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We used the OMEGA imaging spectrometer aboard Mars Express to study the evolution of the water vapor abundance over the Hellas basin, as a function of the seasonal cycle. The H 2 O column density is found to range from very low values (between southern fall and winter) up to more than 15 pr- μ m during southern spring and summer. The general behavior is consistent with the expected seasonal cycle of water vapor on Mars, as previously observed by TES and modeled. In particular, the maximum water vapor content is observed around the southern solstice, and is significantly less than its northern couterpart. However, there is a noticeable discrepancy around the northern spring equinox ($L_{\rm s} = 330$–60°), where the observed H 2 O column densities are significantly lower than the values predicted by the GCM. Our data show an abrupt enhancement of the water vapor column density (from 3 to 16 pr- μ m) on a timescale of 3 days, for $L_{\rm s} = 251$–254°. Such an increase, not predicted by the GCM, was also occasionally observed by TES over Hellas during previous martian years at the same season; however, its origin remains to be understood.

A study of the Martian water vapor over Hellas using OMEGA and PFS aboard Mars Express