On my way to the beer store the other night (really) I was rather startled to see a mostly full moon with a concave section “missing” near the left edge. It didn’t look right and a little thought led me to the conclusion that I was witnessing the beginnings of a lunar eclipse. Turns out I was one of the last people to know this was coming.
Once I was certain that the moon wasn’t being eaten by some large interstellar serpent or caddis fly larva, I got to wondering, "What if the moon got smacked by an asteroid and a sizeable chunk was dislodged? It could conceivably look much as it did during the eclipse with about 25% of the visible surface just plain missing.
Assuming enough mass remained to allow gravity to get to work rounding the thing out again, how long would it take for the object to get more or less spherical again?
It’s solid rock, with no molten core, so it’s unlikely that the moon’s own light gravity could make it “flow” back together. Do the same thing to the Earth and the mantle will flow back into an oblate sphere with a new crust hardened over it.
More or less immediately, geologically speaking. A collision of that magnitude would release an enormous quantity of energy, which would melt much, if not most (or even all) of the Moon. In this molten state, it would take mere hours to settle into a largely spherical shape.
:eek: Given that the newly dislodged piece of the moon, along with whatever dislodged it, will be headed toward the biggest thing in the neighborhood, I would be a bit more worried about the soon-to-be not round Earth, rather than a suddenly not-round moon.
That probably would not happen. The moon isn’t held up there with string, remember; it doesn’t crash into the Earth because its inertia overcomes Earth’s gravity. If the Moon got smucked by an asteroid and was cut in two, the two halves would still have that momentum. They might have very different orbits though.
Even if you manage to make a section of the Moon disappear without melting the rest, the yield strength of rock (and any other normal material) is not high enough, IIRC, to allow a very non-symmetric Moon. The Moon’s gravity is large enough that the edges of the new crater will yield under gravity and fall in to make the Moon nearly round again.
People building condominiums on the moon would cause the same effect, sending the moon crashing into the earth, destroying most of human kind and triggering the rule of the morlochs.
I was mostly responding to posts like Q.E.D.'s and not (obviously) doing a very good job at responding to yours.
First, a disclaimer: I’m not a geologist (or selenologist), just a physicist with a back of an envelope handy, so these aren’t expert opinions. But, starting from the cookie-with-a-bite-taken-out-of-it picture you describe, I don’t think it should take long (hours to days) for the gross shape to be mostly smoothed out. The crater edges will simply crumble (or otherwise yield) under the stresses, and rockslides will fill in most of the crater. How much of it? Well, the pressure under a column of rock of height h and density d is g h d (g the Moon’s gravitational acceleration, ~1.6 m/s[sup]2[/sup]). Yield stress P for rock is something like 140 MPa (from here–also a good link for the general subject of rock deformation and yielding), and let’s assume a rock density of ~ 5 kg / m[sup]3[/sup]. This means that a rock column higher than about P / (d g), about 20 km, cannot support itself. (You can build higher if you build cones or pyramids, but this result should give the right order-of-magnitude limit.) These rockslides and moonquakes will occur with decreasing frequency as the shape gets closer to round (i.e., when the crater height gets close to the yield limit estimated above).
I was mostly responding to posts like Q.E.D.'s and not (obviously) doing a very good job at responding to yours.