She sat down beside him. She pressed her feet into the face of the boulder and pushed, alongside Henry.

“Rock rolling,” Henry said between grunts of effort. “No geology field trip is complete until you’ve sent a boulder crashing down into a caldera, or a forested hillside—”

The boulder came out of its regolith socket with a grind he felt through his knees. With an eerie grace, the rock tipped forward. He tried to keep pushing, but it was gone, and there was no pressure under his feet; he slid a little way down the slope.

He leaned forward to see. As the rock started to fall it was a little like watching some huge inflatable, on Earth, bounding slowly down a hillside; but at length, as the low gravity worked in the resistance-free vacuum, the rock picked up speed. He watched it until it had plunged out of sight, in the deep shadow of the rille. It left a trail in the regolith, a line of shallow craters that looked as if they had been there for a billion years.

He listened for a while, but there was, of course, no noise, no crack as it reached that remote bottom.

“Um,” said Henry. “Kind of fast. Suddenly I feel vertiginous.”

On his butt he worked his way back up the slope, and stood up, yards from the eroded rim. He had left a track like a sand worm in the regolith. When he stood up his butt and legs were coated with dark grey dust; he tried to beat it out but only succeeded in grinding the stain deeper into the fabric.

Geena was surveying the area. She pointed. One set of tracks continued from this point, deeper into the rille.

The ghost of Jays Malone was close here, he thought.

She said, “You ready?”

“Let’s get it over.”

She took the rope from her shoulders, and knotted it professionally around Henry’s waist, taking care not to snag his backpack or his chest controls. Then she wrapped a length of it around a Chevy-sized rock, and took some slack herself.

For a moment they faced each other. Henry could see himself reflected in her gold visor, slumped forward in a simian pose under the weight of his backpack. But he could not see Geena’s face.

Behind her, he could see the camera on the Lunar Rover fixed on them, watching analytically.

He ought to say something. But this was Geena, for God’s sake. They were divorced. In full view of the world, what were they supposed to say now, as he prepared to confront an alien life form?

She said: “I’ll be here.”

“I know.” He licked his lips.

He thought he could see her nod, inside her helmet.

He picked up his tools and turned.