No abstract

The spin axis of Mercury is in a Cassini state (Figure 1). The latter describes a configuration in which the planet's spin axis and orbit normal remain coplanar to and precess about the normal to the Laplace plane (Colombo, 1966;Peale, 1969Peale, , 2006. The precession is retrograde, and the latest estimate of its period is 325, 513 ± 10, 713 years (Baland et al., 2017). Figure 2 shows the orientation in space of the spin axis reported in several recent studies, expressed at the J2000 epoch as is the usual convention. A visual inspection of Figure 2 reveals that within measurement errors, Mercury's spin axis aligns with the plane defined by the Laplace pole and orbit normal, a plane which we refer to as the Cassini plane, confirming that Mercury occupies a Cassini state.The retrograde precession of the Cassini plane implies that the line that depicts its location in Figure 2 is displaced toward the bottom left as a function of time. Hence, a spin pole located to the top right (bottom left) with respect to this line is behind (ahead of) the expected Cassini state orientation and corresponds to a phase lag (phase lead). We denote the offset from the Cassini plane by an angle ζ m , defined positive for a phase lag (see Figure 1b). Table 1 gives the spin pole orientations from the recent measurements that are plotted in Figure 2 as well as their phase lag angles ζ m , calculated by the method described in Appendix A. For all spin pole measurements, the 1σ error on the phase lag is either larger than the phase lag itself, or of similar magnitude. This confirms that within measurement errors, Mercury's spin pole indeed occupies a Cassini state. The magnitude of the phase lags in Table 1 provides a quantitative measure of the deviation from an exact Cassini state. For all spin pole

This a preprint and has not been peer reviewed. Data may be preliminary.

No abstract