I think this will be of interest: http://www.sciencedaily.com/releases/2010/09/100922124546.htm
Some interesting dynamics going on here. If some of the CO2 ice gets temporarily covered each season by a thin layer of water ice, you might expect some areas (shaded areas in north-facing troughs?) to retain CO2 ice throughout the summer and become permanently incorporated into the North Pole Layered Deposits (NPLD). This suggests that the NPLD might contain significant amounts of CO2 ice, but I don't think the spectroscopic evidence indicates this (?). Hmm.
Tom
The thin layer of ice might spectroscopically mask the CO2. (Like as a paint covering).
I wonder if something that penetrates deeper might notice a change, like a RADAR experiment that could measure dielectric constant. (At 0 C, dielectric constant of H2O = 88, CO2 = 1.6. [http://www.asiinstr.com/technical/Dielectric%20Constants.htm])
Update: I just looked at Shane Byrne's review article in Annual Review of Earth and Planetary Science (2009). He says (p. 541) that "buried CO2 ice and CO2 clathrate hydrate have been eliminated as important components [of the NPLD] on the basis of thermal and mechanical strength arguments. He cites paper by Mellon (1996) and Nye et. a, (2000).
Subsurface temperatures quickly approach year-average temps: at ~20 meters, temperature barely changes. I think even at Mars poles year-average temperature is above CO2 sublimation point.
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