And now, just a few decades after humans first reached it, we’ve visibly wounded it, she thought. Whatever the outcome of all this in human terms, it must be a tragedy for the Moon.

But some of the vapour was dissipating, great wisps of it branching away from the Moon. Perhaps it was escaping from the Moon’s gravity well altogether.

Maybe the new atmosphere wouldn’t stick.

She watched anxiously as the flower of steam blossomed on the surface of the Moon.

Now Sixt was using the Air Force glasses. “Oh, my God,” he kept saying. “Oh, my God.” Over and over.

Jays sat down in a rickety old chair that had once, it seemed, belonged to Edwin P. Hubble, who had used this telescope to observe distant galaxies, and so figure out that the universe is expanding. Jays craned his head back, and pressed his eye to the cylindrical eyepiece.

It took him a few moments to figure out how to see. He had to keep one eye closed, of course, and even then he had to align his head correctly, or his view would be occluded by the rim of the eyepiece.

A gibbous disc floated into his view. It was a washed-out grey with a splash of white at one part of the edge.

It was, of course, the Moon.

And he could see the vapour fountaining from its invisible source on the Moon’s far side. Some of it was escaping the Moon’s gravity. But most of it was falling back to the surface, and creeping sluggishly over the face of the Moon.

Right now, the vapour formed a loose cap, sitting over the South Pole region. It was growing, but with almost imperceptible slowness. It was like watching a mould spread across a smear of nutrient in a petri dish.

But it wasn’t growing uniformly. It seemed to be pooling, in the deeper craters and valleys incised into the Moon, before flooding on. In fact, it seemed to be flowing generally north-east — into the mouth of the Man in the Moon — avoiding the brighter area in the south-west corner of the visible face.

He knew the reason for that. The brighter area was the lunar highlands, older and higher than the grey areas, the lava-flooded maria. The volatiles Meacher had liberated were pouring over the Moon’s surface like fog, seeking out the low points, the crater pits and the valleys, the lava seas that flooded the great basins.

In a deep mare to the south — that must be Mare Nubium, he thought — he could almost see the surge of the air as it flowed, a bowl of atmosphere sloshing against eroded rim mountains like dishwater; and at the leading edges of the flood there were waves, hundred-mile crests distinctly visible, reflecting back from the basin’s walls like ripples in a bathtub, moving with a slug-like slowness.

It was, he thought, the first stirring of a new geography.