Melting of Io by Tidal Dissipation
S.J. Peale, P. Cassen, R.T. Reynolds.
Science, 1979, vol. 203, p.892 - 894.

"These calculations suggest that Io might currently be the most intensely heated terrestrial-type body in the solar system"

Tidally Forced Viscous Heating in a Partially Molten Io
M. N. Ross & G. Schubert
Icarus, 1985, 64, p. 391 - 400

Partially molten asthenosphere model:

Partially molten interior model