Mesospheric CO2 ice clouds on Mars observed by Planetary Fourier Spectrometer onboard Mars Express

Abstract

We have investigated mesospheric CO<SUB>2</SUB> ice clouds on Mars through analysis of near-infrared spectra acquired by Planetary Fourier Spectrometer (PFS) onboard the Mars Express (MEx) from MY 27 to MY 32. With the highest spectral resolution achieved thus far in the relevant spectral range among remote-sensing experiments orbiting Mars, PFS enables precise identification of the scattering peak of CO<SUB>2</SUB> ice at the bottom of the 4.3 μm CO<SUB>2</SUB> band. A total of 111 occurrences of CO<SUB>2</SUB> ice cloud features have been detected over the period investigated. Data from the OMEGA imaging spectrometer onboard MEx confirm all of PFS detections from times when OMEGA operated simultaneously with PFS. The spatial and seasonal distributions of the CO<SUB>2</SUB> ice clouds detected by PFS are consistent with previous observations by other instruments. We find CO<SUB>2</SUB> ice clouds between Ls = 0° and 140° in distinct longitudinal corridors around the equatorial region (± 20°N). Moreover, CO<SUB>2</SUB> ice clouds were preferentially detected at the observational LT range between 15-16 h in MY 29. However, observational biases prevent from distinguishing local time dependency from inter-annual variation. PFS also enables us to investigate the shape of mesospheric CO<SUB>2</SUB> ice cloud spectral features in detail. In all cases, peaks were found between 4.240 and 4.265 μm. Relatively small secondary peaks were occasionally observed around 4.28 μm (8 occurrences). These spectral features cannot be reproduced using our radiative transfer model, which may be because the available CO<SUB>2</SUB> ice refractive indices are inappropriate for the mesospheric temperatures of Mars, or because of the assumption in our model that the CO<SUB>2</SUB> ice crystals are spherical and composed by pure CO<SUB>2</SUB> ice.