  The spectral signature of water ice was observed on Martian south polar cap
in 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 was
used to discover water ice deposited during southern summer on the polar cap
(Langevin et al., 2007). However, temporal and spatial variations of these
water ice signatures have remained unexplored, and the origins of these water
deposits remains an important scientific question. To investigate this
question, we have used observations from the Compact Reconnaissance Imaging
Spectrometer for Mars (CRISM) instrument on the Mars Reconnaissance Orbiter
(MRO) spacecraft of the southern cap during austral summer over four Martian
years to search for variations in the amount of water ice.
  We report below that for each year we have observed the cap, the magnitude of
the 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 in
disagreement with the current best simulations of deposition of water ice on
the south polar cap (Montmessin et al., 2007).
  This increase in water ice signatures is most likely caused by deposition of
atmospheric H2O ice and a set of unusual conditions makes the quantification of
this transport flux using CRISM close to ideal. We calculate a 'minimum
apparent' 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 percent
of the total Martian atmospheric water budget. We compare our 'minimal
apparent' quantification with previous estimates.
  This deposition process may also have implications for the formation and
stability of the southern CO2 ice cap, and therefore play a significant role in
the climate budget of modern day Mars.
