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J. Liu, M.I. Richardson (California Institute of Technology), R.J. Wilson (Geophysical Fluid Dynamics Lab., NOAA)
Comprehensive inter-comparison of thermal infrared data collected by Mariner 9, Viking, and Mars Global Surveyor (MGS) is presented, with a specific focus on air temperatures, dust opacities, and water ice opacities. Emphasis is placed upon creating a uniform data set so as to most effectively reduce inter-instrument biases and offsets. We show that the globally-averaged martian atmosphere executes an exceedingly repeatable annual cycle of air temperature, closing in northern spring and summer to within a Kelvin. The annual cycle shows a strong asymmetry about the equinoxes, with northern summer showing relatively low temperatures and essentially no short-term (tens of days) variability. Viking and MGS air temperatures are essentially indistinguishable, suggesting that the Viking and MGS eras are characterized by exactly the same climatic state. Southern summer is characterized by strong dust storm activity, and we note that the period around Ls=225 is characterized by very high dust opacities associated with dust storm development or decay in every year thus far observed by spacecraft. Dust opacity shows a highly repeatable annual cycle, closing to essentially the same values each year in northern spring and summer, with Viking and MGS opacities being very similar. We show that both Viking and MGS data sets show significant (and similar) polar cap edge dust storm activity. The origins of the various major dust storms can be identified in the thermal infrared data from Viking and MGS, including the "flushing" of dust from the northern autumn baroclinic zone into the southern hemisphere tropics, which has also been identified in visible imaging. Water ice opacities have been retrieved from Viking infrared data for the first time. We show that the cloud belt structure and evolution is essentially the same in each of the multiple years observed by Viking and MGS. Relatively subtle spatial features recur in the cloud belt from year-to-year, suggesting the influence of surface topography and thermophysical properties, and consistent supply of water vapor. The seasonal evolution of the tropical cloud belt through northern spring and summer is shown, with the only significant deviations between years occurring from Ls=140-160, where opacities fall in the second MGS year associated with a small dust storm. Polar hood clouds are observed in the Viking and MGS observations with similar timing and extent. It would seem that the martian atmosphere executes a very repeatable annual cycle of atmospheric phenomena, with the only significant exception being the occurrence of major dust events. After such dust events, the atmosphere rapidly relaxes to its stable, repeatable state.
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.