When will the last unambiguous evidence of mankind disappear?
Humanity just disappears tomorrow, let's say it is like the Thanos-snap but it kills everyone. How long until the last unambiguous evidence of our existence disappears? What was this last relic? The faint spectres of electromagnetic signals which will travel space forever, odd isotopes ratios and material compositions and slightly odd geological patters are considered ambiguous. A Voyager probe which still looks like something designed by intelligence and not like a very metal-rich asteroid is unambiguous.
Assume that another civilisation's exploration mission with near-future tech and the capability to examine the solar system for 100 years comes looking. They got the time and huge resources on hand and will study the major objects until they know about them as much as we currently know about Earth.
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Enriched Uranium
Uranium 235 has a half-life of 703,800,000 years. The concentration of U-235 relative to U-238 is fairly low (0.72%) in nature. Humanity has managed to increase that quite a bit. Perhaps a bit more than we should. In any case, even if the surrounding concrete, steel and other materials of research & power nuclear reactors, nuclear submarines, nuclear aircraft carriers and other nuclear devices (not so much the nuclear weapons - if they use plutonium then the half-life is much shorter at a mere 24,100 years) have disintegrated due to various natural processes, the enriched uranium will still be detectable as a non-natural object, for a very long time, easily billions of years.
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