Seasonal Cycles and the Gravity Field of Mars
M. T. Zuber (MIT), D. E. Smith (NASA/GSFC)
Motions within the atmosphere of Mars cause small but measurable effects
on the gravity field and planetary dynamics. To estimate the magnitude
of expected effects in order to assess their detectability, we have used
the NASA Ames General Circulation Model (GCM) in a simulation of a typical
Mars year. Mean atmospheric conditions were computed for twenty 33-day
months from the GCM, which also utilized a recent geopotential topography
model that has improved long wavelength control and fits the Viking
lander site elevations better than previous models. From the GCM we
determined the pressure field, precipitation and wind fields over the
surface of Mars, and then calculated their effect on the low degree terms
of the gravity field, the length of the Martian day, and the position of
the pole of rotation. Approximately 25% of the CO in the Mars
atmosphere moves from one pole to the other as the seasons change;
some of it condenses out as ice forming an additional mass layer on the
surface, thus increasing the mass at the winter pole at the expense of
mass at the equator and summer pole. This mass re-distribution changes
the gravitational (dynamic) flattening ( ) and the "pear shape"
( ) terms in the gravity field of Mars. Our preliminary analysis of
the magnitudes of these effects indicates precipitation of CO at the
winter pole is the single most important effect on the gravity field,
while winds have the most important effect on the rotation. The polar
cap-atmosphere mass exchange may possibly be detectable from Mars Global
Surveyor X-band tracking observations.