Reshaping the Earth's sky

For a story of mine, I'm planning some rather drastic alterations to Earth's immediate surroundings due to a battle. Yes, I know that sort of thing pretty much instantly invalidates what I'm about to say, but please hear me out first.

The changes I want to make are two-fold.

• 1. At one point, the baddie gets smacked into the Moon hard enough to shatter it. The Moon, I mean. The fragments settle into Earth orbit and eventually coalesce into a ring system similar to Saturn's, but much smaller. It naturally wouldn't happen on a human timescale but over centuries, maybe even millennia - but by about 8000 years later, the rings are there.
• 2. When the above fails to kill the baddie, the hero teleports Ganymede over and smacks it into the baddie but Ganymede, being twice as heavy as the Moon, survives the collision in one piece (albeit with a severely defaced surface akin to Miranda) and loses enough kinetic energy in the collision to get captured by Earth's gravity into a stable orbit just beyond Earth's Roche limit (which is 21,823 kilometers/13,560 miles for a satellite with Ganymede's density). At this distance, Ganymede is 13.572° wide in the sky, about 27 times the visible size of the full moon.

My questions:

• 1. Considering that planetary ring systems don't last very long unless being actively stabilized by moons, would these rings still be there 8000 years later or would Ganymede's gravity eject them out of the Earth-Ganymede system?
• 2. Would that distance be close enough for Ganymede to develop a thicker atmosphere by stealing some of Earth's? From what I can tell, it wouldn't come close to even Mars' atmosphere, but is it possible?
• 3. How severe would Earth's loss of angular momentum be from Ganymede orbiting it at such an extreme proximity?
• 4. Can Earth's shorelines expect to be hit with tsunami-strength tides from Ganymede's proximity?
• 5. Considering that Ganymede's distance from Earth would place it well within Earth's outer Van Allen belt and Ganymede has its own magnetic field as well, how badly would Ganymede interfere with Earth's ability to deflect solar radiation away from itself and how badly would Ganymede be irradiated in the process?

I think the biggest problem would be Ganymede's sudden appearance. Had Ganymede approached the Earth in the conventional way, there would be time for both bodies to internally adjust to each other's tidal forces. If Ganymede suddenly appears in low Earth orbit, the sudden shock might very well fracture Ganymede and cause extreme tectonic movements on the surface of the Earth. I would highly doubt anybody on Earth would survive.

That latter part isn't a problem. Earth just got hit with massive orbital bombardment that irradiated the surface beyond habitability and killed all macroscopic life outside the oceans anyway.

Oh, joy. Just the sort of story I'd want to read...

Good thing humanity developed space travel, huh?

But yeah, Earth becomes a Death World. So the tides being crazy strong isn't an issue.

edited 22nd Dec '16 1:27:36 AM by amitakartok

Destroying the moon would take something like 1X10^29 joules of energy and putting that much kinetic energy into a human sized mass would put it in the relativistic field. Unfortunately, collisions and relativistic speed start doing weird things to physics like causing nuclear reactions and summoning higgs bosons. This becomes relevant but that's as far as I can go for scientific descriptions.

Other than the inevitable X-rays generated from supercompressing the matter at the point of impact, which I don't think would be enough to sterilize the planet, you'd also have a rain of moon fragments fall in an apocalyptic firestorm mauling any civilization on the planet. Of course, once Ganymede enters the picture life becomes REALLY unpleasant as continent scouring tides not only scrape the land clean but trigger geological activity as the lithosphere rises with the tide. Side effect of being right at the Roche limit, really. The moon put some absolutely narly tides on the Earth when it was closer. Having an even bigger moon being even closer would naturally have an even more terrifying effect on the surface.

No ring would form. Debris would either be pulled to Ganymede or the Earth as Ganymede is too big to merely break up clumps in the ring.

How far would Ganymede have to be to allow the rings?

Hard to say. At least as far as Earth is from the current moon. Probably farther. You need enough space for an orbit that isn't torn apart by tidal forces. Or, more precisely, a set of orbits.

In fact, I'm not sure if it's possible for a planet with a large moon to form a ring system. Other planetary rings, such as the ones around Jupiter and Saturn, are gravitationally dominated by the gas giant. Tidal forces would disturb the rings, making them more oblong than circular.