# If Theia did collide with Earth, would it have produced rings? If so, would we expect to still see at least some remnants today?

Before I get into the actual question, I'll just specify two things:

1. Few billion years ago, a planet as big as Mars named Theia, is hypothesized to have crashed into the Earth, leading to the formation of the Moon.
2. When an asteroid or moon, passes the Roche Limit, it breaks apart and forms rings around the primary body.

These two things got me thinking,

1. If the event happened, and Theia passed the Roche Limit (because it did crash into earth), then would it have lead to the formation of rings of the Earth?
2. If so, would we expect to still see at least some remnants today?

Different but related questions about Theia and the Giant Impact Hypothesis:

• The giant impact occurred about 4.5 billion years ago, not a few million years ago. Dec 8, 2021 at 4:23
• I've edited your question to differentiate it from the other question and added some additional links, then voted to reopen. I think this can have a good and unique answer or three now. :-)
– uhoh
Dec 8, 2021 at 11:35
• @uhoh thanks a lot, I really hope this question gets reopened, as I haven't got the answer to my question. Thanks a lot :) Dec 8, 2021 at 12:09
• No, the Moon's gravitational influence would probably eject the debris. Dec 8, 2021 at 13:38
• Re When an asteroid or moon, passes the Roche Limit, it breaks apart and forms rings around the primary body. This is a commonly misunderstood concept of the Roche limit. If that was true, it would be impossible to have spacecraft in low Earth orbit. The Roche Limit applies to objects that are only held together by gravity. Objects that have chemical bonds between particles (e.g., rock or ice or artificial satellite) can stay intact well inside the Roche limit. Dec 9, 2021 at 5:20

First things first, but in this case, second things first.

1. When an asteroid or moon, passes the Roche Limit, it breaks apart and forms rings around the primary body.

This is a widely believed misconception of the Roche limit. The Roche limit pertains to objects that are held together by gravitation only. Once chemical bonds come into play the Roche limit is not a limit. A solid or liquid object held together by chemical bonds as well as by self-gravitation can hold itself together well inside the Roche limit.

If there is any validity to the giant impact hypothesis, the object (or multiple objects in the case of variants of the giant impact hypothesis that posit more than one giant impact), such objects would have impacted the Earth nearly intact.

These two things got me thinking,

1. If the event happened, and Theia passed the Roche Limit (because it did crash into earth), then would it have lead to the formation of rings of the Earth?

The Roche limit did come into play after the collision. The Moon had to have formed outside the Roche limit from the debris field left by the giant impact. There is an even more significant limit regarding where our Moon could have formed, and that is the geostationary altitude at the time of the Moon's formation. Tidal forces make objects orbiting closer to geostationary altitude migrate inward, but make objects orbiting beyond to geostationary altitude migrate outward. Our Moon wouldn't exist had it formed at less than geostationary altitude.

Geostationary altitude at the time that the Moon formed would have been much closer to Earth than today, perhaps only ~10600 km from the center of the Earth if the Earth was rotating at one revolution every three hours, or ~16800 km if the Earth was rotating at one revolution every six hours. (That three to six hours range represents estimates for how fast the Earth just after the giant impact.)

If the giant impact hypothesis is at all correct, a debris disk ("ring system") would have formed shortly after the collision, where "shortly" means a few decades, maybe a few centuries.

1. If so, would we expect to still see at least some remnants today?

No. The nascent Moon would have grabbed much of the debris disk. Some of what was left would have been ejected quickly from the Earth-Moon system, some would have quickly been driven back to the Earth, and what little remained would have been eventually ejected from the system, decayed back to the Earth, or crashed into the Moon. 4.4 billion years is a long, long time.

tl;dr Maybe it made rings, but they’re certainly not around today.

The whole reason the Roche limit exists is because of tidal forces, and the whole reason tidal forces exist is because we choose to work in non-inertial reference frames. If you’re not orbiting an object, there’s no tidal forces from that object. Just as a meteor or asteroid can pass the Roche limit and hit us without being affected too much by tidal forces, so, I imagine, would Theia. Now if it started some kind of weird binary spin with Earth and slowly spun together or something like that, it would be a different story, but if it was a more or less head on collision than tidal forces aren’t going to do much pre-collision. (I use the phrase orbit here generally speaking; I include open orbits that involved a curved trajectory because of an object)

However the aftermath of the collision is what could potentially offer the chance for rings to form; you have excess material from the collision that is thrown into orbit, maybe some of it lands itself within the Roche limit and a stable orbit of sorts.

James K in “Why Earth does not have rings” explains it well when he says that, even if in a scenario like this rings when are formed, they would be short lived due to outer atmosphere drag and perturbations.

So in essence it looks like most of the evidence would be gone, although perhaps there’s something I’m not considering that would point to this event.

• Atmospheric drag? Certainly not beyond 1000km in orbit.. and the moon is at 380k km... Dec 11, 2021 at 16:01
• This may be of interest to the op: en.m.wikipedia.org/wiki/Rings_of_Saturn . There's some evidence to suggest Saturn's rings are 100 million years old and will be gone in 300 million years, so the timescales involved could be fairly short in terms of the total age of solar system.
– eps
Dec 11, 2021 at 17:11
• @AtmosphericPrisonEscape I think James K (who originally had the idea of drag affecting ring stability) means to reference the fact that while there’s certainly no short time scale drag, over the time scales we’re talking about the little amount of atmosphere out there could have an influence, if anything else than for adding perturbations that are likely a strong candidate for removing objects. Dec 11, 2021 at 20:41
• @AtmosphericPrisonEscape The Moon is at 385k km ... today. When it first formed it was much, much closer to the Earth. Regarding the Earth's atmosphere, it extends, in an extremely arefied form, to beyond to the Moon's current location. Our artificial MEO satellites will stay in orbit for tens of thousands of years, and our GEO satellites for millions of years. While those are long times by human standards, they are short times compared to the 4.5 billion years the Earth has existed. Dec 12, 2021 at 9:35
• @DavidHammen: Lifetimes of ~10k yrs with proton densities of $10^3 cm^{-3}$? Are you sure those calculations didn't use the wrong Stokes regime for computations of momentum loss instead of the Epstein regime? Dec 12, 2021 at 16:51