Castillo et al. (2005) suggested that Iapetus and the other icy satellites of Saturn formed relatively quickly after the formation of the Saturnian subnebula, and thus were rich in short-lived radionuclides. These radionuclides, like aluminium-26 and iron-60, have short half-lives and would produce interior heating relatively quickly. Without the short-lived variety, Enceladus's complement of long-lived radionuclides would not have been enough to prevent rapid freezing of the interior, even with Enceladus's comparatively high rock–mass fraction, given its small size. Given Enceladus's relatively high rock–mass fraction, the proposed enhancement in 26Al and 60Fe would result in a differentiated body, with an icy mantle and a rocky core. Subsequent radioactive and tidal heating would raise the temperature of the core to 1,000 K, enough to melt the inner mantle. However, for Enceladus to still be active, part of the core must have also melted, forming magma chambers that would flex under the strain of Saturn's tides. Tidal heating, such as from the resonance with Dione or from libration, would then have sustained these hot spots in the core and would power the current geological activity.