[1] The Phoenix Mars Lander detected a larger number of short (∼20 s) pressure drops that probably indicate the passage of convective vortices or dust devils. Near-continuous pressure measurements have allowed for monitoring the frequency of these events, and data from other instruments and orbiting spacecraft give information on how these pressure events relate to the seasons and weather phenomena at the Phoenix landing site. Here 502 vortices were identified with a pressure drop larger than 0.3 Pa occurring in the 151 sol mission (L s 76 to 148). The diurnal distributions show a peak in convective vortices around noon, agreeing with current theory and previous observations. The few events detected at night might have been mechanically forced by turbulent eddies caused by the nearby Heimdal crater. A general increase with major peaks in the convective vortex activity occurs during the mission, around L s = 111. This correlates with changes in midsol surface heat flux, increasing wind speeds at the landing site, and increases in vortex density. Comparisons with orbiter imaging show that in contrast to the lower latitudes on Mars, the dust devil activity at the Phoenix landing site is influenced more by active weather events passing by the area than by local forcing.
[1] Wind speeds and directions were measured on the Phoenix Lander by a mechanical anemometer, the so-called Telltale wind indicator. Analysis of images of the instrument taken with the onboard imager allowed for evaluation of wind speeds and directions. Daily characteristics of the wind data are highly turbulent behavior during midday due to daytime turbulence with more stable conditions during nighttime. From L s ∼77°-123°winds were generally ∼4 m s −1 from the east, with 360°rotation during midday. From L s ∼123°-148°d aytime wind speeds increased to an average of 6-10 m s −1 and were generally from the west. The highest wind speed recorded was 16 m s −1 seen on L s ∼147°. Estimates of the surface roughness height are calculated from the smearing of the Kapton part of the Telltale during image exposure due to a 3 Hz turbulence and nighttime wind variability. These estimates yield 6 ± 3 mm and 5 ± 3 mm, respectively. The Telltale wind data are used to suggest that Heimdal crater is a source of nighttime temperature fluctuations. Deviations between temperatures measured at various heights are explained as being due to winds passing over the Phoenix Lander. Events concerning sample delivery and frost formation are described and discussed. Two different mechanisms of dust lifting affecting the Phoenix site are proposed based on observations made with Mars Color Imager on Mars Reconnaissance Orbiter and the Telltale. The first is related to evaporation of the seasonal CO 2 ice and is observed up to L s ∼95°. These events are not associated with increased wind speeds. The second mechanism is observed after L s ∼111°and is related to the passing of weather systems characterized by condensate clouds in orbital images and higher wind speeds as measured with the Telltale.
[1] The Phoenix and Mars Reconnaissance Orbiter (MRO) missions collaborated in an unprecedented campaign to observe the northern polar region summer atmosphere throughout the Phoenix mission (25 May to 2 November 2008; L s = 76°-150°) and slightly beyond (∼L s = 158°). Five atmospherically related campaigns were defined a priori and were executed on 37 separate Martian days (sols). Phoenix and MRO observed the atmosphere nearly simultaneously. We describe the observation strategy and history, the participating experiments, and some initial results. We find that there is general agreement between measurements from different instruments and platforms and that complementary measurements provide a consistent picture of the atmosphere. Seasonal water abundance behavior matches with historical measurements. Winds aloft, as measured by cloud motions, showed the same seasonally consistent, diurnal rotation as the winds measured at the lander, during the first part of the mission (L s = 76°-118°). A diurnal cycle recorded from L s ∼ 108.3°-109.1°, in which a dust front was approaching the Phoenix Lander, is examined in detail. Cloud heights measured on subsequent orbits showed that in areas of active lifting, dust can be lofted quite high in the atmosphere, doubling in height over 2 h. The combination of experiments also revealed that there were discrete vertical layers of water ice and dust. Water vapor column abundances compared to near-surface water vapor pressure indicate that water is not well mixed from the surface to a cloud condensation height and that the depth of the layer that exchanges diurnally with the surface is 0.5-1 km.
[1] The Telltale wind indicator is a mechanical anemometer designed to operate on the Martian surface as part of the meteorological package on the NASA Phoenix lander. It consists of a lightweight cylinder suspended by Kevlar fibers and is deflected under the action of wind. Imaging of the Telltale deflection allows the wind speed and direction to be quantified and image blur caused by its oscillations provides information about wind turbulence. The Telltale will primarily support surface operations by documenting the wind conditions to improve the efficiency of sample delivery to instruments on the lander deck. During the latter stages of the mission the Telltale investigation will focus on meteorological studies.
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