Internal forcing of Mercury's long period free librations

► The longitudinal libration of Mercury's mantle may contain a decadal signal. ► This may be produced by zonal flows in the fluid core, entraining inner core motion. ► A mantle libration is then generated by gravitational torque from the inner core. ► Resonance occurs when timescale of zonal flows approaches a free mode of libration. ► A decadal mantle libration of 1 arcsec requires flows of the order of 1 km/yr.