Under a near-intact crust, the sphere was riddled with pockets and chambers. The core was picked out by a hard, dark blue knot. “A rotten apple.”

“It’s the deep structure of the Moon.”

“Right.”

He explained how he’d produced this image.

The Moon was a quiet planet.

It wasn’t just the lack of air. The Moon had some seismic activity. In fact the heaviest quakes were imposed by the Earth, every month, “deep forming Moonquakes” caused by the dark tides Earth raised in the rocks of the waterless Moon. And there were occasional “shallow Moonquakes” — smaller, isolated events, of which nobody knew the cause. There were even occasional landslides, caused by impacts or quakes.

But the Moon’s seismic violence was only a hundred-millionth part of the energy that racked the Earth.

On the airless Moon there could be no sound, of course. But the Moon, paradoxically, transmitted sound well, through its solid structure. The Moon had “high Q’, Henry told her. That meant that when you hit the Moon, as he had done with his implanted charges, it would ring like a bell. And if you set a seismometer on the surface, it could pick up your footsteps as you lumbered away in your spacesuit.

What all this meant was that Henry’s networks of seismometers were a powerful tool for unravelling the meaning of the rocky waves that passed through the Moon’s interior.

“…I have a database and analysis program here called BOB II,” he said. He brought up lists of commands. “A neat piece of work. Command-drive, interactive. It’s adapted from the data analysis suite VDAP uses — the volcano disaster people — for the real-time analysis of time series data of seismic events in crisis situations. And—”

“Henry, I don’t care about the software. Tell me what this means.”

For answer he reduced the Moon’s image, and paired it with another. “This is what we called a puffball rock, from Edinburgh. Just a couple of feet or so across. Rotten with Moonseed, ready to blow.”

It was an irregular elliptical shape, something like a potato. But it showed the same pattern of pockets through its structure as the Moon.

“You’re telling me the Moon is a puffball rock.”

“I’m telling you the Moonseed has infested it, as it did rocks on Earth.” There were differences of detail, but the patterns of the two infestations were the same. “You can see the relative scaling of these chambers — the way they cluster, like bubbles butting up against one another. We find the same in smaller rocks. Even dust fragments.”

“…The Moonseed is all the way through the Moon,” she said. “That’s what this means.”