Anyhow, nowhere did it cause him a problem; the Moon, as a geological field site, would, it seemed, be an accommodating place to work.

In fact, he felt an odd ache as he looked down at the dust billowing around his feet. He wanted to take off his gloves and run his bare fingers through the soil, connecting with the Moon. But that was, of course, impossible; he was the alien here, encased in his bubble of Earth murk, and he must stay that way.

He walked further.

He bent and, with both hands, pulled a big rock out of the ground. He had to push his fingers into the crackling surface, smashing up agglutinates, rock fragments glued together by solar wind particles, to get his hands around the rock.

From above the rock looked smooth, almost flat against the ground, like a glacier deposit. But when he dug it out he found its underside, buried in the regolith, was sharp and angular, and maybe ten times as bulky as the portion that had shown above ground, like an iceberg. And the buried surfaces were sheer, lacking the sheen of zap pits and impact glass of the exposed section.

This rock, casually dropped here after some ancient impact, had been eroded flat by an aeon of micrometeorite rain.

He brushed off the dust and held the rock up to his faceplate.

This was a breccia, a compound lump of rock whose fragments had been crushed, ground, melted, mixed and then bound together in a shock melt. When he turned it in the flat sunlight he could see the sparkle of glass, the recrystallized minerals that were holding this lump together.

This rock, in fact, almost looked like a vesicular basalt — a pumice, riddled with bubbles left by gas. But the heat that formed it came not from volcanism but from the energy of the catastrophic fall of a giant impactor. And he thought he could see that this breccia was in fact itself made up of earlier breccias, breccias nested in breccias like biblical generations, remnants of still earlier impacts. In this one chunk there might be pieces of ancient anorthosite crust, mare lavas, even solidified dregs of the original magma ocean.

He weighed the lump of breccia in his gloved hand. Its weight was barely discernible in the feeble gravity. And yet, just looking at it, he felt echoes of the almost inconceivable violence which had shaped the Moon’s early history, sensed the processes which had formed this rock since, processes unlike any on Earth.

And now Geena was calling him, telling him to come with her to the Apollo site.

He put the rock back where he had found it, back where it had lain for a billion years, and loped away through the morning sunlight.

Side by side, Geena and Henry crossed the few hundred yards to the Apollo site. They kept quiet, concentrating on learning how to walk.