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The Mercury year is between 1 and 2 days. Has it ever been greater, and will it eventually become "tidally locked" with the Sun?

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  • $\begingroup$ Do you mean the Mercury year is between 1 and 2 Mercurian days? $\endgroup$ Jun 17, 2023 at 14:53

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Mercury is tidally locked; it has a 3:2 resonance with the Sun, where it rotates three times for every two orbits. It is well understood that Mercury is tidally locked, but modern explanations for how it could’ve come to be are currently unsatisfactory (see this publication from Nature https://www.nature.com/articles/nature02609).

It will probably never leave the resonance it is in now in favor of some 1:1 resonance for the simple fact that it’s current state has the negative feedback of being in a potential energy well; to leave it for a different resonance would take some significant outside interaction, and may not be possible (the lack of certainty of whether this could be possible rises from the lack of certainty in how Mercury came to this specific resonance in the first place)

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The way how Mercury got captured into the 3:2 spin-orbit resonance is explained in great detail in this work (of which I'm a co-author), which is also available in ArXiV.

Planets or moons entrapped in higher spin-orbit resonances stay there forever, unless tidal heating changes their rheology -- in which case they become hot and plastic, and escape from the resonance, and continue their tidal despinning toward synchronism or pseudosynchronism, as explained in this paper. (This is what happened to the inner planets of TRAPPIST-1.)

Tidal heat generated in Mercury is insufficient for a thermal runaway. So I see no apparent reason for Mercury to leave its present spin state.

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    $\begingroup$ Thanks for that info! Please include attribution to any cited works, especially when you're an author of that work. That's a general Stack Exchange policy, see physics.meta.stackexchange.com/q/582/123208 $\endgroup$
    – PM 2Ring
    Mar 8, 2023 at 6:35

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