The Valles Marineris hemisphere of Mars (left) and a mosaic of the Schiaparelli hemisphere of Mars (right).
If you are interested in getting a quick overview of our Mars explorer history, this article from ESA will do the trick. Most of the knowledge that we have of Mars' interior (read more at Geojim56's) comes from planetary geodesy and geodesy A combination of gravity and rotation data tells us a lot about the interior of a celestial body. And I wouldn't be me if I didn't add that the better geodetic systems and knowledge of the Earth we have, the more we will be able to learn about the planets! :-)