SEASONAL MASS MOTIONS ON MARS

The seasonal cycle on Mars involves major changes in global atmospheric pressure and the deposition and sublimation of carbon dioxide at the planet's winter pole. Approximately 30% of the total volatile material is involved in this process and one of the consequences of this redistribution of the mass is a change in the gravity field of the planet that causes observable effects on the motion of spacecraft in Mars orbit. The basic process is one in which atmospheric CO2 is deposited on the winter pole with a corresponding reduction in atmospheric pressure followed by a sublimation back into the atmosphere and an increase in pressure. This process causes a semiannual variation in the mass of the atmosphere but because the orbit of Mars is eccentric the southern hemisphere winters are longer and the pressure variation asymmetric. Tracking by the Deep Space Network (DSN) of the Mars Global Surveyor (MGS) spacecraft over 4 Mars years and more recently of the Mars Reconnaissance Orbiter (MRO) spacecraft have been used to study the changes in the seasonal mass deposited in the polar regions each winter. The observed variations show the clear seasonal cycle, similar to the predicted deposition by Mars global circulation models but also suggest additional processes may be involved in the movement of mass during a Mars year. The seasonal effect of loading on the crust and the movement of carbon dioxide in and out of the regolith with the seasons are possible explanations.