Characterization of Mars' Water Vapor Cycles Using Different Band Depths

Since 2006, the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument onboard the Mars Reconnaissance Orbiter (MRO) has contributed towards the exploration of Mars by searching for the residue of minerals that form in the presence of water. Its observations allow for the column-integrated abundance of water vapor to be retrieved; previous works that utilized this ability found the water vapor band at 2.6 μm to be the strongest and most reliable for retrievals (Smith et al., 2009). However, a more recent work (Khayat et al., 2019) extended retrievals over polar regions covered by surface ice, finding that the 2.6 μm band is greatly affected by the deep CO2 and water ice spectral signatures. This made atmospheric water vapor retrievals nearly impossible to collect reliably. The 1.4 μm water vapor band, however, was not fully influenced by any atmospheric properties of carbon dioxide and not fully mixed with surface ice features of H2O or CO2.

To explore this phenomena of previous works, radiative transfer retrievals of column-integrated water vapor via CRISM observations were conducted using these two water vapor absorption bands. 37,992 observations were retrieved using the 2.6 μm band, later truncated to match and be compared to the 7,484 observations taken from using the 1.4 μm band. The 2.6 μm band measurements of this region in the summer season found water vapor levels reaching 54 precipitable micrometers (pr–μm), whereas the 1.4 μm band retrievals found abundances reaching 87 pr–μm. The distinctive seasonal and geographical trends of water were well depicted within both datasets, seen through the asymmetry in water vapor abundance between the northern and southern hemispheres and the transport of water from the north polar cap to the equator observed during northern summer/fall seasons. This behavior aligns with trends found using other existing techniques, such as the Thermal Emission Spectrometer (TES) on the Mars Global Surveyor (Smith 2004). Directly comparing the two retrieval algorithms' water vapor column abundance measurements revealed an R2 value of 0.85. This comparison between the two bands over ice-free surfaces helps confirm that the 1.4 μm band replicates the water cycle given by the 2.6 μm band, allowing for further exploration and extrapolation of the 1.4 μm band over icy areas.