An assessment of the global, seasonal, and interannual spacecraft record of Martian climate in the thermal infrared

Intercomparison 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 on creating a uniform data set to most effectively reduce interinstrument biases and offsets. The annual cycle consistently shows a strong asymmetry about the equinoxes, with northern spring and summer exhibiting relatively low temperatures, very high year-to-year repeatability, and essentially no short-term (tens of days) variability. The globally averaged Martian nighttime air temperatures close annually to within a Kelvin during northern spring and summer. Daytime temperatures show more variability (3-6 K). The difference in repeatability of daytime versus nighttime temperatures is not understood. Viking and MGS air temperatures are essentially indistinguishable for this period, suggesting that the Viking and MGS eras are characterized by essentially the same climatic state. Southern summer is characterized by strong dust storm activity and hence strong year-to-year air temperature variability. Dust opacity shows a remarkable degree of interannual variability in southern spring and summer, associated with the intermittent activity of regional and planet-encircling dust storms, but exhibits high year-to-year repeatability in northern spring and summer. Specifically, late northern spring and early northern summer dust opacities appear to be completely insensitive to the occurrence (or not) of major dust storms in the previous southern spring or summer. We show that both Viking and MGS data sets exhibit 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 transport of dust from the northern autumn baroclinic zone into the southern hemisphere tropics, which has also been identified in visible imaging. We also note that the period around L-s=225degrees is characterized by very high dust opacities associated with dust storm development or decay in every year thus far observed by spacecraft. Water ice opacities have been retrieved from Viking infrared data for the first time. We show that the northern spring and summer tropical cloud belt structure and evolution are 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 a reasonably 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 L-s=140degrees to 160degrees, where opacities fall in the second MGS year associated with a small dust storm. Polar hood clouds are observed in Viking and MGS observations with similar timing and extent. Interactions between dust and water ice were highlighted in the Hellas basin region during the southern spring 1977a and 2001 dust storms. The observations demonstrate that the Martian atmosphere executes a very repeatable'' annual cycle of atmospheric phenomena. However, a major part of this cycle is the occurrence of highly variable and potentially major dust storm events. After such dust storm events the atmosphere rapidly relaxes to its stable, repeatable state.