They arrived at the edge of the canyons in the middle of that afternoon, exactly where Jamie had wanted, at the juncture of three broad fissures in the ground that reminded him of arroyos carved out of the desert by wildly rushing waters.

But bigger. Gigantic. Like the Grand Canyon, except that there was no river down at their distant bottom. Jamie stood on the level ground where the three huge gullies joined together and he could barely see the other side. Peering down into their depths, Jamie guessed the canyon floors must be more than a kilometer below him, perhaps a full mile down, nothing but red-tinged rock cracked and seamed by eons of heating in the sun and freezing every night.

He felt suddenly small, insignificant, like an ant poised on the lip of a normal arroyo in New Mexico. For a dizzying moment he was afraid he would topple over and fall in.

The ground up here had fewer rocks strewn across it, as if it had been swept clean at one time and the rocks had only partially returned. Strange, Jamie thought. We’re closer to the heavily cratered territory to the south, yet there’s not as much impact debris here as farther north.

He returned his attention to the canyons, feeling an excitement trembling within him that he had never known before. The first man to look into a Martian canyon! There might be a billion years of the planet’s history written into those rocks down there. Two billion. Maybe four. It was scary.

The canyon wall was a nearly vertical drop. The thought of climbing down that rock wall thrilled him and frightened him at the same time. The bottom was so far down! Yet he could see it with absolute clarity. The thin air had not the faintest hint of haze in it.

To his geologist’s eye it seemed clear that this labyrinth of canyons had been caused by a splintering of the ground, a gridwork of faults in the underlying rock that had weakened the crust, cracked it. When water had flowed here, however long ago it was, it had followed along those cracks, widening and deepening them. Or more likely the permafrost beneath the crust melts from time to time and undermines the ground until it collapses.

"Is that the way it happened?" Jamie asked the silent arroyos in a barely vocalized whisper. "How long ago was it?"

The twisted gullies remained mute.

The more Jamie stared into the deep ravines the more he realized that there had been no great rushing flood here. Mars is a gentle world, he told himself. The ground doesn’t quake. There are no storms. If there ever was a flood on this planet it didn’t happen here.

He straightened up and looked across the huge gulf toward the other side of the canyon. Our ignorance is even wider, he knew. Every geologist on Earth could spend a lifetime here and still it wouldn’t be enough to get all the information these tired old canyons have to yield. All I’ve got is the rest of today and tomorrow. Unless I can get Mikhail to change the excursion plan.

He turned to the Russian, who was standing between him and the rover, looking down into the canyon. The rover’s bright aluminum finish was coated with reddish dust now, especially around the wheels and fenders. It made the vehicle look as if it were rusting.

Fighting down a tiny irrational fear that nagged at the back of his mind, Jamie called, "Mikhail, I’ve got to climb down to the bottom. I’ll need your help."

The Russian, in his red hard suit, started walking toward Jamie. "That is an unnecessary risk."

Jamie made himself laugh. "I’ve done a lot of rock climbing. And in full gravity, too."

"It is an unnecessary risk," Vosnesensky repeated.

"Then why did the mission planners allow us to stow climbing gear in the rover? Come on, Mikhail, with the winch and all it won’t be much of a risk at all. If you think I’m in danger you can haul me up whether I like it or not."

"The sun is setting. It will be too cold to work. Tomorrow you can have the whole day."

"I’m okay in the suit. We’ve got three-four hours before sunset," Jamie said. "Besides, the sun’s hitting this side of the canyon now. Tomorrow morning this side’ll be in shadow."

It was impossible to see the Russian’s face behind the gold-tinted visor of his helmet. He was silent for a long time, obviously thinking, weighing the options. Finally he said, "Very well. But when I say to come up, you do not argue."

"Deal," said Jamie.

Jamie spent the next hour inching slowly down the sheer rock face of the canyon wall, stopping every ten meters or so to chip out samples. He wore a climber’s harness over his hard suit, attached to the electrical winch at the canyon rim by a thin cable of composites stronger than steel. Jamie himself controlled the winch with a set of buttons built into the harness, although Vosnesensky could override him by using the controls on the winch itself or even hauling him up manually, if necessary.

The rock’s not stratified, Jamie saw. Seems to be all the same, all the way down to the bottom. That puzzled him. One thick slab of undifferentiated stone? How can that be? He remembered a novel he had read years ago, a scene where an infantry division had been assembled on a parade ground that was described as solid iron one mile thick. Had that scene been set on Mars? Jamie could not remember.

This is different from the area around the dome. There’s never been an ocean here to lay down silt deposits and have them turn into rock layers over the years. I’m looking at the actual mantle of the planet, the original material that made up the planet from its very beginning. One enormous slab of rock that must go down not just one lousy mile, it must be a hundred miles deep! Or even more!

Jamie dangled in midair, twisting slightly in the harness, staring at the reddish gray wall before his eyes. This stuff has been here since the planet was born, since it cooled off and solidified. It could be more than four billion years old! He was panting as if he had run a mile, as if he had just found the most precious diamond in the universe.

There was nothing like this on Earth. Mantle rock was always buried beneath miles of crust. Even the ocean beds were covered with sediments. You never saw exposed mantle rock on Earth. But Mars is different, Jamie said to himself. The old assumptions don’t apply here.

It’s not differentiated, he realized. That’s why there’s so much iron in the sand on the surface. The iron never sank into the core the way it did on Earth. It’s spread all over the surface. Why? How?

Up above, Vosnesensky took an automated sensor beacon from the rover’s cargo bin and busied himself setting it up. The anemometer immediately began turning, fast enough to surprise him. The air was so thin that even a stiff breeze was negligible. Toshima will be happy to have another station reporting to him, Vosnesensky said to himself as he turned on the isotope-powered telemetry radio.

Then he walked back to the winch. Planting his short legs as firmly as the machine’s on the dusty red ground, he took hours’ worth of video shots of the entire area.

Jamie took pictures too, with the still camera he carried in the equipment belt around his waist.

As he neared the bottom Jamie searched for signs of the actual fault line that had created the canyon. In vain. Eons of dust laid down by the winds that yearly billowed up into planetwide sandstorms had covered the canyon floor. Jamie smiled to himself, hanging in the climber’s harness. Give Mars another billion years or two and the canyons will be all filled in.

He did not like to look up while he dangled in the harness. The rock wall loomed above him, much too high and steep to climb. The other walls were kilometers away, but the deeper down Jamie went the closer they seemed to squeeze in on him. It made him feel trapped, frightened in a deep unreasoning part of his brain. So Jamie busied himself chipping away at the rock as he descended and scanning the floor below for any evidence of the fundamental crack in the ground that had started this canyon. He never found it.