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I want a species of alien to leave their mark throughout the galaxy with stone carvings and temples. Taking influence from ancient religious stone carvings and decoratively sculptured temples carved into the rock, this species may even take this form of design into their asteroid space crafts, having detailed carvings of stories and gods into the rock like a giant temple spaceship.

For this question I just want to think about the locations in a galaxy that the species can leave giant sculptures (from a few metres to hundreds or more metres tall possibly) and have as little damage or wear as possible? Would somewhere in the asteroid belt or possibly a moon around a specific type of planet be best, maybe underground in a planet with no geothermal activity? A void may be a good location but I am after locations within a galaxy.

I am assuming the smaller the details the easier for the sculpture to wear down and loose the fine details so statues can be around one hundred metres tall and attached to the rock it was carved from if this helps longevity.

What locations in a galaxy would be best for stone carvings to survive the longest with minimal damage or wear? And are there any estimates for how long they can remain recognisable?

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    $\begingroup$ Do you want the sculptures to stay on the surface of an Earth-like planet, stay buried underground, or on the surface of an asteroid/planet without atmosphere? $\endgroup$
    – Alexander
    Commented Jul 6, 2020 at 17:30
  • $\begingroup$ @Alexander where ever they will stay preserved for the longest, I would imagine an Earth-like planets surface would last the least amount of time but other options seem to have better longevity. $\endgroup$
    – user69935
    Commented Jul 6, 2020 at 20:37
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    $\begingroup$ Buried under the surface of a planet with no geologic activity will give you many billions of years (as Renan had suggested) - but then there will be a question of discovering these sculptures. $\endgroup$
    – Alexander
    Commented Jul 6, 2020 at 20:40
  • $\begingroup$ How long would the species carve these rocks? Cultures and ways of expressing them change over time. $\endgroup$
    – NomadMaker
    Commented Jul 7, 2020 at 1:15
  • $\begingroup$ @NomadMaker True, they would have to be creating the same styles of carvings for hundreds of thousands to millions of years, long enough to spread around a galaxy without FTL. And a reason why the religion wasn't replaced or evidence of regional differences in the art. $\endgroup$
    – user69935
    Commented Jul 7, 2020 at 10:05

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The rim of galaxies is where there's the least radiation and supernovae. You may consider that for material that is sensitive to radiation.

If the structures are made of gold or platinum alloys (which are very non-reactive and resistant to corrosion), and placed underground in planets without an atmosphere, they may last practically forever. The only thing wearing them down will be atomic decay, but 197Au is considered stable (i.e.: should have a half life of at least 1.67 × 1034 years, approximately 1,000,000,000,000,000,000,000,000 the current age of the universe).

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    $\begingroup$ Underground gold and platinum sculptures and structures sounds epic, thanks. $\endgroup$
    – user69935
    Commented Jul 6, 2020 at 14:22
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    $\begingroup$ Planet needs to geologically inactive as well. $\endgroup$
    – RBarryYoung
    Commented Jul 6, 2020 at 23:19
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It is estimated that the landing stages of the Apollo lunar landers will be recognizable as man made objects for 250 million years, and that is even considering the exposure to a day/night cycle of temperature extremes, direct exposure to solar and galactic radiation and an endless dusting from micrometeorites. So simply placing an object or sculpture on the surface of an airless moon or planet is a good start.

The moon isn't going to be ideal for longer term preservation since the Sun is gradually heating up. In one billion years, the Earth will become uninhabitable, and in 5 billion years, the Earth and moon will either be swallowed up by the Sun as it expands into a Red Giant, or be close enough to essentially be melted. Building temples on airless moons or planets orbiting Red Dwarf stars can extend the lifespan from billions of years to a trillion or more, although I'm not clear if a material like rock would not suffer significant erosion after such a time span.

If the aliens are willing to forego rock, they can perhaps create stable patterns on the event horizons of black holes, which will last into deep time, measured as perhaps 10^127 years into the future, at which point the last remaining black holes may have evaporated into Hawking radiation into an unimaginably cold, vast universe. If anything is still around to admire the patterns at 10^126 years, it will be something far beyond anything we can imagine as living today...

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  • $\begingroup$ Interesting, how would they create a pattern on a black holes horizon? $\endgroup$
    – user69935
    Commented Jul 6, 2020 at 21:07
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    $\begingroup$ With a great deal of hand waving. There have actually been speculations about using black hole event horizons as a form of computer disk (need to find the link again), so the pattern of information would be the record the aliens want to leave: newscientist.com/article/… and scientificamerican.com/article/black-hole-computers-2007-04 $\endgroup$
    – Thucydides
    Commented Jul 6, 2020 at 22:17
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Well, if they want giant temple spaceships... why not just go

EXTRRA LARRGE!

Look for rocky, airless moons and carve gigantic symbols into the surface. Imagine a Cerne Abbas Giant hundreds of miles tall with outlines five miles wide and deep!

Imagine a whole moon, artfully smoothed and covered with texts, sacred and profane, in glyphs ten miles tall!

There are craters on the Moon almost four billion years old. That's plenty of time for this civilisation to show off to the Johnny come latelies!

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    $\begingroup$ Note that gravity and tidal forces (to say nothing of atmosphere) can severely limit the height and depth of such structures, "hundreds of miles tall" can only refer to same-elevation figures, and 5 miles deep might be pushing it, e.g. hk-phy.org/articles/mount_high/mount_high_e.html $\endgroup$
    – Cireo
    Commented Jul 6, 2020 at 23:58
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Rogue planets. Find one travelling through the interstellar void and mark it up. Because they're roaming around, they don't have issues with worrying what happens to the star in a solar system.

Other than that, planets around low-mass red dwarfs in systems without much of an Oort Cloud. A 0.1 solar mass red dwarf is estimated to have a lifespan of 10 trillion (not a typo) years, and a minimal Oort cloud reduces the risk of impacts mucking up the temple.

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  • $\begingroup$ Good idea, i had thought that too around red dwarfs for their longevity, would having a smaller ort cloud or asteroids belts have any negative effects to a solar system? $\endgroup$
    – user69935
    Commented Jul 6, 2020 at 16:50
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    $\begingroup$ @Randy: Less Oort cloud probably just means there were fewer heavier-than-helium elements in the proto-stellar disc, i.e. it formed earlier in the life of the universe than the Sun did. I don't believe it will affect the stability of the stellar system in any way, although it might mean you have to bring your own rocky planet with you rather than expecting to find one in situ. $\endgroup$
    – Steve Jessop
    Commented Jul 7, 2020 at 3:05
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    $\begingroup$ If you're going to bring your own rocky planet, why not just put a "cloud" of them out in the galactic halo. A rocky planet in that environment ought to last until proton decay takes over the aging process of the universe. Few black holes to eat your sculptures, but gravitationally bound to a galaxy so less prone to wandering into a void. Might have to consider galactic collisions ejecting some, but that's why you make a bunch... $\endgroup$
    – Zeiss Ikon
    Commented Jul 7, 2020 at 16:49
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Instead of attempting a mega-monument at a single safe location, your species might leave lots of dormant, but durable monuments at lots of places!

I suggest that create landmarks from self-assembling nanomaterials and nanobots, colloquially famous as grey-goo, but actually well within its creators control.

We can already make nanoscale level structures that can take on various shapes, from abstract geometrical shapes to cages for transporting drugs to cancer sites. Some folks at the Technion Institute even succeeded in inscribing the entire Hebrew Bible on a Silicon grain the size of a pinhead in an hour!

Nano-sized cages Credit: Nature (2019). DOI: 10.1038/s41586-019-1185-4

Various nanoparticle cages, Image Credit: Nature (2019). DOI: 10.1038/s41586-019-1185-4

A molecular machine walking along a path. Credit: Wikimedia Commons

A molecular machine walking along a path. See the video on wikimedia commons here.

Your alien civilization, having much more developed science and engineering, and the blessings of the author, would have little difficulty in using these to create artifacts that are several meters or even dozens of miles in size. Moreover, just like real-life cutting-edge materials, these structures can automatically repair themselves from arbitrary damage eons after the creators are gone, probably by incorporating living organisms or nanobots into its structure.

The big advantage of this that once the techniques are developed, they are ridiculous easy and cheap to use at scale (or they can be insanely expensive if you want, perhaps by requiring some rare element). In real life, it would take a materials scientist more than a few hours to make small amounts of such materials in any reasonably stocked chemical laboratory with an autoclave. Your aliens can probably send probes to systems across the galaxy. A probe chooses a nice-looking planet, readies its chemicals and nanobots, and lands on the surface. It finds or bores a deep hole, or even a cave and injects some of its payload, ensuring that a vast underground complex full of carvings would be ready within weeks. It chooses a high cliff and starts spraying, ensuring that permanent statues are now present as long as the cliff lasts. And so on.

Even without FTL, your civilization can leave its marks on widely separated parts of the galaxy, ensuring that at least some of them survive no matter what. Later civilizations can conveniently find them to drive the story.

An even more interesting element is the fact that nanoparticles can change their shape based on things like temperature, the presence of certain chemicals, or even an application of pressure. Proteins in the body are a famous example, and humans have already use this to make molecule sized grippers and cranes.

A moderate dose of handwavium can allow your species to make monuments that only appear when certain rituals are performed. This can conveniently include lighting fires for heat, and making blood or other offerings on a particular spot.

Basically, you get Magic$^{\text{TM}}$ that runs on Science!

This can make for good drama, with a scientifically-minded character seemingly-foolishly insisting on finding a rational explanation in the face of 'obvious' miracles, only to be vindicated (or not) later.

Of course, even with such a highly durable materials you would not want to build your monuments just anywhere. You want to choose planets around stars that will last long, and won't go out with a bang. Red dwarfs are excellent since they have low mass and therefore last really long. They are also well below the Chandrashekhar Limit. The flip side is that many of them are flare stars, and tend to get really bright. Depending on factors like the constitution of your building material, the distance of the planet from the star, and the presence and composition of an atmosphere, it is possible that the radiation may damage the bacteria that repair your structure. Your aliens may solve this problem by choosing stars that don't do this, or planets that are further away from their stars. Or they can miscalculate, so that the monument survives for a long time and then starts disintegrating when their counter-measures fail.

Just make sure you're not near a massive star, or something like a Black Hole or neutron star!

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