▁ ▁Many ▁features ▁that ▁look ▁like ▁the ▁ping os ▁on ▁the ▁Earth ▁are ▁found ▁in ▁U top ia ▁Plan it ia ▁( ~ 3 5 - 5 0 ° ▁N ; ▁~ 8 0 - 1 1 5 ° ▁E ). ▁ ▁An cient ▁rivers ▁and ▁streams ▁ ▁There ▁is ▁great ▁deal ▁of ▁evidence ▁that ▁water ▁once ▁flow ed ▁in ▁river ▁valle ys ▁on ▁Mars . ▁ ▁Pictures ▁from ▁orbit ▁show ▁w inding ▁valle ys , ▁branch ed ▁valle ys , ▁and ▁even ▁me and ers ▁with ▁ox bow ▁la kes . ▁ ▁Some ▁are ▁visible ▁in ▁the ▁pictures ▁below . ▁ ▁Stream lined ▁shapes ▁Stream lined ▁shapes ▁represent ▁more ▁evidence ▁of ▁past ▁flow ing ▁water ▁on ▁Mars . ▁ ▁Water ▁sh aped ▁features ▁into ▁stream lined ▁shapes . ▁ ▁New ▁Cr ater ▁ ▁Sand ▁d unes ▁ ▁Many ▁locations ▁on ▁Mars ▁have ▁sand ▁d unes . ▁The ▁d unes ▁are ▁covered ▁by ▁a ▁season al ▁carbon ▁dio x ide ▁fro st ▁that ▁forms ▁in ▁early ▁aut umn ▁and ▁remains ▁until ▁late ▁spring . ▁ ▁Many ▁mart ian ▁d unes ▁strongly ▁res emble ▁terrest rial ▁d unes ▁but ▁images ▁acquired ▁by ▁the ▁High - Resol ution ▁Im aging ▁Science ▁Ex periment ▁on ▁the ▁Mars ▁Re con na issance ▁Or b iter ▁have ▁shown ▁that ▁mart ian ▁d unes ▁in ▁the ▁north ▁polar ▁region ▁are ▁subject ▁to ▁modification ▁via ▁gra in flow ▁triggered ▁by ▁season al ▁CO 2 ▁sub lim ation , ▁a ▁process ▁not ▁seen ▁on ▁Earth . ▁Many ▁d unes ▁are ▁black