[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] Thermocouples at three levels on a 1 m mast on the deck of the Phoenix Lander provided temperature data throughout the 151 sol Phoenix mission. Air temperatures showed a large diurnal cycle which showed little sol to sol variation, especially over the first 90 sols of the mission. Daytime temperatures at the top (2 m) level typically rose to about 243 K (À30°C) in early afternoon and had large (10°) turbulent fluctuations. These are analyzed and used to estimate heat fluxes. Late afternoon conditions were relatively calm with minimal temperature fluctuations but CFD computations show that heating from the lander deck and instruments have influenced temperatures measured at the lowest level (0.25 m above the deck) on the mast.

The Nadir and Occultation for MArs Discovery (NOMAD) instrument on board ExoMars Trace Gas Orbiter measured a large increase in water vapor at altitudes in the range of 40-100 km during the 2018 global dust storm on Mars. Using a three-dimensional general circulation model, we examine the mechanism responsible for the enhancement of water vapor in the upper atmosphere. Experiments with different prescribed vertical profiles of dust show that when more dust is present higher in the atmosphere, the temperature increases, and the amount of water ascending over the tropics is not limited by saturation until reaching heights of 70-100 km. The warmer temperatures allow more water to ascend to the mesosphere. Photochemical simulations show a strong increase in high-altitude atomic hydrogen following the high-altitude water vapor increase by a few days. Plain Language Summary The ExoMars Trace Gas Orbiter (TGO) is currently in orbit aroundMars measuring the composition of the atmosphere. TGO was able to make observations before, during, and after a large planet-encircling dust storm that occurred from June to August 2018. The TGO measurements provide the first opportunity to observe how water vapor is distributed with height in the atmosphere during a global-scale dust event. It was found that there was a large increase in water vapor very high (40-100 km) in the atmosphere during the dust storm. Using a three-dimensional numerical model of the Mars atmosphere, we found that when dust from the storm is transported up to levels above~40 km, it warms the atmosphere due to solar absorption, and this in turn prevents ice clouds from forming at heights of 40-60 km and allows more water vapor to ascend to greater heights in the atmosphere. This is of interest in terms of the planetary evolution, as water molecules at greater heights are more readily dissociated by sunlight and lost from the atmosphere. This is an important factor for understanding the changes that have occurred since the period when surface features on Mars indicate that liquid water was present.

[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] In situ surface pressures measured at 2 s intervals during the 150 sol Phoenix mission are presented and seasonal variations discussed. The lightweight Barocap®/Thermocap® pressure sensor system performed moderately well. However, the original data processing routine had problems because the thermal environment of the sensor was subject to more rapid variations than had been expected. Hence, the data processing routine was updated after Phoenix landed. Further evaluation and the development of a correction are needed since the temperature dependences of the Barocap sensor heads have drifted after the calibration of the sensor. The inaccuracy caused by this appears when the temperature of the unit rises above 0°C. This frequently affects data in the afternoons and precludes a full study of diurnal pressure variations at this time. Short-term fluctuations, on time scales of order 20 s are unaffected and are reported in a separate paper in this issue. Seasonal variations are not significantly affected by this problem and show general agreement with previous measurements from Mars. During the 151 sol mission the surface pressure dropped from around 860 Pa to a minimum (daily average) of 724 Pa on sol 140 (Ls 143). This local minimum occurred several sols earlier than expected based on GCM studies and Viking data. Since battery power was lost on sol 151 we are not sure if the timing of the minimum that we saw could have been advanced by a low-pressure meteorological event. On sol 95 (Ls 122), we also saw a relatively lowpressure feature. This was accompanied by a large number of vertical vortex events, characterized by short, localized (in time), low-pressure perturbations.Citation: Taylor, P. A., et al. (2010), On pressure measurement and seasonal pressure variations during the Phoenix mission,