Three Martian Years of MGS-TES Aerosol Limb Sounding: Vertical Distributions, Diurnal Variations, and the Polar Night.

We apply a forward radiative transfer model to Mars Global Surveyor Thermal Emission Spectrometer (MGS-TES) limb-pointed radiance spectra, retrieving, for each limb-scan sequence, dust and water ice aerosol mixing ratios at six levels in the martian atmosphere between 10 and 60 km altitude. MGS-TES typically performed a limb-scan sequence every 10 degrees of latitude along its polar, sun-synchronous, 2pm orbit, and MGS orbit tracks were separated by approximately 30 degrees in longitude. Thus, we derive a three-dimensional aerosol data set, with a resolution of roughly 30 degrees in longitude, 10 degrees in latitude, and 10 km in altitude, for every day and every night of the three martian years of MGS mapping operations. In comparison to the familiar nadir-sounding TES aerosol retrievals [e.g. Smith, 2004, Icarus, 167, 148.], our limb- sounding retrievals have very sparse sampling in latitude (1 per 10 degrees, versus roughly 70 per 10 degrees), and thus, in addition to poorer resolution, maps based on the limb-sounding retrievals have less statistical precision due to a much smaller number of samples available for any given region and time period. However, the limb-sounding retrievals have three key advantages: they provide vertical resolution; they provide more accurate opacity estimates, because the vertical profile is derived rather than assumed and because scattering is included in the model; and, they are effective when the surface is cold, such as at night, and over the winter pole. To illustrate the value of the new information provided by the MGS-TES limb-sounding data set, we focus on two prominent features: the equatorial cloud belt and the polar hood. The equatorial cloud belt, as seen in the limb- sounding data set, persists throughout the martian year, but the altitude of maximum mixing ratio increases from 30 to 45 km from aphelion to perihelion solstice, and the maximum mixing ratio decreases by a factor of three. The diurnal pattern of the equatorial cloud belt is also seen to vary: near perihelion the equatorial cloud belt intensifies and shifts slightly southward at night; while in the equinox seasons it exhibits a more complicated diurnal behavior in which the daytime equatorial ice optical depth maximum bifurcates and shifts to northern and southern mid-latitudes at night. The northern winter polar hood is revealed to be most optically thick in an annulus surrounding the pole, with a maximum optical thickness near 70 degrees north latitude. It is apparently isolated from the dusty conditions occurring at the same time over the rest of the planet: the limb-sounding data set observes sharp drop in dust mixing ratio at 50 degrees north latitude. The optical depth of the northern winter polar hood is substantial, peaking at more than 1.5 at 825 cm-1 in mid-fall. In contrast, the southern winter polar hood is barely recognizable, with an optical depth of less than 0.1.