Then also, at the end of the Hesperian had come the brief warm wet period, when water had occasionally run on the surface. These days most areologists thought that this period had been quite wet but not really very warm, annual averages of well under 273Âdeg; Kelvin still allowing for surface water sometimes, replenished by hydrothermal convection rather than precipitation. This period had lasted for only a hundred million years or so, according to current estimates, and it had been followed by billions of years of winds, in the arid cold Amazonian Age, which had lasted right up to the point of their arrival. “Is there a name for the age starting with m-1?” Sax asked.

“The Holocene.”

And then lastly, everything had been scoured by two billion years of ceaseless wind, scoured so hard that the older craters were completely rimless, everything stripped at by the relentless winds strata by strata, leaving behind a wilderness of rock. Not chaos, technically speaking, but wild, speaking its unimaginable age in polyglot profusion, in rimless craters and etched mesas, dips, hummocks, escarpments, and oh so many blocky pitted rocks.

Often they stopped the rover and walked around. Even small mesas seemed to tower over them. Sax found himself staying near their rover, but nevertheless he came upon all kinds of interesting features. Once he discovered a rover-shaped rock, cracked vertically all the way through. To the left of the block, off to the west, he had a view to a distant horizon, the rocky land out there a smooth yellow glaze. To the right, the waist-high wall of some old fault, pocked as if by cuneiform. Then a sand drift bordered by ankle-high rocks, some of them pyramidal dark basaltic ventifacts, others lighter pitted granulated rocks. There a balanced shattercone, big as any dolmen. There a sand tail. There a crude circle of ejecta, like an almost completely weathered Stonehenge. There a deep snake-shaped hollow— the fragment of a watercourse, perhaps— behind it another gentle rise— then a distant prominence like a lion’s head. The prominence next to it was like the lion’s body.