There's a story I'm working on where a retrograde colony is forced to settle on a planet that has been terraformed centuries earlier. This colony has forgotten all advanced technology and live in a medieval state, so I'm trying to figure out if these individuals would encounter any traces of the terraforming that took place, and if so, how these traces would look like. So, my question is if a planet was terraformed by an advanced ancient civilization, would there be any "fingerprints" left behind eons after, and if so, how would these artifacts look like? I already envisioned remains such as machinery or satellites in the sky, but I'm curious to see what other people can come up with. EDIT: To give more detail, the flora and fauna of the planet would be very similar to Earth's - the civilization who did the terraforming are human, just as the colonists are, so they both originate from our solar system. I'd imagine the terraforming mostly consisted of introducing plants as well as bioengineered animals that shared similarities with the extinct organisms of our own planet. Specifically, the planet is dominated by small reptile-birds and old world mammals.
$\begingroup$ Other than they very obviously didn't evolve on that world? That the geological record of the world is very obviously at variance with present conditions? That everything on that world is young and unsettled? BTW, how was the planet before terraforming? Did it have life? What happened to it? How thorough was the terraforming? What did the terraforming specifically consist of? $\endgroup$– AlexPApr 22, 2020 at 21:26
1$\begingroup$ This really depends on how terraforming was done. It could have been a semi-natural process lasting millions of years, quick "engineering", with or without bringing new species, or super-quick "planetary cloning". $\endgroup$– AlexanderApr 22, 2020 at 21:29
$\begingroup$ So the biosphere will be a mix of native and Earth-based organisms? This sounds like one popular fantasy book series, but just giving the name of the series here would be a big spoiler... $\endgroup$– AlexanderApr 22, 2020 at 23:13
$\begingroup$ Oh I'm not sure what series you speak of. The organisms themselves would be derived from Earth-based organisms - none of the organisms would have existed before the terraforming. $\endgroup$– ani benApr 23, 2020 at 15:19
There are a few important factors at play:
How wide spread was the ancient civilization?
If the civilization which teraformed the planet was a global one and its inhabitants just suddenly disappeared, then it would take tens of thousands of years for just the above ground aspects to disappear completely. This wouldn't stop future civilizations from directly coming into contact with evidence of cities, vehicles..etc with a little digging.
This articles gives a decent overview of timelines involved for human constructs to disappear back into the environment: [ https://www.discovermagazine.com/planet-earth/earth-without-people ]
How advanced was the ancient civilization
I'll use our modern civilization as reference...
For a time line of ~ 1000s of years since the ancient civilization there would be abundant evidence everywhere. Especially in arid places, the remnants of buildings and structures would last without losing much of their form.
For a time line of ~ 10000s of years much of the evidence on the surface (except for the largest of mega engineered structures) would have disappeared, but with a little digging (mines for instance) a medieval civilization would most likely come across some form of evidence that they weren't the first civilization. Evidence for the largest buildings and massive concrete structures would most likely be detectable on the time scales of tens of thousands of years if they are not destroyed in a cataclysmic process (glaciers, meteors, earthquakes...etc).
On a time scale of ~ 100,000s yrs almost all traces from the surface would have completely disappeared. Evidence would remain the how the land was changed, but without digging things up, its highly unlikely any surface structures would remain. Advanced medieval civilizations would likely come across buried evidence, however they would lack the knowledge and technology to deduce much about the things they found.
~ 1,000,000s yrs its doubtful that even our current civilization would find much evidence. There would be anomalies (high concentration of radioactive substances or synthesized chemical compounds for instance) but there would be no above ground structures left whatsoever and most buried structures would have degraded completely. Presumably certain large underground structures such as tunnel networks, vaults and radioactive waste storage might still be around but in what form depends heavily on geological activity, especially tectonic movement, and most would probably be unrecognizable to their current form.
If a civilization was more advanced than ours, enough so that they teraformed a planet, it is conceivable that they constructed extremely large machinery (with areas spanning ~ 100s km$^2$) which aided in the teraforming processes and which might be evidenced for much longer than any current structure we have built on Earth.
In the first ~ 10,000 yrs all satellite orbits would decay and they would be burned up upon reentry. The only way I see an ancient civilization leaving anything unattended for ~10,000s or more years with current technology, and still being immediately recognizable to a medieval civilization, is if they intentionally constructed a mega-structure with longevity in mind.
Any form of information transfer (books, hard-drives...etc) as well as common everyday items would be destroyed beyond use within ~100 - ~1000s of years. So while they might come across artifacts depending upon how long the civilization had been gone for, they wouldn't be able to learn much more than that they were not the first.
Fossil record discontinuity.
The fossil record before and after the terraforming event would be very different. If there were life forms on the planet before terraforming, those forms might be present in the fossil record before the terraforming and disappear after with new forms appearing. Or the fossil records might start very abruptly with no fossils before. One might take that as evidence of Creation which would not be wrong in this case.
Fossil records also contains record of the chemical history of the planet. For example the Great Oxygenation Event of Earth was billions of years ago, and we can find evidence of it in the form of deposits of precipitated iron, which turned to rust when the oxygen came.