1
$\begingroup$

I've been toying around with the idea of a tidally locked Earth. Spent the better majority of today figuring out weather patterns, habitable zones, etc, but I missed one big detail: How the hell did the Earth get tidally locked? At first I toyed with the idea of a meteor hitting the planet with the impact countering the direction of the spin and it magically slowing to a halt. Then I remembered that if a meteor hit the Earth with enough force to stop its spin, pretty much every damn thing on the planet would be dead. I'm generally trying to avoid that.
Another idea I played with was some weird giant gyroscope experiment gone awry, but that sounds like a bit of a cop out.

Is there any mechanism by which the Earth could become tidally locked to the Sun within the span of 100 years?

$\endgroup$
  • 1
    $\begingroup$ Not spinning due to synchronization of rotation and orbit is quite different than actually being tidaly locked. Which one do you really want? $\endgroup$ – Mołot May 15 '17 at 9:46
  • $\begingroup$ You have a secondary problem to tidally-locking the Earth in the first place: keeping it locked. Tidal forces from the Moon currently act to slow the Earth's rotation, but if the Earth were to effectively stop rotating, the reverse would occur. It would be a slow process (over thousands or millions of years), but eventually it would accelerate the Earth's rotation enough to disrupt tidal locking with the sun. So if you want to keep the Earth tidally locked, you'll need to find some way to get rid of the Moon entirely. $\endgroup$ – Salda007 May 15 '17 at 9:48
  • $\begingroup$ You may want to look at this SciFi "The Long Afternoon of Earth Mass Market Paperback – January 1, 1962 by Brian W. Aldiss (Author)" - "amazon.com/Long-Afternoon-Earth-Brian-Aldiss/dp/0451085752" $\endgroup$ – Enigma Maitreya May 15 '17 at 16:23
  • $\begingroup$ "Although written 20 years earlier, "The Long Afternoon of Earth" bears a strong resemblance to Aldiss' later "Helliconia" series. The main point of these books is to explore a world in which astronomical influences have drastically altered the climate. "Afternoon" is set billions of years in the Earth's future, when tidal forces have slowed Earth's rotation to the point where one side of the planet always faces the aging Sun, while the other side is in perpetual darkness. The book takes us on a tour of this world, to see how the creatures of Earth have adapted to this profound change." $\endgroup$ – Enigma Maitreya May 15 '17 at 16:23
3
$\begingroup$

I'm pretty sure it has to be a gradual halt. If Earth just stopped spinning all of a sudden, you wouldn't stop moving, and would get flung into the wall, or a tree at a very high speed. The atmosphere would also not stop moving and destroy everything with the most powerful tornado force winds.

Naturally, time is the first way that Earth's rotation could stop is by waiting millions, maybe billions of years, thanks to the moon, the rotation is slowing already. But, life will probably not exist by the time Earth stops spinning this way.

The easiest way would be to turn Earth into a system with another object that is equal in mass. So, find a way to increase the mass of the moon.

You're going to have to wave your magic wand for this.

$\endgroup$
  • $\begingroup$ Yup, I figured as much. Interesting idea though, off-setting the Earths spin with something else. Perhaps some large body could enter the solar system and sort of find itself orbiting around Earth, somehow influencing its spin? $\endgroup$ – wollington May 15 '17 at 2:03
  • $\begingroup$ You could try to go at that angle. Just think about the unlikeliness of a large Earth-sized object entering the solar system and somehow starting to orbit Earth. Also without destroying the moon. $\endgroup$ – Phillip McLaurin May 15 '17 at 2:07
  • $\begingroup$ Exactly, that's kinda what I'm hung up on. Perhaps I'm going about it wrong. Maybe there should be some doomsday event with an asteroid and humanity somehow survives via moon colonization and they end up coming back some day. I'm not sure. $\endgroup$ – wollington May 15 '17 at 2:09
  • 1
    $\begingroup$ That seems better, but I'm pretty sure that even a large asteroid impacting the Earth would mess up the moon in some way. Perhaps deep, underground colonization on Earth, emerging after the debris has settled? $\endgroup$ – Phillip McLaurin May 15 '17 at 2:12
  • $\begingroup$ That was another thought, but I feel like that's been done so damn many times before. However, I do like the idea better than a Moon colony because although I don't think an asteroid hitting Earth would effect the Moons orbit so wildly that returning would be impossible, it'd certainly add needless complication. Maybe an underground vault system is the best way to go. $\endgroup$ – wollington May 15 '17 at 2:15
1
$\begingroup$

The energy requirements for this are pretty ridiculously high. The rotational energy has to go somewhere, and it will probably turn into heat and cook the Earth. If you can somehow store the energy in the Earth's core, you might be able to avoid catastrophe.

The earth has a rotational kinetic energy of 2×10^29 J. A tidally locked planet has negligible rotational energy. In order to stop the rotation in 100 years, you need to bleed that energy off at a rate of 2×10^27 J/year or 7×10^19 W. Lets take a look at how much power that is.

Currently, the entire human race produces 3×10^12 W of electricity, one ten millionth of the required energy. The Earth gets 1.74×10^17 W of energy from the sun, 1% of the energy required.

It should not come as a surprise that the energy requirements for changing the motion of a planet are astronomical. Planets are astronomical bodies. 100 years is less than an eye blink in astronomical time.

Hypothetically, you somehow find a way to turn the rotational energy of the earth into useful energy. I don't know how you did it. You probably used a machine powered by pure handwavium. Kinetic energy has a nasty tendency to turn into heat energy. This is one of the consequences of the second law of thermodynamics. If your hypothetical rotational energy harvester actually gets up and running, it somehow has to get 99.9% of that energy away from the Earth's surface. If 1% stays behind, it will produce heat equivalent to the heat coming from the sun, and the Earth will cook.

Instead of sending this energy to space, we could use a machine powered by pure handwavium to store the rotational energy in the Earth's core or mantle. The Earth has a mass of 6×10^24 kg, which means there are 33000 J of rotational energy per kilogram of mass. For simplicity, I will assume that the Earth's crust has negligible mass. I will also assume that the Earth's mantle and core have a specific heat of roughly 500 J/kg/C, roughly the same as iron at room temperature. That means that it takes 500J of heat to increase the temperature of 1kg by 1 degree C. This assumption is mostly just a guess. I can't find great numbers for specific heat of the Earth's mantle. If we store all of that energy in the Earth's core and mantle, its temperature will increase by 66C. This is not huge, because the mantle between 500-900C already. This might lead to increased earthquakes, but that is way way better than turning the surface into an oven.

Earth's magnetism comes from its core. Maybe the mechanism you use to slow the Earth has some connection to magnetism.

$\endgroup$
0
$\begingroup$

So as of now the earth's rotation is already slowing, at a rate of about 2ms a year, mostly due to the moon's tides so in about 5 Billion years the earth will stop rotating and eventually start spinning the other way!

The only way for the earth to become tidally locked would be complete removal of the moon and some sort of catastrophic event (such as a meteor strike) that would remove the majority of %99.72 earth's angular momentum, an incident that would probably kill all life on earth.

The only non-catastrophic event that could have this effect is a large body coming into orbit around the earth that would have the same effect as the moon only in greater proportion. [EDIT] This would cause massive waves flooding most of the continents but it MIGHT not kill everyone]

$\endgroup$
0
$\begingroup$

Within the span of 100 years? Not really.

Earth's rotational kinetic energy is about 2.138×10^29 J. As a contrast, the asteroid that killed the dinosaurs 65 MYA had a kinetic energy of about 4x10^23 J.

Basically you would need the equivalent energy of 500,000 dinosaur-destroying asteroids hitting at the exact same angle... needless to say even if you somehow managed to accomplish this through aliens or some act of god, the earth would be a a ball of lava.

A way you could actually sortof accomplish this is by having a moon-sized object ejected from some other star system with the perfect amount of kinetic energy and angle of impact. The odds of this happening are basically zero. and the Earth would be a lava ball. but that's basically the best answer aside from magic.


A direction you might want to consider is an Earth-like planet orbiting a red dwarf star. Red dwarf stars are much smaller and dimmer, meaning that the habitable zone is very close to the star. Because the habitable zone is so close, any planets within this habitable zone are almost always going to be tidally locked.

With this you can have an Earth-like planet that has just always been tidally locked.

$\endgroup$
  • $\begingroup$ Out of curiosity, where did you find the figure for Earth's rotational kinetic energy, or how did you derive it? It wouldn't be difficult to calculate Earth's orbital kinetic energy, but I don't think I've ever seen any data on the rotational kinetic energy. You can edit your answer to clarify that point. $\endgroup$ – a CVn May 15 '17 at 19:59
  • $\begingroup$ I'd also like to know. I was doing some research to find the mass and velocity needed for an asteroid to stop the Earths rotation but I unfortunately couldn't find any data on the Earths rotational energy. But then again maybe I'm just bad at Google. $\endgroup$ – wollington May 16 '17 at 1:32

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.