High-altitude dust layers on Mars: Observations with the Thermal Emission Spectrometer

Limb-scanning observations of Martian atmospheric dust with the Thermal Emission Spectrometer (TES) over 3Marsyears indicate two distinct altitude layers with persistent maxima in the dust mixing ratio vertical profile. The first, lower maximum in the dust distribution profile is the high-altitude tropical dust maximum (HATDM) centered at 20-30km, previously detected by the Mars Climate Sounder (MCS). Through the observation period, the HATDM followed a repeatable seasonal cycle with a brief absence in early northern spring and reached its highest altitudes and largest amplitude during the dust storm season in southern spring and summer. The HATDM is likely maintained during the day by a combination of convective and topographic updrafts and then degraded at night by scavenging from water ice clouds. The second, upper maximum in the dust distribution profile, which we refer, for convenience, to as the upper dust maximum (UDM), is centered at 45-65km and is only detected in daytime observations. We see additional evidence of its presence in the limited number of MCS aerosol opacity retrievals available at these altitudes. Comparable dust mixing ratios are nearly absent from this altitude range at night. This upper maximum is generally a northern hemisphere phenomenon, peaking in amplitude in northern summer and nearly absent from the TES observational domain during the dust storm season. We suggest topographic updrafts over Martian volcanoes, small particle size, diurnal transport associated with thermal tides, and scavenging by water ice as probable key factors in the creation of the UDM.