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

The sun was behind him. The far walls were in full sunlight, and now Jays was sure he could see layers: distinct strata in the rock, poking through the light dust coating.

Excited, he described what he saw. “Okay, Joe, I think I can see from top to bottom, one distinct layer about ten per cent, which has multiple layers within it. And another at about forty per cent down, which looks like a solid unit of a somewhat harder, tan-coloured rock, but it’s covered with fines and talus. We haven’t seen to the bottom; I think I’ll get the chance to go further down…”

He felt his heart thump. The rille layers were a record of the Moon’s volcanism, the strata left by ancient basalt floods, driven by an internal heat that had all but died almost as soon as the Moon formed.

The only other volcanic remnants Apollo crews had found had been dug out by impacts, shattered and melted and reformed, scattered over the surface, heavily processed. In the rille walls, though, he was facing true lunar bedrock. What he had come for.

Samples of basalt from the maria — the lunar seas, like Apollo 11’s Tranquillity — would take you back as far as the age of mare volcanism, when founts of lava had flooded the great impact basins. But if you wanted to look further you had to go find bedrock: dusty windows on even greater antiquity, all the way back to the birth of the Moon.

Bedrock was the core of the mission, as far as Jays was concerned. And a big fat sample of bedrock, maybe from deep inside that old rille, would be his trophy fish.

He felt his soul expanding.

Nobody had ever seen this sight before, nobody. And, no matter who came after him, for whatever purpose, no matter how much smarter they were than him, they could never take that away. Schröter’s Valley would forever be a part of him.

He went over a crest, and was now descending into the rille itself. But there was no sharp drop-off; like every other surface here the rille wall was eroded to smoothness, and the footing was secure, the regolith layer thin.

For a moment he thought he glimpsed a stretch of the very bottom of the rille. Something shining there. But that was impossible, of course. It had to be a trick of the light. A scuff on his faceplate.