Ter-Diurnal Atmospheric Tide on Mars

Cyclic absorption of solar radiation generates oscillations in atmospheric fields. These oscillations are called atmospheric or thermal tides, which are furthermore modified by topography and surface properties. This leads to a complex mix of sun-synchronous and non-sun-synchronous tides that propagate around the planet eastward and westward. This study focuses on analyzing the ter-diurnal component (period of 8 hr) from surface pressure observations by Mars Science Laboratory (MSL), InSight, Viking Lander (VL) 1, and VL2. General Circulation Model (GCM) results are used to provide a global context for interpreting the observed ter-diurnal tide properties. MSL and InSight have a clear and similar seasonal cycle, with local amplitude peaks at around solar longitude (Ls) 60°, Ls 130°, and Ls 320°. The amplitude peak at Ls 320° is related to the annual dust storm, while the Ls 230° dust storm is not detected by either platforms. During global dust storms, MSL, VL1, and VL2 detect their highest amplitudes. The GCM predicts the weakest amplitudes at the equinoxes, while the strongest ones are predicted in summer for both hemispheres. GCM amplitudes tend to differ from the observations but match slightly better around the aphelion season. During this time, model results suggest that the two most prominent modes are the sun-synchronous ter-diurnal tide and an eastward propagating resonantly enhanced Kelvin wave. Simulations with and without the effect of radiative heating by water ice clouds indicate the clouds may play a significant role in forcing the ter-diurnal tide during northern hemisphere summer season.