these blocks. Numerous shallower canyons, up to 200 feet deep, crisscross the plateau. German naturalist Uwe George, author of In the Deserts of This Earth, says, "The canyons run through the plateau like the streets of a large city, with 'apartment houses' between them." Within this city are dead ends, intersections, tunnels, caves, stone needles 100 feet high, and bridges arcing like overpasses over the streets. These bizarre formations were caused by wind and water erosion, the water erosion taking place long ago when the Sahara had a wetter climate, long replaced by wind erosion in this extremely arid region. Since the wind carries sand and dust, high winds, according to Uwe George, act like "a sandblasting machine." It's certainly reasonable to find such formations on Tatooine. They also suggest that there may once have been more water on Tatooine than there is now.

Before we try to figure out what Tatooine may have been like in the past, let's examine its current condition. As far as we can tell, the entire planet appears to be a desert. Earth has a much more varied climate, with only 30 percent of the continents being desert. Deserts form in particular areas because of a variety of factors. Most terrestrial deserts he near the Tropic of Cancer or Tropic of Capricorn, where the sun's rays are at their strongest during the Northern Hemisphere's summer and the Southern Hemisphere's summer, respectively. Patterns of air circulation, ocean currents, and elevation also contribute to the formation of deserts. Once formed, deserts tend to expand. Strong winds dry out the topsoil of neighboring lands and blow it away. Sandstorms cause even more dramatic erosion. They can sand the paint off a car and wear the landscape down to bare rock. The deserts on Earth are expanding in this way, with the Sahara growing by forty square miles each day.

Even if Earth's continents did become huge deserts, though, our planet would still have one major difference from Tatooine: oceans. We don't see any large bodies of water on Tatooine, and unless they're hiding on the far side of the planet, we have to assume there are none. Dr. Condie finds the lack of surface water hard to accept. "To have a whole planet that is a desert and yet to have moisture in the atmosphere I think would be difficult."

As we discussed earlier, scientists believe most of the free water on Earth came from water locked in the rocks, and since Tatooine generally seems quite similar in composition to Earth, we can assume that Tatooine similarly has water locked in its rock.

On Earth, that water was liberated by volcanic action. On Tatooine, then, less volcanic action may have freed smaller quantities of water.

Why would Tatooine have had less volcanic activity? Perhaps Tatooine has smaller quantities of radioactive elements that serve to heat it. Or if the composition of Tatooine is about the same as Earth, then perhaps Tatooine is less massive than Earth. A less massive planet would have less internal heat. An example of such a planet is Mars, which is only about half the size of Earth. Mars has some volcanic activity, but it has been decreasing steadily, and Mars has never been as geologically active as Earth. Thus it's not too difficult to explain why a planet would have less free water than Earth. With this theory, Tatooine always would have had less free water, and it always will have less free water. Signs of water erosion, then, would simply be the result of irregular downpours, as occur in the desert.

In proposing ways Tatooine differs from Earth, though, we have to be very careful. As discussed earlier, terrestrial life survives on Earth because of many different factors in delicate balance. A planet that differs significantly from Earth in any way will likely not be able to support human life. For example, if Tatooine had the geologic activity of Mars, human life could not survive on it. Remember that currents in the liquid outer core create a planet's magnetic field. Because of its minimal internal activity, Mars has a very weak magnetic field, which does not keep damaging high-energy charged particles away from the surface as Earth's magnetic field does. So we must propose only incremental differences between Tatooine and Earth, slight changes that might be just enough to create the differences we see. Tatooine, after all, is much more like Earth than any of the planets in our solar system.

While we're doing this, please be aware that any theories we consider will be very speculative. A planet is an extremely complex system, with many different factors interacting, and we don't yet really understand how they all affect each other. After all, when it comes to planets, we only have a few examples to work with, and only one that we've studied in detail.

If we want to theorize that Tatooine had more water earlier in its history - and accounting for signs of erosion is only one reason we might want to do this - we need to work a little harder. One of our local planets provides a handy comparison. While Mars is now a desert planet, it is covered with signs of water erosion. Scientists believe Mars once may have been warmer and wetter. But what happened to the water? Current theories involve several different processes occurring together. One of these in particular might be relevant to Tatooine.

Scientists believe that some of the water on Mars has simply evaporated from its atmosphere into space. A planet holds an atmosphere to it by gravity. The molecules in the atmosphere are constantly moving around, colliding with each other. The higher the temperature, the faster the molecules move and the more they collide with each other. In these collisions, a lighter molecule can pick up more speed than a heavier one. A lighter molecule may then actually attain the escape velocity required to overcome the planet's gravitational attraction. It will escape the atmosphere and head off into space.

Molecules are constantly leaking from the atmosphere of any planet into space. The less massive the planet, the lower the required escape velocity. Dr. John Schilling, research engineer at SPARTA, Inc., explains, "The occasional molecule in the upper atmosphere picks up enough speed in a 'lucky' series of collisions to escape into interplanetary space." The rate of evaporation depends on the mass of the particular molecule, the mass of the planet, and the air temperature. These factors allow light elements like hydrogen and helium to escape from Earth's atmosphere. In the weaker gravity of Mars, molecules as massive as water can sometimes escape, even though the temperature, and so the velocity of the molecules, is lower.

We really don't want Tatooine to have gravity as weak as that of Mars. Mars, after all, has only a very thin atmosphere and can't support complex life. If water molecules can escape the atmosphere too easily, we won't have any water left, and we'll lose other elements as well.

Yet we don't need to lose an entire water molecule to lose water. Ultraviolet radiation from the twin suns can strike a water molecule in the upper atmosphere and break it into hydrogen and oxygen. The light hydrogen could escape, while the heavier oxygen remains trapped in the atmosphere. The water has virtually been 'lost, though the entire molecule did not leave the planet. This process occurs on Earth, though at a very low rate, since our atmosphere acts as a shield against most ultraviolet radiation. Only if a molecule makes it to the upper part of the atmosphere will it likely be hit by ultraviolet rays.

If we theorize that Tatooine is a bit less massive than Earth, then it might start out with less free water in the first place. Water molecules in the atmosphere couldn't rise higher in the lighter gravity, more easily reaching the upper portion where they would be exposed to ultraviolet radiation. We could even imagine that Tatooine's twin suns put out a bit more ultraviolet radiation than our sun - though not too much, unless you want Luke and Anakin to get skin cancer. Another factor could contribute as well. If Tatooine does have a magnetic field just a bit weaker than Earth's, it would offer slightly less protection from high-energy charged particles in space. These particles could also break apart water molecules. Thus we could explain a very slow loss of water from the atmosphere, leading to a gradual drying of the planet over billions of years. Humans happened to find and colonize Tatooine during the few tens of millions of years when it had lost most of its moisture but still had enough remaining to make it habitable.

So if we're theorizing that Tatooine is a "bit less massive" than Earth, how massive is that? Mars, at about half the size of Earth and just 10 percent the mass, seems too small. Dr. Michael Burns, a theoretical astrophysicist and president of Science, Math, and Engineering, Inc., believes a planet about two-thirds the size of Earth, with a mass about 30 percent that of Earth, might fit the bill.

Even though Tatooine may be very slowly losing water, at the time we see it, it still retains a fair amount. Moisture exists in the air of Tatooine. We see clouds in the atmosphere, and Luke lives on Owen and Beru's moisture farm, which, according to the Star Wars Encyclopedia, condenses moisture from the air. What water Tatooine has, then, would circulate through Tatooine as it does in the desert. Water vapor in the atmosphere would give rise to ghost rains and rare downpours that erode the landscape and then sink into the ground. Those downpours would provide groundwater that could be accessed through deep wells and that would perhaps, in depressions in the ground, give rise to the occasional oasis. The Sahara has significant quantities of groundwater. Some of Tatooine's rain and groundwater would then evaporate into the atmosphere, continuing the cycle.

It's very expensive to pump water up from significant depths, so perhaps farming the moisture from air would be cheaper and easier. Dr. Condie points out that "In some areas on Earth you have a hard time getting water out of the ground. The water table may be low or nonexistent. Yet in those cases you just go to a well, or pipe in the water from elsewhere." If water is this difficult to pump everywhere on Tatooine, though, and condensation has somehow been made easy, that could be a better solution. If many farmers are doing it, this could lead to a significant depletion of the slight humidity in the air. Luckily, Tatooine seems only sparsely populated.

I said earlier that positing a wetter history for Tatooine was helpful for more than just explaining erosion. If the theory that life on Earth originated near underwater hydrothermal vents is true, then it's hard to imagine how life began on Tatooine. We can imagine primitive underground life developing, as scientists think might exist on Mars, but what about more complex life? Dr. Jakosky says, "If you're going to get big organisms, I think you need standing bodies of water. Microbes can live in the pore space of rocks in the crust. But anything bigger needs room to move." So if Tatooine had more water on its surface at one time, that would make it easier to understand how complex life might have evolved. What kind of life? You'll have to wait for the next chapter to find out.

Twenty years ago, most scientists would have said that planets are probably rare, habitable planets very rare, and habitable planets with all the ingredients for life extremely, extremely rare. Now, planets appear to be quite common in the universe, habitable planets a fair fraction of these, and the ingredients necessary for life widespread. Life may be developing, living, and dying in star systems all around us. Although we might need some technological assistance when we visit, with a little help we should be able to survive on a wide variety of worlds. So in this huge population explosion, exactly what sort of neighbors might we find?

Extract taken without permission from Cavelos, Jeanne The Science of Star Wars
New York, New York: St. Martin's Press, 1999