Solar infrared occultation observations by SPICAM experiment on Mars-Express: Simultaneous measurements of the vertical distributions of H ₂O, CO ₂ and aerosol

Abstract

The infrared AOTF spectrometer is a part of the SPICAM experiment onboard the Mars-Express ESA mission. The instrument has a capability of solar occultations and operates in the spectral range of 1-1.7 μm with a spectral resolution of ∼3.5 cm ^-1. We report results from 24 orbits obtained during MY28 at L 130°-160°, and the latitude range of 40°-55° N. For these orbits the atmospheric density from 1.43 μm CO ₂ band, water vapor mixing ratio based on 1.38 μm absorption, and aerosol opacities were retrieved simultaneously. The vertical resolution of measurements is better than 3.5 km. Aerosol vertical extinction profiles were obtained at 10 wavelengths in the altitude range from 10 to 60 km. The interpretation using Mie scattering theory with adopted refraction indices of dust and H ₂O ice allows to retrieve particle size ( r∼0.5-1 μm) and number density (∼1 cm ^-3 at 15-30 km) profiles. The haze top is generally below 40 km, except the longitude range of 320°-50° E, where high-altitude clouds at 50-60 km were detected. Optical properties of these clouds are compatible with ice particles (effective radius r=0.1-0.3 μm, number density N∼10 cm) distributed with variance ν=0.1-0.2 μm. The vertical optical depth of the clouds is below 0.001 at 1 μm. The atmospheric density profiles are retrieved from CO ₂ band in the altitude range of 10-90 km, and H ₂O mixing ratio is determined at 15-50 km. Unless a supersaturation of the water vapor occurs in the martian atmosphere, the H ₂O mixing ratio indicates ∼5 K warmer atmosphere at 25-45 km than predicted by models.