Moon hit strong enough to put it in a slow spin?

Question

I’m building a world nearly equal to earth; i need a catastrophic event that make the moon start spinning breaking the tidal lock, a really slow spin, like 500 years for a full spin. i thought about a strong impact with great angle. It will be possible while retain its orbit mostly unchanged? Every scientific objection are welcomed

Answer 1

A lunar day, equal to lunar orbital period, is ≈28 (Earth) days, adding a 1/500 yr^-1 to it corresponds to a relative change of (1/(500×365))/(1/28), which is about 150 ppm. Such a small deviation will likely be compensated by the orbital resonance attractor in the spin-orbit phase space, even given a low eccentricity of the Moon orbit. My feeling is that the Moon would return to the 1:1 resonance barely after 1-5 axial spins at the increased (or decreased) rotation rate. Since the Moon orbit eccentricity is very low (≈0.06), the resonant band of the spin-orbit phase space is quite narrow, and, as you likely know, separated from other resonant bands by chaotic bands. It is possible that the rotation speed will enter the a band of a chaotic regime, so that the rotation rate would not decrease smoothly back to where it were. The tidal heating will also vary then. In the final phase, the process shifts from the chaotic regime to librations of decreasing amplitude which may be protracted; surprisingly, this part is modeled by a dumped pendulum equations quite well.

The heat dissipated in the process could in principle start some volcanic activity on the Moon, and will certainly initiate outgassing in the heating rock, but I cannot speak to this, unfortunately. The excess energy, about 150 ppm relative, i.e. 1.5×10^-4 of the total rotational energy, is quite low. The largest unknown here is the time over which the energy dissipates. My feeling is the process should be quick, on the order of 10^2 to 10^3 years. Unfortunately, I'm not familiar with energy models when establishing an orbital resonance; I studied only the dynamics of it, where energy “just gets lost,” so I have no intuition how much heating this will cause, and what the heating profile over time would be.

Sorry that this is a little bit handwavy. This is rather just my thoughts about how complicated the situation is, and that its outcome may be predictable or not, depending on how far off 1:1 resonance the system will be thrown, i.e. whether it will reach the chaotic band in the phase space. The Moon will not necessarily disintegrate from a glancing blow, as @l-dutch helpfully estimated. Such a blow will impart both spin and orbital velocity change, and the exact figure will depend on the angle of impact and the mass and speed of the impactor. A head-on impact will change orbital velocity without affecting spin, which will also disturb the resonance. Any impact, central or glancing, actually will. The impact itself will also produce significant amount of heat in an instant, which should also be accounted for. An off-equator blow will also send the rotation axis tumbling, which will eventually stabilize in the presence of Sun's gravity, that also should not be discounted. Planetary dynamics is already horribly complex even when these effects are ignored. I'd turn to numeric modeling rather than trying to write and solve equations accounting for all factors; they aren't likely even solvable (welcome to the wonderful underworld of the systems of PDEs!). But an impact that throws the Moon out of resonance by just 150 ppm is not catastrophic at all for the Earth-Moon system, and things will “get back to normal” almost instantly on the astronomic timescale, although the new “normal” may end up a bit different, due to a shifted rotation axis of the Moon and a different eccentricity and, albeit very slightly, radius of its orbit.

As a wild guess, does The Universe Sandbox has a model of tidal resonance?

Since you have a wide literary licence, you may assume any sensible regime in the range that I tried to describe, even if rather qualitative. If you want lunar volcanism, it's possible. If you want the impact ejeca reaching the Earth, it's also possible. A recovery time of hundreds to tens of thousand years is reasonable. Finally, if you want the Moon to end up rotating on a tilted axis with precession after the event, it's not only possible but very likely. If you do not, that's also does not violate physics, just requires a more or less precise nearly-equatorial momentum transfer, which is also realistic, given the relatively low energy gain.

Answer 2

The rotational kinetic energy of the moon is $3 \cdot 10^{23}$ J. In order to change the rotational speed of the moon, you will need to play with an energy of that order of magnitude.

The gravitational binding energy of the moon is instead $1 \cdot 10^{29}$ J, which is about 1 million times higher.

Therefore you can "safely" spin up the moon by grazing its surface with asteroid impacts.

Answer 3

The Moon rotates around the common Earth-Moon barycenter in a given time T1, and it also revolves around its own axis in a time T2.

Being tidally locked just means that T1 = T2.

And one solution to make it not tidally locked would be to vary T2. This would require to either brake or increase the Moon's rotation.

A different - actually, exactly opposite - approach would be to vary T1 by altering the Moon's orbit. This you can do by crashing a very large comet against the Moon, thus increasing (or decreasing) its orbital speed. The Moon's rotation would remain unchanged, and therefore would fall out of sync with the new duration of the Lunar month.

To decrease the effects of the impact on the Moon's integrity you might imagine the comet having recently broken up in, say, a close encounter with Jupiter. The impact would take place on a very large portion of the Moon's surface (a more catastrophic version of this impact is depicted in Jack DeVitt's Moonfall.

Even in a non-completely-catastrophic scenario (say, something like Bob Shaw's The Ceres Solution) you would get lots of ejecta, a considerable portion of which would certainly re-enter Earth's atmosphere, as well as very probably triggering a Kessler chain reaction.

A less catastrophic scenario would have the Moon narrowly missed by a much larger celestial body than a comet: large enough to "pull" the Moon outwards in its orbit. This would inevitably gravitationally influence the Earth and its orbit also, so most timebases would need to be recalibrated.

Answer 4

Volcanic eruptions can release enough energy to be comparable to the kinetic energy of the moon's motion.

Perhaps a glancing hit by a massive meteor could have ripped off the surface of this moon, and the intense tectonic response could spew enough lava into space to make the moon spin? The moon might have a glowing spot for a hundred or so years before it settled down.

Our moon isn't tectonically active, but yours could be.