Bare metal on the Moon is tougher than it sounds, yet still almost certainly doomed on billion‑year horizons. Vacuum halts oxidation, and no atmosphere means no rain, wind, or biological decay, so corrosion chemistry barely runs at all compared with Earth.
The real executioners are micrometeoroids and temperature cycling. Hypervelocity dust grains and small impactors, hitting at tens of kilometers per second, sandblast and crater any exposed hull; thermal expansion and contraction between blistering day and deep‑cold night drive fatigue cracks through alloys and composites. Over immense spans, even centimeter‑scale impacts can progressively fragment a lander, while impact gardening and regolith transport slowly bury or disperse the fragments beneath churned soil.
So a harsh verdict follows for intact detectability. To stay recognizable and resolvable after such durations, a visiting civilization would need deliberate macro‑scale engineering: megastructure‑level mass, armor of thick rock or regolith shielding, perhaps emplacement inside a lava tube to avoid direct bombardment, and materials engineered against creep and sublimation, such as refractory ceramics or metallic glass. Planetary geology adds another constraint: large impacts repeatedly rework the upper crust, so even shielded artifacts risk being faulted, buried, or melted. Given those requirements, a casual, Apollo‑class visitor is almost ruled out by the timescale alone; what might endure would look less like a lander and more like infrastructure carved into the Moon itself.
That hypothetical relic, half structure and half modified terrain, would blur the line between artifact and geology long before any later astronomers tried to find it.
