A publication accepted by JGR describes how the orbit of Mars around the sun could affect the deposition of water-ice at the Martian South Pole. Possibly, the buildup and erosion of water ice is coupled to the precession cycle of Mars. This was deduced from Mars Express observations and a model of the Martian climate.
It is also described that the erosion of water ice at the Martian South Pole was stopped by the formation of CO2-ice only about 1000 years ago. This CO2 now suits as a cold trap and enables the formation of water ice in a small area around the pole (south of 87°S).
All the details can be found in this ESA press release:
'On the origin of perennial water ice at the South Pole of Mars: a precession-controlled mechanism?', by F. Montmessin, R. M. Haberle, F. Forget, Y. Langevin, R. T. Clancy and J.-P. Bibring, accepted for publication in the JGR Planets journal, and presented at the 7th International Conference on Mars, Pasadena, California (9-13 July 2007)