# Would it be possible to detect traces of a civilisation after a world-shattering collision?

If the Earth was destroyed by a cataclysmic event such as a large mass colliding with it, such that it fractured into multiple pieces, would any trace of human civilisation remain intact ?

I.e., if some alien race discovered the remnants and thoroughly examined them, would they find anything that would suggest that a civilisation once existed on that world?

If so, would these traces include substantial things like buildings or structures remaining intact on pieces of the fractured earth?

• What level of technology exists on Earth at the time of the impact? – user10945 Mar 7 '17 at 10:23
• Fractured? There's no way that would happen. You're severely underestimating the amount of energy necessary to "shatter" a planet. Long before it got anywhere close to shattering, you'd melt at least the crust of the planet, entirely eliminating any structures. You might still get a few guesses based on just the elements (e.g. "there's a pool of something that might have at some point been a nuclear reactor"), but no structures, and no way to be sure this is evidence for civilization (e.g. we've already seen what we think were ancient natural nuclear reactors). – Luaan Mar 7 '17 at 11:38
• And even with "mere" shattering, it's hard to imagine how anything might survive - if you just magically split the earth in eight pieces of the same size, they would start flowing to form their own spheres very quickly. The new "planets" would quickly go through massive upheaval, completely destroying anything on the surface and burying the remnants hundreds of kilometers deep. And of course, eventually they would collide with each other again, forming a new Earth. – Luaan Mar 7 '17 at 11:41
• Interesting. Everything we have built on this planet is made of stuff we found on this planet. So, we have taken existing stuff and forced a level of organisation on it. Only 3 percent of the earth's surface is urbanised, the percentage of the earth's volume that has been "organised" must be infinitesimal. The chances of finding those bits of organised but otherwise samey matter on a planet smashed to bits - with the bits turning inside out and smacking into each other - are pretty small. IMO. – Grimm The Opiner Mar 7 '17 at 14:38
• Perhaps there might be a better chance of the various items that have been put into orbit (i.e. man-made satellites) surviving intact? Their orbits would presumably get messed up, but they might at least survive long enough for one or more of the planetary fragments (or a nearby moon) to have some kind of "surface" for it to crash into. – Steve Mar 7 '17 at 14:58

## Artifacts on the Moon

Even if Earth's surface will be completely cleaned, there is a chance that alien scientists would notice some signs of civilization left on the Moon or a near planet.

If the catastrophe was not big enogh to disrupt Earth gravitation field it is also possible that some of more than 3000 satellites will be still orbiting the planet.

• If the Earth goes splat chances are our side of the moon is going to pick up some more craters. – user25818 Mar 7 '17 at 17:31
• The satellites are a strong recommendation. They should be easy to find because of their radar signatures. Also some satellites may have orbits which are stable for multiple millions of years. An unusual example of this is LAGEOS 1 and 2 ilrs.cddis.eosdis.nasa.gov/missions/satellite_missions/… – OrangePeel52 Mar 7 '17 at 20:04
• @OrangePeel52, Satellites will be relatively difficult to find. Even if the earthsplat didn't knock them all out of orbit, space is big and satellites are small. Over time, they will drive out of their orbits. So, if it is recent, sure. If it's been thousands of years? There might not be many left nearby. – ShadoCat Mar 7 '17 at 21:13
• What about the Horizon space probe? Voyager? The Mars Rover? We've left quite a bit of debris in other places, and the moon would be a good place to start, +1! – Anoplexian Mar 7 '17 at 22:40
• Would it be logical to conclude that if artefacts remained intact on the moon or nearby planets that, due to their level of technology, they could have only come from somewhere nearby, and therefore the planet that has been destroyed ? – cjt Mar 8 '17 at 10:55

# Small traces, nothing big

An impact like that would not "fracture" the Earth, it would "splat" it, like if you shot a big drop of water with a BB gun. On large scales like that, there is nothing that is "solid"; everything behaves like fluid or putty. You cannot even drill 10 kilometer down into the "solid" crust of the Earth without the bedrock closing in on the hole as if it was a malleable material being squeezed.

With an event like that, every place on Earth will have an earthquake that is entirely off the scales. The crust of the Earth will ripple and flex in completely unimaginable ways. This will shatter any and all man-made structures.

And even if you would get some really large chunks, these will eventually crumble under their own gravity and form new spheroids. Nothing will remain standing because the very ground we stand everything on will have shaken so violently that no structure will remain intact.

That said...

Even a pile of rubble is a trace. If your aliens' scientists are worth their salt, they will notice very curious distributions of materials. There will be billions of blotches and spots all over the place where they find concentrations of compounds that just cannot form on their own.

The one thing that can stop this is if the partitioning is so violent that the extremely thin crust that we live on gets crushed, blended and mixed into the hot contents of the rest of the planet as the chunks collapse under their own gravity. Unfortunately — for you — it is not at all unlikely that this is exactly what will happen.

• This however is not unlikely is that what you meant to say? – Grimm The Opiner Mar 7 '17 at 14:06
• I Guess not! : ) – Grimm The Opiner Mar 7 '17 at 14:30
• Ooorah double negative. – kingledion Mar 7 '17 at 18:17
• Water? The gravitational binding energy of many elements exceeds their ionization energy. And an impact isn't going to have a huge bias towards depositing kinetic energy as KE and not heat. We get plasma, not liquid, from such an impact. – Yakk Mar 8 '17 at 16:34

# The Earth won't fracture

Any impact large enough to 'fracture' the Earth, would first melt it. For example, the impact that made the moon likely liquefied the entire crust. In that case, there will be nothing identifiable on the Earth. Other answers mention this that and the other thing that might be recognizable; but liquefaction and a vigorous stirring (since there is no other way to describe being hit with a moon sized object at 4 km/s) would mix all evidence of humanity in with quintillions of tons of water and rock and mantle until nothing could be identified, with one exception mentioned below.

# Things not on Earth might remain.

Another comment points out that the liquefaction of Earth would likely not leave much standing on the near side of the moon. However, there are a few probes lost in the vast-ness of deep space, and some broken-down rovers on Mars that will be there for some time. Not that long, in the case of Mars, due to the dust storms.

# Isotopes would be the only identifier

While you could not identify a layer of activity to associate with an 'Anthropocene', given the vigorous mixing mentioned above, alien geologists would know something was up. We humans have made non-trivial amounts of some elements that should not be there. The fission yield curve shows what nuclear wastes are produced in what relative abundances. Of the seven long lived fission products in nuclear waste, Technetium-99, Cesium-135, and Zirconium-93 are each relatively common.

Alien geologists, assuming they showed up in the next 1-5 million years, would wonder where all these unexpected isotopes on an otherwise barren world came from. Or maybe they won't wonder because they are Daleks and they sent the planet-liquidizer.

• This is by far the best answer. The Earth's crust will be melted, nothing manufactured will survive. Indeed, evidence of life may well be obliterated: certainly, no fossils will be left. But the isotopes will persist. – nigel222 Mar 8 '17 at 10:46
• One other thought: radioactive Zirconium is especially interesting. Zirconium is the first thing to crystallize out of molten rock (as Zircon). Thereafter it is extremely hard and durable with a very high melting point. We know how old Earth is because of detailed studies of natural zircons and the Uranium (at origin) and Lead (as decay product only) which they contain. So its almost certain that the first thing that will be studied are zircons, wherein the unnatural Zirconium-93 isotope will be strongly concentrated, and its decay products trapped. – nigel222 Mar 8 '17 at 11:02
• Melt is the wrong word. We get plasma, not fluid, if I did my math right: gpe of 55 g of iron is roughly the ionization energy to free 2 of its electrons. – Yakk Mar 8 '17 at 16:39
• The crust will melt, at which temperature? What about ceramic or tungsten objects? You may find them embedded into solidified lava later on. – galinette Mar 8 '17 at 17:56
• @galinette It's a good bet nothing would survive - the temperatures should be high enough to form plasma, and Earth would very visibly glow for quite a while. Even if something did avoid melting (there might be pockets here and there), it would need to have lower density than the crust - otherwise it would just sink through the molten mass. Tungsten has a very high density, and most ceramics are also denser than continental crust. Not to mention that there would be some serious upheavals that could easily bury the former surface under kilometers of debris. – Luaan Mar 9 '17 at 10:04

It strongly depends on which level of details they can examine the sample.

There are many traces that indirectly point to a civilazation.

• an abnormal concentration of metals in a rock can suggest that the rock was a metallurgic debris, implying that some civilization extracted metals from ores.
• polimeric materials (yep, plastics) or glasses can also point to an evoluted race capable of manipulating materials (no PVC, TFE or borosilicate glass is naturally formed, afaik)

So these traces would suggest that a civilization existed. Large structures would remain, again, not as big thing (no Coliseum or Big Ben for them to look at), but as a distinct chemical trace ( a skyscraper would maybe leave a lump of molten steel derived from its supporting frame, together with methamorphic rocks originated from the concrete and building stones)

• Please turn plastic to plasma, mix it with a few tonnes of Earth's mantle/crust also turned to plasma, and find the plastic. Your estimate of the impact energies seems ... off. – Yakk Mar 8 '17 at 16:35
• yes for a bottle of Coke. maybe not for an oil processing plant which is operating at full capacity, that will leave an anomalous Carbon concentration... – L.Dutch - Reinstate Monica Mar 8 '17 at 17:14
• The total mass of all human-made things is roughly 30 trillion tonnes. The total mass of Earth is 6e24 kg. All human goods produced comes to 0.0000005% of the mass of the Earth. I would call the oil processing plant a drop in the bucket, but a in a 1 Litre bucket that oil plant is a 0.01 cm diameter mote. And everything like that we made comes from Earth itself, just a mild rearrangement of surface materials. – Yakk Mar 8 '17 at 18:27

# Yes, but you would need astrogeologists to study the remains.

Sufficiently advanced civilizations leave a geological record. Let me quote from Anthropocene (Wikipedia):

The Anthropocene has no agreed start-date, but one proposal, based on atmospheric evidence, is to fix the start with the Industrial Revolution (late eighteenth century)

[...]

Fossil record

Increases in erosion due to farming and other operations will be reflected by changes in sediment composition and increases in deposition rates elsewhere.

[...]

Trace elements

In terms of trace elements, there are distinct signatures left by modern societies. For example, in the Upper Fremont Glacier in Wyoming, there is a layer of chlorine present in ice cores from 1960s atomic weapon testing programs, as well as a layer of mercury associated with coal plants in the 1980s. From 1945 to 1951, nuclear fallout is found locally around atomic device test sites, whereas from 1952 to 1980, tests of thermonuclear devices have left a clear, global signal of excess 14C, 239Pu, and other artificial radionuclides.

If it would be possible to examine the composition of the fragments, geologists could extrapolate geological events related to technological eras. This would be similar to today's geologists knowing the composition of the atmosphere millions of years ago just using the fossil record.

Please note that traces of the anthropocene exist all over the world, not only in inhabited areas.

The question would be whether the cataclysmic event would completely obliterate the geological record (imagine nuking an archaeological site), or add on top of it (imagine dropping napalm on an archaeological site, with enough napalm residue to make an extra layer on top).

A planet being ripped into an asteroid belt due to high gravitational tidal forces is very much like the formation of a planetary ring. Imagine you could send some geologists up to Saturn, and bring back some chunks of its ring. Would there be big enough fragments of the crust to see the geological eras? Or would the fragmentation melt everything together? The harder the cataclysmic event, the harder the geology/archaeology gets.

• Carbon-14 is produced in higher quantities in nuclear testing than naturally, but it does still exist naturally. It's unlikely that the higher concentration would be noticeable. However plutonium is not produced naturally at all, so any amount detected is strong evidence. – thegreatemu Mar 7 '17 at 16:16
• I beg to differ. Plutonium (as well as other heavy elements) does exist naturally, yet its half-life is very short in a geological time scale. They could also be produced by cosmic gamma radiation. But you are right in that alien geologists would use radiometric dating, and an anthropocene era should be easy to tell apart. – IvanSanchez Mar 7 '17 at 16:58
• xenoastrogeologists? – WW. Mar 8 '17 at 7:15

On the surface, probably not.

While a comment noted that an impact large enough to "fragment" a planet will behave more like a "splat", the implications of this have largely been missed. Such an impact will produce an enormous cloud of ejecta (a significant fraction of the planetary mass) which will mostly fall back to the surface. This will have two effects.

First, the impacts will produce an enormous amount of heat, and a lot of the impact areas will become molten, erasing any signs of man-made artifacts.

Second, even a small fraction of the planet's mass will bury the remnant surface very deep. For instance, assuming uniform density, 1% of the earth's mass will produce a blanket about 13 miles deep. Given the scale of the ejection event, a fairly uniform coverage should be assumed.

As a result, any visiting aliens had better be interested in relatively deep (much deeper than we can do) exploration of the pathetic remnant of a once-living planet. Exposure of core material at high temperature will have locked up all the atmospheric oxygen, and for quite some time the surface temperatures will be high enough to keep the water as vapor (that is, steam).

Of course, you may have noticed a possible loophole in the opening sentence of this answer. The ejecta will not entirely return to the surface, and for a shorter or longer period some of it will orbit the earth - we'd probably have a very nice set of rings for a few millenia - and if the visitors are lucky they might encounter artifacts in orbit. The odds on this are not great, but I don't see how they would be zero, either. You'd expect to see fragments of refined metal, and small, dense objects might survive the ejection process in a recognizable form. This might apply to objects such as steel tools and jewelry. Platinum, for instance, is quite hard, strong, and melts at very high temperatures.

EDIT Even a badly damaged Rolex would be a pretty sure sign that someone had been on the planet.

Unless, of course, the Blind Watchmaker debate is universal. END EDIT

• Its not the ejecta that would ruin the surface, it is the liquefaction of the Earth's crust that would do it. – kingledion Mar 7 '17 at 18:15
• Regardless of the damage done by the actual impact, the returning ejecta will both liquify and bury the original surface. – WhatRoughBeast Mar 7 '17 at 21:25

If the Earth is destroyed to the point of melting the crust and destroying all possible artifacts, there are still those that were not on the Earth at the time.

Besides artifacts on the Moon, Mars, Venus, Titan, and various radioactive radio-sending beacons heading slowing to the Oort cloud, there is a cloud of sattelites including many in a rather distant Geosynchronous orbit. These may be scattered rather than vaporized. Even if the expanding cloud batters them, they will also move away and at least some of them may remain as recognisable anomolies in the debris.

Assuming current level technology, the first thing the aliens will find on their scanners would be radiation spots from the melted down 447 nuclear reactors that are currently active.

This would be a pretty unmistakable sign of civilisation.

• You do realize that this is next to nothing compared to what goes on in the center of the Earth itself, right? If the Earth has been fractured, the core of the planet is exposed to the universe. Also it would have to be a fairly quick discovery — in the first 1,000 years or so — because otherwise all the "hot" items that would make an immediate impression have then decayed and we are left with the elements with long half-lives that consequently have much lower intensity. – MichaelK Mar 7 '17 at 11:37
• groans Gods I hate the "Nuclear Is Black Magic Lava!!" trope so much. People just lose all their faculties of discernment of what is realistic and what is not when the word "nuclear" enters the conversation. Here: Chernobyl Recovery and Development Programme – MichaelK Mar 7 '17 at 11:46
• People are living in the Chernobyl exclusion area, and so do animals. It's still a health hazard, but nowhere near to make it "uninhabitable". The reactor itself is another matter, but in the kind of cataclysm the OP describes, there's no way it would stay in one spot anyway - all the waste would be spread out over considerable volume, making it unrecognisable. – Luaan Mar 7 '17 at 11:49
• Historic uranium production en.wikipedia.org/wiki/List_of_countries_by_uranium_reserves is 2.8 million tonnes, 2.8x10^9 kg Abundance in the crust is 2.7 ppm en.wikipedia.org/wiki/… Mass of crust (lithosphere) 1.365x10^23 kg madsci.org/posts/archives/1999-11/943288749.Es.r.html so mass of uranium in lithosphere is 3.6x10^17 kg, or 130 million times more than the world's stock. The world's stock of radioactive material is not quite a drop in the ocean, but less than a drop in a bathtub. – Pete Kirkham Mar 7 '17 at 13:07
• @bloer Not true, Pu-244 is primal, though in exceedingly small quantities. More to the point, long lived radioactive wastes are both longer lived and more abundant than any other Pu isotopes. – kingledion Mar 7 '17 at 18:13

## Apollo Space Program Artifacts

There are actually some fairly significant artifacts which would be much easier to discover than anything on the Moon, Mars or things like puny inter planetary probes...

There are five S-IVB Saturn V third stages in heliocentric orbits, and they are all obviously artificial to anyone looking for them. Being that they are twice the size of a London bus, they would be the most likely artifact to be found by anyone surveying the solar system.

Such a collision would eject significant mass from surface. Nearly all of that mass would fall on the surfaces of Earth and Moon. This would melt and reform both surfaces removing all traces of us on either surface. It would also destroy all satellites orbiting Earth or Moon. This is supposed to be how the Moon was created. (Probably without the part about the traces of advanced civilization being erased?)

The collision would do nothing about traces outside the Earth-Moon system, so given some time a record of our existence could be launched to stable orbit in space. Earths L4 and L5 points, maybe? So if you want the aliens to find traces of us, it is easy to do. Kind of interesting to think of what we would choose to record of ourselves and what the aliens would think of it, really.

...if some alien race discovered the remnants and thoroughly examined them

As the OP implies, let's assume no limits on the practical abilities of our aliens to filter the debris (i.e. they are arbitrarily good at forensic study).

Diamond (melting point ~4500K) should survive immersion in magma at the temperature of the mantle (~4000K). Therefore, they will find unnaturally cloven gem-stones. The golden ring will melt, the hand that wore it will vaporize, but the gem will remain. Love, truly is eternal.

• Melting point isn't the only thing that matters, though. Diamonds burn and are thermodynamically unstable (eventually turning into graphite, though it might take millions of years). Good luck reconstructing the diamond from a trace of carbon dioxide. Even if it somehow avoided both melting and burning (and in a cataclysm like this, the temperature might be far higher than the mantle's for a while), it's not certain the shape would be preserved - diamonds are hard, but they are also somewhat brittle. If they actually found a brilliant, it would be rather interesting evidence, though :) – Luaan Mar 9 '17 at 10:19
• I didn't say they would all survive. It's quite conceivable that on the opposite side of the impact zone, we would be subjected only (!) to cataclysmic earthquakes and volcanic upwelling of lava. After a crust had solidified and eroded down a bit, you might indeed find the Koh-i-Noor sticking out of a rock. – Oscar Bravo Mar 9 '17 at 10:27

The proportion of the different - stable - isotopes tend to be the same in the whole Universe.

Nuclear reactions change these proportions, and thus they remain visible forever even after all of them decayed, even after trillions of years.

Although our radioactivity changes only a very small part, compared to the mass of the whole Earth. If somebody would mix the whole volume of the Earth, we couldn't measure anything in it.

It depends the technology earth had at that point. But radio-waves would remain, sadly most of the first human things they may have heard, would be Hitler. Still our legacy may remain and maybe would be possible to pinpoint earth's position. Plus our many satellites and debris would scatter.We would always have the probe we are sending to the edge of solar system (forgot the name) it contains several artifacts from earth including a map to find earth,blood and also music!

• Pioneer, V’ger, and now New Horizons. – JDługosz Mar 9 '17 at 9:44
• @JDługosz you may edit the answer if you want, that is what i was looking for – PeterKima Mar 9 '17 at 9:54
• Romantic, but probably untrue. It seems that the unintentional radio emissions we've produced so far are far too weak to be reasonably detected at a distance larger than tens of light years. It's a good chance that unless somebody already heard Hitler, they wouldn't in the future - they'd be too far. Even for directed high powered radio telescopes transmitting and receiving, the range is very low. Voyager and Pioneer would survive, but it's hard to imagine how they might be found - they're just tiny specks of dust in interstellar distances. Not entirely impossible, but still very unlikely. – Luaan Mar 9 '17 at 10:13
• @Luaan Donte true, Layman would agree with you as i think you are talking about inverse square law. But the signal would still be there. Faint at best ("For a civilization only a couple hundred light-years away, trying to listen to our broadcasts would be like trying to detect the small ripple from a pebble dropped in the pacific ocean off the coast of California from Japan") planetary.org/blogs/emily-lakdawalla/2012/3390.html is a good read on that although somethings are not 100% right. But then again we are always improving and as unlikely as it would be world-shattering is also – PeterKima Mar 9 '17 at 10:35
• @Luaan Still it would be there, like a faint last breath masked by background. – PeterKima Mar 9 '17 at 10:38