Paul, the dust storm is substantially covering the polar cap on one side of Mars, but not so much on the other.
Check out this image by expert Mars imager Clyde Foster, he was lamenting the sheer dustiness of the polar cap. Without knowing the time of the image, I'd guess yours might be of a comparable longitude on Mars. The polar cap is quite a lot clearer on the
Valles Marineris side, though it's still a little 'dirty'. The boundary between obscured and not so obscured is is somewhere near Hellas, as
in Christophe Pellier's image. I'd guess the issue is very likely to be on Mars rather than your processing!
Search the Mars images in
PVOL to see a good collection of recent images.