Edmond Hamilton 
Captain Future #26: Moon of the Unforgotten [EPUB ebook] 

Curt Newton and Otho plumb the perilous secrets of the Jovian Moon Europa—where Ezra Gurney, friend of the Futuremen, has fallen prey to a mystic cult!
The Captain Future saga follows the super-science pulp hero Curt Newton, along with his companions, The Futuremen: Grag the giant robot, Otho the android, and Simon Wright the living brain in a box. Together, they travel the solar system in series of classic pulp adventures, many of which written by the author of The Legion of Super-Heroes, Edmond Hamilton.