re: Why couldn't the disappearing white matter be CO2?
At the temperatures and pressures at the phoenix site CO2 should be a gas, so if the white material was CO2 surviving protected beneath the surface, and it was brought out into the sunlight it should have dissapeared much quicker than it did. Water ice is quasi-stable: the conditions are right for it, but there is still some sublimation, and that probably gets redeposited on to the ground or into the soil, not back onto the chunk it came from, so the exposed chunks dissapear but much more slowly.