Talk:Phobos/@comment-14475-20160225034554/@comment-2001:980:3B71:1:6DAC:D58C:BB5B:5AB5-20181117125804

Mars's is never correct, when ending with an S it gets the -es suffix... However, the exception to this (according to the Oxford style guide) is for given names in which case it gets an apostrophe only, so the correct form is Mars'.