Active Volcanism on Io: Global Distribution and Variations in Activity - Lopes-Gautier, R.; McEwen, A.; Smythe, W.B.; Geissler, P.E.; Kamp, L.; Davies, A.; Spencer, J.R.; Keszthelyi, L.; Carlson, R.; Leader, F.E.; Mehlman, R.; Soderblom, L. (Icarus, 1999)