Can a planet still function with a damaged moon?

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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.