 PHOENIX LIDAR OBSERVATIONS OF DUST, CLOUDS, AND PRECIPITATION ON MARS   J. Whiteway1, L. Komguem1, C. Dickinson1, C. Cook1, T. Duck2, P. Taylor1, R. Davy1, J. Seabrook1, D. Fisher3, A. Carswell4, M. Daly5, V. Popovici1, and the Phoenix Science Team. 1York University, Toronto, Ontario, M6J 1P3 (Whiteway@yorku.ca). 2Dalhousie University, Halifax, Nova Scotia. 3 Natural Resources Canada, Ottawa, Ontario, 4Optech Inc., Vaughan, Ontario. 5MDA Space Missions, Brampton, Ontario.   Introduction: The Phoenix mission [1] obtained measurements from the surface of Mars through the midsummer peak in the abundance of atmospheric water vapor. During this period it was possible to observe the local processes that contribute to the water cycle. This included the measurements of near surface humidity [2], and temperature [3].  A unique instrument on the Phoenix mission was a lidar [4] that detected the backscatter of pulsed laser light emitted upward into the atmosphere. It measured the vertical distribution of atmospheric dust and water ice clouds.   Dust in the Planetary Boundary Layer: Figure 1 shows the height distribution of extinction coefficient derived from the lidar measurements on sol 45 (Ls = 98o). The extinction coefficient is the fractional reduction in the laser pulse energy per unit length due to scattering by dust in this case. The extinction coefficient can also be considered as the effective cross sectional area of scattering material per unit volume, so it is proportional to the mass of scattering material. The profile measured on sol 45 (Fig. 1) is typical for moderate dust loading with no clouds. The dust is distributed evenly with height up to 4 km due to the vertical mixing by convection and turbulence during daytime within the Planetary Boundary Layer (PBL).  The height of the PBL was variable between 4 km and 6 km. The atmospheric dust loading reached a peak around mid-summer and then generally decreased.  Figure 1 also shows the measured profile of extinction coefficient on sol 97 (Ls = 126 o) where the dust loading is reduced by more than a factor of three in comparison with sol 45.    Clouds and Precipitation: During the period around summer solstice, clouds were observed sporadically and mainly above heights of 10 km. Moving into late summer, 50 sols after solstice (Ls = 117 o) the lidar detected a regular pattern of cloud formation each night within the planetary boundary layer. A shallow surface based cloud formed at around midnight (Mars local solar time) and a second cloud layer formed after 1 am at heights between 3 km and 6 km.  The observed clouds formed at an estimated temperature of around    -65o C, so they are composed of water ice crystals. For each sol in late summer the water ice clouds persisted throughout the night and then dissipated when the atmosphere warmed sufficiently during daytime. As the summer progressed toward autumn, the clouds persisted longer into the morning hours and extended further toward the ground. Clouds were not detected in the afternoon or evening.   Figure 2 shows the contour plot of the height distribution of the laser backscatter over the hour of measurements from 04:23 to 05:21 on sol 99 (Ls = 127o). This indicates the outline and internal structure of the cloud that drifted above the Phoenix landing site. The most striking features are the vertical streaks at the base of the cloud after 05:00. This pattern is consistent with ice crystals precipitating from the cloud, and eventually sublimating in the dry air below the cloud. The essential point here is that precipitation moves water downward toward the surface from heights above 4 km.    Discussion: The Phoenix lidar measurements of atmospheric dust indicate that the planetary boundary layer (PBL) on Mars is well mixed up to a height of 4 km by the daytime turbulence and convection during summer above the northern polar region. The lidar also observed that water ice clouds form within the PBL each night in late summer and that ice crystals precipitate toward the surface. The PBL is saturated with water at night.  Moisture that sublimates from ice crystals in the air and from the surface during daytime is mixed up to the top of the PBL. In the early morning hours, clouds are forming at ground level and at heights around 4 km since these are the coldest parts of the planetary boundary layer. The cloud is capped at the top of the PBL because daytime turbulent mixing does not transport moisture above that height. Water is transported downward by precipitation at night, and then upward to the top of the PBL by turbulent mixing during the day. As the height of the PBL decreases while the season advances toward autumn, water will be confined within the PBL by precipitation.   References:  [1] Smith, P., et al. (2008), JGR, 113, E00A18. [2] Hudson et al. (2008), LPS this issue. [3] Taylor, P., et al. (2008), JGR, 113.  [4] Whiteway, J., et al. (2008), JGR, 113, E00A08.    Figure 1.  Profiles of extinction coefficient derived from lidar measurements on sols 45 (Ls = 98 o), and 97 (Ls = 126o).        Figure 2. Contour plot of lidar backscatter coefficient for measurements on sols 99 (Ls= 127 o). The backscatter coefficient is the fraction of the laser pulse that is scattered back to the lidar per unit length and solid angle.    
