Moonquakes

[Guest lucky]

This question has its origins in this thread. I'm indebted to austenandrews and others for making some great suggestions and giving me several ideas.

 

Hypothetical 1:

A section of the moon's center measuring ~ 800 cubic km suddenly vanishes. The disappearance does not cause any excess energy to radiate into the moon's crust.

 

Hypothetical 2:

The size and mass of the object is probably too small to have any significant impact on the moon's orbit.

 

Now, since the center of the moon has suddenly disappeared, the likelihood of moonquakes is quite high. Aside from the possibility of large chunks of the moon raining down on earth, would these quakes throw off the moon's orbit?

 

N.B.: For whatever it's worth, I'm a GM who's designing a Galactic Heroes campaign. I'm still in the "laying the foundations" phase and I'm trying to work out some plot threads.

[Agemegos]

Re: Moonquakes

 

The size and mass of the object is probably too small to have any significant impact on the moon's orbit.

 

No, the smallness of the object isn't the issue. The issue is that this sort of event is not of a type to have any direct or appreciable effect on the Moon's orbit no matter how great the mass or volume involved were.

 

Now, since the center of the moon has suddenly disappeared, the likelihood of moonquakes is quite high.

 

Yes, it is a certainty.

 

Aside from the possibility of large chunks of the moon raining down on earth

 

There is no such possibility.

 

would these quakes throw off the moon's orbit?

 

No, and this is not an issue of scale. Quakes are not the kind of thing that has an effect on the orbit of the object quaking.

 

The re-arrangment of anomalous masses within the Moon might have a very small effect on the third-order components of its gravitational field. This might produce tiny changes in the orbits of satellites in close orbits near the Moon's surface, and over a sufficent number of years it might be possible to map the changes in the Moon's gravitational anomalies by extremely accurate measurement of the orbits of such satellites.

 

I am afraid taht you are barking up the wrong tree. If you want to have the Moon's orbit change to any perceptible degree you are going to have to give it a very powerful shove. Or else ignore the physics.

[austenandrews]

Re: Moonquakes

 

There is no such possibility.

What makes you say that? I grant you it won't fling mountains into space, but there have been volcanic eruptions on Earth that tossed rocks halfway to the moon. And the moon has, what, a fifth the escape velocity, not even counting the Earth's own pull? I'm no seismologist, but I suspect a moonquake of decameter-order displacement could easily give Earth a pretty spectacular meteor bombardment.

[Agemegos]

Re: Moonquakes

 

What makes you say that? I grant you it won't fling mountains into space' date=' but there have been volcanic eruptions on Earth that tossed rocks halfway to the moon. And the moon has, what, a fifth the escape velocity, not even counting the Earth's own pull? I'm no seismologist, but I suspect a moonquake of decameter-order displacement could easily give Earth a pretty spectacular meteor bombardment.[/quote']

 

Perhaps we disagree on what to call a sizeable chunk of the Moon. A few cubic kilometres of rock fragments and dust is a substantial ejectum on the volcanic scale, but not a sizeable chunk of a 22-billion-cubic-kilometre body.

 

Just put it down to a semantic misunderstanding.

 

Besides: decametre displacements? The event will reduce the Moon's average radius by 21 mm. Total lateral adjustment is two pi times that: 130 mm of compression spread out around the Moon. Any decametre displacements at the surface are going to result only from seismic stresses already near the surface---do you think the Moon really has decametre seismic disequilibria? If so. why are they not being released by the stresses of tidal kneading? I am pretty sure that the eccentricity of the Moon's orbit produces tidal kneading through a range greater than 21 mm.

[austenandrews]

Re: Moonquakes

 

Just put it down to a semantic misunderstanding.

Agreed. I was mainly going off comments from the other thread.

 

Besides: decametre displacements? The event will reduce the Moon's average radius by 21 mm. Total lateral adjustment is two pi times that: 130 mm of compression spread out around the Moon. Any decametre displacements at the surface are going to result only from seismic stresses already near the surface---do you think the Moon really has decametre seismic disequilibria? If so. why are they not being released by the stresses of tidal kneading? I am pretty sure that the eccentricity of the Moon's orbit produces tidal kneading through a range greater than 21 mm.

I'm positing the worst-case scenario, of course, that a small percentage of the max possible core displacement would make it to the surface. I confess I don't actually know how 500 billion megatonnes of iron would collapse on itself but I would think you could vector it so that tidal deformity would increase the damage.

 

But you make a good point, the stresses wouldn't shoot to the surface quickly. More likely we'd have to figure a really nasty crack at the core-lithosphere boundary (where a Google check tells me deep moonquakes originate) maybe shot full of molten iron like a wedge from the initial collapse. Then the crack could make its way upward after the iron flowed back to the core. That's possible, correct? I would think the moon's geological inactivity might even make it probable. If so, how quickly could we get a big displacement to the surface? Come on, work with me here, we're trying to add some cosmic spectacle to lucky's game.

[Dr. Anomaly]

Re: Moonquakes

 

Just a side note: though major moonquakes wouldn't affect the moon's orbit, they may well affect its rate of rotation by a very tiny (but measurable) amount. Not only will the diameter of the moon be ever-so-slightly smaller due to the implosion of surrounding material into the sudden emptiness at its core (meaning the remaining mass is packed into a slightly smaller volume and therefore must increase speed of rotation very slightly) but we had an example here on Earth just recently with the major earthquake/tsunami to illustrate this.

 

The speed-up won't have any real human-scale affect on things, but the slight difference will be measurable.

 

Further side note: these so-called "glitches" -- quakes due to cooling that result in a slight decrease in body radius -- are one thing that cause the sudden, slight, but measurable change in the rotation rate of neutron stars.

[Guest lucky]

Re: Moonquakes

 

I am afraid taht you are barking up the wrong tree. If you want to have the Moon's orbit change to any perceptible degree you are going to have to give it a very powerful shove. Or else ignore the physics.

There's been a misunderstanding here: I am NOT looking for an excuse to pull the moon down; I'm trying to figure out what would happen if a sudden space appeared at the center of the moon. The previous thread led me to the conclusion that moonquakes would result. Now I'm trying to figure out what the effects of these moonquakes would be.

 

Basically--and I *know* that this part is bad science--I'm setting up a scenario where Earth's moon is actually the accumulated detritus around a space ship. The ship was placed in Earth's orbit a long time ago and has gradually "snowballed" into the moon as we now see it.

 

The ship, owing to its advanced construction, is intact. When a deep drilling team reaches it, a team of investigators (the PCs) inadvertently gain access to its interior and accidentally activate the FTL Drive which causes the entire ship to instantaneously teleport across the galaxy. Owing to its massive size, this will cause some geological disturbances on the lunar surface. I wanna know what they are and if I can exploit 'em for dramatic purposes.

[tancred]

Re: Moonquakes

 

Somebody else has been reading David Weber, I see.

[Guest lucky]

Re: Moonquakes

 

Somebody else has been reading David Weber' date=' I see. [/quote']

Okay, you're the second person to mention this 'Weber' guy--did I miss a memo? Who the hell is David Weber?

[Supreme]

Re: Moonquakes

 

What makes you say that? I grant you it won't fling mountains into space' date=' but there have been volcanic eruptions on Earth that tossed rocks halfway to the moon. And the moon has, what, a fifth the escape velocity, not even counting the Earth's own pull? I'm no seismologist, but I suspect a moonquake of decameter-order displacement could easily give Earth a pretty spectacular meteor bombardment.[/quote']

Except that these aren't quakes caused by volcanism (sp?) which is a big build up of energy. These are quakes caused by the settling of material. I don't see anything other than a few pebbles coming to Earth (which would still cause a visible, but not destructive light-show).

 

Wouldn't this settling cause the Moon's rotation to change? The Moon suddenly becomes more compact with the same rotational energy. We'd finally start to see the dark side of the Moon from Earth.

[Nyrath]

Re: Moonquakes

 

Okay' date=' you're the second person to mention this 'Weber' guy--did I miss a memo? Who the hell is David Weber?[/quote']

David Weber is the famous SF author of the Honor Harrington series of "Hornblower in Outer Space" books.

 

The first book in the series, "On Basilisk Station" is available as a free download at http://www.baen.com/library/

If you like space navy combat books, you'll love Honor Harrington.

 

However, the reference above is to the "Dahak" series: "Mutineer's Moon" and "The Armageddon Inheritance". These are space operas with a vague similarity to the Perry Rhodan books. The main point is that it turns out our moon is actually a disgused alien super dreadnaught which has been in orbit since about 50,000 BC.

[Supreme]

Re: Moonquakes

 

David Weber is the famous SF author of the Honor Harrington series of "Hornblower in Outer Space" books.

 

The first book in the series, "On Basilisk Station" is available as a free download at http://www.baen.com/library/

If you like space navy combat books, you'll love Honor Harrington.

 

However, the reference above is to the "Dahak" series: "Mutineer's Moon" and "The Armageddon Inheritance". These are space operas with a vague similarity to the Perry Rhodan books. The main point is that it turns out our moon is actually a disgused alien super dreadnaught which has been in orbit since about 50,000 BC.

Then what the hell are we paying all those astronomers and lunar geologists good money for???

[Guest lucky]

Re: Moonquakes

 

The main point is that it turns out our moon is actually a disgused alien super dreadnaught which has been in orbit since about 50' date='000 BC.[/quote']

Really? Well... crud. There goes my idea to turn this into a bestselling novel.

Thanks for the info, maybe I'll try reading these books before I start my campaign.

[tancred]

Re: Moonquakes

 

David Weber is the famous SF author of the Honor Harrington series of "Hornblower in Outer Space" books.

 

The first book in the series, "On Basilisk Station" is available as a free download at http://www.baen.com/library/

If you like space navy combat books, you'll love Honor Harrington.

 

However, the reference above is to the "Dahak" series: "Mutineer's Moon" and "The Armageddon Inheritance". These are space operas with a vague similarity to the Perry Rhodan books. The main point is that it turns out our moon is actually a disgused alien super dreadnaught which has been in orbit since about 50,000 BC.

There's a 3rd book too, Heirs of Empire I believe is its title.

 

Run, don't walk, to your bookstore and snatch up Mr. Weber. He is, IMHO, one of, if not THE best science fiction writer still alive and writing today.

Weber has a gift for fleshing out characters; he makes them people you wish you could meet. He's also quite skilled at snappy dialogue; you'll find yourself laughing even in the middle of his most serious books.

 

His fantasy books didn't inspire me as much, but I can highly recommend all of his SF books.

[austenandrews]

Re: Moonquakes

 

Then what the hell are we paying all those astronomers and lunar geologists good money for???

Maybe someone else is paying them more.

[Dr. Anomaly]

Re: Moonquakes

 

Wouldn't this settling cause the Moon's rotation to change? The Moon suddenly becomes more compact with the same rotational energy. We'd finally start to see the dark side of the Moon from Earth.

Yes it would...as I mentioned in a post earlier in this thread. However, as for seeing the dark side...not in any currently living human's lifetime. The chance in rotation rate will be too small to be noticable on a human scale, though it will certainly be measurable with sensitive instruments.

[austenandrews]

Re: Moonquakes

 

Except that these aren't quakes caused by volcanism (sp?) which is a big build up of energy. These are quakes caused by the settling of material.

But the settling of a tremendous amount of material. I'm proposing a huge buildup of energy as the moon's weight tries to close a giant crack. The moon wasn't always geologically inactive. The "fossils" of old volcanoes must still exist as somewhat defined heterogenous structures in the crust, providing pathways for an anomalously big release of potential energy. If memory serves, the moon's Earth-facing "seas" are darker igneous rock, so perhaps the extinct volcanoes that formed them can spit one last watermelon seed at the heavens.

 

(Or maybe I should take my ten-dollar layman's words and find work as a technical consultant on The Core. )

 

I don't see anything other than a few pebbles coming to Earth (which would still cause a visible, but not destructive light-show).

I wasn't thinking of an Armageddon-style bombardment. More like a really dispersed shotgun blast, with softball-size pellets. Or maybe you can bundle the energy into hurling a single house-size rock that can explode Tunguska-style over a suitably dramatic target? I doubt the stresses of ejection would allow that option, though.

[Guest lucky]

Re: Moonquakes

 

See, that seems more plausible to me. I have a hard time swallowing the idea that an "extinction-class" moonchunk would break off and slam into Manhattan, but something like your "shotgun pellet" idea sounds totally workable...

 

Hell, why put the cart in front of the horse by worrying about the Earths surface getting bombarded? Wouldn't the ejected moon particles hit our satellites and space stations first? What kind of damage would/could that do?

[Agemegos]

Re: Moonquakes

 

There's been a misunderstanding here: I am NOT looking for an excuse to pull the moon down; I'm trying to figure out what would happen if a sudden space appeared at the center of the moon. The previous thread led me to the conclusion that moonquakes would result. Now I'm trying to figure out what the effects of these moonquakes would be.

 

Okay, good.

 

The answer is that the Moon's surface will settle by 0.84 inch on average, there will be landslides on the surface erasing some smaller craters, and may be a few moonquakes about the sizes of the ones normally triggered by tidal kneading. The Moon's rotational speed will increase by a few parts per billion as the result of its decrease in moment of inertia: nutation will increase slightly, but tidal stabilisation will prevent it actually rotating.

 

Short story: significant readings from seismometers on the Moon. Increased nutation detectable by careful astronomers. Nothing perceptible to the common man or the naked eye (on Earth).

[Agemegos]

Re: Moonquakes

 

Yes it would...as I mentioned in a post earlier in this thread. However' date=' as for seeing the dark side...not in any currently living human's lifetime. The chance in rotation rate will be too small to be noticable on a human scale, though it will certainly be measurable with sensitive instruments.[/quote']

 

Indeed. I would be very surprised if teh increase in angular speed were enough to overcome tide-locking. We'd see a slight oscillation, not actual rotation.

[austenandrews]

Re: Moonquakes

 

The answer is that the Moon's surface will settle by 0.84 inch on average' date=' there will be landslides on the surface erasing some smaller craters, and may be a few moonquakes about the sizes of the ones normally triggered by tidal kneading.[/quote']

So you think there's no chance of a crack in the lithosphere propagating significantly to the surface? Not even in a body as rigid as the moon?

[Dr. Anomaly]

Re: Moonquakes

 

So you think there's no chance of a crack in the lithosphere propagating significantly to the surface? Not even in a body as rigid as the moon?

Probably not, for a couple of reasons.

 

First, the Moon is cold to the core, but you have to keep in mind the effect of pressure on material at that depth. Even though the Moon is "solid" all the way to the core, the pressure once you get more than a couple of hundred miles down (exact figure for a body the mass & density of the Moon escapes my memory just now, I can compute it if need be) makes the rock and metal act more like a thick "tar", if you will...though it will transmit shocks, it doesn't do so by cracking. There is a sort of barrier between the tarry-state solids and the more usual solids of the crust and outer mantle caused by a poor transmissive matching between the two "types" of substance. This would tend to slow down the shockwaves as they cross the boundary, and further would tend to pulverize (crumble) the more normal, brittle mantle/crust material once they did cross. The odds of an actual crack making it all the way to the surface are slim indeed.

 

The second reason is that the shockwaves would lose a bit more velocity while traversing the just-pulverised material, which would momentarily act as a sort of fluid. The lost energy would emerge as heat, and stand a good chance of melting some of the lithosphere rather than cracking it. You might see an upwelling of newly-formed magma through existing deep cracks -- caused by old meteor impacts and such -- but probably nothing as spectacular as a lava "plume" or volcanic eruption. Most likely if the just-liquified hot rock did manage to reach the surface, you'd get a couple of new, small maria as it oozed to fill in a few old craters and then solidified again.

[austenandrews]

Re: Moonquakes

 

Could you arrange a lava dome out of that?

[Dr. Anomaly]

Re: Moonquakes

 

Could you arrange a lava dome out of that?

Given the relatively low gravity of the moon, I don't think that would be out of the question.

[Guest lucky]

Re: Moonquakes

 

What's a lava dome?