He reached the walls of the rille. They were shallow, but now they were slick with mud, and their profile seemed to have changed. Further down the rille he could see evidence of landslips, great swathes of mud which had come shearing off the rille walls.

And, down the centre of a valley cut a billion years before by a flow of lava, a rivulet of water was gathering. It was the start of a river which would gather, Henry knew, until it pooled with dozens of others in the great sea that must be forming in the Oceanus Procellarum.

Geena shivered.

He turned to her. “You okay?”

“I think so.”

Her voice sounded thin… and then he realized that he was hearing her, her voice faintly transmitted by the thin new air.

“How about that. I can hear you.”

“What?”

“Never mind. You shivered.”

“It’s just being out here,” she said. “In the open.”

“And not on Earth. I understand. We’re the first humans to walk around unsheltered like this, on another world, in all our history. We’ve had nothing to prepare us for this. If you weren’t scared—”

“It would prove I’m even more unimaginative than you think already?”

“I didn’t mean that.”

“You’re so patronizing, Henry.”

She walked away from him, her feet leaving great glopping craters in the mud. After three paces she slipped, and landed on her butt with a slow-motion splash; gobbets of mud sprayed up around her.

Henry knew, absolutely, imperatively, he must not laugh.

She growled. “Probably the low gravity. Reduced friction.”

“Probably.”

“Watch your step, Henry.”

“Copy that.”

They powered up the comms unit, and set up its low-gain antenna. They put headphones to their ears, and Geena started sending an automated telemetry feed; Henry could hear static, and the chatter of the telemetry.

For long minutes, there was no reply.

“Don’t give up,” Geena said doggedly.

“Yeah. There’s a lot of electrical activity in this atmosphere. It must be hard to punch a signal through—”

The static broke up. It was replaced by a chaotic noise.

At first Henry thought they were picking up the crackle of lightning from some remote storm — could an ionosphere have formed here so quickly? Then his mind resolved the sound.

Cheering.

It was human voices, raised in cheers and whoops, either at Korolyov, or Houston, or both; he couldn’t tell, and right now didn’t care.

His eyes prickled. As if he was the one who had just found a planet full of people who were alive after all, and not the other way around. Damn it.

They climbed the rille wall.