"The ancient history of the two planets is similar in many ways, and it is very fascinating that in the course of evolution the two worlds have taken such different paths," explains astrobiology expert Amy Williams of the University of Florida. "If there really is no life on Mars, why isn't there? What has changed? What has happened? Why wouldn't there be? And, if there was in the past, what happened then?" Dictionary Click on a word to look it up.