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The spectral signature of water ice was observed on Martian south polar capin 2004 by the Observatoire pour l'Mineralogie, l'Eau les Glaces et l'Activite(OMEGA) (Bibring et al., 2004). Three years later, the OMEGA instrument wasused to discover water ice deposited during southern summer on the polar cap(Langevin et al., 2007). However, temporal and spatial variations of thesewater ice signatures have remained unexplored, and the origins of these waterdeposits remains an important scientific question. To investigate thisquestion, we have used observations from the Compact Reconnaissance ImagingSpectrometer for Mars (CRISM) instrument on the Mars Reconnaissance Orbiter(MRO) spacecraft of the southern cap during austral summer over four Martianyears to search for variations in the amount of water ice.  We report below that for each year we have observed the cap, the magnitude ofthe H2O ice signature on the southern cap has risen steadily throughout summer,particularly on the west end of the cap. The spatial extent of deposition is indisagreement with the current best simulations of deposition of water ice onthe south polar cap (Montmessin et al., 2007).  This increase in water ice signatures is most likely caused by deposition ofatmospheric H2O ice and a set of unusual conditions makes the quantification ofthis transport flux using CRISM close to ideal. We calculate a 'minimumapparent' amount of deposition corresponding to a thin H2O ice layer of 0.2mm(with 70 percent porosity). This amount of H2O ice deposition is 0.6-6 percentof the total Martian atmospheric water budget. We compare our 'minimalapparent' quantification with previous estimates.  This deposition process may also have implications for the formation andstability of the southern CO2 ice cap, and therefore play a significant role inthe climate budget of modern day Mars.

Interannual observations and quantification of summertime H2O ice deposition on the Martian CO2 ice south polar cap