Or maybe even more tantalising, how did the previous sentient species deal with it. Or to reverse that question, is there something a hypothetical species, akin to our own, could have constructed that could have withstood Snowball Earth ? :)

It sounds easy enough, objects in space, large structures, unnatural concentrations of radioactive materials, sure, but over millions of years with kilometers of the top crust simply shaved off and recycled through the core of the planet?

It's amusing our notion of "glacial" loses meaning over such time-spans. How can it do anything, since glaciers just lie there. But over millions of years, it's a high-speed efficient grinding machine the size of a planet.

Conversely, as to how we might deal with the situation, ludicrous though it might seem with our advances and technology, maybe we simply can't. Even a passive beacon on the moon which wouldn't have suffered from Snowball Moon; it seems plausible enough, but it has to withstand at least 250 million years of the moon's night/day cycles. Is there something we could build right now that could withstand that?

Maybe? Probably? But for now I'm content the Snowball Earth hypothesis spices up a lot of Lovecraft stories. "And with strange aeons even death may die” indeed.

The society would have to have never discovered radioactivity to go undetected this long. In comparison it will take millions of years to wipe out human traces ever since the first atomic testing was done.

Billions for sure - we know because the natural reactor at Oklo 1.7 billion years ago is still very much detected.

https://en.wikipedia.org/wiki/Natural_nuclear_fission_reacto...

Very cool. Glad to learn about that!

Even things on dead planet like the moon can't survive indefinitely because meteors either striking it or dusting it, even if we etched the moon with nuclear blasts contour would get fuzzy and disappear over million years.

One option would be to put in one Lagrange point some high quantity of radioactive material. You'd need to pick quantity and material well tho, because material with short hair life would cool off to get and become just a rock and materials with too long half life would be very hard to detect due their low emissions.

Give it a shape nature would have had time to replicate like an empty cylinder and it would likely be the best way to leave artifact for million years into the future. Won't be able to store any meaningful information beyond "someone was here" and future people coming to bertrand teapot style of argumentation against God would have a hell of a shock upon rediscovery

Almost all of the moon's surface is older than the ~500MYA scale we're talking about. It has the occasional impact, like Earth does, but the overwhelming majority of those craters you see are from the early bombardment. The dark splotches visible on the near side, the maria, are mostly flat volcanic plains laid down toward the end of that period about 3BYA, and are visibly lacking in craters.

No, a base on the moon would still be there.

That’s the large impacts. The Moon gets hit by ~2800 kg of meteor material per day, though this likely has been decreasing over time.

It’s about one ‘bullet’ going ~38,000 mph per 379 square kilometers. But times 365 days a year you’re at 1 bullet per square km/year. Over 500 million years that’s 500 bullets per square meter which would heavily damage if not outright destroy any structure and cover it in lunar dust.

PS: The 38,000 mph issue is hard to conceive of assuming similar weight it’s ~2,500 times the kinetic energy of a 9mm bullet. Military rail guns are aiming for Mach 7+ which is ~1/40 the kenetic energy of a similarly massed meteor.

Yeah, but those aren't going to destroy a structure, just put holes in it the way they put little holes in the surface. You can look at the moon landing videos and see surfaces that have been consistently flat and undisturbed at scales greater than cm for billions of years.

It kind of depends on what you mean by structure. Buildings would not survive. The great perimids would look like odd hills after this stuff.

Moon is x impacts per day * 365 days * 500 million / 37.9 million square kilometers = 4815 days worth of bombardment per km squared. Each day represents 2800 kg so each square meter gets hit with ~ 13.5 kg if stuff going 17,000 m/s.

1/2 Mass * velocity squared. 13.5 kg * 17,000^2 = 1950750000j or 1/2 a ton of TNT per square meter. Except the force is conctrated in one direction so it’s closer to 1 to 3 tons of TNT for every single square meter.

Now it’s diffent because these impacts don’t happen at the same time. But they also don’t happen from a single direction so taller structure would be hit along their surface.

(Edit: this was written to respond to your original comment about the Sears tower, not the one that you have up now which is about ancient egyptian physics or something. I'm too lazy to type up another response. I'll just leave it at "you're wrong" and depart)

> But if you put say the sears tower there 500 million years ago it would not be recognizable as a structure

That's just not true. Look at the Apollo 11 photos. Those exposed rocks are 3+ billion years old (six times older than the timescale we're talking about) and they're certainly not covered in a mound of dust. A 500m-scale structure on the moon would absolutely still be standing, absent really really bad luck.

Look at the size of those rocks. Fist sized chunks of rubble are not a structure. After the surface solidified it was continuously bombarded and broken into tiny fragments. That same thing would happen to buildings on those time scales.

Picture a cooling pile of molten rock that’s solidified, now you can’t pick that up. You can only pick up fragments that are blown apart and looking at those pictures their are plenty of fragments you need to dig to find the solid rock.

https://munsell.com/color-blog/apollo-lunar-regolith-core-sa.... That core sample is over a foot down and did not hit sold bedrock.

Additionally, useful surface structures probably would be made in part of lightweight materials transported from Earth; a useful base would likely be largely subsurface. There's no particular reason why an entrance to such a base need be durable or monumental, or even directly visible with human eyes (or artificial aids thereto) from many kilometres above (let alone from Earth).

Apart from radio beacons and similar navigational aids, there's good reason to think a predecessor terrestrial spacefaring civilization would see very differently from mammals closely related to humans.

(Along those lines, one can imagine a predecessor Earth civilization whose leading species is comfortable in a temperature or pressure that humans could not tolerate, and that their comfortable working conditions would drive the location and composition of a lunar base for them. Even our own species had (and has) mountain-dwelling civilizations in environments challenging to subtropical coastal people. If they had been in the Space Race, mountain-dwellers might have had some advantages in needing to haul around less atmosphere (being comfortable with a higher cabin altitude), and they're the same species as the first Americans and Soviets in space.)

You would probably find this article interesting: https://www.theatlantic.com/science/archive/2018/04/are-we-e...

It starts out as a hypothetical between a couple of researchers that resulted in a paper on the longest-lasting effects of human civilizations.

It's a good read, but the summary is that human civilization has resulted in atmospheric changes which will be detectable 100 million years from now, and we've got that much atmospheric evidence buried in geology without similar evidence for a past civilization.

> how did the previous sentient species deal with it

I like your thinking :-) It's the kind of fantasizing that I'm convinced is at the core of human progress, based on hardly more than a number of currently unknowable "what if"s.

It is, as you point out difficult to know if such a species could have existed. But if they did and if they tried to survive, we might be fairly certain that they failed barring something even more fantastical, currently only to be found in the darkest corners of youtube.

The real utility of this line of thought is IMO how we might try to survive the same conditions, since there is no real assurance that it could never happen again. We don't have the answer yet, agree.

It’s also at the core of human art.

This remind me of "the ice people" https://en.m.wikipedia.org/wiki/The_Ice_People_(Barjavel_nov...

At the time scale of billions of years, the question of whether there was sentient life on Earth is roughly equivalent to asking if there is sentient life on other planets, since Earth is very nearly (but not quite) another planet at that time scale. If sentient life arose that quickly that means it is even easier than we thought it was to arise, by quite a bit. The fact we don't see any in the sky would strongly suggest (but not prove) to me that there was not a previous civilization. (The most parsimonious explanation as to why there aren't a lot of civilizations in the sky is that for yet fuzzily-understood reasons, they can't arise much more quickly. [1])

Plus, while the narrative that "evolution increases the complexity over time" isn't as strong as some may suppose, it does provide a ceiling to complexity over time, and I'd say most of the evidence that we have would strongly suggest that there isn't any feasible way that the biology of a billion years ago could have supported intelligent life, because that ceiling is just too low. Today, there seems to be several possible feasible lines for intelligent life (birds, sea mammals, octopus) in the "short-term" multi-million-year future, with homonids getting there first, but a billion years ago I don't think that was the case. I suspect your average ant nowadays is more intelligent that much of what was around back then.

Oh, and, yes, we could build something that would survive on the moon. In particular, we're not obligated to put only one thing on the moon; that's an artifact in thinking with the exact precise technology of today, where's it's really expensive. In another 50-100 years it's quite plausible that putting up such a momument would be considered on the scale of an ambitious art project, and it's not hard to imagine dozens and hundreds of such monuments being put on the moon over time. Local materials can be used for bulk, and the only upper limit on the size of such a momument would be the energy available for whatever it is you wanted to do with it. Moving around local materials and building the equivalent of a pyramid would be almost feasible today ("almost" here being "less than a factor of magnitude of development away"), for instance, and melting & refreezing mass quantities of lunar surface for some visually-obvious effect is eminently practical if you have fusion. (And if you have fusion that can use the He3 on the moon, downright cheap.) It's not hard to conceive of monuments miles in size, or if you assume self-repairing or -replicating machinery, you can make something visible from Earth feasibly. Even with only solar power, that just slows the project down. Had that been done billions of years ago, some of them (or some of it) may have been destroyed by meteors but the vast majority would still be there. In another 100-200 years of reasonably-projected technological advance we'd be able to consider completely resculpting the effective appearance of the surface of the moon, for any number of reasons and in any number of ways. The moon shows no evidence of this, which would be enough to say that if there is a sentient civilization in Earth's past, it would put the Great Filter back in front of us again. (Albeit much more weakly, since only one civilization may have died before resculpting the moon.)

It's still faintly possible that we'll explore the moon in more depth that we have today and find "something" interesting. The fact that it's soooo untouched at a bulk scale suggests to me it's not likely, but it's possible some civilization got to where we are today, +/-50 years, and then died back, leaving just a trace on the moon, just as we have so far.

(IMHO, the most recent paper on the Fermi paradox from a couple of months ago puts the Great Filter behind us fairly firmly, by showing there's no compelling reason to assume the sky should be full of civilizations.)

[1]: It is well-understood that you need at least some time for complex elements to develop, and in enough quantity to form useful planets and such, so you do need at least some time for some stellar generations, but there is no currently-known reason that I'm aware of that the sky should not be inhabited by a multi-billion-year old civilization. On a cosmic scale, Earth isn't that late to the party in some sense, but looking at its history it still seems like you could shave some billions of years off of its history and still get an "Earth-like planet" somewhere else.

Just finished the End of the World podcast. Episodes 1-3 talk a lot about the Fermi paradox, the Great Filter, etc. Recommend a listen.

Do you have a link to the Fermi paradox article you mentioned?

A big lump of Uranium-236 would probably survive that long.