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Inspired by this question: How long will traces of the moon landings (e.g. footsteps, the hardware) or other human activity on the moon approximately survive to be recognized as unambiguous evidence of human existence?

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In theory, the footprints on the moon can last as long as the moon itself. However the moon is always being bombarded by micro-meteorites and charged particles from the Sun, putting the life-span of these footprints at around 10 - 100 million years. According to this article, lunar rocks erode at a rate of about 0.04 inches/1 million years.

From past studies of moon rocks collected by astronauts during the Apollo missions, researchers have learned that the rocks erode at a rate of about 0.04 inches every 1 million years. "In human terms, it may seem like forever, but in geologic terms, probably there will be no traces of the Apollo exploration in, let's say, ten to a hundred million years," Robinson said.

But of course these footprints will be unrecognizable long before ten million years. They will probably we recognizable for a couple thousand years.

The American flags on the moon are now all white from the intense radiation of the Sun which faded the colours and are also starting to disintegrate. This passage is found here.

The natural disintegration of the flag's material in the harsh conditions on the moon's surface is to be expected.

Other equipment such as experiments and the LM descent stage will last a very long time. They will decay from micro-meteorites and charged particles but they will definitely be recognizable for a couple of ten thousand years. But traces of fragile metal sheets can be seen in hundred thousands to millions of years which is still evidence of human existence.

Sources:

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    $\begingroup$ I expect the part that will last the longest is either the radiothermal generator for the ALSEP experiments (it's a large, solid lump of metal), or the storage cask for the RTG fuel element (it's designed to survive Earth reentry). $\endgroup$ – Mark Jul 27 at 1:46
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    $\begingroup$ If there will be a close hit by a meteroite of some meters in size, the ejected moon dust may hide all footprints. The time to such a single event is not predictable. $\endgroup$ – Uwe Jul 27 at 14:51

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