Is Mercury Tidally Locked?

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If you’ve looked even slightly into what tidal locking is you’d probably be well aware that our moon is tidally locked to Earth, which in simple terms means we only see one side of the Moon’s face at all times.

It’s often believed that due to Mercury’s relatively close proximity to the Sun, it would also be a tidally locked entity however, this isn’t quite the case.

Mercury is locked to the sun following a 3:2 orbital resonance, meaning it rotates 3 times around its axis every 2 times it orbits the Sun or 1.5 every full orbital cycle. Therefore, even though the Sun’s gravitational pull does affect Mercury (which I’ll explain in a little more detail below) it technically is not tidally locked to the Sun.

Can Planets Be Tidally Locked ?

Yes they can, in fact we know that the now dwarf planet Pluto and its moon Charon are tidally locked to each other whilst the Earth is likely to tidally lock itself to the Moon too considering every year its spinning slower around its axis, at a rate of about 1.4 milliseconds every century.

Which theoretically means our Earth will be tidally locked to the moon in a few billion years from now. Although it is more likely to be caught in the inflation of the Sun near the end of its life cycle over becoming locked.

What Is Orbital Resonance?

Orbital resonance is when two celestial bodies exert regular gravitational burst upon each other resulting in either a stable or unstable resonance between the 2 or more celestial bodies.

Some examples include Pluto and Neptunes resonance of 2:3 and the resonance of Jupiter’s moons Ganymede, Europa and Io of 1:2:4.

Jupiter's moons Tidally Locked
This how the tidal locking is for Io, Europa and Ganymede

For entities with an unstable resonance, one example being Saturn’s inner moons, they can cause objects within their vicinity to be ejected, which is why the inner most moons of Saturn have created gaps within the gas giants’ rings.

On the other hand, a 1:1 resonance between a body and moon is one element scientists have used to define a planet. When two celestial have a 1:1 resonance this means that debris won’t be able to fall within the orbit of either celestial object as the debris will either get flinged out or crash on to the surface of either entity.

Is Mercury Tidally Locked To The Sun?

No Mercury is not tidally locked to the sun as it follows a 3:2 orbital pattern as mentioned earlier on. However, it does make sense that some would assume that Mercury is tidally locked as older books (before 1965) from scientists did make these assumptions. Now of course we know it isn’t.

You can say the Mercury is under an elliptical tidal lock due to its 3:2 spin resonance with the sun. As for the reasons why, we don’t exactly know but, there are few theories that scientists have looked into.

Why Does Mercury Have A 3:2 Orbital Spin Resonance?

One theory that was tossed aside suggested that the friction from Mercury’s core mantle could be the reason however, due to the pinpoint accuracy of the viscosity values required of the planet, many deemed this theory to be highly unlikely.

Another theory suggests Mercury initially evolved from a much higher eccentricity then we observe today, which simply means the planet likely has started following a far more normal form of resonance in comparison to what it may have been many many years ago.

And the final theory that’s still somewhat plausible has to do with the planet evolving from a retrograde rotation which suggests Mercury used to orbit in the opposite direction to the sun spinning on its axis and has now potentially decelerated due to the tidal torques resulting in the planets current resonance.

Of course these are just theories as scientists still aren’t all too familiar with the dissipation process of planets and stars but, going by the consistent numbers that have been developed over the years, the above 3 theories are the most probable reasons for why Mercury follows an elliptical tidal orbit and not a 1:1 tidal lock.


In summary Mercury is not tidally locked to the sun as it’s currently spinning around its axis once every 1.5 times its orbit. On top of that there isn’t any concrete theory as to why Mercury is on a 3:2 spin resonance due to the lack of information scientist have about the energy dissipation of planets.

I guess you could say there is some potential in Mercury eventually tidally locking to the Sun however, in our current climate and lack of knowledge on where Mercury may have come from (in regards to its original orbit) just leaves us with more questions than answers.

Hopefully this article was helpful in figuring our if Mercury is genuinely locked tidally to our Sun. if you’ve made it this far I just want to say… thanks for reading.

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