What do you need to generate ice? Water. Mars does have water, yes. Enough water to cover the entire planet in ice? Uhm, nope. A lot of the water that used to be on Mars dissipated into the vacuum of space when its atmosphere degenerated. Now, we are left with only small remnants of water (comparatively) in the forms of clusters of ice at the poles of the red planet. All that remains on the main surface is a very cold red-sand desert where the markings of water still remain. It's fascinating stuff.