  The orbital motions and spin-axis rotations of extended bodies are
traditionally considered to be coupled only by tidal mechanisms. The orbit-spin
coupling hypothesis supplies an additional mechanism. A reversing torque on
rotating extended bodies is identified. The torque effects an exchange of
angular momentum between the reservoirs of the orbital and rotational motions.
The axis of the torque is constrained to lie within the equatorial plane of the
subject body. Hypothesis testing to date has focused on the response to the
putative torque of the Martian atmosphere. Atmospheric global circulation model
simulations reveal that an episodic strengthening and weakening of meridional
overturning circulations should be observable and is diagnostic in connection
with the triggering of Martian planet-encircling dust storms. Spacecraft
observations obtained during the earliest days of the 2018 Martian global dust
storm document a strong intensification of atmospheric meridional motions as
predicted under this hypothesis. We review implications for atmospheric
physics, for investigations of planetary orbital evolution with rotational
energy dissipation, and for theories of gravitation.
