I've always wondered about this. Let's say civilization evolved a billion years ago on Mars or Venus to the level we are now. Would we ever be able to detect it? If they put up satellites, could some still be orbiting, or would they all have degraded by now? What if they really wanted to leave some sign they were there? What could they do? Venus would be really hard unless satellites work. But maybe Mars?
I imagine there would be residue of artificially created nuclear materials that would still be detectable a billion years later. Some half lives are measured in sextillion/quintillion years.
I don’t have a reference handy but I remember reading there is a candidate period around 55 million years ago. The temperature rose rapidly and there are elevated heavy metals and isotopes in the crust from that time period.
