Atmospheric dynamics at the Phoenix landing site as seen by the Surface Stereo Imager

The Surface Stereo Imager has made observations of dust blowing aloft and clouds near the horizon at the Phoenix landing site. These subtle features are apparent because of the high signal-to-noise ratio of the camera which allows for the removal of a mean frame from multiple images captured in rapid succession and the ability to conduct simultaneous capture through different filters in each camera eye. By examining the ratios between two filters, it was possible to determine in a relative sense how the water ice content of the atmosphere changed over the mission and on a diurnal time scale. The direction of travel and speed of features aloft near the zenith has been inferred and agree well with the diurnal pattern of near-surface wind direction from the Telltale. Direct observation of cumulus-like cloud near the surface suggests convection of water vaporrich air, but only until midday, requiring a mechanism to inhibit cloud formation in the early afternoon. The spectral ratios agree well with the observation of cloud and indicate a general increase in water ice toward the end of the mission as well as a strong diurnal pattern. However, even in periods of high water ice content, there is still a great deal of variability and days when dense clouds are absent. Also, different cloud layers are occasionally observed moving in different directions, indicating occasional wind shear aloft. Features observed had estimated minimum optical depths up to 0.11.