What would happen if the moon broke up?
May 25, 2015 10:59 AM   Subscribe

Neal Stephenson's new novel, Seveneves, starts with the moon breaking up into seven pieces (spoilers for the first 50 pages of the book inside).

This leads to continued fragmentation of the moon, culminating in a massive meteor bombardment of earth. I'm assuming that Neal Stephenson has done his homework and that the physics behind this is plausible, but I'd like more details. Assuming that the lunar breakup described did happen, how long would the ensuing fragmentation take? What fraction of the lunar mass would end up hitting earth? Links to any modeling or papers on the subject welcome. Bonus: links to descriptions of the effect of the resulting meteor bombardment (I remember hearing about this as a possible cause of the extinction of the dinosaurs on Radiolab, but I'd like to see the original paper).
posted by pombe to Science & Nature (12 answers total) 5 users marked this as a favorite
If you take the disputable possibility of a breakup for granted, the effects would include:
-- a lot of small pieces crashing into the earth, wreaking havoc including tidal waves and craters
-- normal tides would be disrupted, with resulting effects on biological habitats and surfing
-- the stabilizing effect of the moon's presence on the earth's rotation would be disrupted, resulting in unpredictable wobbles and consequent further effects on tides and climate, and potential changes to the earth's orbit around the sun
-- much less light at night, depending on how small the pieces got, so good news for sky-watchers.
posted by beagle at 11:44 AM on May 25, 2015

The book's premise is not plausible in a way that I can imagine. The Moon is held together by its own gravity, so what would be the mechanism for it to "break up"? You would have to impart enough energy to overcome the Moon's gravitational binding energy. Assuming the Moon to be a uniform sphere, that would be 1.234E+29 joules of energy. That more than 3 million times the 2004 Indian Ocean earthquake or 300,000 times the energy of the impact that created the Chicxulub Crater in the Yucatán Peninsula. This is not going to happen without a Death Star shooting the Moon. So, without a proposed mechanism, it is hard to talk about how long it would take.

What fraction of the lunar mass would end up hitting earth?

Assuming the Moon were to come apart in a uniform manner, very little of its matter would be on a trajectory to hit Earth. Imagine a sphere expanding outward. Matter gets spread out very quickly. Assuming the Moon blows up uniformly at its closest to the Earth (about 356,400 km), that's a sphere with an area of 1.59619E+18 square meters. The surface area of the Earth facing the moon is 1.275e11 square meters, so 1/7.99e-8 of the Moon's mass should hit the Earth. That would be 5.86e15 kg. Assuming even disbursement, that is 3.6 grams per square meter of the Earth's surface at a variety of impact angles. That's not very exciting. An exciting story would have huge pieces of rock heading towards Earth, but you really need a meteor for that. The Moon "breaking" just isn't plausible.

Not a bombardment side effect, but the effect of the Moon's gravity on Earth would obviously be lost.

I am sure someone will correct me if I got any of this wrong, of course.

People don't realize that the Moon is quite far away. All of the other planets could fit within the Moon's orbit about Earth.
posted by Tanizaki at 11:50 AM on May 25, 2015 [8 favorites]

According to the current theory on its formation, the moon was formed following an event in which a pre-Earth asteroid mass the size of Mars was hit by another one around the same size. In the cataclysm that followed, a giant, very hot, spinning cloud of rock, iron, nickel (which are the major components of asteroids, generally speaking) sat around in its new orbit (which is our orbit around the sun now). All that energy has to go somewhere.

Each of a bajillion little rocks and metal droplets and such orbiting a bajillion-1 other objects. The interactions of all these pieces caused them to run into each other, sometimes stick together, or sometimes just settle together from mutual gravity, and sometimes events conspired to flight bits out and away from the cloud of crap. The heavy stuff (metal) formed a large giant ball. The rock took longer to for into anything, as it's lighter stuff. Most of it joined the metal ball (and thus Earth was formed), and some of it found its way to a stable orbit around the Earth, and all that stuff coalesced into a second ball, now the Moon.

The Agent of the novel didn't just pierce the moon, it added a huge amount of energy to it. The 7 chunks stored all that energy as gravitational potential energy. If you pulled the moon into 2 pieces, and expended energy to accomplish that, most of that energy would be released as kinetic energy when the two halves of the moon pulled themselves back together.

It comes down to how much energy was added to the moon by the Agent. Most of that energy would go into heating the moon, but since moon is lightweight rock (or at least 6 of the 7 fragments are-- "Peach Pit" is the small metal core of the moon) the rocks would fragment when that kinetic energy is applied to them, and some stuff will by sprayed far and fast from the Moon into various independent orbits. Some majority percentage of that won't have the velocity to escape the Earth-Moon gravitational system, and of that huge percentage, most of it will fall to Earth eventually, because Earth is the big attractor.

Will all that crap heat the atmosphere? From here, I can refer you to something more concrete: Chicxulub. When I grew up, the story of the Chicxulub asteroid that caused the mass extinction of the dinosaurs was the story of a white-hot impact in Central America followed by a kind of nuclear winter-induced climate change. There's a new theory, based on studies of kinetic impacts, that suggests that the impact threw so much rock, both from the asteroid as well as from the Yukatan, into the air, and into space, which then fell back down all over the Earth, that the heating of the entire planet took place within hours of the impact. This Radiolab story, which is difficult to source but does feature reputable scientists, gives a kind of radio dramatization of the event.
posted by Sunburnt at 12:35 PM on May 25, 2015 [3 favorites]

When I read the free 25 page excerpt, my first objection was to how the moon was broken up into so few large pieces. It would seem to require quite a gentle push of evenly distributed force, so that the moon came apart like a gently pulled pastry.

That excerpt is here.

Greg Bear came up with a decent way to assemble a matter/antimatter device at the center of a planet (outside of how to maintain degenerate matter densities at sub-neutron star masses) in The Forge of God.

The amount of antimatter annihilated that will exactly balance the energy of gravity mentioned by Tanizaki is a cube about 100 meters to a side of 10 g/cm material. That's enough energy to vaporize a sphere of iron a thousand miles in diameter. The moon's iron core is about 250 miles in diameter, so maybe no "Peach Pit" could survive.

I would expect that the chunks formerly known as Moon would form a a very slowly expanding debris field, and that the vaporized portion would expand to a cloud of significantly larger size and perniciousness.

Of course, it could be part of the story that the work was done by a technology indistinguishable from magic. I won't find out until I've gnawed my way out of the Baroque Cycle.
posted by the Real Dan at 1:41 PM on May 25, 2015

I'd expect the slowly expanding debris field suggested above, Adding that it would eventually form a ring, with larger chunks corralling the smaller particles into bands, like Saturn's ringlets. My understanding is that with the moon being far outside the sorts of low density gasses that slow down satellites in lower "near earth" orbits, that part from whatever forces blew the moo apart, there wouldn't be any reason for large chunks to immediately start raining down.

I love reading about astronomy but I failed grade 10 physics, so I basically pulled all this out of my ass.
posted by bonobothegreat at 1:54 PM on May 25, 2015

I've done some further reading, and there's a bit of an expository interlude that Stephensen jams in when one character is in peril on the X-37 (around 38% into the book; I don't have a page number because I'm reading on a tablet, but it's the chapter called "Hard Rain").

It's a few paragraphs, beginning with slicing a cone, but here's a relevant one:

What was true of the solar system as a whole had also been true of the Earth-moon system. The moon had circled the Earth in a nearly circular orbit. From time to time, a wandering stone from deep space would blunder in through a libration point and get captured into a geocentric orbit, but sooner or later it would hit the moon, hit the Earth, or be ejected by a close encounter with one of those bodies. Thus had the moon swept Earth’s skies for billions of years and protected it from most big meteor strikes, making it a suitable place for the development of complex ecosystems and civilizations.

The paragraph that follows describes how the process applies to a new debris cloud, and how that, in turn, imperils the character on the X-37.
posted by Sunburnt at 8:32 PM on May 25, 2015

In case anyone is wondering, I finished the book last week (VAGUE SPOILERS FOLLOW), and there was never any explanation for the Agent. I seem to recall one character suggesting a tiny black hole passing through the moon, but I think another scientist in the book shot that theory down as being unlikely/impossible.

The plausibility of the whole thing and the likelihood of meteor bombardment come down to what in the world the Agent was, and I'm not aware of anything we currently know of that could break apart the moon like that. To get through the book without getting hung up on the question, I just told myself that maybe it was some bizarro high energy particle or quantum bullshit, because theoretically anything is possible, just varying levels of likely.
posted by yasaman at 10:43 AM on May 26, 2015

This is Neal we are talking about here...crazy meticulous detail guy....for you excerpt readers the moon starts to behave how you would think as it heads towards turning into a ring around the planet.

I'd recommend you read the book. The moon breaks into millions of pieces right off the bat, it is just the big 7 are the most visible.
posted by th3ph17 at 10:48 AM on May 26, 2015 [1 favorite]

The wiki article, and its sources, on the Geology of the Moon might be interesting and informative. Not as much dairy as we'd all hoped, but it's interesting nonetheless, and we've learned a lot from retrieving boxes of rocks, and also dropping things onto it at high speed and recording the moonquakes that resulted. (The entire lunar surface rings like a bell when you strike it with the empty third stage of a Saturn V rocket, evidence that the moon's mostly solid, unlike the mostly-liquid Earth.)

We've also studied moon rocks that landed on Earth after the moon took a bit hit. It's difficult to tie a given rock down here to a specific impact up there, but none are in doubt that impacts of a certain size up there lead to meteors down here.
posted by Sunburnt at 2:05 PM on May 26, 2015

Response by poster: The main question that bothers me is, where does the energy come from to move significant parts of the moon's mass out of lunar orbit? In the book's description it seems like the seven pieces of the moon are not left with a great deal of kinetic energy following disruption by the agent, so it seems like they ought to re-coalesce into a solid body.

Also, here's a link to a recent article discussing atmospheric heating following the Chixculub cratering event. This appears to be what Radiolab was discussing.
posted by pombe at 5:05 PM on May 26, 2015

The main question that bothers me is, where does the energy come from to move significant parts of the moon's mass out of lunar orbit?

According to the synopsis I have read, "a strange, unknown source", which is another term for "magic". Energy requirements are a common blunder in fiction. For example, in Game of Thrones, the fire that a dragon breathes out must have first entered the dragon the form of food energy. The flame blasts we see and hear about would require the dragon to spend several days at the buffet to fuel such flames.

Your instinct is correct regarding re-coalescing. According to the giant impact hypothesis, the Moon coalesced from a ring that circled Earth after Earth's impact with a hypothetical planet called Theia. It would take a very long time for the debris to re-coalesce, though.

The linked article's comments about "debris into low earth orbit" and "the possibility that it might instead become an impenetrable ceiling only a hundred or so miles above our heads" don't make sense to me because the Moon is not in low earth orbit in the slightest. I should read the book before having an opinion but the Moon is moving away from Earth at about 2 inches per year, so I don't see how we get from that to chunks of the Moon falling to Earth.
posted by Tanizaki at 12:19 PM on May 27, 2015

> it seems like the seven pieces of the moon are not left with a great deal of kinetic energy following disruption by the agent

They didn't, but they had a truly massive amount of potential energy, specifically gravitational potential energy.

If we assumed that the moon broke into 7 pieces of equal mass (which would be on the order of 10^22 kg), and they were blown apart from each other by a number of miles, the gravitational potential energy GPE will = mgh-- 10^22kg * gravity * height. Now, g is kinda low here and a little tricky to calculate (it depends on geometry)-- but let's say the moon somehow broke into 7 equal-massed spheres for some reason. The surface gravity of each one is g=-GM/(r^2), radius at the surface is about 900,000m, so surface gravity is about 0.81 N/kg = m/s^2.

So, GPE = 10^22kg * 0.81 N/kg * height (m) gives me 8.1x10^24 joules for every kilometer of separation, and that's the potential for 1 part to each other part. If somehow all of these pieces were 1km from every other part, that would be the 8.1x10^24 joules times 6!, that is, 6 factorial: 6x5x4x3x2x1 (to calculate all the mutual attractions in a 7-body system). I get 5.8x10^27 joules, or 5.8x10^18 gigajoules, a unit I mention because a "ton of TNT" is a unit of energy equal to 4.184 gigajoules, so we have 1.4x10^18 tons of TNT. The largest nuke ever exploded was the Soviet device Tsar Bomba which was dialed in at 50Mt of TNT, so in this highly idealized case, that's 2.8x10^10 Tsar Bombas of gravitational potential energy between all 7 parts. That's an assload of energy. As the distances go up, so does the potential energy.

Every time the pieces fall towards one another and hit, more potential energy is converted into kinetic energy, until it gets down to a kind of baseline level where all those tiny chunks of moon have a lot of kinetic energy and zip in every possible direction.
posted by Sunburnt at 5:48 PM on August 3, 2015

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