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We used recently produced Solar System ephemerides, which incorporate twoyears of ranging observations to the MESSENGER spacecraft, to extract thesecular orbital elements for Mercury and associated uncertainties. As Mercuryis in a stable 3:2 spin-orbit resonance these values constitute an importantreference for the planet's measured rotational parameters, which in turnstrongly bear on physical interpretation of Mercury's interior structure. Inparticular, we derive a mean orbital period of 87.96934962 $\pm$ 0.37days and (assuming a perfect resonance) a spin rate of 6.138506839 $\pm$0.28 degree/day. The difference between this rotation rate and thecurrently adopted rotation rate (Archinal et al, 2011) corresponds to alongitudinal displacement of approx. 67 m per year at the equator. Moreover, wepresent a basic approach for the calculation of the orientation of theinstantaneous Laplace and Cassini planes of Mercury. The analysis allows us toassess the uncertainties in physical parameters of the planet when derived fromobservations of Mercury's rotation.

Mercury's resonant rotation from secular orbital elements