The Vertical Distribution of Dust in the Martian atmosphere as Observed by the Mars Climate Sounder

Introduction: Observations of the spatial and temporal variability and optical properties of atmospheric dust have been a part of almost every major spacecraft mission sent to Mars. One important achievement of this observational program has been the creation of multiannual datasets of dust (and water ice) column opacity with near-global coverage and repeat cycling of approximately two weeks by the Thermal Emission Spectrometer (TES) on Mars Global Surveyor (MGS) and the Thermal Emission Imaging System (THEMIS) on Mars Odyssey [Smith, 2004, 2009]. Dust column opacity measurements can provide valuable information about surface radiative balance and the gross structure of meteorological systems associated with dust, but the vertical distribution is more sensitive than column opacity to the mechanisms of dust lifting, transport, and removal. The vertical distribution of dust has been observed by a variety of orbital and surface landed instruments [e.g., Conrath, 1975; Anderson and Leovy, 1978; Jaquin et al., 1986; Chassefiere et al., 1995; Lemmon et al., 2004; Montmessin et al., 2006; Rannou et al., 2006; Clancy et al., 2009], though these observations have not been as systematic (in one way or another) as those that make up the TES and THEMIS column opacity records. The Mars Climate Sounder (MCS) on Mars Reconnaissance Orbiter (MRO) has been making global, vertically resolved observations of infrared radiance from Mars’s limb, nadir, and off-nadir in nine broadband channels sensitive to dust, temperature, and other aerosol (see McCleese et al. [2007] for description of the instrument and observing strategy.) Simultaneous retrievals from MCS limb observations of vertical profiles of temperature, dust, and water ice are now available [Kleinbohl et al., 2009]. In terms of length of record, coverage, frequency of repeat cycling, and ability to distinguish dust opacity from water ice opacity; this dataset now should be the vertical analog to the TES and THEMIS column opacity datasets. Here, we discuss major features of the observed vertical dust distribution from the beginning of MCS science operations during early northern summer of Mars Year (MY) 28 to just after northern summer solstice of MY 30. Interested readers should consult Heavens et al. [2010a, 2010b] for methodological details and interpretation. Density-Scaled Opacity (DSO): Temperature, pressure, and opacity retrievals from MCS observations can be used to calculate the density-scaled opacity (the quotient of opacity and density), which is a good proxy for both the mass mixing ratio and the heating/cooling rates per mass due to dust under optically thin conditions. Since mass-mixing ratio is the quantity conserved in transport processes, the vertical dust distribution will be reported in terms of density-scaled opacity.