A Wizard of Mars — Diane Duane

Young wizards Kit and Nita are faced with strange events when a life form from another era emerges on Mars. Though the Martians seem friendly, they have a plan that could change the shape of more than one world. As the shadow of interplanetary war stretches over both worlds, Kit and Nita must fight to master the strange and ancient synergy binding them to Mars and its last inhabitants. If they don't succeed, the history that left Mars lifeless will repeat itself on Earth.