From which, we begin to suspect that the composition of the permanent South polar cap is not pure CO2, but is CO2-clathrate (70% H20, 30% CO2). This is not a whole different from the permafrost above 60 N which is ~ 50% rock/dust and 50% H2O. In both cases, the non-H components are not measured.

By fiddling with the ice to dust ratio of permafrost, and perhaps including some sheets of pure water ice in the south polar cap, the numbers can be made to match more closely.

However, I caution that the data so far is preliminary. I will be able to say much more in ~ 10 days when I've attended the LPSC where the Odyssey team will be making a set-piece presentation and will be open to questions about CO2, H2O, and clathrate.