▁only ▁direction ▁the ▁tra pped ▁water ▁can ▁flow ▁is ▁hor izont ally . ▁The ▁water ▁could ▁then ▁flow ▁out ▁onto ▁the ▁surface ▁when ▁it ▁reaches ▁a ▁break , ▁like ▁a ▁cr ater ▁wall . ▁Aqu if ers ▁are ▁quite ▁common ▁on ▁Earth . ▁A ▁good ▁example ▁is ▁" We eping ▁Rock " ▁in ▁Z ion ▁National ▁Park ▁Ut ah . ▁▁▁ ▁On ▁the ▁other ▁hand , ▁much ▁of ▁the ▁surface ▁of ▁Mars ▁is ▁covered ▁by ▁a ▁thick ▁smooth ▁mant le ▁that ▁is ▁thought ▁to ▁be ▁a ▁mixture ▁of ▁ice ▁and ▁dust . ▁This ▁ice - rich ▁mant le , ▁a ▁few ▁yards ▁thick , ▁smooth s ▁the ▁land , ▁but ▁in ▁places ▁it ▁has ▁a ▁b um py ▁texture , ▁res emb ling ▁the ▁surface ▁of ▁a ▁basketball . ▁Under ▁certain ▁conditions ▁the ▁ice ▁could ▁m elt ▁and ▁flow ▁down ▁the ▁s lop es ▁to ▁create ▁g ul lies . ▁Because ▁there ▁are ▁few ▁cr aters ▁on ▁this ▁mant le , ▁the ▁mant le ▁is ▁relatively ▁young . ▁An ▁excellent ▁view ▁of ▁this ▁mant le ▁is ▁shown ▁below ▁in ▁the ▁picture ▁of ▁the ▁P to le ma eus ▁Cr ater ▁R im , ▁as ▁seen ▁by ▁Hi RI SE . ▁ ▁Ch anges ▁in ▁Mars ' s ▁orbit ▁and ▁t ilt ▁cause ▁significant ▁changes ▁in ▁the ▁distribution ▁of ▁water ▁ice ▁from ▁polar ▁regions ▁down ▁to ▁lat itudes ▁equivalent ▁to ▁Texas . ▁During ▁certain ▁climate ▁periods ▁water ▁v apor ▁leaves ▁polar ▁ice ▁and ▁enters ▁the ▁atmosphere . ▁The ▁water ▁comes ▁back ▁to ▁ground ▁at