Maiden Voyage is written in the key of D Dorian.
According to the Theorytab database, it is the most popular key among Dorian keys and the 29th most popular among all keys. The D Dorian scale is similar to the D Minor scale except that its 6th note is a half step higher (B). Chord progressions in Dorian have a characteristic sound due to the major quality of the chord built on the 4th scale degree.
See the D Dorian Cheat Sheet for popular chords, chord progressions, downloadable midi files and more!