Mars equatorial mesospheric clouds: Global occurrence and physical properties from Mars Global Surveyor Thermal Emission Spectrometer and Mars Orbiter Camera limb observations

We report the occurrence of a new type of cloud in the Mars dayside (1300–1400 Local Time) atmosphere, apparent as high‐altitude (60 to 80 km), vertically discrete aerosol scattering layers. Mars Global Surveyor (MGS) limb observations from the Thermal Emission Spectrometer (TES) visible channel indicate peak frequencies at the beginning and end of the aphelion northern summer season ( L S = 30° and 150°), where they are confined to equatorial (15°S–15°N) latitudes and two longitude ranges (40°E–2°W and 50°W–120°W). Limb images from the MGS Mars Orbiter Camera (MOC) indicate significant horizontal variations in these Mars equatorial mesospheric (MEM) clouds on 20–50 km scales. On the basis of the distribution of projected limb heights, MEM clouds exhibit peak optical depths over 70–75 km altitudes, that are substantial (nadir τ vis of order 0.01) for the low‐pressure region of formation (∼1 μbar ). Averaged TES limb infrared spectral (5–40 μ m) and solarband radiance profiles corresponding to MEM occurrence indicate dust aerosols ( r eff = 1.5–2.0 μ m, τ vis ∼ 0.4) at 0–25 km altitudes, capped by water ice clouds ( r eff = 1.5–2.0 μ m, τ vis ∼ 0.2) at 20–45 km altitudes. The lack of detectable infrared radiances at MEM cloud heights precludes distinction of water versus CO 2 ice, but indicates ≤1 μ m particle sizes for water or ≤1.5 μ m for CO 2 ice compositions. More recent Mars Express observations point toward Mars mesospheric CO 2 clouds, although current dynamical and radiative models do not indicate sufficiently cold temperatures at MEM cloud locations to produce daytime CO 2 saturation conditions.