The composition of Jupiter: sign of a (relatively) late formation in a chemically evolved protosolar disc

It has been proposed that the enrichment in noble gases found by Galileo in the atmosphere of Jupiter can be explained by their delivery inside cold planetesimals. We propose instead that this is a sign that the planet formed in a chemically evolved disc and that noble gases were acquired mostly in gaseous form during the envelope capture phase of the planet. We show that the combined settling of grains to the disc mid-plane in the cold outer layers, the condensation of noble gases on to these grains at temperatures below 20–30 K, and the evaporation from high disc altitudes effectively lead to a progressive, moderate enrichment of the disc. The fact that noble gases are vaporized from the grains in the hot inner disc regions (e.g. Jupiter formation region) is not a concern because a negative temperature gradient prevents convection from carrying the species into the evaporating region. We show that the ∼2 times solar enrichment of Ar, Kr and Xe in Jupiter is hence naturally explained by a continuous growth of the planet governed by viscous diffusion in the protosolar disc in conjunction with an evaporation of the disc and its progressive enrichment on a million-year time-scale.