Yes, but I was talking about Venus, not Mercury.
Previous discussion https://boards.straightdope.com/sdmb/showthread.php?t=735272
The Venus spin/rotation seems to be tidaly locked by the Earth in a 29:7 ratio with some complexity due to other neighbors and the outer planets which tend to orbit the solar systems barycentric when the inter planets paths are best described as heliocentric.
Oddly enough the effects of the Earth-Venus resonance group and other groups are what cause the solar cycles, probably through the tidal force but I think that is still unknown.
Resonance groups are a bit harder to explain without the language of math, but while the cycles are more complex then was previously thought they do seem to fit in with these models in a fashion that only looks chaotic due to looking at them in isolation. That and some unknown information related to the mass of the outer planets and unknowns that are mostly likely due to a lack of information in the Kuiper belt.
You have to consider the 29:7 Venus-Earth, the 3:1 Venus-Mars and other relationships to see that the rotational periods are pretty close to what we observe. Not that this is solved, and there are issues like the lack of resonance Mars and Jupiter but Venus would not be tidally locked because of the interactions between the Mercury/Venus/Earth/Mars neighbors and the outer systems.
I myself resorted to the spherical cow when talking about the tidal locking of the Earth to the Moon, but the Earth-Moon system would be locked (given enough time) due to their closeness to being a binary-planet system, and the Earth-Moon system would still rotate under that model related to other interactions like the resonance between the Earth-Venus system.
Here is a fairly accessible paper that may provide more information.
The term “tidal locking” is a bit overloaded though, and note that the above is once again talking about resonances of systems as tidal locking.
The dynamo effect is what causes the solar cycles. It’s magnetic, not gravitational.
My understanding is that both gravitational and electromagnetic planetary forces seem to have correlation to the electromagnetic properties of the heliosphere, and that a complete physical explanation is still needed.
That said it looks like my papers on this subject all date to 2014 and earlier so I wouldn’t mind an update.
But feel free to strike that part from my post, the neighborhood harmonics are still seem highly related to the rotational periods and do account for the lack of the more simplistic heliocentric form of tidal locking.
There is this recent report that says that the variability of Venus’ rotation is due to winds blowing on its mountains.
The issue is the conservation of angular momentum (usually). There just hasn’t been time to transfer the angular momentum of most planets to something else, e.g., the size of the orbit.
Tidal drag from the Sun can slow down the Earth’s rotation but it’s going to take a while, esp. with the Moon also having a effect. And the angular momentum gets transfered to a higher orbit which changes what rotational rate would make a tidal lock. And eventually Jupiter’s going to have an effect on Earth moving around. And, and, and.
4+ billion years is just a drop in the bucket.
Mercury is sort of a special case. The highly elliptical orbit means the libration interferes with tidal locking. Not really sure what happened to Venus to give it a slow, retrograde spin.
Similar to tidal locking are orbital resonances. E.g., Neptune and Pluto are in a 3:2 orbital resonance. Given the small size of the objects and the distances involved that might seem impressive. But again, it comes down to angular momentum. Their sizes means that it’s much easier to swap (orbital) angular momentum with such objects. They slow down/speed up (and therefore move in or out) so that eventually a resonance can take over and lock them in.
Yes. From here https://en.wikipedia.org/wiki/Tidal_locking#Timescale the time to achieve tidal lock is proportional to the sixth power of the distance from the Sun to the planet (or the planet to the moon), which is a big factor (Mercury is about 150 times further from the Sun than the Moon is from Earth, and 150^6 is about 10 trillion.
Note that the page does cover how Venus is locked, and how Mercury may be locked to the Earth.
We need additional modern space missions to Venus, Neptune and Uranus before this will hit the 5 sigma level required in this area. The discovery of the gravity waves (note not gravitational waves) due to mountains above is an example, but unfortunately that data which was found by the Akatsuki probe was salvaged from a mission with a failed orbital insertion. It is amazing that they did manage to salvage part of the mission, but the important part is that a variation in the rotation is caused by “friction” but not an ultimate slowdown outside of the “traction” that these bulges offer to the tidal force.
The angular momentum transform between a planet and atmosphere has losses, but the planet as a whole still conserves it’s total angular velocity (ignoring differences in heat loses). With tidal locking, either in the traditional model like the Moon is locked to the Earth or how Mercury is locked to the the Sun that angular velocity is transferred either to the two body system or to the rotation of the other body.
The differential caused by gravity waves and a thick atmosphere may provide more traction for the tidal force to transfer the angular momentum to another body or it may just transfer that energy to the atmosphere without transferring that energy to the n-body system or another body.
There is a terminology conflict in some of this and I am not stating any poster is making any claims but it would be easy for a reader to believe that this is an example of lifting oneself with one’s own bootstraps. The Tractive Force is pretty complex, and while this video doesn’t account for the atmospheric gravity waves of aspects like the mountain ranges it may show how this works, but note that this tractive force even without the mountain ranges applies several tons of force per kilometer on the Jet Stream.
This is a very complex and unsolved problem, and really needs the language of math to even approach but if you stop the animation and look at the force vectors it will show that they are larger overall in a direction counter to rotation in the earth-moon-sun system.
The interaction of Gravity waves, Inertial gravity waves, Kelvin waves, and Rossby waves get complex really quick. But it is their ability to better transfer angular momentum to other parts of the system external to the planet that is important to tidal locking.
I thought it was more of a dragging force…one body dragged backward, one dragged forward.
Yep, that is the common misunderstanding that I referenced before that has been with us for well over 100 years.
Most likely it is caused by elementary textbooks copying other elementary text books without validating the assumptions.
Here is another page that will go into it in more depth in addition to the ones above.
But it is the tractive force, or the forces tangential to the surface that cause this effect.
Here is another video that tries to explain the way this works without resorting to those common but flawed analogies.
And yes even NOAA uses this incorrect simplification,
The reserves the more important force to another page.
It has been a problem for a long time.
Sorry I was slow to check back. I’ll have to take the time to interpret the explanation which is more than I thought it would be.