“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.”

“But I’ll bet the landscape isn’t too easy to negotiate.”

“No. Some of the smaller canyons there are a couple of miles deep. If you had several months to survey the place, and some kind of flying machine—”

“But we don’t,” Stone said. “Okay, Natalie. That leaves two places. Both on the border between the old stuff in the southern hemisphere and the volcanic plains in the north.”

“Yes. This one in the eastern hemisphere” — on the opposite side of the world from Tharsis — “is called Nilosyrtis Mensa. It is what we call ‘fretted’ terrain.” She dug out a photograph, this one a mosaic in black and white. It showed a surface uniformly crumpled.

“Christ,” Stone said. “It looks like beaten copper.”

“We think the older, southern terrain has been eroded, here on the border, leaving this irregular, grooved landscape.”

“Looks bloody difficult to land on,” Bleeker said.

“Yes, and you’d need long traverses to achieve systematic surveys.”

“All right. So that leaves one site.”

The final flag was at the western fringe of the Tharsis Bulge, close to the border of the north and south terrains. It was in the middle of a green stripe that cut north to south across the Valles. The green, together with the blue ribbon of the Valles, made a rough upright cross, straddling the equator.

“This is a region shaped by running water. Apparently. There are channels that seem to flow out of the Valles Marineris, and across the northern plains.”

Stone smiled. “So these are the famous water-carved features you tell us about in the Singing Wheel.”