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 S eptember 1997 (L-s 184 degrees), has permitted extensive observations over more than a Martian year. Initially, thin (normal optical depth <0.06 at 82 5 cm(-1)) ice clouds and hazes were widespread, showing a distinct latitudi nal gradient. With the onset of a regional dust storm at L-s = 224 degrees, ice clouds vanished in the southern hemisphere, to reappear gradually afte r 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 pres ent in early northern summer. Apparently characteristic of the northern sum mer season, it vanished between L-s = 140 degrees and 150 degrees. The lati tudinal extent of this feature is apparently controlled by the ascending br anch of the Hadley circulation. The most opaque clouds (optical depth simil ar to0.6) were found above the summits of major volcanic features; these sh owed spatial structure possibly associated with wave activity. Variety amon g low-lying late morning clouds suggests localized differences in circulati on and microclimates. Limb observations showed extensive optically thin (op tical depth <0.04) stratiform clouds at altitudes up to 55 km. Considerable latitude and altitude variations were evident in ice clouds in early north ern spring (L-s = 25 degrees); near 30 km, thin clouds extended from just n orth of the equator to similar to 45 degreesN, nearly to the north polar vo rtex. A water ice haze was present in the north polar night (L-s = 30 degre es) at altitudes up to 40 km. Because little dust was present this probably provided heterogeneous nucleation sites for the formation of CO2 clouds an d snowfall at altitudes below similar to 20 km, where atmospheric temperatu res dropped to the CO2 condensation point. The relatively invariant spectra l shape of the water ice cloud feature over space and time indicates that i ce particle radii are generally between 1 and 4 mum.