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After tossing this question in my head for a while, I can't really come up with the best results.

The situation is someone or something has split the earth in half, through the core and has moved the halves away from each other, far enough away they can no longer really interact with each other. Assume both halves are on the same orbit as Earth would be if it were unsplit, and ignore any forces that would be required to split and move the Earth-halves.

The Question:

Can life, especially human life, survive on the halves of the Earth?

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  • $\begingroup$ Considering how many people die with each of those tiny (in comparison) earthquakes... you should consider asking instead what of the immediate aftermath kills the most people and what region longest . For that of course you have to define where you split. Btw what happens to the moon ? $\endgroup$
    – Raditz_35
    Apr 13, 2018 at 16:21
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    $\begingroup$ You're thinking about the new season of Marvels Agents of Shield aren't you. $\endgroup$
    – Pelinore
    Apr 13, 2018 at 16:27
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    $\begingroup$ @Raditz_35 I am not sure of the exact mechanics, but basically gravity pulls on the matter (the ground in this case) and tries to get it as close to the center of gravity as possible, and if it is not shaped like a sphere, it will collapse down into a lower area (such as towards the old surface of the earth, or towards the old core. $\endgroup$
    – OneSurvivor
    Apr 13, 2018 at 16:36
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    $\begingroup$ yeah you will very quickly have two small round planets not two half spheres, even if you just magically placed the halves there they will reliquify and be squeezed into ball just due to gravity, AKA hydrostatic equilibrium. ceres is a sphere and it is only bout 1000 km across, each of your halves is 6 times this at their narrowest, they will farm spheres sos fast they may blow larges chunk of their surface in the process. $\endgroup$
    – John
    Apr 13, 2018 at 17:52
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    $\begingroup$ "Human life" is an easy no. "Life" as defined by a dictionary; I'd have to go with: probably. $\endgroup$
    – Mazura
    Apr 13, 2018 at 22:02

6 Answers 6

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Probably not. If the halves were dome-shaped, then gravity would try to shift it down into an orb, causing catastrophic events, landslides, etc. Everything would be pulled to the middle. Even if the gravity problem was ignored, the moon might get caught in a different orbit, causing fluctuations in the tide. Without rotation, the Earth would be disrupted. The rotating iron core generates Earth's magnetic field. If the rotation of the core was disrupted, the magnetic field would cease. Then the sun would strip away the atmosphere and burn the surface, blasting life with a constant barrage of radiation (SPF 5000, anybody?) CME's (coronal mass ejections, essentially concentrated radiation that acts like an electro-magnetic pulse (EMP))would fry electronics, and everything would die. Assuming you had a magical field of magnetism, however, you would still have the problem of there being no night or day. Any way you cut the Earth, and any way you cut it, it won't work.

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Shortly said: no.

More detailed answer: once you split the Earth in two, each part will crumble under gravity to get back to a spherical shape. This alone would make the surface hostile to life.

Then since you have half the mass you also have a much lower gravity, meaning that the atmosphere as it is today will be gone by the time the half earth would have cooled down to life decent values.

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  • $\begingroup$ A half sized earth would have less mass, but the surface would be closer to the mass center. If $r_e$ is the radius before the split, $r_r = r_e\cdot2^{-\frac{1}{3}} \approx 0.7937*r_e$ is the radius of the reduced planet, and the resulting gravity on its surface is $g_r = \frac{r_e^2}{r_r^2}\cdot\frac{1}{2}\cdot g_e \approx 0.7937\cdot g_e$ I guess, such a reduction would not cause immediate loss of the entire atmosphere. I fully agree on the crumbling part of your answer, though. $\endgroup$
    – cmaster - reinstate monica
    Apr 14, 2018 at 7:27
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No. Among other things, the oceans and atmosphere would pour over the newly created edge and flow down the gradient until they equalized, resulting in a number of effects:

  • Creating mighty wind and currents.
  • Reducing the amount of atmosphere and water available on the (former) surface.
  • Causing a great cataclysm as the cooler air and ocean reach the hot inner core of the earth, which is now exposed and radiating and likely to deform as noted by L.Dutch and Anonymous in their answers.
  • The lighter external crust which currently "floats" on the thicker, viscous interior of the earth and which is presently driven across the surface by convection would a) no longer be constrained along edges, and begin to spread over the edge, which itself would be deforming towards a more spherical shape b) be pulled apart by the new transverse forces.
  • These storms of air, water, steam, and earthquakes would likely scrape the surface free of topsoil, eliminating prospects for farming, while coating the surface in a fresh layer of ash and sediment.
  • Over any longer period, the earth would wobble erratically as it sought a new equilibrium; as Anonymous noted, you'd no longer have an interior dynamo to generate a magnetic field. The two halves would also develop a new and erratic orbital relationship with the moon, and I think the details of that relationship would depend on how far apart the two pieces were and where the moon was in its orbit and what angle the was bisected at at the time of the split.
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  • $\begingroup$ That's an excellent answer for a first post. Congratulations! Welcome to worldbuilding.se. If you haven't already, please take our tour and review our help center pages. $\endgroup$
    – JBH
    Apr 13, 2018 at 23:38
  • $\begingroup$ The Earth would quite literally become two balls of plasma due to the heat caused by the halves collapsing. $\endgroup$
    – forest
    Apr 14, 2018 at 3:11
  • $\begingroup$ @forest I'd have to see some math on that. You'd see some gravitational heating due to kinetic collapse, but how much more significant would that be than exposure to the earth's already 6,000K core? $\endgroup$
    – jeffronicus
    Apr 14, 2018 at 16:13
  • $\begingroup$ @jeffronicus I imagine the core itself wouldn't change much given that it's already roughly at the center of gravity. The collapse will be more energetic as the mantle is destroyed. $\endgroup$
    – forest
    Apr 15, 2018 at 5:37
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Here are some educated guesses from a physicist about what would happen if the Earth were split in two. I'm assuming that the splitting happens magically and instantaneously, with one half just suddenly disappearing and reappearing somewhere else. Although this leads to quite extreme effects it's actually the best-case scenario - any other way of splitting the planet would add energy to the system, making it even worse.

I'm also assuming the split happens along the Grenwich Meridian, so that it splits England, Africa, both poles and the Pacific Ocean into two, while the Americas and East Asia end up in the middle of their own hemispheres.

For someone in central America or Indonesia, the very first thing they'll notice is that gravity is suddenly half of its normal strength. This will probably feel like a terrible earthquake, as if the ground has suddenly started to drop away underneath them. This is just an illusion, though - it hasn't. Yet.

For someone closer to the edge of the hemisphere, gravity has not only reduced but also changed direction slightly, since it now points towards the centre of gravity of the hemisphere instead of to what used to be the centre of the Earth. However, this is the least of their problems and they will not have much time to think about it, because of the enormous shockwave that's passing over the land at the speed of sound, destroying everything. This occurs because the air at the edge of the hemisphere is suddenly in contact with a vacuum and is rushing into it. The air that goes over the edge will then start falling towards the core of the Earth.

Speaking of the core, this has now exploded. It was previously under a huge amount of pressure, so much so that the inner core is solid iron, despite being hotter than the outer core, which is liquid. Once that pressure is released it will turn back into a liquid, expanding as it does so. I'm not sure exactly what the volume change is, but I'm sure there are now unimaginable amounts of white hot liquid iron rushing out into space at 6000C. This explosion can't be seen from the surface, but it causes a shockwave of its own, which will travel up through the mantle and manifest as a huge earthquake, which will destroy everything the atmospheric shockwave doesn't get. For places far from the edge, the earthquake will arrive before the atmospheric shockwave, because it has less distance to travel through the Earth.

After that, the half-Earth will start collapse under its own gravity to turn back into a sphere half the size of the original. The Earth's core is now entirely liquid and the mantle, while largely solid, doesn't have anywhere near enough strength to hold itself in a hemisphere shape without collapsing. I don't know how long this collapse will take but it will be well underway within hours if not before. If you watch simulations of the Moon-forming impact you'll see that the Earth behaves like a liquid when it is subject to forces on that scale.

The process of collapse turns gravitational potential energy into heat, with the result that once it's finished the half-Earth will have a surface of molten rock and an atmosphere made mostly of hot gases that were previously trapped in the mantle and core.

In short, most people will be dead within hours, and long-term survival is completely impossible.

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When you mix life with 10,000°C plasma, all you get is more plasma.

No, it would be absolutely impossible to survive for a very, very long time after this event. Each half would collapse in on itself violently. In general, if you think you have an idea of what kind of forces are involved, multiply that by a few orders of magnitude, and then some. The last time anything of a similar magnitude happened (the Theia collision), the entire Earth was turned into a ball of plasma.

Imagine ripping a huge mountain from the surface of the Earth, and dropping it from high in the upper atmosphere. Now imagine taking two mountains. Now three. That alone would likely sterilize Earth and would put Chicxulub impactor to shame, and yet we're not talking about some wimpy mountains or asteroids, but a massive chunk of the Earth collapsing in on itself.

Millions of years later, you would find two dry planets with no atmosphere.

Those are the immediate effects lasting a very, very long time. But what about long-term effects? What if you come back to one of the Earth halves several million years later? You'd find a dry, moon-like planet with a rather homogeneous composition (much like the moon itself). The heat of the collapse will have burnt away the atmosphere, and no atmosphere means the oceans will evaporate, so no water either. Any atmosphere that was left after the planet cooled will be blown away by solar winds, as the Earth's magnetism will have been completely eliminated, destroying the protective magnetosphere. Without terraforming, it will be uninhabitable.

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  • $\begingroup$ "The heat of the collapse will have burnt away the atmosphere" <-- that makes no sense. The products of combustion are also gases, so it would just change the composition, not remove the atmosphere. It would also leave the nitrogen unaffected. Moreover, the collapse of the planet would release plenty of other gases from the mantle, so I would expect if anything a much thicker atmosphere after the event. The question of whether that atmosphere would be blown away by the solar wind is much less clear cut. The gravity would be less but the escape rate depends on a lot of other factors as well. $\endgroup$
    – N. Virgo
    Apr 14, 2018 at 7:46
  • $\begingroup$ In particular, it's not at all obvious that there would be no dynamo. As you point out the collapse process adds a lot of heat, so you would still have a molten core, and the surface would still be cooling, so you'd still have convection. It's not well understood how planetary dynamos form so there is wiggle room here, but it does seem that all the ingredients would be in place to simply form a new one. $\endgroup$
    – N. Virgo
    Apr 14, 2018 at 7:48
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    $\begingroup$ Also, nothing would become a "10,000 degree plasma." Gravitational collapse would add heat but not nearly that much. I doubt the core would increase in temperature much beyond its current 6,000 degrees. The surface would quite likely melt (due to mixing as well as heating) but we're talking molten rock, not plasma. $\endgroup$
    – N. Virgo
    Apr 14, 2018 at 7:50
  • $\begingroup$ @Nathaniel It has nothing to do with combustion. Perhaps "burn" was the wrong word to use. And assuming the energies are similar to that of the Theia collision, it would absolutely bring it up to that temperature, at least for a short time. The energy greatly exceeds that of a nuclear explosion (which is in the millions of degrees, albeit only for a short time). $\endgroup$
    – forest
    Apr 15, 2018 at 5:34
  • $\begingroup$ Well if it doesn't have to do with combustion then what does it have to do with? Regarding the temperature: the energies involved would not be as large as the Theia collision, because there is no kinetic energy from another body as there is in that case. That said, some parts of the collapsing half-Earth system might reach very high temperatures - but not all of it. It certainly isn't true that "the entire Earth was turned into a ball of plasma" during the Theia collision. It was turned into a ball of mostly molten rock. $\endgroup$
    – N. Virgo
    Apr 15, 2018 at 6:11
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Surprisingly yes life can survive, not human life sadly but life nonetheless. Now let me introduce you to a group of creatures known as extremeophiles. These babies can survive condition toxic to us and most life forms. Moist are microbes(excluding the tartegrade of coarse) and are capable of living in places that avoid the new deadly obstacles brought by a split earth or are just good at handling it. Now I'm gonna go out on a limb here and say possibly some undersea hydrothermal vent creatures (yeti crabs, tube worms, etc...) could survive as well being so far from the disaster zone.

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