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.

Keywords

• Planetary interior