And this guy was likely to be designated the Ares mission specialist. My God.

Phil Stone waved Bleeker quiet. “Go on, Natalie. We’re committed to the science. You’ve got us.”

“All right. Now,” she plowed on, “the probes have shown us that in the case of Mars we have two main types of landscape. The yellow stuff in the south is heavily cratered, and looks ancient. And this pink stuff, to the north, is made up of smooth, young plains. The planet bulges out below the equator; most of the south is above the mean altitude, and most of the north is below.”

“You say ‘ancient’ and ‘young,’ ” Stone said. “Meaning?”

“ ‘Young’ is maybe half a billion years old. The plains are volcanic — frozen lava fields. And the ancient cratered stuff is three to four billion years old. That’s almost as old as the planet itself…”

Bleeker said, “So let’s get back to the flags. I guess those seven Hammer and Sickles are the sites the Soviets have identified as prime interest.”

“Yes. You can see—”

“So screw that,” Stone said easily. “Let’s look at the good old American selections. Those two white stripes at the top and bottom of your chart — I guess we’re looking at the polar caps.”

“Yeah.”

“I see no flags up there.”

“No. We have to rule out high latitudes for the first mission.” Spacecraft arriving from Earth would naturally settle into a parking orbit around Mars with not much inclination to the equator; changing the orbit to reach the poles would take a lot of extra energy. “But it’s a shame; the poles are interesting.”

“What are the caps? Water ice?”

“Maybe. The orbit of Mars is more elliptical than Earth’s. And that distorts the seasons. In the south you get a short, hot summer, but a long, cool winter. And the makeup of the caps seems to differ as well. We think the cap in the north is water ice, yes. But the southern cap is probably carbon dioxide — dry ice.

“There are a lot of puzzles about the poles.” She walked across the room to a blowup photograph; it showed a thick band of layering in brownish terrain.

“What the hell’s that?” Bleeker asked. “It looks like melted chocolate.”

“These are bands of thick-layered deposits, thirty or forty feet thick, that surround the poles for hundreds of miles; they are made up of dust and ice, mixed up, laid down by the Martian winds. The bands tell us that the deposition process must vary, over the years. Or the millennia, anyhow. But what caused the variation? We’ve got three possible mechanisms. First, maybe the eccentricity of Mars’s orbit changes.”

“Why should it?” Stone asked.

“Mars is a lot closer to Jupiter than we are; Jupiter’s mass is capable of a lot of perturbation. Or maybe the tilt of the planet’s axis changes.”

“I can see how that would happen,” Stone said. “That bottom-heavy southern hemisphere would make a hell of a difference to Mars’s moment of inertia. The whole damn thing must wobble like a spinning top.”

She smiled. “On geological time scales, yes.”

“And what’s your third mechanism?”

“That the heat output of the sun changes, in some way we don’t understand.”

Bleeker frowned. “But that would change the Earth’s climate.”

“That’s right. And that’s why the layering is a good reason for going to the poles someday. Mars is like a dusty mirror, Phil, Adam; every time we look into it, we learn something about the Earth.”

They were silent for a moment, digesting that.

York felt pleased with herself. Even if they learned nothing else, if she could puncture their complacency, make them think about the significance of the flight they were likely to take, she’d have achieved something.

She glanced again at her polar blowup. It was actually of much lower quality than the images taken by later generations of probes, which had concentrated on equatorial landing-site mapping. Because of the Mars landing program, paradoxically, much less was known about the planet as a whole than might otherwise be possible.

And it was in the hands of guys like these to make it all worthwhile.

Adam Bleeker said, “I’d guess the high-latitude problems would also rule out the site you’ve marked far to the south there, Natalie.”

“I guess. But it’s another interesting site. That’s the Amphitrites Patera: an ancient volcano, much older than the volcanic plains in the northern hemisphere. We don’t fully understand how it was formed. Maybe the vulcanism there was sparked off by the huge impacts which created the massive impact craters in the south. You see these mustard yellow spots in the center of the southern fields: that’s Argyre and Hellas — huge, ancient impact basins, more than three billion years old. Hellas is bigger than anything we’ve found on the Moon — bigger even than the Mare Imbrium, for example. Hellas is where the Soviets put down Mars 9.”

Stone whistled. “That’s what you get for setting up shop next door to the asteroid belt, I guess.”

Argyre held a Stars and Stripes.

“You’re suggesting we should try for Argyre?” Bleeker asked.

“It’s a possible. Argyre is obviously very ancient, and very deep. But the basins are surrounded by concentric rings — mountain chains, actually — which would be hard to negotiate or land on.

“Now,” she went on, “you can see that the rest of the action is in the western hemisphere. This scarlet area, sprawling over into the north, is the Tharsis Bulge: on average, more than five miles above the surrounding terrain. And these crimson spots are the great shield volcanoes.” She pointed. “Ascraeus, Pavonis, and Arsia Mons; and here, to the northwest, is Olympus Mons: 370 miles across its base, with a caldera fifty miles wide. Olympus is so big it pokes its way out of most of the atmosphere. So you get orographic clouds, formed when the air has to move up the slopes…”

“Sure,” Bleeker said, “but I hear Olympus is so huge that it wouldn’t be so spectacular from the ground.”

She shrugged. “Maybe. Look at this.” She hunted about on the pin board on one wall, until she found the image she wanted. She passed it to the astronauts. It was a perspective view of a huge volcano; a cliff, sharp and well delineated, marked out its nearer rim. “That’s a computer image, an oblique view, faked up from Mariner data.”

Stone pointed to the cliff. “How high is that?”

“The scarp? Oh, three miles.”

“Jesus. A three-mile-high cliff?”

“Give or take.”

They were both staring at the cliff image. Bleeker held up his hands in mock surrender.

She suppressed a grin. Astronauts were easy to impress if you pushed the right gosh-wow buttons.

Stone said, “I see you have a couple of flags on top of those big volcanoes.”

“Yeah. Olympus Mons is the youngest, and the tallest; and the youngest lava flows on Mars emanate from it. But Olympus is seventeen miles high—”

“Too high for aerobraking,” Bleeker said. “And I guess that would rule out the other Tharsis volcanoes also.”

“Okay,” Stone said. “To the east of Tharsis I see a ragged blue streak, stretching along the equator. I guess that’s the Mariner valley.”

“Yes. Valles Marineris. The great canyons: two and a half thousand miles long, four miles deep, and over a hundred miles wide. We know that the Valles system wasn’t formed by water. A lot of the individual ‘canyons’ are boxed in. So water couldn’t have gotten in or out of them; we’re looking at geological faulting here, like the Rift Valley in Africa.”

“The whole valley looks as if it’s flowing out of your Tharsis Bulge,” Bleeker said.

“Yeah. And we don’t think that can be a coincidence. Maybe when the bulge was uplifted, magma withdrew from around it, which would have cracked the surface. There would have been earthquakes and extensive faulting.”

“I see we could maybe go for the Valles Marineris itself,” Stone said.

“Maybe,” York said. “This flag is actually in a tributary called the Candor Chasma; we’ve seen layers in the canyon walls here, so we’d be able to get clues to the canyons’ origins.”