I don’t know how accurate it is, but I’ve heard that if humans were to suddenly go extinct, almost all traces of our civilization would be gone in 50,000 years from now.
With that in mind, how would we even hunt for past alien civilizations on other planets? Will every planetary expedition also require a global mining operation that hopes to uncover preserved artifacts of the past? Or are there easier ways of doing it?
My thought would be that artificial satellites, orbiting abandoned spacecraft and interstellar probes whose point of origin can be traced back to a specific solar system would probably one of the first signs of an advanced civilization.
While most signs of a civilization would vanish, some would survive for a long time. Stainless steel, for example, can last for tens of thousands of years.
This is a good point and begs another question – is even tens of thousands of years long enough to yield a satisfactory search result?
Given the age of the galaxy in the universe, it’s more likely that were separated by tens or hundreds of millions of years, than just a few 10,000 years.
I’d say any sign of life on the surface is a good indicator of the evolution of intelligent life sometime in the past.
Some substances are definite signs of intelligent life. Pure aluminum, for example, is almost non-existent in nature. So if you search a planet and find a roll of aluminum foil, you know there was a technological civilization on the planet at some point.
How about gold?
Gold is a great conductor and any advanced civilization would use it. So if someone hit a motherload of gold bars that had been refined, its a pretty good evidence of an advanced civilization.
Our civilization has had an obvious effect on the plant and animal life on this planet. We’ve introduced species into areas where they didn’t evolve, we’ve eliminated species, and we’ve reduced various ecosystems. So even if all humans and human artifacts disappeared tomorrow, our presence would be obvious to an alien observer who studied the planet’s ecology.
But I wonder how long that would last? Domestic species would die out or go feral. Wild species will fill in various ecological niches. Species would continue to evolve and differentiate. At some point, would the post-human ecology be indistinguishable from a natural ecology?
or just look for buried obelisks on the moon 
Almost.
But objects such as granite gravestones would easily last far longer. Projectile points shaped by early humans more than 50k years ago look much the same today as they did when discarded.
Along the lines, what could we use to contact a future civilization millions of years from now just to let them know that we were once here.
I don’t know about contact but it terms of human presence? We’ve left some truly unmistakable engineering acheivements.
Things like enormous quarries where stone has been cut for 200 years straight down? That’s something that will be evident for a long time. Certainly more than 50,000 years. And it will be an hard sign to ignore that someone was thinking and using here in the past.
Radioactive waste. Stuff remains active and detectable for tens of millions of years. Any advanced alien civilization would probably leave behind at least a little detectable nuclear waste.
I would think one could postulate a massive extinction event brought about by mankind’s disappearance. It would be known as the Moovian discontinuity as alien scientists wonder what could have caused approximately 1 billion cows to die in a five year period.
The sheep will be pretty much f-ed as well.
And if the foil’s hat-shaped, we’d know that like our own species, the aliens often fell prey to crazy conspiracy theories.
Another good way to determine the presence of long-lost civilizations is by their garbage heaps. Many archaeological finds are discovered by examining the piles of garbage left behind by previous cultures – often buried and undisturbed for thousands of years. Even today, examining modern landfills can reveal tantalizing secrets about our recent past.
Yeah, I’m pretty sure a satellite in geostationary orbit will stay in orbit for a really long time due to negligible atmospheric friction.
One thing is sure; you’d need to be ve-e-wy, ve-e-wy quiet.
I declare the Moovian discontinuity and the Elmer Fudd reference co-winners of the thread. 