▁varied ▁conditions . ▁Real ▁differences ▁may ▁be ▁the ▁result ▁of ▁Mercur y ' s ▁size , ▁gravit ational ▁field , ▁proxim ity ▁to ▁the ▁Sun , ▁internal ▁composition ▁and ▁structure , ▁or ▁timing ▁of ▁major ▁vol can ic ▁episodes ▁relative ▁to ▁the ▁decrease ▁in ▁impact ▁cr aters . ▁Sur face ▁differences ▁include ▁the ▁pres ervation ▁of ▁secondary ▁cr aters ▁around ▁older ▁cr aters ▁and ▁bas ins , ▁and ▁the ▁absence ▁of ▁recogn izable ▁text ured ▁and ▁line ated ▁e ject a ▁blank ets ▁such ▁as ▁those ▁surrounding ▁the ▁Im b rium ▁and ▁Orient ale ▁Bas ins ▁on ▁the ▁Moon . ▁Possible ▁differences ▁in ▁vol can ic ▁features ▁include ▁the ▁absence ▁of ▁w ides p read ▁dark ▁mare - type ▁depos its , ▁vol can ic ▁dom es ▁and ▁con es , ▁and ▁sin uous ▁r illes . ▁Where as ▁pla ins ▁and ▁terra ▁divisions ▁may ▁be ▁distinguished ▁on ▁Mercur y , ▁the ▁distinct ▁lun ar ▁dich ot omy ▁of ▁mare ▁and ▁high lands ▁is ▁not ▁present ▁on ▁that ▁half ▁of ▁Mercur y ▁observed ▁by ▁Mar iner ▁ 1 0 . ▁ ▁S ources ▁▁▁ ▁P rep ared ▁for ▁the ▁National ▁Aer onaut ics ▁and ▁Space ▁Administration ▁by ▁the ▁U . S . ▁Department ▁of ▁the ▁Inter ior , ▁U . S . ▁Ge ological ▁Survey . ▁Pub lished ▁in ▁hard copy ▁as ▁US GS ▁Mis cell aneous ▁Investig ations ▁Series ▁ ▁Map ▁I – 1 2 3 3 , ▁as ▁part ▁of ▁the ▁Atlas ▁of ▁Mercur y , ▁ 1 : 5 ,