▁and ▁more ▁nearly ▁res emble ▁those ▁of ▁the ▁lun ar ▁mar ia . ▁Although ▁cr ater ▁counts ▁are ▁more ▁reliable ▁because ▁there ▁are ▁fewer ▁second aries ▁than ▁in ▁the ▁cr ater ed ▁pla ins , ▁resolution ▁is ▁a ▁serious ▁constraint ▁to ▁developing ▁cr ater ▁counts ▁on ▁the ▁relatively ▁small ▁tract s ▁of ▁smooth ▁pla ins . ▁Pre lim inary ▁counts ▁made ▁on ▁a ▁few ▁of ▁the ▁more ▁extensive ▁occurr ences ▁of ▁smooth ▁pla ins ▁show ▁a ▁cum ulative ▁cr ater ▁frequency ▁of ▁about ▁ 7 . 5 ▁× ▁ 1 0 2 / 1 0 6 ▁km 2 ▁for ▁cr aters ▁ ▁larger ▁than ▁approximately ▁ 2 . 5   km . ▁This ▁frequency ▁is ▁compar able ▁to ▁that ▁of ▁the ▁lun ar ▁mar ia ▁near ▁the ▁Apol lo ▁ 1 1 ▁landing ▁site ▁( G ree ley ▁and ▁G ault , ▁ 1 9 7 0 ; ▁Ne uk ume ▁et ▁al ., ▁ 1 9 7 5 ; ▁Meyer ▁and ▁Gro lier , ▁ 1 9 7 7 ). ▁Like ▁that ▁of ▁the ▁cr ater ed ▁pla ins , ▁the ▁al bed o ▁of ▁the ▁smooth ▁pla ins ▁is ▁intermediate ▁compared ▁to ▁other ▁units ▁on ▁Mercur y ▁but ▁is ▁high ▁compared ▁to ▁that ▁of ▁the ▁mare ▁bas alt ▁on ▁the ▁Moon . ▁ ▁A ▁few ▁patch es ▁of ▁very ▁dark ▁material ▁occur ▁in ▁the ▁western ▁part ▁of ▁the ▁quad rangle ▁where ▁the ▁sun ▁angle ▁is ▁high ▁and ▁al bed o ▁contrast s ▁are ▁en hanced . ▁The ▁largest ▁of ▁these ▁dark