Thus, laden with their gear, they loped away, towards the rille.

Henry stopped periodically, setting in place miniature acoustic flow monitors and seismometers, sensitive to high-frequency vibrations. This was part of the monitoring network he was going to build up around the rille, and whatever lay within it.

For a while they were still tracking the Apollo astronauts” exploration. But now they came to a point beyond which there were no Rover tracks, only footprints: two sets, tracking up and down the incline. Henry could see how the Apollo astronauts” tracks had diverged, as they loped about the hill, taking what samples they could in their haste. But he and Geena had only one purpose now.

They marched directly up the slope, ignoring the meanders of their predecessors, following the line of steepest ascent.

It was a difficult climb. The dust was thick: the slope was almost bare of rock, and the dust and rubble was churned up, mixed and messy dust that gave the mountain its smoothed-over shape. Maybe it looked attractive from afar, but on foot it was difficult terrain. What it meant was that with every step he took dust fell away from his feet, like soft sand, as if he was climbing the side of a dune.

He was out of breath in a few steps.

Still, he persisted.

He paused for a breath. He turned and looked back at the skeletal Rover. It looked like an ugly toy: squat and low, sitting there in a churned-up circle of dust. Its orange fenders and gold insulation were the brightest things on the surface of the Moon. A few yards behind him, Geena was labouring up the slope after him, her arms full of gear, her red commander’s armbands bright.

…He was on the Moon, he remembered suddenly; this was no routine hike.

The return of perspective was unwelcome.

He remembered some of the early, now lost, theories of the Moon’s surface. One geologist called Thomas Gold had warned that the Moon would be covered in a layer of fluffy dust dozens of feet thick. Armstrong and Aldrin would have to drop coloured weights to the surface before they landed; if the weights sank, they would have to abort their landing immediately, before their LM was swallowed. Gold had clung to those views even after unmanned craft had safely settled on the surface, but happily for Apollo 11 he had been proven wrong…

Maybe.

Now that he was approaching the nest of the Moonseed, Henry wondered whether Gold had been more correct than he knew. What if the layers of basaltic strata beneath his feet, infested by Moonseed, were indeed Gold’s dust?

He continued.

He reached a flat crest, and came suddenly on the rille: Schröter’s Valley. It was a gap in the landscape in front of them. It wound into the distance, its walls curving smoothly through shadows and sunlight.

As he walked further, the surface of the mare sloped gently towards the rille rim, and the regolith was getting visibly thinner. The rille walls themselves sloped at maybe twenty-five or thirty degrees.

He stopped, where the slope was still gentle.

The sun was behind him. The far walls were in full sunlight, and Henry could see layers: distinct layers of rock, poking through the light dust coating. They looked like layers of sedimentary rocks on Earth, sandstones or shales, laid down by ancient oceans, the myriad deaths of sea creatures. But what he was seeing, here, had nothing to do with water, or life. The story of the Moon, laid out for him here, was different.

These layers were lava flows. Over hundreds of millions of years, a succession of outpourings had flowed out of the Moon’s interior, covering and recovering the valley floor, building up the ground here.

But then, pulsing out of Cobra’s Head, a lava river had coursed down the slope of the older landscape, a brief band of light cutting savagely into the older layers. The flow cooled from the edges, the hardening rim confining the central channel. Eventually the channel even roofed over with hardening rock, and the lava stream cut deeply into the underlying mare basalt.

But the brief eruption of heat subsided rapidly. The remnant of the lava drained away and cooled, leaving a tunnel in the rock. Along much of its length the roofed-over tunnel collapsed, exposing its floor to the sunlight.

This will do, he thought.

Henry walked along the rille edge, until he came to a place where a boulder, four or five feet tall, was embedded in the inner wall of the rim. He sat down in the dirt, resting on his hands; the regolith crunched beneath his butt. He put his feet flat against the rock and started to push. It was hard to get any traction; the friction between his butt and the ground was so low he kept sliding backwards. Eventually he found a way to brace his arms at an angle behind him, and get more purchase.

Geena joined him. “What in hell are you doing?”

His exertions weren’t budging the rock, but they were lifting him up off the ground, to which the low G only casually stuck him. “Help me. It’s a tradition.”

“More science, Henry?”

“Hell, no. Come on.”

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.

He walked forward, towards the rille. He went over a smooth crest, and started descending into the rille itself. But there was no sharp drop-off; like every other surface here the rim was eroded to smoothness, and the footing was secure. The rope trailed behind him, reluctant to uncurl and lie flat in this weak gravity.