EJECTA — A plain that keeps four billion years of impacts — and dates itself by how many it holds.

The ground is the only witness left.

On a world with no wind, no water, and no drifting plates, nothing erases an impact except the next impact. Every scar simply waits.

So the plain becomes a ledger. A fresh crater lands bright, throwing pale rays of ejecta across the older ground; over time the rays darken to the grey of everything around them and the crater joins the crowd. Land enough of them and new craters begin falling on top of old ones — overprinting the rims, half-erasing the bowls, leaving faint circular ghosts, or palimpsests, where a crater used to be.

Watch one patch and it is chaos: a bright flash here, a rim clipped there. But the surface as a whole is keeping an honest count, and that count is a clock.

Count the scars, read the age.

We cannot carbon-date the Moon. But we can count. An old surface has been exposed to the rain of debris for longer, so it carries more craters per square kilometre — and that density maps to an age.

The panel does exactly this. It tallies craters above one kilometre across, plots their size-frequency distribution — many small, few large, the classic falling line — and reads a model age off the accumulated count. Early on the age climbs fast; the young solar system was a shooting gallery, the Late Heavy Bombardment still pelting every world in reach.

Then it slows. Once craters are packed rim to rim, each new one destroys about as many as it makes. The count stops rising — saturation equilibrium — and the surface can no longer tell you whether it is four billion years old or five. It has simply run out of room to remember.

Left running, the plain fills, saturates, and a rare basin-forming impact resurfaces a patch — and there, counting starts again from young.