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Suppose human race in its current state disappears at once, every human being, leaving everything the humanity has created. All the other forms of life remain.

  • How long will the civilization traces still be detectable by another civilization which accidentally comes to the Earth? What if it has technical progress level same to ours and what if they have much more advanced technologies?

  • What are the traces of our civilization which will remain detectable longest? Will it be spacecrafts on the orbit? And what apart of them?

By detectable let's mean state of an object which allows one to say for sure that it's been created by intelligent civilization.

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Billions of years.

Future visitors will find rock strata with clear signs of the Anthropocene epoch. It will contain novel minerals that incorporate plastics, glass, and other refined materials.

Further detailed searches will locate fossils. Even if a car, for example, has been competely eroded, it will show traces of the metal shell that was originally burried in the mud. Organic matter will rot but synthetic polymer will not, and will be encapsulated: larger samples can be found with enough effort.

Increases in erosion due to farming and other operations will be reflected by changes in sediment composition and increases in deposition rates elsewhere. In land areas with a depositional regime, engineered structures will tend to be buried and preserved, along with litter and debris. Changes in biodiversity will also be reflected as will species introductions. Litter and debris thrown from boats or carried by rivers and creeks will accumulate in the marine environment, particularly in coastal areas.

Then there are the footprints and machines on the moon, which will have a datable thickness of dust coving everything.

So I have to say that as long as the Earth exists, our imprint will remain detectable. That will be about 5 billion more years, before the final great recycling.

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  • $\begingroup$ The signs would be there, but can an alien civilization detect them successfully? $\endgroup$ – Youstay Igo Sep 12 '15 at 15:19
  • $\begingroup$ That wouldn't be the case if there is significant glacial activity to scrape the surface clean or lots of flooding to cover coastal areas. Deep mines might be preserved but would they be recognized as artifacts of intelligent activity or though to be some geologic process? Even the moon eventually will be resurfaced via meteorite impacts. We don't even know how long plastics will take to degrade (since we haven't had them that long), perhaps there are biological or geological processes that can degrade them much faster than we think. $\endgroup$ – Jason K May 26 '16 at 15:02
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    $\begingroup$ Scraping the surface via glaciation will just move the gunk to another location, even concentrating it. Seafloor sediment will still occur. $\endgroup$ – JDługosz May 26 '16 at 19:26
  • $\begingroup$ I agree though that if the future explorers are looking in a single region, the amount of evidence can vary substantially. Only through a global survey of the geologic record will the presence of this epoch (and all the othes) be known and characterized for certain. $\endgroup$ – JDługosz May 26 '16 at 20:07
  • $\begingroup$ Last but not least all the decay products of enriched uranium and other radioisotopes will be clearly detectable for millions and billions of years... $\endgroup$ – fgysin Jan 10 '17 at 12:01
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This depends on which structures created by the humans are left standing. We are talking about cities here. Skyscrappers. Mills, factories, production units. Libraries.

How many (and in which condition) of these are standing? And where, too? Long after the human race (I'm counting 50,000 years here) is extinct, an alien spaceship lands on earth. Even after 50,000 years of storms, cyclones, rainfalls, scorching heat, meteorite impact events and earthquakes, you would be amazed to know how much of our current structures would be immediately detectable. Not detectable as in "oh wah! look this is the statue of liberty!" but as in "here are some fragments of a large figurine. most parts are missing but here is the right foot and some portion of the left shoulder."

With dating methods, they can easily date the remains and know that an advanced intelligent race dwelled on this planet nearly 50,000 years ago. They could excavate a museum and find some fragmentary pieces of earlier civilizations which would date to 55,000 years or so (egyptians). In regions where fossilization conditions are favorable, they might excavate human skulls and limbs and form a good understanding of our anatomy.

This is if the alien civilization was smart enough and was technically advanced enough (nearly our level with 10% margin).


If they don't have any dating methods available, then they can definitely deduce that a very advanced (they don't know dating techs so definitely our current tech progress is greater than theirs when they land) race used to live on this planet. But they will not be able to deduce when they went extinct.

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  • $\begingroup$ They will be able to get a rough idea of age in the same way that Victorian geologists could get a rough idea of the age of fossils, by summing the thickness of all the overlying rock strata up to their recent. (Victorian geology estimated trilobites to be several hundred million years old. Physicists said that the sun could not possibly stay hot for that long. The Geologists were right! ) $\endgroup$ – nigel222 Jan 9 '17 at 12:28
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Millions, or Possibly Billions of Years.

Air, water, and tectonics will eventually devour everything left on the face of the earth, and I suspect that anything in LEO will eventually fall out of orbit due to drag from the attenuated atmosphere.

That said, we have left a whole lot of junk lying around on the moon, some of it surrounded by hundreds of human footprints. With no atmosphere to speak of, that junk could last a long time. For bonus points, you could have your hypothetical aliens notice light bouncing off one of the retroflective mirrors we left behind.

The folks at space.com seem to think that the moon will last until the sun changes into a red giant. This gives you an upper bound of 5.4 billion years. By that point, however, all the other life on earth will likely be gone as well.

A more reasonable upper limit may be far less, however. The sun will get really hot in another 1.1 billion years (see prior link), and we don't really have any long-term data on the effects of unshielded exposure to the solar wind. Still, I think a few million years would be a safe bet, and even if you can't figure out where it came from, evidence of space travel would seem to imply "civilization" of some sort.

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  • $\begingroup$ Provided the moons final demise is the red giant sun and not being hit by a large rock at speed. $\endgroup$ – RhysW Jan 9 '17 at 10:06
  • $\begingroup$ We will leave many traces here on Earth for hundreds of millions of years. Every manufactured object dumped in landfill or simply abandoned is a potential technological fossil, just as every animal or plant can form a biological fossil. Metals, plastics and especially ceramics do not decay quickly like flesh and bones, so they are far more likely to form impressions in sediment that then becomes rock. Also large scale civil engineering will leave otherwise inexplicable discontinuities in rock strata. $\endgroup$ – nigel222 Jan 9 '17 at 12:03
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Buildings will remain, at least as traces. there's a nice documentary on that, you should take a look at it. As far as I remember, it didn't consider the most valuable part of our civilization anyway. From the information point of view, our culture is doomed for the most part. Books are perishable, in a geological time scale. Depending on the material, they will last from 10 to 10000 years. Even more CDs, DVDs, hard drives. in about 100 years or so, all of these data media will disintegrate. what will be left? clay tablets, epigraphs, and anything that is written on resistent materials. There are many examples, some of these are very surprising, but very rare. look for Jordan_Lead_Codices on wiki. If aliens, or even future humans from 30k years in the future would find these, what they could understand? we are still having troubles understanding many dead languages which left some kind sort of written corpus.

Another point of view is, what if the only things that will remain of our civilization relate to just one aspect? Egyptians in example, didn't build their homes with stone, not even the palaces for the most part. So we only know about them through their tombs and some religious monument. What would aliens know of us if, in example, only military bunkers and research facilities deep underground would remain?

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  • $\begingroup$ Most modern buildings have concrete foundations. Many use bricks. These can fossilise. So they'll be able to get a pretty good idea of a whole town or city. Some cities will be lost to erosion but those built on land that is moving downwards over geological time will not. $\endgroup$ – nigel222 Jan 9 '17 at 12:13

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