This picture shows a cutaway view of the possible internal structure of Io. The images were created from a mosaic of images obtained by NASA's Galileo spacecraft. Io is the most volcanically active body in the solarsystem. The interior characteristics of the moon are inferred from gravity field and magnetic field measurements by the Galileo spacecraft. Io's radius is 1821 km, similar to the 1738 km radius of our Moon; Io has a metallic (iron, nickel) core (shown in gray) drawn to the correct relative size. The core is surrounded by a rock shell (shown in brown). Io's rock or silicate shell extends to the surface.