Atmospheric noble gas isotope and bulk K/U ratios as a constraint on the early evolution of Venus and Earth

While Venus and Earth were accumulating mass within the solar nebular these protoplanets also captured significant hydrogen dominated atmospheres by picking up gas from the circumstellar disk during the formation of the Solar System. These primordial atmospheres were then quickly lost by hydrodynamic escape of H-atoms, dragging heavier elements with it at different rates, leading to changes in their isotopic and elemental ratios which is reflected in the present-day atmospheric noble gas isotope and elemental ratios of Venus and Earth.Our simulations show that these ratios can be best explained if the Sun was born between a weakly and moderately active star and if Venus and Earth had grown to 85-100% and 53-58%, respectively, of their current masses by the time the nebula gas dissipated approximately 3.5 Myr after formation of the Sun.