The northern ice pack is water ice. The southern ice pack is CO2 ice. We’ve known this for a long time. They have suspected sumerged aquafirs of ice/water because of the erosion patterns observed on the suface. So they think it’s been frozen, and buried over the past several billion years of being blasted by ateroids and meteors.

The northern hemisphere of Mars was hit by a BIG ateroid a long time ago, and is several kilometers shallower than the southern hemiphere- so if it was covered with water, the whole top of it would be covered by an ocean. This is thought to be the impact that ended the presense of surface water.