Atmospheric rotational effects on Mars based on the NASA Ames general circulation model

Abstract

[1] Atmospheric rotational effects on Mars are computed and analyzed. Both axial (ÁLOD) and equatorial effects (polar motion) are evaluated. Surface values of stress and pressure from the NASA Ames general circulation model are used as inputs to compute the topographic, stress, and gravitational torques. Time series for the ice caps moments of inertia and the axial component of atmospheric angular momentum provide inputs for a separate computation of axial effects. Stress torque dominates the torque budget. This is different from the results obtained from Earth atmospheric models, in which pressure effects are paramount. Axial torque and ice caps yield annual and semiannual ÁLOD amplitudes of 0.747 and 0.121 ms, respectively. Torque-induced polar motion reaches a maximum of 16.26 mm in the second half of the Martian northern hemisphere winter. Changes in LOD using the angular momentum approach are 0.187 and 0.136 ms for the annual and semiannual harmonics. The expected precision of the planned NetLander Ionospheric and Geodesic Experiment (NEIGE) should detect the main harmonics in the ÁLOD time series. Annual and semiannual polar motion harmonics induced by atmospheric torque are below the level of NEIGE detectability.

The effects of seasonal mass redistribution and interior structure on Length-of-Day variations of Mars

“…Nevertheless the associated errors remain high (about 40% for annual amplitudes and 50% for the semi-annual amplitudes). The errors given for the gravity solution of J 2 are of about the same order of magnitude (Yoder et al, 2003;Smith et al, 2001).…”

“…Seasonal zonal winds which are the primary cause of DLOD on Earth, have been thought to be much less important on Mars (Folkner et al, 1997). Because of the lack of direct measurements, their only estimates are from Martian GCMs, which suggest, nevertheless, annual amplitudes as large as one third of the total DLOD (Van den Acker et al, 2002;Sanchez et al, 2003). This makes necessary the evaluation of zonal wind effects and more cumbersome the determination of the interior structure from DLOD measurements.…”

“…Smith et al (2001) reported temporal changes in the elevation of Martian polar caps and the planetÕs global mass redistribution due to the seasonal CO 2 cycle, using the data from MOLA. They showed elevation changes up to 1.5 ± 0.25 m in the North cap and 0.9 ± 0.30 m in the South cap.…”

“…On the basis of surface pressure data from Viking landers, Cazenave and Balmino (1981) and Chao and Rubincam (1990) estimated the seasonal variations of LOD. With the help of Global Circulation Models (GCM) of Martian atmosphere, Defraigne et al (2000), Van den Acker et al (2002) and Sanchez et al (2003) improved the previous DLOD estimates by including the atmospheric wind influence and a more realistic surface topography. In the present study, we compare the direct measurements of DLOD from the Viking and Pathfinder landers (Folkner et al, 1997) with the LOD variations calculated from the seasonal mass redistribution given by the MOLA laser altimeter onboard the MGS spacecraft (Smith et al, 2001), the High Energy Neutron Detector (HEND) onboard the Mars Odyssey spacecraft (Litvak et al, 2004), and the Doppler tracking of the MGS spacecraft (Smith et al, 2001;Yoder et al, 2003).…”

“…MarsÕ surface mass distribution varies at seasonal time scales due to condensation, sublimation and precipitation of CO 2 , the primary constituent of the atmosphere. As much as a third of the atmosphere has been estimated to take place in this seasonal CO 2 cycle (Smith et al, 2001). The sizes and masses of both polar caps as well as the atmospheric pressure change, resulting in seasonal variations of the Martian inertia tensor.…”