MARS EQUATORIAL MESOSPHERIC CLOUDS

We report the occurrence of a new type of cloud in the Mars atmosphere, apparent as high altitude (60 to 80 km mesospheric), vertically discrete aerosol scattering layers. Mars Global Surveyor (MGS) limb observations from the Thermal Emission Spectrometer (TES) over 2-3 pm local times (LT) indicate distinctive temporal and spatial distributions that suggest strong dynamical influences in the formation of these Mars equatorial mesospheric (MEM) clouds. They exhibit high occurrence frequencies at the beginning and end of the aphelion northern summer season on Mars (centered at Ls = 30° and 150°), where they are confined to equatorial (15S-15N) latitudes and two relatively narrow longitude ranges (330-20W and 50-120W). Limb images from the MGS Mars Orbital Camera (MOC) over 1-3 pm LT indicate significant horizontal variations in these mesospheric clouds on scales of 25-50 km, which couple with the limb viewing geometry to introduce ambiguity in the altitude of individual cloud identifications. Based on the observed distribution of projected limb heights, MEM clouds exhibit peak optical depths that are quite significant (τvis ~ 0.01) for the low-pressure region of formation (~ 1 μbar), within a fairly narrow altitude range of 70-80 km. Undetectable 5-40 μm signal within TES IR limb spectra precludes direct compositional information (i.e., water versus CO2 ice). However, the observed upper limit for infrared scattering in the context of the observed visible scattering (TES solar band and MOC blue filter) indicates small aerosol particle sizes (Reff ≤ 0.5 μm) for water or CO2 ice compositions. If MEM clouds are water ice, vigorous dynamical transport of water vapor to the 70-80 km altitude of cloud formation is required to produce the substantial cloud optical depths observed. If the mesospheric clouds are CO2 ice, very cold (≤ 105 K) atmospheric temperatures are required for the season and location of their occurrence. Spatially resolved temperature measurements of the Mars mesosphere are very limited, but the Pathfinder entry profile (Lat=19N, Long=34W, Ls =142°, LT= 4 am) exhibited cold, CO2 saturation temperatures around 80 km for a location and season consistent with MEM cloud occurrence. Furthermore, disk-average 70-80 km Mar atmospheric temperatures from groundbased sub-millimeter observations indicate minimum values (daytime, 40S-40N, ≤ 120 K) for the solar longitude ranges of MEM clouds, as do European Climate Database (EMCDsee Forget et al, 1999) model temperature fields at 82 km. Introduction: One of the more interesting aspects of the atmospheric temperature profiles retrieved during the Pathfinder descent entry was the detection of cold 80 km altitude temperatures below the saturation conditions for CO2 ice formation (Schofield et al., 1997). Clancy and Sandor (1998) suggested the relatively frequent occurrence of CO2 ice clouds in the 70-80 km altitude region, on the basis of observed (Pathfinder and sub-millimeter ground-based) cold temperatures, Pathfinder images of blue (small size) ice clouds in the predawn sky (Smith et al., 1997), and Mariner 6 and 7 near-IR (4.3 micron) identification of CO2 ice in equatorial limb tangent views around Ls=200° in 1969 (Herr and Pimental, 1970 – but which may be due to nonLTE CO2 emission). Before MGS observations, it remained unclear whether CO2 or water ice aerosols are ever present at > 60 km altitudes. Dust aerosols had been identified at such high altitudes in Mariner 9 (Anderson and Leovy, 1978), Viking (Jaquin et al., 1986) and MGS (Clancy et al., 2003) limb radiances, associated with the 1971, 1977, and 2001 global dust storms, respectively. The highest detached ice cloud identified from Viking limb data occurred at a projected tangent altitude of 55 km, at 16S, 72W and Ls = 176° (Jaquin et al., 1986; recently modeled by Montmessin et al., 2002).