As water crystalizes it will absorb oxygen interstitially, causing it to expand. Thus, it floats, and it releases O2 as it melts. It is also softer. Water which crystalizes at higher altitudes is more dense and much harder due to the lack of oxygen to incorporate.

I got to wondering about that after encountering a lot of ice on Mt. Orizaba in Mexico. That ice, about 18,000 feet, was as hard as a rock and it would take a lot of whacking and pounding to chip a step. Scraping would have polished it nicely, would not have dented it. I sharpened the pick on my ice axe hoping against hope that had I slipped it could hold. I still have no confidence it could have unless I fell with my full weight on it within the first 1/2 second. It had a hard time penetrating.

I am completely sure those folks know what they are doing and have allowed for those eventualities. I haven't seen anything yet which would seem to let them chip away, but I'm certain it's a non-issue. It could well be it's not 100% water ice but some mixture which is bound to be much softer.

Arf