I want to create a civilization that were living on earth when the planet was a much smaller protoplanet or a planetesimal. If earth grew over billions of years, could a civilization have existed while earth grew around them? Like maybe 1,000 miles below our current surface. Obviously these lifeforms would have to look very different than humans to survive that deep, but is this a dumb or crazy idea?
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7$\begingroup$ Earth formed within 100 million years after the Sun lit up. There is no "billions of years" gap. And for the first 500 million years of its existence Earth did not have a solid crust, because it was molten. Earth became sort-of similar to what it is now about 3.5 billion years ago, about 1 billion years after its formation; mind you, it still had a rather unfriendly atmosphere consisting mostly of nitrogen with a lot of carbon dioxide and no oxygen... And 1000 miles down the temperature is in the thousands of degrees centigrade. $\endgroup$– AlexPCommented Jun 8 at 11:38
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3$\begingroup$ @Pelinore: 4.6 billion years ago the Sun lights up. 4.5 billion years ago Earth forms, with about 90% of the mass it has today; Theia hits soon after, bringing the missing 10% of the mass and forming the Moon. 4 billion years ago the crust cools down enough to become solid; oceans form; second atmosphere, mostly nitrogen with a lot of carbon dioxide; the Late Heavy Bombardment. 3.5 billion years ago life appears. 2.5 billion years ago the Great Oxidation begins. 2.0 billion years ago the Great Oxidation ends, oxygen in the atmosphere is about 10% of what we have now. $\endgroup$– AlexPCommented Jun 8 at 12:17
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5$\begingroup$ @Joachim By neutrino-synthesis of the luminiferous ether, obviously! $\endgroup$– Escaped dental patient.Commented Jun 9 at 5:24
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2$\begingroup$ Are you asking about: mass coming from space? material blown in over time? avalanches/land slides? as what happened in historical cities where old buildings are knocked down leveled off and new stuff built on top? The question language seems to suggest gaining mass from space but that is incredibly destructive /apocalyptic for meaningful amounts of mass. That is question doesn't really say where the mass is coming from which makes a big difference. $\endgroup$– Gault DrakkorCommented Jun 10 at 19:01
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2$\begingroup$ If dust rains down “over millions of years” to become new soil, would anyone notice? How much accumulation have you in mind? Cities do exist whose oldest buildings' ground floor is well below the current grade. $\endgroup$– Anton SherwoodCommented Jun 12 at 20:46
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2 Answers
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Yes!
As we all know, planets such as our Earth expand in size as they age and develop. Thought to be caused by thermal expansion in the deep chthonic zones of the planet, it was reasoned that this expansion broke up the planetwide ur continent into the various land masses we see now.
Charles Darwin, during his great voyage of discovery noted that a huge portion of South America: "uplifted to its present height by a succession of elevations which acted over the whole of this space with nearly an equal force." And also that this was due to "the gradual expansion of some central mass [of the Earth] acting by intervals on the outer crust" with the "elevations being concentric with form of globe (or certainly nearly so)." Unknown to these ancients, we can surmise that smaller earthy planets like Mars are either younger or have not yet undergone a thermal expansion era, for they are smaller than Earth, or else are much older or more vigorous in their thermal expansion eras, as we can see with the many super-earths found in other star systems.
Clearly, over such long periods of geologic time, other forces shall also be at work, from orogeny to erosion, earthquakes, the formation of river valleys and the movements and meltings of ice sheets.
Any civilisation that has existed on Earth since the time it was very small will have to have exhibited prodigious capacities to react and evolve with the changing conditions of the expanding Earth. I would suspect that, early in their history, these people abandoned the surface world to its relatively rapid changes and remained at their original surface level. For, who needs rivers that wander all over the place and mountain ranges that rise and erode away in the blink of an eye!
No, any stable civilisation craves continuity. And the denizens of the ancient surface, hot and melty, constantly pocked by asteroids and perpetually pestered by other planetesimals were themselves likely to be creatures of extreme heat. Possibly stony or volcanic in their biology. Their wizards might have foreseen the gradual expansion of Earth, its crust cooling and breaking up into untenably frigid crustal zones.
These peoples and creatures would have remained in place, seeking the more commodious climes of the deep chthonic realms of Earth. Those places we call the mantle and even the outer core. Far away from the frigid and stone-freezing climes of the surface, they inhabit a deep oceanic world of liquid nickel-iron.
Their aeon long lives have almost for ever been separated from our fleeting moments upon the surface. We know nothing of them, and they have all but forgotten the stars that wheel over the world above. Yet some brave explorers have heeded the ancient lore and sought for ways to see them, making the long journey up through the rivers of magma. There, perhaps, one of us might see one of them leaping up from the lake of magma in some volcano as they seek to see the forgotten Sun.
Notes: The first part is based on the Expanding Earth Theory, which has never completely disappeared from science. Though now considered debunked, it was in active consideration even into the 1960s. The second part is based on a story from my own worldbuilding, about a race of people who do in fact inhabit the shores of the Nickel-Iron Ocean deep beneath the inhospitable surface!
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$\begingroup$ @Pelinore -- And this is Worldbuilding after all, and not Geology! $\endgroup$ Commented Jun 8 at 12:15
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5$\begingroup$ Your last paragraph should go to the top of this answer, then I'd upvote it as a good yarn. As it is, the junk science goes first and people might not scroll down to the disclaimer. $\endgroup$– o.m.Commented Jun 8 at 15:48
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4$\begingroup$ Yeah, I'll second that your disclaimer should be at the top. I started reading down and it was like what-the-junk is someone pushing here?! "As we all know", there's no need to make psuedo-science appear as if it's legit. $\endgroup$ Commented Jun 8 at 17:33
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3$\begingroup$ +1 I'm a big fan of bring this site back toward worldbuilding and away from physics - we already have a place for that! $\endgroup$– CharlieBCommented Jun 8 at 22:33
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1$\begingroup$ Disclaimer at the top defeats the purpose. This is not GeologyLITE.SE nor Physics.SE. I was actually hesitant to put a disclaimer at all. I did consider it though. $\endgroup$ Commented Jun 9 at 4:14
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If the setting is even remotely hard science, the planet will not grow significantly without adding mass. And adding mass means asteroid impacts which wreck the ecosphere.
So the answer is no.
Inflating planets would mean they get hollow, and by definition planets cannot be hollow in their core. A planet is, among other things, a body which is forced into a spherical shape by the own gravity.
