▁thin ▁atmosphere . ▁ ▁Event ually , ▁small ▁cra cks ▁become ▁large ▁can y ons ▁or ▁tr ough s . ▁ ▁Small ▁cra cks ▁often ▁contain ▁small ▁p its ▁and ▁ch ains ▁of ▁p its ; ▁these ▁are ▁thought ▁to ▁be ▁from ▁sub lim ation ▁of ▁ice ▁in ▁the ▁ground . ▁ ▁Lar ge ▁areas ▁of ▁the ▁Mart ian ▁surface ▁are ▁loaded ▁with ▁ice ▁that ▁is ▁protected ▁by ▁a ▁meters ▁thick ▁layer ▁of ▁dust ▁and ▁other ▁material . ▁ ▁However , ▁if ▁cra cks ▁appear , ▁a ▁fresh ▁surface ▁will ▁expose ▁ice ▁to ▁the ▁thin ▁atmosphere . ▁▁ ▁In ▁a ▁short ▁time , ▁the ▁ice ▁will ▁disappear ▁into ▁the ▁cold , ▁thin ▁atmosphere ▁in ▁a ▁process ▁called ▁sub lim ation . ▁ ▁D ry ▁ice ▁behav es ▁in ▁a ▁similar ▁fashion ▁on ▁the ▁Earth . ▁ ▁On ▁Mars ▁sub lim ation ▁has ▁been ▁observed ▁when ▁the ▁Phoenix ▁land er ▁un cover ed ▁ch unks ▁of ▁ice ▁that ▁disappeared ▁in ▁a ▁few ▁days . ▁ ▁In ▁addition , ▁Hi RI SE ▁has ▁seen ▁fresh ▁cr aters ▁with ▁ice ▁at ▁the ▁bottom . ▁ ▁After ▁a ▁time , ▁Hi RI SE ▁saw ▁the ▁ice ▁depos it ▁disappear . ▁ ▁The ▁upper ▁pla ins ▁unit ▁is ▁thought ▁to ▁have ▁fallen ▁from ▁the ▁sky . ▁ ▁It ▁d rap es ▁various ▁surfaces , ▁as ▁if ▁it ▁fell ▁even ly . ▁ ▁As ▁is ▁the ▁case ▁for ▁other ▁mant le ▁depos its , ▁the ▁upper ▁pla ins ▁unit ▁has ▁layers , ▁is ▁fine - gra ined , ▁and