Feet scrambling, the suited figure fell and tumbled. He came to rest just a few meters downslope--a short fall, but the damage was done. He heard it: something crunched.

Inside the suit the man took a cautious breath; everything seemed fine. Probably just two rocks rubbing together, he thought, struggling upright. Moments later he was at the top. From there he could see the 2020 Mars Expedition's six-person living quarters and experimental water-extraction plant. They looked small and fragile below on the vast crater floor.

A few hundred kilometers north, hidden beyond the sharp curve of Mars's nearby horizon, was a polar ice cap--three kilometers thick and full of dusty, frozen water. But the explorer, a hydrologist, was bound only for a few nearby hillocks to dig soil samples. He planned to test just how water-rich this area was for supporting a possible full-scale permanent colony.

The camp commander--a biologist--had warned him: no scientist should travel alone outside the crater's wall. Mission rules required a "buddy." But he wasn't going all that far, he thought.

He descended the crater's outer wall in slow bounds. At his heels trundled a waist-high robotic rover. "The Husky" they called it--a proper dune buggy with springy wire wheels and treads, laden with several latched titanium boxes, a supply bag, and what looked like a miniature distillery.

Once in a while on the downward slope, the explorer skidded a bit on loose gravel and kicked up puffs of talcum-fine reddish dust that remained suspended for minutes in the thin martian air. It clung to everything. It sure would be nicer to ride, he thought, as he rubbed the dust off his faceplate. But in 2010, NASA engineers had calculated that precious expedition weight could be saved if most rovers were built only large enough to haul equipment and not passengers. Short journeys were done on foot.

At half-past 12 to the minute, Martian time, he arrived at a hillock crisscrossed with a network of small, dry rivulets meandering away down its north side. The rover's on-board neutron spectrometer was beeping: it had picked up a massive concentration of hydrogen--probably water-ice--not far beneath his feet. He was pleased at the speed he had made. At this rate, he should be able to get samples from several hillocks and return to camp by six, where a hot supper would be waiting.

Thirsty from his vigorous hike, he sipped at his spacesuit's water supply. It gurgled like a soda straw drawing up the last of a milkshake from the bottom of a glass. Puzzled, he tried again. He had filled it that very morning--it ought to work. Then he remembered. That crunching sound back at the crater, it must have been his water bottle. He reached around and unscrewed the cylinder. It was cracked ... and empty.

The explorer cursed his luck. Here he was; he had had an accident; and he was alone. That biologist back at camp was right after all. And he was sure to hear about it for the next two years.

He licked his dry lips. Even in a spacesuit, dehydration was an ever-present threat in the thin, desiccated arctic air of Mars. Thinking about it made him thirstier. Work the problem, he reminded himself. He reached into the Husky's supply bag and pulled out a small roll of duct tape--not standard issue, but no one left camp without one. Moments later the bottle was water-tight again. It was still empty, though, and he needed to refill it. But how?

He glanced down, and only his helmet stopped him from slapping his forehead. Of course! What he needed was right there, underfoot.

He activated the rover's drill, which looked like a miniature post hole digger, and started drilling into the reddish soil. The first 20 centimeters went quickly--it had the loosely cemented texture of powdery sand that had once been wetted and then dried. Half a meter deep, however, the drill slowed as it encountered layers as dense as clay. The explorer pulled out the drill and knelt to examine the hole.

Mars dirt looked nothing like the permafrost layers of tundra in Earth's living Arctic where ice gleamed as distinct crystals within the soil. What he saw here reminded him more of compacted volcanic ash. (He had run his bare hand across some once on the slopes of Iceland's primitive basaltic volcano Mount Hekla. On Mars, he dared not remove his glove.) It looked awfully dry. Even so, its sorbet-fine crunchy texture suggested it was maybe 40 to 50 percent ice by volume.

Good thing, the explorer thought--he had never experienced such thirst. And, besides, he didn't want to admit defeat.

Carefully, he dumped the dense sample into the rover's gas-analyzer oven, where it was hermetically sealed and pressurized. The chamber looked pitifully small--after all it was intended only for scientific analysis. But he hoped it would yield at least enough water to alleviate his growing thirst.

The oven's plutonium-fueled heater gave out 100 watts of continuous power. Setting it to slow melt would create a muddy slurry, useless for drinking. So he set it to stepwise heating--to flash the soil's temperature to 200�C, immediately vaporizing any ice. Through a small window, he watched as delicious droplets condensed on cooling plates and rolled into a sample-collection cup. Mouth parched, he cherished the sight.

He paused. Was this distilled water even safe to drink? What minerals might still be dissolved in it? Were there even, perhaps, Martian organisms? Back at camp were all the Expedition's apparatus for testing and sterilization--and even then they didn't use Mars water for drinking.

But the sight of the filling cup put a wild idea into his head. I just won't swallow, he thought. Or maybe I'll be the first to drink Mars water.

He unlocked the oven's external door, retrieved the cup, and moved to drain it into his suit's water bottle. But not fast enough. The water began fizzing and steaming angrily, leaping over the rim and then freezing in a tiny cloud of ice crystals. In moments, the cup was completely dry.

He struggled for calm. Liquid water was highly unstable in Mars's vacuum-like atmosphere. He knew that, but he didn't realize how rapidly it would boil.

Could he make it back with no water? He glanced around at the alien terrain and felt a little scared. It struck him that he should radio for help to see if someone could meet him with extra water halfway.

"You were right, you were right," the explorer mumbled as he flipped the switch. Sure enough, it was the biologist who answered.

Special Note: Current knowledge about the approximate nature of ice-laden Martian soils and the operation of the stepwise gas-analyzer oven was provided by Dr. James Garvin, lead Scientist for Mars exploration at NASA Headquarters, based on results from Mars Odyssey and previous spacecraft. The fictional explorer, of course, is a 21st-century reincarnation of the nameless pig-headed miner in Jack London's famous 1908 short story "To Build a Fire"-while the freight rover is an adaptation of the man's husky.

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