Mission Update: MGS Thermal Emission Spectrometer Atmospheric Studies

Observations covering one half martian year allow us to follow the development of the northern winter polar vortex. This high speed west wind builds up from fall ("early October" in a calendar seasonally equivalent to the terrestrial calendar) to maximum strength in winter ("late December"). As spring approaches ("late March"), it gradually declines. At maximum strength its winds exceed 160 m/s (360 miles per hour). It also acts as an effective barrier to the northward transport of atmospheric dust; during its most active phase, only condensates (water and CO2 ices) were observed in its core. Detailed study of this effect is important to determine the accumulation of deposits on the permanent polar cap.

Figure 2: Atmospheric Dust Variations (Southern Spring, Summer)

Historically, southern spring and summer are the seasons of maximum dust storm activity. In midspring, two months after MGS entered martian orbit, a major regional dust storm erupted. TES observations spanned its entire development , which will permit detailed study of the mechanisms that drive these systems. The storm originated near 30 degrees south latitude, and rapidly spread southward, as well as to the east and west. The figure shows how the amount of dust in the atmosphere changed at two different latitudes. As measured by the optical depth at wavelengths near 10 micrometers, longitudinally averaged dustiness increased dramatically (and by significantly more in the active regions of the storm at high southerly latitudes (60S-75S). Surprisingly, transport of dust north across the equator was also quite efficient, though no storm activity, per se, occurred. However, this intense flow of dust was unable to penetrate the north polar vortex. At high southerly latitudes, variable activity occurred until summer solstice. Another, lesser storm occurred in midsummer. Localized small storms also developed, particularly near the edge of the retreating south polar cap. Our latest observations indicate that dust activity has declined substantially, with a corresponding clearing of the atmosphere. Study of dust storm mechanisms and the long term variability of their occurrence is important in understanding the accumulation of materials in the permanent polar caps.

Figure 3: Low Latitude, High Altitude Ice Clouds (Northern Spring)

Though there is very little water vapor in the martian atmosphere, atmospheric conditions are cold enough to produce extensive cloudiness. This figure shows the distribution of water ice clouds with latitude on a day in early northern spring (purple=clear, blue=very thin=optical depth~0.01,toward green=increasingly thick to total optical depth in vertical~0.06 at infrared wavelength of 45 micrometers). Overall, cloudiness decreases from the equator toward the pole. A variable, "low altitude" haze extends from ~15 km toward the surface at all latitudes; these pressure levels are where cirrus clouds are found on Earth. A much higher stratified cloud layer, extending about half way to the pole, is present near 30 km, at pressures where terrestrial noctilucent clouds occur. As water continues to be released fom the sublimating seasonal north polar cap, additional clouds can be expected to occur. Clouds can be used as tracers to investigate the long term transport of water to and from the permanent polar caps.

The TES instrument was built by Santa Barbara Remote Sensing and is operated by Philip R. Christensen, of Arizona State University, Tempe, AZ. The MGS mission is managed for NASA by the Jet Propulsion Laboratory, Pasadena CA.

Contact: John Pearl, Goddard Space Flight Center; (301)286-8487; john.pearl@gsfc.nasa.gov