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First the situation, then the question. Here's the situation:

An Earth-like planet had a civilization similar to ours, but when it emitted its first radio signals, a nearby alien observation post did an evolutionary reset of the planet by enveloping it in a shroud that wiped out the civilization via the old greenhouse treatment. Many years later, a human craft sets down on the planet and discovers the ruins of the civilization. This is where it gets tricky.

Since the planet is in the same general galactic neighbourhood as Earth, and since, in my story, there aren't many planets in the galaxy where advanced races ever develop, it feels too coincidental for the humans to find a planet where there are relatively recent ruins.

So I'd like to push the destruction as far back in time as I can while still leaving recognizable ruins for the humans to discover. I wish I could push it back millions of years, but there wouldn't be any ruins left and the humans aren't going to look for any esoteric traces deep in the ground.

So the question is:

Given that the civilization was technologically similar to ours mid-20th century, and given that the planet is being overheated by a planetary shroud, how many years back can I push the start of the destruction process and still have recognizable ruins when the humans arrive?

I know the pyramids and other stone structures have lasted thousands of years, but they're typically in arid lands unlike the wet and noxious world I envisage the ruins in my story will have to deal with. It doesn't seem like ruins would last very long in a greenhouse world, unfortunately. I'm guessing a few thousand years at most. Tell I'm wrong, please.

By the way, I realize the answer depends, in part, on how quickly the shroud overheats the planet, but all I can say to that is that the process needs to be done in a way that allows only simple life forms to ultimately survive it.

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    $\begingroup$ You want tropical pyramids? Try Mayan $\endgroup$ – candied_orange Jul 20 '16 at 3:51
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Many ancient sites have some level of traffic or occupation that keeps much of nature's incursions away. See Chernobyl for what can happen in 30 years. But it is still recognisable.

Ancient sites in South and East Asia were rarely just abandoned. They were often sacked and damaged first.

Thailand

  • Ayutthaya 13th-18th century

  • Sukhothai 13th-15th century

  • Lopburi occupied for more than 1,000 years

Indian Subcontinent - Sirkap excavated after "discovery" - Nalanda - Madhya Pradesh

So there are discoverable and noticeable ruins that last centuries. Although they'd require long lasting construction materials such as stone. Remember many cities have stone foundations and ground levels before skyscrapers.

Remember also that destruction by greenhouse effect makes some parts of the planet wetter and hotter, but desertification will occur in other parts.

I think it's totally reasonable to discover

  • easily recognisable ruins for 800 to 1000 years.
  • Buried or covered mounds that have surprisingly straight lines and right angles for up to 5,000 years
  • Satellite survey with near infra read and could show buried straight lines and right angles for longer.
  • Satellite survey with ground penetrating technology could show 25,000 years of history. Especially mid 20th century rubbish dumps.
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    $\begingroup$ Good answer. It confirms a lot of what I was thinking. I was imagining them finding suspiciously regular-shaped mounds. But as you say - and I agree - that would only buy me a few thousand years. As for the satellite surveys, the humans will visit the planet in a small craft that's designed for short planetary recon missions, so probably it wouldn't be able to do such surveys, so that would rule out finding the really old ruins. Then again, the story is set some decades in the future, so maybe the craft could have such a capability. Hmm...intriguing. Thanks. $\endgroup$ – Arbutus Jul 21 '16 at 2:20
  • $\begingroup$ Satellites we have today with near infrared are 400kms up and rather small so if you have the technology for small planetary survey you can have the technology for ground penetrating radar. But if your story needs our heroes to find a ruined Big Ben, or Capitol Hill, make it so. $\endgroup$ – paulzag Jul 21 '16 at 13:16
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Short answer: ceramics! Especially modern bricks.

Ceramics are created by firing stuff in a kiln at temperatures way beyond any greenhouse effect, so such piffling heat is not going to bother them. They are designed to take the weight of all the rest of the wall above them, so are physically tough.

Brick buildings will eventually fall down, and bricks will be subject to erosion and frost shattering like any other rock (bricks are basically artificial rocks). However, there are going to be lots of places where the humans are finding tons and tons of suspiciously rectangular 'stones', and the stumps of brick walls may still be standing. Some of the stones will have things like "London brick company" stamped into them!

Anything else made of ceramic - roof tiles, crockery, some paving slabs, bathroom sinks and toilets - will also behave like a (more delicate) rock. There will be shattered and eroded bits of these all over the place too.

In fact, even if it happened millions of years ago, and all the bricks eroded to fragments, a geologist would still (eventually) figure out they were artificial. The geologist would find some weird conglomerate with these odd inclusions (the brick fragments) in it. They would analyse them are discover they were formed from clay minerals at stupidly high temperatures, 1 atmosphere pressure and in a high oxygen environment. That's not a combination that occurs much (at all?) in nature - usually to get the high temperatures, you are also buried in deep in the earth at high pressure, and there is no free oxygen.

Not ceramic, but breeze blocks/cinder blocks are also designed to be physically tough. They are concrete, which is (sort of) artificial limestone, so water will very slowly dissolve them, as well as all the erosion which will affect bricks. But in certain environments, again there will be all these rectangular rocks...

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  • $\begingroup$ The OP said "I could push it back millions of years, but there wouldn't be any ruins left and the humans aren't going to look for any esoteric traces deep in the ground". So analysis of sedimentary rock formation isn't what he's looking for. $\endgroup$ – paulzag Jul 20 '16 at 12:15
  • $\begingroup$ As Paul points out, the situation of my story makes it unlikely the humans would be able to analyze any really old traces, like the conglomerates you mention. But them discovering something like cinder blocks would be possible. More food for thought. Thanks. $\endgroup$ – Arbutus Jul 21 '16 at 2:30

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