▁because ▁they ▁are ▁derived ▁from ▁the ▁dark ▁vol can ic ▁rock ▁bas alt . ▁Ext r ater rest rial ▁sand ▁se as ▁such ▁as ▁those ▁found ▁on ▁Mars ▁are ▁referred ▁to ▁as ▁" und ae " ▁from ▁the ▁Latin ▁for ▁waves . ▁ ▁Land ing ▁site ▁ ▁Some ▁of ▁the ▁targets ▁suggested ▁became ▁possible ▁sites ▁for ▁a ▁R over ▁Mission ▁in ▁ 2 0 2 0 . ▁ ▁The ▁targets ▁were ▁in ▁Fir so ff ▁( cr ater ) ▁and ▁Hold en ▁Cr ater . ▁ ▁These ▁locations ▁were ▁picked ▁as ▁two ▁of ▁ 2 6 ▁locations ▁considered ▁for ▁a ▁mission ▁that ▁will ▁look ▁for ▁signs ▁of ▁life ▁and ▁gather ▁samples ▁for ▁a ▁later ▁return ▁to ▁Earth . ▁ ▁Land s cape ▁features ▁ ▁Dark ▁slope ▁stre aks ▁ ▁Rec urrent ▁slope ▁line ae ▁ ▁Rec urrent ▁slope ▁line ae ▁are ▁small ▁dark ▁stre aks ▁on ▁s lop es ▁that ▁el ong ate ▁in ▁warm ▁seasons . ▁ ▁They ▁may ▁be ▁evidence ▁of ▁liquid ▁water . ▁ ▁L ay ers ▁Many ▁places ▁on ▁Mars ▁show ▁rocks ▁arranged ▁in ▁layers . ▁Rock ▁can ▁form ▁layers ▁in ▁a ▁variety ▁of ▁ways . ▁ ▁Vol cano es , ▁wind , ▁or ▁water ▁can ▁produce ▁layers . ▁ ▁L ay ers ▁can ▁be ▁hard ened ▁by ▁the ▁action ▁of ▁ground water . ▁ ▁, ▁ ▁This ▁group ▁of ▁layers ▁that ▁are ▁found ▁in ▁a ▁cr ater ▁all ▁come ▁from ▁the ▁Arab ia ▁quad rangle . ▁ ▁This ▁next ▁group ▁of ▁layer ed ▁terrain ▁comes ▁from ▁the ▁Lou