Elon Musk is known to be active on twitter with out of this world to know what the followers think and make them thinking more. I really dont think Mars is flat after soo many explorations and space missions.  There are two rovers by mars already doing their work known as Curiosity and Insight by NASA. Other countries like China and India are too preparing for missions at Mars. Mars is certainly known to have life supporting atmosphere and is known to be Earth like but absolutely it is not flat.