Mars Global Surveyor Thermal Emission Spectrometer (TES) observations of dust opacity during aerobraking and science phasing

The Mars Global Surveyor (MGS) arrived at Mars in September 1997 near Mars' s southern spring equinox and has now provided monitoring of conditions in the Mars atmosphere for more than half a Mars year. The large majority of t he spectra taken by the Thermal Emission Spectrometer (TES) are in a nadir geometry (downward looking mode) where Mars is observed through the atmosph ere. Most of these contain the distinct spectral signature of atmospheric d ust. For these nadir-geometry spectra we retrieve column-integrated infrare d aerosol (dust) opacities. TES observations during the aerobraking and sci ence-phasing portions of the MGS mission cover the seasonal range L-s = 184 degrees-28 degrees. Excellent spatial coverage was obtained in the souther n hemisphere. Northern hemisphere coverage is generally limited to narrow s trips taken during the periapsis pass but is still very valuable. At the be ginning of the mission the 9-mu m dust opacity at midsouthern latitudes was low (0.15-0.25). As the season advanced through southern spring and into s ummer, TES observed several regional dust storms (including the Noachis dus t storm of November 1997) where peak 9-mu m dust opacities approached or ex ceeded unity, as well as numerous smaller local storms. Both large and smal l dust storms exhibited significant changes in both spatial coverage and in tensity over a timescale of a day. Throughout southern spring and summer th e region at the edge of the retreating southern seasonal polar ice cap was observed to be consistently more dusty than other latitudes.