▁their ▁present ▁positions . ▁Jup iter ▁thus ▁would ▁have ▁consum ed ▁much ▁of ▁the ▁material ▁that ▁would ▁have ▁created ▁a ▁bigger ▁Mars . ▁The ▁same ▁simulations ▁also ▁reproduce ▁the ▁characteristics ▁of ▁the ▁modern ▁astero id ▁bel t , ▁with ▁dry ▁astero ids ▁and ▁water - rich ▁objects ▁similar ▁to ▁com ets . ▁ ▁However , ▁it ▁is ▁unclear ▁whether ▁conditions ▁in ▁the ▁solar ▁ne b ula ▁would ▁have ▁allowed ▁Jup iter ▁and ▁Sat urn ▁to ▁move ▁back ▁to ▁their ▁current ▁positions , ▁and ▁according ▁to ▁current ▁estimates ▁this ▁possibility ▁appears ▁unlikely . ▁Moreover , ▁alternative ▁explan ations ▁for ▁the ▁small ▁mass ▁of ▁Mars ▁exist . ▁ ▁Late ▁He avy ▁Bomb ard ment ▁and ▁after ▁ ▁Gra v it ational ▁dis ruption ▁from ▁the ▁outer ▁plan ets ' ▁migration ▁would ▁have ▁sent ▁large ▁numbers ▁of ▁astero ids ▁into ▁the ▁inner ▁Sol ar ▁System , ▁sever ely ▁de ple ting ▁the ▁original ▁bel t ▁until ▁it ▁reached ▁today ' s ▁extremely ▁low ▁mass . ▁This ▁event ▁may ▁have ▁triggered ▁the ▁Late ▁He avy ▁Bomb ard ment ▁that ▁occurred ▁approximately ▁ 4   b ill ion ▁years ▁ago , ▁ 5 0 0 – 6 0 0   mill ion ▁years ▁after ▁the ▁formation ▁of ▁the ▁Sol ar ▁System . ▁This ▁period ▁of ▁heavy ▁bomb ard ment ▁last ed ▁several ▁hundred ▁million ▁years ▁and ▁is ▁evident ▁in ▁the ▁cr ater ing ▁still ▁visible ▁on ▁ge olog ically ▁dead ▁bodies ▁of ▁the ▁inner ▁Sol ar ▁System ▁such ▁as ▁the ▁Moon ▁and ▁Mercur y . ▁The ▁oldest ▁known