Io is composed primarily of ferrous rock.
 The minor constituents of Io's atmosphere, such as NaCl, SO, O, and S derive either from: direct volcanic outgassing; photodissociation, or chemical breakdown caused by solar ultraviolet radiation, from SO2; or the sputtering of surface deposits by charged particles from Jupiter's magnetosphere
 Models based on the Voyager and Galileo measurements of Io's mass, radius, and quadrupole gravitational coefficients (numerical values related to how mass is distributed within an object) suggest that its interior is differentiated between a silicate-rich crust and mantle and an iron- or iron-sulfide-rich core

