Diurnal variability and spatial distributions of martian atmospheric water ice from mars express PFS/SWC observations

Water ice clouds are an important component of the Martian atmospheric water cycle. However, the diurnal variability of water ice clouds remains poorly constrained by observations and discrepancies are existed in the mechanisms and theories regarding their relationship with the cycling of water vapor. This study presents a multiple Martian-year climatology of atmospheric water ice clouds on Mars, from observations by the Planetary Fourier Spectrometer Short Wavelength Channel (PFS/SWC) onboard Mars Express. The water ice aerosol optical depth is retrieved in the 3.3 μm spectral band, covering approximately eight Martian years (MY27-34). The results delineate the well-known seasonal patterns: the aphelion cloud belt (ACB) in low latitudes, the north polar hood (NPH), and the south polar hood (SPH). Spatial analysis reveals that within the ACB, peak optical depths are longitudinally associated with major topographic highs like the Tharsis region. Comparative analysis with THEMIS and CRISM datasets shows consistent seasonal evolution but highlights quantitative discrepancies attributed to local time differences, underscoring significant diurnal variability. PFS’s unique local time coverage allows us to characterize this diurnal cycle, finding typically higher morning and lower afternoon opacities, with the strongest amplitude in the aphelion season. Spatially, enhanced water ice optical depths are observed over the Tharsis bulge, Elysium Mons, and Arabia Terra, correlating with regions of low thermal inertia and orographic lifting. Comparison with water vapor column abundances retrieved from PFS/SWC reveals a coupled seasonal exchange between ice and vapor, particularly during the aphelion season. Discrepancies between our retrievals and outputs from the Mars Climate Database (MCD) suggest that the real Martian water cycles proceed more slowly than current general circulation model simulations predict. This multi-annual dataset provides critical observational constraints for validating global climate models and advancing our understanding of the Martian water cycles.