“Venus had no moon, Earth one, Jupiter four. Mars should have had two. Since they couldn’t see them, they were probably small. And close. Therefore fast.”

Peter laughed. “Swift must have been a smart man.”

“Or his source. But it was still blind luck. The sequence being a coincidence.”

They stopped on another spallation ridge, from which they could see the rim of Swift Crater, as a nearly buried ridge on the next horizon. A small gray rocket plane stood on the black dust like a miracle. Above them Mars filled most of the sky, a vast orange world. Night was falling across the eastern crescent. Isidis was directly above them, and though he could not make out Burroughs, the plains to the north of it were patched with great white splotches. Glaciers meeting up to become ice lakes, and the beginnings of an ice sea. Oceanus Borealis. A corrugated layer of clouds lay pasted right against the land, reminding him suddenly of what Earth had looked like from the Ares. That was a cold front, coming down Syrtis Major. The pattern of white clouds was just what it would have been on Terra. Spiraling waves of condensation particles.