I'm thinking of having a world that has a moon that's been partially split (so not fully halved) with the debris slowly separating from it. At night you'd see the split moon mainly and lots of various-sized mirror reflectors around that. How would a moon like this effect the planet, tides, etc?

Assuming this is one moon for the one world that will take a very long time to slowly separate into at least two halves, with a lot of debris surrounding it, maybe even a ring around the halves, what planetary changes would be involved at the least and greatest states of this disintegration of their moon?

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    $\begingroup$ Under normal circumstances the moon would remerge with itself so what is splitting it apart? $\endgroup$
    – Soan
    May 8, 2019 at 16:43
  • $\begingroup$ reminds me of the lack of suspension of disbelief I get when I see the moon in the RWBY series. good question! $\endgroup$
    – IT Alex
    May 8, 2019 at 16:59
  • $\begingroup$ THUNDARR THE BARBARIAN! : fromthewastes11811.files.wordpress.com/2014/11/… $\endgroup$ May 8, 2019 at 17:02
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    $\begingroup$ Either it blew up (pieces launched at escape velocity) which is not your scenario, or it's really close to the planet, in which case it causes some weird problems like regular tsunamis. I think human life can survive though. $\endgroup$
    – adrian
    May 9, 2019 at 1:00

3 Answers 3


In order for the fragments not to simply reform into a single body from their own gravity, the damaged moon would have to meet certain criteria.

Either it would have to be within the planet's Roche limit, or the fragments would have to have been launched at above the moon's escape velocity. Since you want a slowly evolving situation, the latter can be ruled out, so we need to assume the moon in question is so close to its primary that it was on the verge of tidal destruction anyway -- and when the "damage" (pretty much have to be an impact) took place, the fragements had tidal forces separating them with barely more force than the gravity that would tend to bring them together.

If the timing was just right the fragments of the moon would drift apart over a period of months, years, perhaps even generations. The smaller bits that were blasted clear by the impact would already have begun to form a ring system, though it would be faint at first -- but as the major fragments, already fractured by the main impact, begin to tidally separate the collisions between them would increase the count of small fragments, filling in a ring (more like those of Jupiter or Neptune, rather than the huge gaudy system Saturn shows).

The effects of all this on the primary would be, first, a heavy meteorite bombardment near the moon's orbital plane (likely the planet's equator as well); this will continue for a long time, as fragments decay orbit. Second, there would be a huge change in the planetary tides -- assuming the moon was large enough to raise noticeable tides in the first place, those tides would gradually lessen as the peaks and valleys of the cycles spread out, eventually vanishing entirely as the moon finishes its transformation into a ring.

As noted by Morris The Cat, these same tidal effect changes would also apply to the primary's crust -- though in the case of a moon already near its Roche limit and a breakup event "barely big enough", these would be gradual rather than catastrophic. You'll still have plenty of trouble with the impact belt (and it'll cause some tsunamis, too); it might still be an extinction event, but the required breakup impact is much smaller in this case and it need not be an instantaeous catastrophe.

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    $\begingroup$ It's also possible that tidal forces may be one of the driving forces of plate tectonics; but this is still debated by scientists. If this correlation is true, your world's plate tectonics would become weaker, causing fewer earthquakes, but also slowing the planet core's dynamo action, which would weaken its protective magnetic field. $\endgroup$
    – cowlinator
    May 9, 2019 at 2:17
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    $\begingroup$ The roche limit for the earth-moon system is roughly 15000 km, the moon that low in the sky would be...interesting $\endgroup$
    – Borgh
    May 9, 2019 at 7:49
  • $\begingroup$ You and I have very different definitions of gaudy. $\endgroup$
    – L0j1k
    May 9, 2019 at 8:38
  • $\begingroup$ @cowlinator Regardless of the end effects (reduction in tectonic activity, loss of magnetic field), all these changes would be rather gradual -- potentially generational vs. overnight -- assuming as above that the moon in question was already on the verge of Roche limit breakup, and the breakup event was the minimum needed. Breakup even could even be just the actual Roche limit tensile failure of the moon's material makeup. $\endgroup$
    – Zeiss Ikon
    May 9, 2019 at 11:07

The book Seveneves has the moon break into 7 parts for no particular reason. It just broke; it did not blow up. The parts bump and grind into each other. A few weeks after it happens one big part breaks into 2 little ones and they break up more and more.

One problem is the little parts of the moon that come down to earth. That actually turns out to be more than a little problem. In the short term the center of mass of the moon stays the same and stuff is unchanged as regards tides. Of course the moon looks different on account it is 7 (then more) big pieces and many little ones.

On thinking about it I am not sure why little pieces came to earth except that this phenomenon drove the whole plot and without it there would be no story. The little pieces are a lot closer to the moon's barycenter than to earth and I think they would all stay close to home. It is hard scifi and I am sure he explained why.

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    $\begingroup$ I was thinking about this book as I was reading this question! +1 $\endgroup$
    – Vnge
    May 8, 2019 at 20:09

It depends a lot on what kind of relationship the planet and the moon had to start with. If we're assuming the same relative masses and distances that the earth and the moon have, then the results would be pretty dramatic.

For starters, let's think about how much energy we're talking about. In order to have the moon split, and STAY split, you'd have to accelerate most of the 7x10 to the 19th power tons of the moon's mass beyond its 2.4 km/s escape velocity.

That's... a pretty stunning quantity of energy and any event capable of creating this would have some effects on your planet too before you even GET to the secondary effects. that kind of energy output would by necessity turn a not-insignificant fraction of the moon directly into light, heat, and other forms of electromagnetic radiation. You'd have some really spectacular aurorae at the very least and probably most or all of the side effects of a direct impact from a Coronal Mass Ejection.

Zeiss Ikon already talked about the meteorite bombardment and the tides, but again if you're starting with something analogous to the Earth/Moon system, you're ALSO going to have earthquakes. The moon applies those tidal forces to the earth's crust too, not just the oceans, so a disruption to those tidal forces is also going to cause a lot of moving and shaking on the fault lines. That means earthquakes AND volcanoes, AND the associated tsunamis in addition to the tidal disruptions you're already getting.

TLDR: It would be a major extinction-level event.

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    $\begingroup$ If the moon in question was already treading on its Roche limit, it would only take enough impact to fragment the body for it to start separating -- essentially jump-starting the breakup that would still have happened as the moon's orbit continued to reduce. So most of the super-high-energy stuff would be avoided in that case, but the tidal effects on the crust -- could be a problem. $\endgroup$
    – Zeiss Ikon
    May 8, 2019 at 17:25
  • $\begingroup$ @ZeissIkon Well, that's an interesting math problem... In our solar system at least all the satellites that are anywhere close to their Roche limit are extremely small (and close) relative to their primaries, and don't apply any significant tidal forces to their primaries. I doubt that blowing up Deimos or Phobos would have much impact on Mars, for example. You'd have to find some kind of balance point where the satellite is close enough to be within the Roche limit and large enough to apply tidal forces, without being SO close and SO large that its presence would be inherently destructive. $\endgroup$ May 8, 2019 at 17:37
  • $\begingroup$ I'm not so certain that the change in tidal forces would have the effect on the planet you describe. To the extent it had any effect at all, it should be a smoothing out of the tidal cycle, producing a bit less amplitude of the high/low extremes, but those extremes would last a bit longer. $\endgroup$ May 8, 2019 at 19:04
  • $\begingroup$ @MontyHarder The moon's tidal force can cause movement of the earth's crust of up to 40cm. That's a lot of energy. en.wikipedia.org/wiki/Earth_tide $\endgroup$ May 8, 2019 at 19:22
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    $\begingroup$ @MorrisTheCat Yes. Now explain to me how breaking the moon into two large chunks and a bunch of smaller ones will cause more movement than the already-existing solid moon causes. It's the same mass, but spread out over a larger area. How does the tidal force increase in that scenario? $\endgroup$ May 8, 2019 at 19:32

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