▁patch es ▁is ▁apparently ▁super posed ▁on ▁the ▁bright ▁ray s ▁of ▁a ▁c 5 ▁cr ater ▁and ▁is ▁therefore ▁very ▁young . ▁ ▁C rat ers ▁and ▁bas ins ▁▁ ▁C rat ers ▁are ▁u bi quit ous ▁features ▁of ▁the ▁mer cur ian ▁surface . ▁For ▁the ▁purpose ▁of ▁mapping , ▁a ▁five fold ▁morph olog ic ▁classification ▁of ▁cr aters ▁( fig . ▁ 1 0 ▁in ▁McC au ley ▁et ▁al ., ▁ 1 9 8 1 ) ▁is ▁the ▁basis ▁for ▁determ ining ▁their ▁relative ▁ages . ▁The ▁young est ▁cr aters ▁( c 5 ) ▁have ▁sharp ▁rim ▁cr ests , ▁text ured ▁e ject a ▁blank ets , ▁and ▁a ▁well - defined ▁field ▁of ▁secondary ▁cr aters . ▁Under ▁fav ou rable ▁light ing ▁conditions , ▁the ▁young est ▁cr aters ▁exhib it ▁bright ▁ray s ▁super posed ▁on ▁all ▁older ▁materials . ▁Old er ▁cr aters ▁have ▁increasing ly ▁de grad ed ▁r ims ▁and ▁lower ▁relief ▁and ▁have ▁lost ▁their ▁secondary ▁cr ater ▁fields . ▁The ▁major ▁differences ▁between ▁mer cur ian ▁and ▁lun ar ▁cr aters ▁are ▁apparently ▁related ▁to ▁the ▁greater ▁gravit ational ▁acceleration ▁and ▁the ▁higher ▁impact ▁veloc ities ▁on ▁Mercur y . ▁Cont inu ous ▁e ject a ▁depos its ▁are ▁less ▁extensive , ▁and ▁secondary ▁cr aters ▁are ▁more ▁sharp ly ▁defined ▁and ▁cluster ed ▁near er ▁their ▁primary ▁cr ater . ▁Also ▁on ▁Mercur y , ▁acc ent u ated ▁secondary ▁cr aters ▁form ▁prominent ▁cr ater