Al DECAY: HEAT PRODUCTION AND A REVISED AGE FOR IAPETUS

Introduction: We revisit the decay energies that have been used for computing the heat produced by the decay of Al in geophysical models. Its decay scheme is complex and the values that have been used have a range of about a factor of three (Table 1). This is a major issue because Al is a primary heat source for planetary objects formed in the early solar system. Based on Schramm et al., [1] and updated with the most recent nuclear constants, we recommend a heat production value of 3.12 MeV per decay and a half-life of 0.717 My. The heat value is a factor of ~2.4 higher than used for Iapetus by Castillo-Rogez et al. [2]. The new value does not change their conclusions but does shift their time of formation of Iapetus by about 1 My, moving it (from ~2.5 5.0) to between ~3.4 and 5.4 My after the formation of the Ca-Al inclusions (CAIs). This range is fully consistent with the growing number of observed protoplanetary disks that have cleared lanes, indicating giant planet formation, in less than 8 My after their formation [3].