INTERANNUAL VARIABILITY OF PLANET-ENCIRCLING DUST STORMS ON MARS

Interannual variability of the atmosphere of Mars is dominated by the occurence, or absence, of planet-encircling dust storms. In this paper we use a recent review of Earth-based telescopic observations of Mars together with Viking orbiter and lander data to estimate the frequenc y of occurrence of planet-encircling dust storms over the past century and to test whether the period spanned by the Mariner 9 and Viking mi ssions to Mars is representative of the decades prior to 1950. In doin g so, we take into account the practical limitations imposed on Earth- based coverage of Mars by the orbital geometries of the two planets. B oth spacecraft and Earth-based observations suggest that planet-encirc ling dust storms on Mars occur during a so-called ''dust storm season' ' in southern spring and summer. Viking data demonstrate decidedly tha t planet-encircling dust storms could have occurred in the past on Mar s without being detected from Earth during years in which Mars was far from Earth during the dust storm season and thus difficult to observe . However, the same historical record indicates that planet-encircling storms were absent during the dust storm seasons monitored during sev eral favorable oppositions prior to 1956 and after 1986. Overall, the chance of a planet-encircling dust storm occurring in any arbitrary Ma rs year is estimated to be approximately one in three (18-55 % at the 95 % level of confidence), if such occurrence is random from year-to-y ear and yet restricted seasonally to southern spring and summer.