B How long would remains be detectable?

[newjerseyrunner]

Say human beings went extinct. How long would it be detectable to a visiting alien that intelligent beings once lived there? I'm not talking about infrastructure that disappears relatively quickly.

For example: we knew an asteroid impacted Earth 65mya long before we found the crater because of a sedimentary layer of ash that envelopes the planet.

In the early 1900s we pumped tons of lead into the air, then during the later half, we dumped tons of nuclear waste into the air. How long would the geology show the telltale signs of a fire-controlling / nuclear capable species?

 

[DaveC426913]

Well, of our workable metals and other resources such as oil, have been extracted from the top few km, refined into pure forms, and distributed across the surface of the Earth.
It'll require subduction to return them, which is hundreds of millions of years.

It depends on how sophisticated their detection systems are.

 

[lifeonmercury]

Say human beings went extinct. How long would it be detectable to a visiting alien that intelligent beings once lived there?

1 billion years

 

[Mono Kakata]

Are you ignoring fossilization and paleontology in general? Unless all human bodies are somehow vaporized, detecting fossilized remains of billions of human beings will be trivially easy, and for millions of years.

The fossil record of life on Earth before and after the impact you refer to is good, for example, and if something like that body hit us now, it would be just as good.

May I suggest you have a look at a college-level introduction to geology/paleontology? Your questions will be answered there.

 

[Rubidium_71]

If aliens are visiting our star system, maybe they would be able to find objects we created and left on other worlds like the Huygens probe if evidence on Earth was lacking. We've also deposited plenty of devices on Mars which could be found in the far future. Like Dave said, it would depend on what their method of detection would be.