Fourth World (The Iamos Trilogy, Book 1)

His past is her future.

Life on Mars isn't all it's cracked up to be when you're Isaak Contreras. Ever since his dad disappeared two years ago, Isaak's been struggling to keep up in school, and he never seems to be able to live up to his mom's high expectations. But everything changes when he finds an ancient coin among his missing father's possessions. The coin makes him a target of both the Martian colonial government and a crazed scientist with a vendetta--and it leads him to a girl from another time named Nadin, who believes that Isaak might just hold the key to saving both their worlds. That is, if they can survive long enough to use it...