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.”