Gemstones, particularly cut diamonds and sapphires will last quite a long time. Probably longer than most stone/flint artifacts even.
Possibly of interest, the 10,000 year clock project.
That may be true, but the materials that we use to build our cities won’t just vanish. Even if a large city is totally destroyed by erosion, it will leave behind sediments that are completely unlike anything in nature. Take a city, pulverize it, and spread it along river beds and alluvial fans. That layer of sediment will be composed of lime and gravel (eroded concrete), tar, bits of ceramics, glass “sand” that’s chemically distinct from natural silica sand, and unusually high levels of iron and other metals. Even if nothing is recognizable, the particular mix of materials will stand out in comparison to natural sediments.
Some cities will be buried before they are completely eroded. In some cases, masses of sand and dust will eventually cover everything up. In other cases, plant life will grow on the rubble, and deposit layers of sand and organic material over everything. In yet other cases, some cities will sink into their river deltas and be covered by sediments from upstream. There probably wouldn’t be much in the way of intact buildings or walls, but there would be oddly square piles of non-natural materials, and possibly intact foundations outlining the footprint of the city. Over geologic time the buried cities would be squished into some new types of sedimentary or metamorphic rocks, but again they would be recognizable as Not Natural.
A lot of the replies in this thread have concentrated on the more recent end of human development, but I’m more interested in the earlier stuff, and how far a civilisation can get without actually leaving much behind.
We can (and do) find early human artifacts from up to hundreds of thousands of years ago, but it seems that the further you go back, the rarer the finds are. Are they going to be even rarer, almost non-existent, in 60 million years?
Eh, impactors of sufficient size will destroy anything we can conceive of, including the Earth itself. I would, therefore argue that since the threat of meteoric destruction hangs over every possible answer to the OP, we can effectively consider it to be immaterial to the conversation (and thus exclude it from consideration).
False.
Luna has no atmosphere to speak of, & micrometeorites will do damage, too.
pfffft. Everyone knows the ‘natural nuclear fission reactor’ is proof of the Atlantean civilization of 1.7 billion years ago. They were so advanced they had nuclear weapons before multi-cellular organisms even evolved. :o
I don’t know a lot about this subject, but the wikipedia entry said it was 3.1% U-235 when this occurred 1.7 billion years ago, vs the 0.72% seen now. I don’t know if 3.1% was the universal % of U-235, but with the half life difference I wouldn’t be surprised.
Would nuclear weapons that are 90% U-235 undergo natural reaction and detonation over the course of a billion years? Or would they just decay into Thorium until they are about 40% U-235, 50% other elements the U-235 decayed into in a billion years (or would one of those decay sequences give off neutrons that ignited the reaction)? It seems either way, it would still signify civilization since the implication will be that fissionable U-235 was isolated from U-238 and stored separately.
A satellite placed into L4 or L5 should remain in place even on very long timeframes, if the existence of Jupiter’s Trojan asteroids is any guide. We’ve got some stuff at L1 and L2, but those aren’t actually stable in the sense of being self-correcting. Nothing at L4 or L5 that I’m aware of, as they’re more costly to get to and don’t have the same advantages as L1 and L2 for observational satellites, but current technology would easily suffice to put stuff into such an orbit if we wanted to.
Honestly, there is no level of technology that can’t be preserved. Seriously, look at the fossil record. We have imprints of leaves and feathers. We can look at embryos inside eggs, and preserved soft tissues. These more delicate features are preserved less frequently than things like bone and wood, but they’re there.
So the very first stone tools will be preserved. The cuts of those tools on bones will be preserved. We’ll find bone, skin and wood that have all be cut, carved and shaped. The more delicate and decomposable the artifact, the less likely it is to be preserved, but if you had a stone age culture living for a geologic timespan, I refuse to believe that evidence wouldn’t survive.
My first thoughts exactly when I read OP. Plus blue cards.