Heating of planetesimals by short-lived radioisotopes 60Fe and 26Al and the effect on the water content of protoplanets

Joseph William Eatson

Whilst the formation of solar-system planets is well understood, the geological history of exoplanets is much less clear. Rocky exoplanets can vary significantly based on the composition and properties of the planetesimals they form from. An important factor influenced by planetesimal composition is water content, where the desiccation of planetesimals impacts the final water content of the resultant planets. While the inner planets of the solar system are comparatively water-poor, planetary formation models suggest that rocky exoplanets engulfed by ice or water could potentially be widespread. Here we attempt to understand how common short-lived radioisotopes (SLRs) affect the water content of planetesimals, and how this would affect the formation of ocean planets. In this work, we simulate small planetesimal bodies using the I2ELVIS marker-in-cell hydrodynamical code, tracking the temperature and the evaporation of water ice trapped in the planetesimal mantle. This model utilises two radiogenic heating sources, 26Al stored in hydrous silicates and 60Fe in either a discrete iron core or grains randomly distributed throughout the planetesimal. We find that 26Al enrichment is the dominant cause of planetesimal heating and therefore an important parameter in constraining the quantity of water after planetary formation. 60Fe can become the primary heating source in the case of extreme levels of enrichment or very iron-rich planetesimals, but this requires extreme formation conditions.