In the world my friend and I are making, the dark lord makes one of the planets uninhabitable by causing a moon, about the size of our own, to fall on it. About 7000 years later, it is unnaturally cooled down via deus-ex-machina. What would be the impacts of this event on the planet itself? The planet is about the size of Earth.

I am asking what effects the impact would have on the natural terrain of an Earth-like planet and how that would look when the land in and outside of the crater is cooled down and oceans re-form 7000 years later, unnaturally. Note: I am asking about how it affects the planet, I would understand it more easily by considering how the planet would look because of the moon-fall after 7000 years when the oceans re-form. Don't worry about how the oceans re-form, its deus-ex-machina and that part doesn't matter. I am asking specifically about the terrain and what it looks like.


I recently found a similar question here, but just like the question that this question I found references, it's answers don't help me and aren't specifically the same question since we are really focusing on opposite sides of the planet. He is focusing on the area opposite the area of impact, and on a much larger time-scale, while I am focusing on the area of impact.


I just decided that my world is in the process of becoming a Q-ball world before the moon-fall. Ie. it is tectonically inactive, though it hasn't entirely lost its magnetic field, erosion hasn't washed away it's mountains yet either. I don't know how this will work, but I am wanting to make it really cold deep-down before the moon-fall (though there is an iron core and it does hold a relatively high density). I don't know if this detail matters.

  • $\begingroup$ Look, someone has to say it, intentional or no: nice pun in the title :) $\endgroup$ – Gilad M Dec 15 '19 at 14:41
  • $\begingroup$ unintentionally $\endgroup$ – skout Dec 16 '19 at 21:45
  • $\begingroup$ You have to specify the size of the moon. A very small moon could be no worse than the Chicxulub impact (the dinosaur killer.) A big moon (such as ours) wipes all life and terrain. $\endgroup$ – Loren Pechtel Dec 18 '19 at 4:38
  • $\begingroup$ Sorry, I meant to specify that the moon is a luna analog and the planet was an earth analog $\endgroup$ – skout Dec 18 '19 at 17:37

What you are asking is basically the giant impact hypothesis:

The giant-impact hypothesis, sometimes called the Big Splash, or the Theia Impact suggests that the Moon formed out of the debris left over from a collision between Earth and an astronomical body the size of Mars, approximately 4.5 billion years ago, in the Hadean eon; about 20 to 100 million years after the Solar System coalesced.

As a consequence of the impact the whole planet would be turned into a molten body of rock, orbited by debris. Being basically a liquid it won't retain any shape present prior the impact.

Upon cooling down you would find no visible traces of the old planet.


Depends on size of impactor and speed of collision. The giant-impact hypothesis has a Mars sized planet hitting the proto earth at high velocity. The energy was enough to remelt both planets, throwing a good part of Earth and Theia into orbit round molten earth. This debries formed moon.

A smaller impactor would leave a crater, have a look at the moon, particulary either Mare Oriental, or Mare Imbrium for what a large but not quite catastrophic impact looks like. The base would fill with lava making a plain (fill with water later), surrounded by a ring (or rings) of mountains. Note impact would trigger a cold period (dust in air and forest fires), look at scenarios for killing off the dinosaurs from Chicxulub crater.

If it was an existing moon orbiting planet that was de-orbited, the moon would break apart before reaching surface at the Roche radius, forming a thick ring. You could then have these debris "rain down" on planet say forming a heavily cratered belt around equator of planet. The last step is a bit artificial, because the ring would probably be made up of fairly small bits, and getting the rain requires ongoing de-orbiting, say by gas in ring ... but you have a super-powerful being anyway.

  • $\begingroup$ Welcome to World Building, nice first post. $\endgroup$ – Renan Dec 16 '19 at 4:23
  • $\begingroup$ I doubt it would break up, the moon is falling as in it's slowing down. This means that the planet's gravity would now be pulling the moon towards it, the moon would pull the planet towards it and it seems to me like what would be happening would be happening far to quickly for the entire object to break around the roche limit. $\endgroup$ – skout Dec 16 '19 at 21:54
  • $\begingroup$ Falling objects speed up ! Lower orbits have higher speed. The break up is caused by tidal forces being greater than the tensile strength of the object. $\endgroup$ – SpaceTasAstro Dec 17 '19 at 23:23
  • $\begingroup$ So I forgot to specify that both the moon and the planet are similar to their counterparts in the earth-moon system. I re-specified that in the question, and I wouldn't have caught it unless someone else pointed it out. Because of this, I do doubt that the moon would be completely torn apart at the Roche limit because I would believe that the impact would happen to fast for an object that large to be torn by tidal forces. So, rephrased, what would happen if our moon fell on our earth? $\endgroup$ – skout Dec 18 '19 at 17:48

After the impact, the entire planet would be covered by fire and lava. Once it cools down, the surface would be changed dramatically.

There would be a giant crater, about the size of the Indian Ocean would take most of the scene, with bordering mountains around the crater. The wind from the blast wave would create hills, crevices, and mountains around the general area.

From the ejecta, there would be several small craters all over the planet, along with a satellite ring around the planet.

The mountains likely wouldn't be that big, but they would be significantly taller than their surroundings, and would cause a weather/climate difference,specifically a colder climate, around the crater. They would also be rather tilted, due to the blast that formed them also having blown them sideways.

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    $\begingroup$ Factually wrong: if the planet is molten like you said, there cannot be a crater left $\endgroup$ – L.Dutch - Reinstate Monica Dec 14 '19 at 5:45
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    $\begingroup$ I agree, at least as long as we're talking about something the size of Earth's moon. If it's one of the Martian moons, though, you might just get a big crater. $\endgroup$ – jamesqf Dec 14 '19 at 19:05
  • $\begingroup$ I mean, are you sure there would be no crater left? after all, the impact takes place only 7000 years before there is a cooldown, an unnatural one, yes, but a cooldown nonetheless. $\endgroup$ – skout Dec 14 '19 at 22:02
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    $\begingroup$ @skout: If the moon in question is around the size of Earth's Moon, the entire planet (or at least what wasn't blasted into orbit) is going to be molten. Not just the surface, but all the way through. There will be no crater, any more than there's a crater left when you throw a rock in the water. Again, see "Giant Impact Hypothesis". $\endgroup$ – jamesqf Dec 16 '19 at 5:59
  • $\begingroup$ Alright, so when I say "crater", I don't think that it wouldn't fill back in, my question is really is "would there be mountains surrounding the impact zone" and "what would the area in the center of the impact zone look like"? if there is no mountains, I would have some re-working to do in my worldbuilding. $\endgroup$ – skout Dec 16 '19 at 21:51

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