  We used recently produced Solar System ephemerides, which incorporate two
years of ranging observations to the MESSENGER spacecraft, to extract the
secular orbital elements for Mercury and associated uncertainties. As Mercury
is in a stable 3:2 spin-orbit resonance these values constitute an important
reference for the planet's measured rotational parameters, which in turn
strongly bear on physical interpretation of Mercury's interior structure. In
particular, we derive a mean orbital period of 87.96934962 $\pm$ 0.00000037
days and (assuming a perfect resonance) a spin rate of 6.138506839 $\pm$
0.000000028 degree/day. The difference between this rotation rate and the
currently adopted rotation rate (Archinal et al, 2011) corresponds to a
longitudinal displacement of approx. 67 m per year at the equator. Moreover, we
present a basic approach for the calculation of the orientation of the
instantaneous Laplace and Cassini planes of Mercury. The analysis allows us to
assess the uncertainties in physical parameters of the planet when derived from
observations of Mercury's rotation.
