Longest lasting structures

[Lusankya]

How likely would you be to get fossils of human artefacts? I mean, let's say that I drop a bicycle/gun/rubix cube in a swamp and it ended up getting covered in silt and fossilised. Would the aliens be able to tell from the fossils that they were crafted? Housing foundations, especially, are easily covered, and sometimes we build in swamps.

And former rubbish dumps would possibly leave an interesting geological signature for posterity - a nice mix of metals and organic waste. Maybe one of the geologists could tell us how likely such a combination is to occur in nature.

And, while this isn't a "structure" - we've pretty much screwed up animal distributions. Even if we disappeared tomorrow, we'll end up with closely related species living on opposite sides of the planet. Unless you have something moving these animals around, how do you explain (say) farm animal distributions without spitting in the face of evolution?

[Skylon]

The stuff on the moon may last, but how obvious is it to find? Surely most of it is covered in dust from when the lander took off?

Only the upper ascent stage of the LEM ever launched off the Moon. The lower descent stage was it's launchpad, therefore the upper stage engine never fired into the dust on the surface. No air means no displacement when the engine fired, so again no dust got kicked up. You can see it in videos of Apollo 15, 16, and 17 when they set up cameras to televise the astronauts returning to orbit.

The later Apollo missions also covered a lot of distance, meaning there was a lot of area for the astronauts to leave crap lying around (and of course, make footprints). According to a NASA site, the Apollo 17 lunar rover covered 30.5 KM.

[erik_t]

Considering that humans have died virtually everywhere on this planet I'd say there is going to be a fair amount of fossilized human skeletons even dozens of millions of years in the future.

I'm not sure how many Fosillised Humans their would be, although their are a fair number of us, we havn't been around for very long and only a very tiny proportion of all deaths would form fossils under special conditions. Also fossils would be hard for a alien to find, they would have to actually look and often dig for them rather than just have they lying around.

Anyway could you tell how inteligent humans were from a fossil?

If you have some rudimentary knowledge of how higher vertebrates heal, then any brain surgery is going to be a massive indicator. So will intentionally-fused vertebrae. Consider our knowledge of trepanning, partially from skulls we've found.

Also of note will be the relatively huge number of osteoporosis-ized spines from all the old folks we keep alive. These people couldn't live to such an old age without a reasonably advanced society.

[Junghalli]

It's worth pointing out that we have been able to find fossils and artifacts of things like Australopithicines and Homo Erectus that lived millions of years and had much less material culture than we did. So I'm fairly confident if the aliens (or a subsequently evolving terrestrial sapient species) did a thorough investigation of Earth's fossil record they'd be able to find at least some traces of us.

My bet on the artifacts that are most likely to still be around in deep time are ceramics and plastics, especially if they get buried. The former already have a chemical composition similar to fossils, and the latter are basically indigestible rubbish to present decomposers (although something might eventually evolve that can break them down). I can easily see lots of buried plastic and ceramic artifacts becoming part of the fossil record.

[Sky Captain]

How about underground structures leaving some strange naturally unexplainable imprints in geological strata? I`m thinking of all those tunnels, pipes, cables, building foundations what are under every major city?