Observations of Martian ice clouds by the Mars Global Surveyor thermal emission spectrometer: The first Martian year

Successful operation of the Mars Global Surveyor spacecraft, beginning in September 1997 (L_s = 184°), has permitted extensive observations over more than a Martian year. Initially, thin (normal optical depth <0.06 at 825 cm^-1) ice clouds and hazes were widespread, showing a distinct latitudinal gradient. With the onset of a regional dust storm at L_s = 224°, ice clouds vanished in the southern hemisphere, to reappear gradually after the decay of the storm. The zonally averaged cloud opacities show little difference between the beginning and end of the first Martian year. A broad low-latitude cloud belt with considerable longitudinal structure was present in early northern summer. Apparently characteristic of the northern summer season, it vanished between L_s = 140° and 150°. The latitudinal extent of this feature is apparently controlled by the ascending branch of the Hadley circulation. The most opaque clouds (optical depth ∼0.6) were found above the summits of major volcanic features; these showed spatial structure possibly associated with wave activity. Variety among low-lying late morning clouds suggests localized differences in circulation and microclimates. Limb observations showed extensive optically thin (optical depth <0.04) stratiform clouds at altitudes up to 55 km. Consederable latitude and altitude variations were in ice clouds in early northern spring (L_s = 25°); near 30 km, thin clouds extended from just north of the equator to ∼45°N, nearly to the north polar vortex. A water ice haze was present in the north polar night (L_s = 30°) at altitudes up to 40 km. Because little dust was present this probably provided heterogeneous nucleation sites for the formation of CO² clouds and snowfall at altitudes below ∼20 km, where atmospheric temperatures dropped to the CO² condensation point. The relatively invariant spectral shape of the water ice cloud feature over space and time indicates that ice particle radii are generally between 1 and 4 μm.