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I'm writing a story where the Moon became a ring.

I've seen plenty of discussion in other questions' comments about feasibility, and how turning the Moon into a ring would cause a true Apocalypse. Fair enough. The Moon only had its periapsis lowered enough to be destroyed without hitting Earth, plate tectonics happened, mega tsunamis, hundreds of thousands years pass, somehow humanity is technomagically rebooted upon the planet. Now there is a planetary ring over their heads.

The thing is, there would be infalling material. The equator would be constantly bombarded by meteorites. I would like to make a plot point about this, in which such a bombardment makes it nearly impossible to cross from one hemisphere to the other.

Suppose we get a 1 kg to 100 kg (edit: upon impact) hitting various points around the equator (say, any 10 square kilometer area is hit by one of these once an hour). Would such a constant bombardment make the planet uninhabitable, or would people, fauna and flora far away (at least a few degrees of latitude away) from this be ok?

I don't care if such a bombardment means the ring won't last a million years. I only need it to be there for a few hundreds of human lifetimes. Edit: to be clear, I don't care if the rate of impacts means the ring isn't stable for millions of years (I also understand stuff coming down also means possibly smaller stuff escaping). But I also don't care if it is stable for hundreds of millions of years. In the end, thr ring stability is not in the scope of the question.

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    $\begingroup$ Would not this related question effects of ring debris have suitable answers? $\endgroup$
    – Gault Drakkor
    Commented Aug 16 at 22:21
  • $\begingroup$ @GaultDrakkor no - the other question's only only answer mostly assumes an icy ring, with a sidenote for a rock dust alternative that is much thinner than what I assume here. I am starting by affirming that we have a case with a severe rain of heavy chunks that hit the ground. $\endgroup$
    – The Square-Cube Law
    Commented Aug 17 at 0:48
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    $\begingroup$ Just to be clear you want 1-100kg chunks hitting atmosphere or you want chunks that are still that size when they eventually hit the ground? $\endgroup$
    – Ash
    Commented Aug 17 at 1:18
  • $\begingroup$ If you're looking at a short-lived ring, does that mean that all/most of the Moon's mass is impacting the Earth over a period of less than a million years (hundreds of thousands of years pre-story plus a few thousand more of storytime)? Thinking of what-if.xkcd.com/162 $\endgroup$
    – KerrAvon2055
    Commented Aug 17 at 3:25
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    $\begingroup$ @ash I mean upon hittong the ground (or bursting within the lower atmosphere), I will edit to clarify. $\endgroup$
    – The Square-Cube Law
    Commented Aug 17 at 3:39

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Some maths:

We just assume the "fuel" of debris is unlimited in terms of the plot.

Computing the number of impacts:

40,000 km circumference at the equator divided by 10km for the impact zone (simplifying to a line here).

4000 impact zones being hit once per hour = 4000 impacts around the equator per hour. Each zone is hit by about 24 impacts per day (24h/day).

For fun: 4000 * 24 = 96,000 impacts per day around the equator, times 365 days = 35,040,000 (35 mio.) impacts per year around the equator.

Impact Severity:

Refering to this crater simulation calculator. Further Info on Wikipedia on Impact Events.

----input---- 
Diameter: 0.1m (a big apple)
Density: 2.5g/cm³ (something like "stone")
Speed: 11km/s ("reversed" earth's escape velocity, meaning just dropped, no extra velocity)
Angle: 90° (worst case, straight line down)
----computed----
Mass: ~1.0kg
Kinetic energy: ~80,000 Joules (~22Wh, approx. one laptop battery charge)
Crater diameter: ~1.3m  (~4ft)
Crater depth: ~0.3m (~1ft)
Ejecta spread: ~3m (~10ft)

This almost scales linearly. At 1m diameter, the mass would be around 1000kg, the crater about 10m in diameter and 3m deep, ejecta spread around 30m. Also increasing the impactor speed to 30km/s did not do much to the results (most notably: spread increasing to about 40m)

Problematic situation:

If the ejecta of the craters is so much material that dust and dirt and other particles are spread through the atmosphere, blocking sunlight in the worst case.

For Reference: Global effects might occur with ejecta spread of 10km and above (that is the flying altitude of passenger airplanes). 100km would mean a global spread throughout the whole atmosphere (at 100km altitude, atmosphere ends and space is supposed to start), most likely guaranteeing a "fallout winter" scenario (That happened with the Chixulub impactor; a 10km asteroid hitting earth 66mio. years ago, the infamous "dinosaur-killer")

Real situation:

Most of the impacts at the equator will hit ocean water (I assume well above 90% of events). This makes a nice "splishy" sound, but there will be no particle ejecta shot into the atmosphere. Marine life will probably file complaints ;)

-- Increased amounts of water vapor might be set free, though. It is hard to predict the global effect of it. Might be just over the ocean, maybe also spreading towards land or leading to more reflected sunlight and global dimming/cooling. The effects are probably not too drastic.

Landmasses at the equator are mostly (northern) south-america, (central) africa and the islands of indonesia (thinly spread land). But the ejecta radius is very small, thus very far from a global effect.

Conclusion:

The results show no significant impact on life on earth globally. It might be annoying for life at the mentioned landmasses, but has only local impact characteristics (about 100m diameter).

Still, given about 24 impacts per day per zone makes it pretty much uninhabitable.

Not considered is spreading of impacts to a wider belt around earth. It will get less with increasing distance from the equator.

Also not considered are secondary effects, for example bush fires (or tsunamis) which might spread uncontrollably.

Edit: Also not considered is the influence of the atmosphere on re-entering debris (heating up, breaking, etc), nor the explosion of those debris somewhere above the surface (I would deem those less impactful on a global scale). See controlgroup's remark in the comments.

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    $\begingroup$ Interesting. I am considering some impactors one or two orders of magnitude more massive to also fall every so often. I see crossing the equator might not be impossible, but would still put people in high risk of death. $\endgroup$
    – The Square-Cube Law
    Commented Aug 17 at 3:48
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    $\begingroup$ You can also make the "curtain" more dense with much more impacts of smaller debris in shorter periods of time. While this will not change much on the global scale, it could still mean significantly more harm to (unarmored) humans (and animals). At a highly simplified scale: you can consider a pistol bullet at 1000Joule being deadly. Impacted debris only of 3mm in size (weighing around 30 milligrams) even exceeds this almost double. $\endgroup$
    – Antares
    Commented Aug 17 at 4:15
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    $\begingroup$ @Antares ... assuming they survive reentry. I agree with you in that such projectiles would be deadly, but they'd have to start as much more massive objects to withstand the force and heat of reentry; I would imagine that a few larger bits would break up into many more 30 mg bullets as they fall to the Earth. $\endgroup$
    – controlgroup
    Commented Aug 18 at 5:13
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    $\begingroup$ @controlgroup: True, of course. In this scenario I do not take re-entry phase, atmospheric drift, or heat-up due to drag, melting, shrinking and splitting up etc. into account. I just assume those particles "arrive" in this size and speed at the surface. Not where they are coming from or how they could be formed. They will be formed because of the law of large numbers is my underlying assumption. $\endgroup$
    – Antares
    Commented Aug 18 at 5:49

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