Abstract

A model of the annual cycle of pressure on Mars has been developed for a 2-year period chosen to include 1 year at Lander 2 and to minimize the effect of great dust storms at the $22^\circ$N Lander 1 site. The model was developed by weighted least squares fitting of the Viking Lander pressure measurements to an annual mean, and fundamental and the first four harmonics of the annual cycle. The very close agreement between the two years suggests that an accurate representation of the annual $\rm CO_2$ condensation- sublimation cycle can be established for such years. The two annual mean pressures are identical to 0.006 mbar out of 7.9 mbar, and the differences in amplitudes for the first five periodic components between the two years range from 0.017 to 0.001 mbar. The phase angles, primarily dependent on solar insolation determined orbital dynamics, differ by $-3.0^\circ\ L_S$for the second harmonic (year 1 minus year 2), and drop to $\le\ 0.7^\circ$ for the fundamental and fourth harmonic. Although the slight year-to-year differences appear to be real, this model is proposed as a `` nominal'' Martian annual pressure cycle and applications are suggested. By analogy, the corresponding first year's representation at Lander 2 is also proposed as the `` nominal'' cycle, although it has not been verified by data from a subsequent year. These models provide a method of removing low frequencies from the annual pressure cycle for spectral analyses of baroclinic, tidal, and normal mode oscillations, and for comparisons of the interannual variability.