“For a while. But Burroughs is at about the lowest point in that little depression. That’s why it was sited there. Because the sides of the bowl gave it long horizons. No. The mesa tops will get covered too. I can’t be sure how long it will take, because I’m not sure of the flow rate. But let’s see, the volume to be filled is about . . .” He tapped away madly, but his eyes were blank, and suddenly Nadia saw that there was another part of his mind doing the calculation faster than the AI, a gestalt envisioning of the situation, staring at infinity, shaking his head back and forth like a blind man. “It could be pretty fast,” he whispered before he was done typing. “If the melt pod is big enough.”

“We have to assume it is.”

He nodded.

They sat there beside each other, staring at Sax’s AI.

Sax said hesitantly, “When I was working in Da Vinci, I tried to think out the possible scenarios. The shapes of things to come. You know? And I worried that something like this might happen. Broken cities. Tents, I thought it would be. Or fires.”

“Yes?” Nadia said, looking at him.

“I thought of an experiment— a plan.”

“Tell me,” Nadia said evenly.

But Sax was reading what looked like a weather update, which had just appeared over the figures scrolling on his screen. Nadia patiently waited him out, and when he looked up from his AI again, she said, “Well?”

“There’s a high-pressure cell, coming down Syrtis from Xanthe. It should be here today. Tomorrow. On Isidis Planitia the air pressure will be about three hundred and forty millibars, with roughly forty-five percent nitrogen, forty percent oxygen, and fifteen percent carbon diox—”

“Sax, I don’t care about the weather!”

“It’s breathable,” he said. He eyed her with that reptile expression of his, like a lizard or a dragon, or some cold posthuman creature, fit to inhabit the vacuum. “Almost breathable. If you filter the CO2. And we can do that. We manufactured face-masks in Da Vinci. They’re made from a zirconium alloy lattice. It’s simple. CO2 molecules are bigger than oxygen or nitrogen molecules, so we made a molecular sieve filter. It’s an active filter too, in that there’s a piezoelectric layer, and the charge generated when the material bends during inhalation and exhalation— powers an active transfer of oxygen through the filter.”

“What about dust?” Nadia said.

“It’s a set of filters, graded by size. First it stops dust, then fines, then CO2.” He looked up at Nadia. “I just thought people might— need to get out of a city. So we made half a million of them. Strap the masks on. The edges are sticky polymer, they stick to skin. Then breathe the open air. Very simple.”

“So we evacuate Burroughs.”