IT WAS A QUARTER of an Earth century before Mars was again visited by humans.  Six years after the Envoy was silent, the drone probe Zombie, sponsored jointly by the Geographic Society and La Société Astronautique Internationale, bridged the void and took up an orbit for the waiting period, then returned.  The photographs taken by the robot vehicle showed a land unattractive by human standards; her recording instruments confirmed the thinness and unsuitability of the Arean atmosphere to human life.