Prof. Jim Bell (of Arizona State University), who is a key leader in projects to take images with NASA's rovers on Mars, discusses the history and current state of our exploration of the red planet.  He summarizes the scientific findings from the Spirit, Opportunity, Curiosity, and Perseverance missions. He puts each mission into the larger context of the questions we are asking -- both about Mars today and about ancient Mars, which could have been far more hospitable for life.