How to make a planet with a chunk missing from it? [closed]

Question

In the forgotten mists of time a Great War was fought. During that war the planet Kutok had a massive gouge (averaging 1,000 miles across and 4,000ft deep) and extending North from the polar cap down to about -50 south latitude.) made in it. Laser weapon, cross-dimensional beam, shipwreck/asteroid impact, or series of kinetic impacts makes no matter. Even a non-military cosmic accident. Whatever you feel would be most feasable. The planet is slightly smaller but also slightly denser than earth, giving it a gravity of 1.05 standard. In the here and now it is habitable with a breathable atmosphere, with the gouge forming the planet's only ocean. So bonus points if you can answer without making the planet permanently unhabitable. If not nbd, terraforming is a thing! The end goal is to have a "Middle Sea" almost bisecting the planet as the sole ocean or sea.

I want to know the following:

1: could such a feature be made with a single event without destroying the planet? If it can't be, how much smaller would the gouge have to be?

2: would the ejecta from such a feature be enough to Form a visible ring around the planet?

3: if yes to 2, how long would it take the ring to form, and assuming no additional matter is added to it how long would the ring last?

4: I assume this gouge would have mountain ranges or hills to either side, is this true? Are there any other geological features you would expect to see?

Answer 1

1: could such a feature be made with a single event without destroying the planet? If it can't be, how much smaller would the gouge have to be?

It is really quite astonishingly hard to destroy a planet. Even smashing two planets together still generally ends up with a planet and a big cloud of debris. It might be bad news for anyone or anything living on the planet at the time, and it might render it uninhabitable for some time as the ejecta settles out and everything cools down. You have used the phrase "mists of time", which implies that the crater was made quite some time ago, probably more than long enough for things to settle down again.

The impact scar is much wider than the Chixulub crater, but might be much shallower. One possible source would be a moon (possibly a large captured asteroid) in an unusual polar orbit that had been destroyed and deorbited, resulting in a brief debris cloud or ring that then fell onto the planet underneath causing many overlapping craters. This is likely to have been catastrophic for the biosphere of the world (if any). There really isn't any way to cause destruction on this scale without wrecking the biosphere... again, this was much bigger than the Chixulub impactor which was either directly or indirectly responsible for a mass extinction and major, long-lasting climate change.

2: would the ejecta from such a feature be enough to Form a visible ring around the planet?

It very much depends on how the feature was formed, but the chances are good that you will not form a debris ring. Ejecta lofted up from the surface will either escape the world's gravity entirely and shoot out into interplanetary space, or will fall back onto the world.

The Theia impact references by nzaman involved a truly gigantic collision with a huge body relative to the size of the Earth which was already on a heliocentric orbit, making it a lot easier to create a cloud of debris in space which could then be captured. The results of the impact included pretty much resurfacing the entire planet, which would have rendered it thoroughly uninhabitable for an exceptionally long period of time... certainly thousands of years and probably a hell of a lot more.

My moon idea might have left debris in orbit, which might reasonably form a ring, but it would be not be ejecta from the planet below, if that was important for you. The ring is likely to be quite faint but it was made from high-albedo material (like ice) it could well be visible from the surface of the world under the right conditions.

3: if yes to 2, how long would it take the ring to form, and assuming no additional matter is added to it how long would the ring last?

If debris could be captured, it would be captured quite promptly. If the source of the ring was an object orbiting the planet, it would take some years for the debris to stretch out over the object's orbital track. The shape of the ring would change quite a lot as time went on as bits of it fell onto the planet below as a result of orbital decay caused by various means or as the size of particles was reduced by collisions possible resulting in erosion of the ring by the solar wind and so on. Ring systems might not last more than a few million years or so, but I'm guessing that's fine for your needs.

4: I assume this gouge would have mountain ranges or hills to either side, is this true? Are there any other geological features you would expect to see?

You do get mounding of ejecta around a crater, so regardless of how the feature was formed (with a few exceptions, like most of the debris was somehow at escape velocity or the atmosphere had been blown off) you should expect to find some. If the feature was formed by impacts of falling objects that hit at an angle, one end of the feature will have a much more pronounced mountain range than the other. A whole series of small impacts might not push much debris outside of the impact area (because a lot of the debris from the central impacts will fall into the craters forkmed by adjacent impacts, etc) and so the resultant hills or mountains will look quite different from those caused by fewer but more massive impacts.

You'd probably see a lot of other associated craters of similar age, probably along the same orbital track as the body that caused the feature in the first place, all the way around the planet.

Answer 2

Your ocean has 10x less water than Mars. For a bunch of reasons about how planets form, you are not going to get an Earth mass world with so little water unless there is something actively scouring away the planet's hydrosphere. So, for this scenario to be believable, you either need to put it into a really low solar orbit like Mercury or you need to make that trench WAY deeper.

Since this is supposed to be a life sustaining planet it will probably be similar in age to Earth (give or take a billion years or so) and in the goldilocks zone, your trench will need to average ~200-380mi deep depending on how much water and atmosphere this even blasted away. Most conceivable weapons and astral bodies would not result in such a shaped scar on a planet under any circumstances, but perhaps it was not a weapon exactly.

Answer: A stanford torus world ship

The video game Halo featured alien torus ships that were about 10,000km around but only a few km wide. If a Covenant like faction knew they were losing the war perhaps they would have flown a Halo Array into an enemy world as a sort of ultimate kamikaze attack. With enough speed the torus would have cleaved into the planet like a giant axe head as it disintegrates splitting the world open. The gouge would be so deep and wide that the mantal's magma would cool down and turn to solid stone before sealing as it is quenched by hundreds of millions of cubic kilometers of ocean water pouring into the giant crack form above. This crack may would seal up quickly in terms of geological time scales, but in the timescale of civilizations, it would become a "permanent" feature of the planet. Because it is a propelled ship and not gravity driven you can play with velocities and diameters until you get to the exact specs you need to make your cut.