26Al in Iapetus - Consequences for the Formation of the Saturnian System

We revisited Iapetus' dynamical evolution models inorder to understand under which conditions the observed non-hydrostatic shape of this satellite (corresponding to a past 17-h geoid) became frozen and was preserved until the present. This study highlights the need of extra sources of energy not considered in previous thermal evolution models for the Saturnian satellites. Thermal evolution models driven by the decay of long-lived radiogenic species do not produce a temperature high enough for a body such as Iapetus, composed only of 20% silicate by mass and 3.561 10e6 km distant from its primary, to despin over the age of the Solar System. Different approaches were considered to increase the internal temperature: alteration of the surface properties to limit heat loss (greenhousing, low emissivity, insulating layer), impact heating, and enrichment in long-lived radiogenic species. None of these approaches can account for the despinning, except enrichment in long-lived radiogenic species. However that option cannot produce models that preserve the 17-h geoid. We demonstrate that short-lived radiogenic species, especially 26Al, as found in the Calcium-Aluminum-Inclusions (CAIs) provide conditions suitable for models that match the observational constraints. The relatively short half-life of 26Al allows the constraint of Iapetus' formation time with high accuracy: 1.0+/-0.2 to 1.6+/-0.4 My after the production of CAIs. This new result provides a strong constraint on the formation of Saturn. Based on current values for the absolute formation time of the CAIs, Saturn's age is estimate as 4.566 5 By. We will discuss implications for geological observations of Iapetus as well as implications for the internal evolution and geology of other Saturnian satellites. Part of this work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract to NASA.