The formal name `` Nix '' , from the Greek goddess of darkness and night , and mother of Charon , was announced on June 21 , 2006 on IAU Circular 8723 , where the designation Pluto II is also given . The formal name `` Nix '' , from the Greek goddess of darkness and night , and mother of Charon , was announced on June 21 , 2006 , on IAU Circular 8723 , where the designation Pluto II is also given .