THE MARTIAN ANNUAL ATMOSPHERIC-PRESSURE CYCLE - YEARS WITHOUT GREAT DUST STORMS

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-degrees-N Lander 1 site. The model was developed by weighted least squares fitting of the Viking L ander pressure measurements to an annual mean, and fundamental and the first four harmonics of the annual cycle. The very close agreement be tween the two years suggests that an accurate representation of the an nual CO2 condensation- sublimation cycle can be established for such y ears. 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 period ic components between the two years range from 0.017 to 0.001 mbar. Th e phase angles, primarily dependent on solar insolation determined orb ital dynamics, differ by -3.0-degrees L(S) for the second harmonic (ye ar 1 minus year 2), and drop to less-than-or-equal-to 0.7-degrees 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 anal ogy, 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 remov ing low frequencies from the annual pressure cycle for spectral analys es of baroclinic, tidal, and normal mode oscillations, and for compari sons of the interannual variability.