Editing Mercury (planet) (section)

=== Spin-orbit resonance ===
[[File:Mercury's orbital resonance.svg|thumb|After one orbit, Mercury has rotated 1.5 times, so after two complete orbits the same hemisphere is again illuminated.]]

For many years it was thought that Mercury was synchronously [[tidally locked]] with the Sun, [[rotating]] once for each orbit and always keeping the same face directed towards the Sun, in the same way that the same side of the Moon always faces Earth. Radar observations in 1965 proved that the planet has a 3:2 spin-orbit resonance, rotating three times for every two revolutions around the Sun. The eccentricity of Mercury's orbit makes this resonance stable—at perihelion, when the solar tide is strongest, the Sun is nearly stationary in Mercury's sky.<ref>{{cite journal |last1=Liu |first1=Han-Shou |last2=O'Keefe |first2=John A. |title=Theory of Rotation for the Planet Mercury |journal=Science |year=1965 |volume=150 |issue=3704 |page=1717 |doi=10.1126/science.150.3704.1717 |pmid=17768871 |bibcode=1965Sci...150.1717L|s2cid=45608770 }}</ref>

The 3:2 resonant tidal locking is stabilized by the variance of the tidal force along Mercury's eccentric orbit, acting on a permanent dipole component of Mercury's mass distribution.<ref name="Colombo" /> In a circular orbit there is no such variance, so the only resonance stabilized in such an orbit is at 1:1 (e.g., Earth–Moon), when the tidal force, stretching a body along the "center-body" line, exerts a torque that aligns the body's axis of least inertia (the "longest" axis, and the axis of the aforementioned dipole) to always point at the center. However, with noticeable eccentricity, like that of Mercury's orbit, the tidal force has a maximum at perihelion and therefore stabilizes resonances, like 3:2, ensuring that the planet points its axis of least inertia roughly at the Sun when passing through perihelion.<ref name="Colombo" />

The original reason astronomers thought it was synchronously locked was that, whenever Mercury was best placed for observation, it was always nearly at the same point in its 3:2 resonance, hence showing the same face. This is because, coincidentally, Mercury's rotation period is almost exactly half of its synodic period with respect to Earth. Due to Mercury's 3:2 spin-orbit resonance, a solar day lasts about 176 Earth days.<ref name="strom" /> A [[sidereal day]] (the period of rotation) lasts about 58.7 Earth days.<ref name="strom" />

Simulations indicate that the orbital eccentricity of Mercury varies [[chaos theory|chaotically]] from nearly zero (circular) to more than 0.45 over millions of years due to [[Perturbation (astronomy)|perturbations]] from the other planets.<ref name="strom" /><ref name="Correia2009">{{cite journal |last1=Correia |first1=Alexandre C. M. |last2=Laskar |first2=Jacques |title=Mercury's capture into the 3/2 spin-orbit resonance including the effect of core–mantle friction |journal=Icarus |year=2009 |doi=10.1016/j.icarus.2008.12.034 |arxiv=0901.1843 |volume=201 |issue=1 |pages=1–11 |bibcode=2009Icar..201....1C|s2cid=14778204 }}</ref> This was thought to explain Mercury's 3:2 spin-orbit resonance (rather than the more usual 1:1), because this state is more likely to arise during a period of high eccentricity.<ref name="Correia">{{cite journal |last1=Correia |first1=Alexandre C. M. |last2=Laskar |first2=Jacques |year=2004 |title=Mercury's capture into the 3/2 spin-orbit resonance as a result of its chaotic dynamics |journal=[[Nature (journal)|Nature]] |volume=429 |pages=848–850 |doi=10.1038/nature02609 |pmid=15215857 |issue=6994 |bibcode=2004Natur.429..848C|s2cid=9289925 }}</ref> However, accurate modeling based on a realistic model of tidal response has demonstrated that Mercury was captured into the 3:2 spin-orbit state at a very early stage of its history, within 20 (more likely, 10) million years after its formation.<ref>{{Cite journal |bibcode=2014Icar..241...26N |last1=Noyelles |first1=B. |last2=Frouard |first2=J. |last3=Makarov |first3=V. V. |last4=Efroimsky |first4=M. |name-list-style=amp |title=Spin-orbit evolution of Mercury revisited |journal=Icarus |pages=26–44 |year=2014 |volume=241 |issue=2014 |doi=10.1016/j.icarus.2014.05.045 |arxiv=1307.0136|s2cid=53690707 }}</ref>

Numerical simulations show that a future [[Secular resonance|secular]] [[Orbital resonance|orbital resonant]] interaction with the perihelion of Jupiter may cause the eccentricity of Mercury's orbit to increase to the point where there is a 1% chance that the orbit will be destabilized in the next five billion years. If this happens, Mercury may fall into the Sun, collide with Venus, be ejected from the Solar System, or even disrupt the rest of the inner Solar System.<ref name="Laskar2008">{{cite journal |last=Laskar |first=Jacques |date=March 18, 2008 |title=Chaotic diffusion in the Solar System |journal=[[Icarus (journal)|Icarus]] |volume=196 |issue=1 |pages=1–15 |bibcode=2008Icar..196....1L |doi=10.1016/j.icarus.2008.02.017 |arxiv=0802.3371|s2cid=11586168 }}</ref><ref name="Laskar2009">{{cite journal |last1=Laskar |first1=Jacques |last2=Gastineau |first2=Mickaël |date=June 11, 2009 |title=Existence of collisional trajectories of Mercury, Mars and Venus with the Earth |journal=[[Nature (journal)|Nature]] |volume=459 |issue=7248 |pages=817–819 |doi=10.1038/nature08096 |bibcode=2009Natur.459..817L |pmid=19516336|s2cid=4416436 }}</ref>