The light detection and ranging instrument on the Phoenix mission observed water-ice clouds in the atmosphere of Mars that were similar to cirrus clouds on Earth. Fall streaks in the cloud structure traced the precipitation of ice crystals toward the ground. Measurements of atmospheric dust indicated that the planetary boundary layer (PBL) on Mars was well mixed, up to heights of around 4 kilometers, by the summer daytime turbulence and convection. The water-ice clouds were detected at the top of the PBL and near the ground each night in late summer after the air temperature started decreasing. The interpretation is that water vapor mixed upward by daytime turbulence and convection forms ice crystal clouds at night that precipitate back toward the surface.
[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.
A time and height dependent eddy diffusion model is used to investigate possible scenarios for the size distribution of dust in the lower atmosphere of Mars. The dust is assumed to either have been advected from a distant source or to have originated locally. In the former case, the atmosphere is assumed to initially contain dust particles with sizes following a modified gamma distribution. Larger particles are deposited relatively rapidly while small particles are well mixed up to the maximum height of the afternoon boundary layer and are deposited more slowly. In other cases, a parameterization of the dust source at the surface is proposed. Model results show that smaller particles are rapidly mixed within the Martian boundary layer, while larger particles (r > 10 µm) are concentrated near the ground with a stronger diurnal cycle. In all simulations we assume that the initial concentration or surface source depend on a modified gamma function distribution. For small particles (crosssectional area weighted mean radius, r eff = 1.6 µm) distributions retain essentially the same form, though with variations in the mean and variance of the area-weighted radius, and the gamma function can be used to represent the particle size distribution reasonably well at most heights within the boundary layer. In the case of a surface source of larger particles (mean radius 50 µm) the modified gamma function does not fit the resulting particle size distribution. All results are normalised by a scaling factor that can be adjusted to correspond to an optical depth for assumed particle optical scattering properties.
[1] Diurnal variation of ground fog and water ice cloud formation at the NASA Phoenix lander site is investigated using a one-dimensional Mars Microphysical Model (MMM) coupled with the results from the one-dimensional University of Helsinki atmospheric boundary layer (ABL) model. Phoenix is scheduled to reach Mars in May 2008 and land in the northern plains (65°-72°N). Observations from Mars Global Surveyor Thermal Emission Spectrometer for the proposed landing site and season L s = 76°-125°have been used for the model initialization, both in the ABL and MMM. The diurnal variations of temperature and eddy diffusion coefficients produced by the uncoupled ABL are then applied to the MMM. Extinction and backscattering coefficients and lidar ratios are presented for the simulated dust and water ice clouds at the Phoenix location. Results of the dust and ice clouds are then used to simulate the Phoenix lidar measurements at two wavelengths, 532 and 1064 nm.
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