Envelope ⁴⁰Ar mole fraction as a function of the rock-to-envelope mass ratio for a giant planet under different assumptions regarding the primordial ⁴⁰Ar enrichment inherited from the PSN. The upper panel assumes a chondritic potassium abundance in the rocky component, whereas the lower panel assumes a potassium abundance depleted by a factor of 10. Colors indicate primordial ⁴⁰Ar enrichments of 1, 10, 50, 100, 500, and 1000 times the adopted protosolar value. The increasing contribution of radiogenic ⁴⁰Ar produced by ⁴⁰K decay causes the curves to rise with increasing rock-to-envelope mass ratio, with this effect being substantially reduced in the potassium-depleted case. Vertical dashed lines mark the present-day heavy-element-to-envelope mass ratios inferred for Jupiter, Saturn, Uranus, and Neptune, here interpreted as rock-to-envelope mass ratios under the end-member assumption that the entire heavy-element inventory is contained in rocky material. The horizontal dotted line indicates the nominal detection limit for in situ measurements.