▁becomes ▁dens er . ▁The ▁dust ▁and ▁sand ▁particles ▁caught ▁in ▁the ▁fro st ▁slowly ▁sink . ▁By ▁the ▁time ▁temper atures ▁rise ▁in ▁the ▁spring , ▁the ▁fro st ▁layer ▁has ▁become ▁a ▁sl ab ▁of ▁semi - trans parent ▁ice ▁about ▁ 3 ▁feet ▁thick , ▁lying ▁on ▁a ▁substr ate ▁of ▁dark ▁sand ▁and ▁dust . ▁ ▁This ▁dark ▁material ▁absor bs ▁light ▁and ▁causes ▁the ▁ice ▁to ▁sub lim ate ▁( turn ▁directly ▁into ▁a ▁gas ). ▁ ▁Event ually ▁much ▁gas ▁accum ul ates ▁and ▁becomes ▁press ur ized . ▁ ▁When ▁it ▁finds ▁a ▁weak ▁spot , ▁the ▁gas ▁esc apes ▁and ▁blow s ▁out ▁the ▁dust . ▁ ▁Spe eds ▁can ▁reach ▁ 1 0 0 ▁miles ▁per ▁hour . ▁ ▁Calcul ations ▁show ▁that ▁the ▁pl umes ▁are ▁ 2 0 – 8 0 ▁meters ▁high . ▁Dark ▁channels ▁can ▁sometimes ▁be ▁seen ; ▁they ▁are ▁called ▁" sp iders ." ▁ ▁The ▁surface ▁appears ▁covered ▁with ▁dark ▁sp ots ▁when ▁this ▁process ▁is ▁occurr ing . ▁ ▁Many ▁ideas ▁have ▁been ▁advanced ▁to ▁explain ▁these ▁features . ▁ ▁These ▁features ▁can ▁be ▁seen ▁in ▁some ▁of ▁the ▁pictures ▁below . ▁ ▁Upper ▁Pla ins ▁Unit ▁Rem n ants ▁of ▁a ▁ 5 0 - 1 0 0 ▁meter ▁thick ▁mant ling , ▁called ▁the ▁upper ▁pla ins ▁unit , ▁has ▁been ▁discovered ▁in ▁the ▁mid - lat itudes ▁of ▁Mars . ▁ ▁First ▁investig ated ▁in ▁the ▁De uter on ilus ▁Mens ae ▁(