No theoretical basis for an orbit–spin coupling inertial force in planetary atmospheres

Correlations between Martian orbital dynamics and the interannual variability of global dust storms (GDS) were reported by Shirley (2015) and Shirley and Mischna (2017). To explain this correlation, Shirley (2017) proposed a theoretical derivation of an inertial force acting on the Martian atmosphere, depending on both the planet's spin rate and its orbital angular momentum. This term, referred to as the “coupling term acceleration” (CTA), has been hypothesized to provide a dynamical forcing mechanism for GDS initiation. In this study, we revisit the orbit–spin coupling hypothesis by re-deriving the inertial forces acting on a parcel of air in the atmosphere of a rotating, orbiting planet. We show that no orbit–spin coupling term naturally appears in the equations of motion. Furthermore, the inertial accelerations associated with the planet's orbital motion are almost entirely compensated by the external gravitational field, leaving only a well-known gravitational tidal term. This result demonstrates that there is no theoretical basis for a direct orbit–spin coupling force acting on planetary atmospheres.