Probing the Rock Mass Fraction and Transport Efficiency inside Uranus Using 40Ar Measurements

Abstract The bulk of Uranus consists of a rock–ice core, but the relative proportions of rock and ice are unknown. Radioactive decay of potassium in the silicates produces 40 Ar. If transport of argon from the core to the gaseous envelope is efficient, a measurement of 40 Ar in the envelope will provide a direct constraint on the rock mass present (assuming a chondritic rock composition). The expected 40 Ar concentrations in this case would be readily detectable by a mass spectrometer carried by a future atmospheric probe. For a given envelope concentration there is a trade-off between the rock mass present and the transport efficiency; this degeneracy could be overcome by making independent determinations of the rock mass (e.g., by gravity and seismology). Primordial 40 Ar is a potential confounding factor, especially if Ar/H 2 is significantly enhanced above solar or if degassing of radiogenic 40 Ar were inefficient. Unfortunately, the primordial 40 Ar/ 36 Ar ratio is very uncertain; better constraints on this ratio through measurement or theory would be very helpful. Pollution of the envelope by silicates is another confounding factor but can be overcome by a measurement of the alkali metals in the envelope.