▁the ▁sky ▁a ▁number ▁of ▁times ▁in ▁the ▁past . ▁▁ ▁In ▁some ▁places ▁a ▁number ▁of ▁layers ▁are ▁visible ▁in ▁the ▁mant le . ▁ ▁It ▁fell ▁as ▁snow ▁and ▁ice - co ated ▁dust . ▁ ▁There ▁is ▁good ▁evidence ▁that ▁this ▁mant le ▁is ▁ice - rich . ▁ ▁The ▁shapes ▁of ▁the ▁pol yg ons ▁common ▁on ▁many ▁surfaces ▁suggest ▁ice - rich ▁soil . ▁ ▁High ▁levels ▁of ▁hydro gen ▁( pro bably ▁from ▁water ) ▁have ▁been ▁found ▁with ▁Mars ▁Od ys sey . ▁ ▁Th erm al ▁measurements ▁from ▁orbit ▁suggest ▁ice . ▁ ▁The ▁Phoenix ▁( space craft ) ▁discovered ▁water ▁ice ▁with ▁made ▁direct ▁observations ▁since ▁it ▁land ed ▁in ▁a ▁field ▁of ▁pol yg ons . ▁ ▁In ▁fact , ▁its ▁landing ▁rock ets ▁exposed ▁pure ▁ice . ▁ ▁Theory ▁had ▁predicted ▁that ▁ice ▁would ▁be ▁found ▁under ▁a ▁few ▁cm ▁of ▁soil . ▁ ▁This ▁mant le ▁layer ▁is ▁called ▁" lat itude ▁dependent ▁mant le " ▁because ▁its ▁occurrence ▁is ▁related ▁to ▁the ▁latitude . ▁ ▁It ▁is ▁this ▁mant le ▁that ▁cra cks ▁and ▁then ▁forms ▁pol yg onal ▁ground . ▁This ▁crack ing ▁of ▁ice - rich ▁ground ▁is ▁predicted ▁based ▁on ▁physical ▁processes . ▁▁▁ ▁, ▁ ▁Pol yg onal ▁pattern ed ▁ground ▁ ▁Pol yg onal , ▁pattern ed ▁ground ▁is ▁quite ▁common ▁in ▁some ▁regions ▁of ▁Mars . ▁ ▁It ▁is ▁commonly ▁believed ▁to ▁be ▁caused ▁by ▁the ▁sub lim ation ▁of ▁ice ▁from ▁the ▁ground