The chocolate was named for the founder of the company, Mr. Mars. This is a unusual but not unheard of surname and it was used by Raymond Chandler as the surname of a gangster "Eddie Mars" in his series of novels like "The Big Sleep". If you check your phone book you might find someone with the same surname. I did.

The planet was named for the Roman god of war, Mars, because of it's reddish colour. It has had many other names.