Castle in the stars. Book three, The knights of Mars

Summary/Review: "When Seraphin, Hans, and Sophie returned from their voyage to the moon, they didn't come home empty-handed--they brought with them aetherite, a miraculous substance that defies gravity! To keep their secret safe from the Prussians, the trio lives in hiding on a remote island in Brittany. Meanwhile, Seraphin's father has founded the International Society of Aether, a group of scientists dedicated to the peaceful pursuit of space exploration. But on the eve of their inaugural meeting, he vanishes without a trace! The Knights of Aether's last hope lies in a daring rescue of King Ludwig and Professor Dulac. They will find them on the farthest outskirts of the Prussian empire--the planet Mars!"--Provided by publisher.